High Voltage Direct Current technology has certain characteristics which make it especially attractive for transmission system applications. HVDC transmission system is useful for long-distance transmission, bulk power delivery and long submarine cable crossings and asynchronous interconnections. The study of faults is essential for reasonable protection design because the faults will induce a significant influence on operation of HVDC transmission system. This paper provides the most dominant and frequent faults on the HVDC systems such as DC Line-to- Ground fault and Line-to-Line fault on DC link and some common types of AC faults occurs in overhead transmission system such as Line-to-Ground fault, Line-to-Line fault and L-L-L fault. In HVDC system, faults on rectifier side or inverter side have major affects on system stability. The various types of faults are considered in the HVDC system which causes due to malfunctions of valves and controllers, misfire and short circuit across the inverter station, flashover and three phase short circuit. The various faults occurs at the converter station of a HVDC system and Controlling action for those faults. Most of the studies have been conducted on line faults. But faults on rectifier or inverter side of a HVDC system have great impact on system stability. Faults considered are fire-through, misfire, and short circuit across the inverter station, flashover, and a three-phase short circuit in the ac system. These investigations are studied using matlab simulink models and the result represented in the form of typical time responses.

1. 1
A
Seminar Report
On
FAULT ANALYSIS IN HVDC & HVAC TRANSMISSION
LINE
SEMINAR
PRESENTED
BY
MR. SHUBHAM MAROTI KALASKAR
(REG. NO. 20140218)
B-TECH IN ELECTRICAL ENGINEERING
ACADEMIC YEAR 2016-2017
Under the Guidance of
PROF. ANISH SALVI
ELECTRICAL ENGINEERING DEPARTMENT
DR. BABASAHEB AMBEDKAR TECHNOLOGICAL UNIVERSITY
LONERE, RAIGAD, MAHARASHTRA-402103
FOR THE ACADEMIC YEAR 2016-17

2. 2
ACKNOWLEDGEMENT
In this society nothing can be accomplished alone. As we began to reflect on
magnitude of this seminar-report, we are overwhelmed by the guidance and support
extended by our teacher, friends and others. There is difficulty in assigning the
hierarchy since it has been true effort from beginning.
We would like to express our sincere gratitude and like to mention that this work
would not have been possible without time to time guidance provided by our
facilitator PROF.ANISH SALVI. We have been greatly benefited by his valuable
suggestions, constant encouragement and patience throughout his work. He has given
his full effort in guiding the team in achieving the goal as well as his encouragement
to maintain our progress in track. From the very onset he has taken a keen interest in
the development of our seminar and we are very grateful for his time, efforts and
timely suggestions.
We are thankful to all teaching and non-teaching staff of department of electrical
engineering of Dr. Babasaheb Ambedkar Technological University, Lonere for
providing us with the necessary documents required for reference from time to time.
We would finally like to thank our university for providing the environment and
infrastructure required for the completion of this seminar.
Mr. ShubhamMarotiKalaskar
(Reg. no.20140218)
Date:
Place: Dr. B.A.T.U. Vidyavihar, Lonere-402103

3. 3
INDEX
Sr.No. Content Page No.
Acknowledgement
List of figure I
List of table II
Abbreviations III
Abstract IV
Chapter 1
1.1 Introduction 1
1.2 Compression the HVDC and the HVAC systems 3
1.3 Advantages and inherent problems associated with
HVDC
4
1.4 Economic comparison 7
1.5 Components of an HVDC transmission system 8
1.6 Types of HVDC systems or configuration 11
1.7 Comparison of HVDC link and HVAC link 15
Chapter 2
2.1 Faults in HVDC system 16
2.1.1 DC line faults 16
2.1.2 DC line to ground fault 17
2.1.3 Short circuits in a converter station 20
Chapter 3
3.1 Fault current effect in power system 21
Chapter 4
4.1 AC fault analysis 28
Chapter 5
5.1 HVDC grid protection objectives and requirements 31

4. 4
5.2 Fault clearing strategies for HVDC grids 32
5.3 Methods of dc line protection 33
Conclusion 38

5. 5
LIST OF FIGURE
SR. NO. TITLE PAGE NO.
1 Basic block diagram of HVDC system 1
2 Mono polar HVDC system 11
3 Bi polar HVDC System 12
4 Homo Polar HVDC System 13
5 An equivalent circuit of DC Line-to-Line fault 17
6 An equivalent circuit of DC Line to Ground Fault 18
7 Fault current (HVAC system). 23
8 Fault current (HVDC system). 23
9 Sending end bus (HVAC system). 23
10 Sending end bus (HVDC system). 24
11 Receiving end bus (HVAC system). 24
12 Receiving end bus (HVAC system). 25
13 Generator bus (HVAC system). 25
14 Generator bus (HVDC system) 25
15 Fault Current (HVDC system) 26
16 Fault Current (HVAC system) 26
17 Fault Current (HVDC system) 26
18 Simulation results for 2-level VSC-HVDC
System (a) fault current during, (b) Fault current
during L-L, (c) fault current during LLL
29
19 Diagram indicating AC fault location 29

6. 6
LIST OF TABLE
SR. NO. TITLE PAGE NO.
1 Table -01: Fault Current
Comparison
27
2 Table-02: Fault current magnitude 29

7. 7
ABBREVIATIONS
AC Alternating current
DC
Direct current
HVDC High Voltage Direct current
GTOs gate turn-off thyristors
(CSCs). Current-Source Converters
(VSCs). Voltage-Source Converters
(IGBTs) Insulated gate bipolar transistor
AC/DC-conversion Convertor
L-G Single line to ground fault
L-L Line- line fault
LLL Three phase to ground fault

8. 8
ABSTRACT
High Voltage Direct Current technology has certain characteristics which
make it especially attractive for transmission system applications. HVDC
transmission system is useful for long-distance transmission, bulk power delivery and
long submarine cable crossings and asynchronous interconnections. The study of
faults is essential for reasonable protection design because the faults will induce a
significant influence on operation of HVDC transmission system. This paper provides
the most dominant and frequent faults on the HVDC systems such as DC Line-to-
Ground fault and Line-to-Line fault on DC link and some common types of AC faults
occurs in overhead transmission system such as Line-to-Ground fault, Line-to-Line
fault and L-L-L fault. In HVDC system, faults on rectifier side or inverter side have
major affects on system stability. The various types of faults are considered in the
HVDC system which causes due to malfunctions of valves and controllers, misfire
and short circuit across the inverter station, flashover and three phase short circuit.
The various faults occurs at the converter station of a HVDC system and
Controlling action for those faults. Most of the studies have been conducted on line
faults. But faults on rectifier or inverter side of a HVDC system have great impact on
system stability. Faults considered are fire-through, misfire, and short circuit across
the inverter station, flashover, and a three-phase short circuit in the ac system. These
investigations are studied using matlab simulink models and the result represented in
the form of typical time responses.

9. 9
Chapter 1
BASIC OF HVDC TRANSMISSION
1.1 INTRODUCTION
 What is HVDC?
The electric power is produced, transmitted and distributed as an AC power. From the
generating stations, power is transmitted to the end user via transmission and
distribution lines. Transmission lines are long and operate at high or extra high
voltages.
But, the amount of AC power transmission through the line is restricted by its
inductance. To overcome this, a High Voltage DC (HVDC) transmission system is
generally employed for high power transmission.
Fig 1: basic block diagram of HVDC system
At the end of the DC power system, DC power is inverted to the AC power and
synchronizes with succeeding AC network. So the entire HVDC consists of three
sections, namely converter station, transmission portion and an inverter station.
The sending end or converter station consists of 6, 12, or 24-pulse thyristor bridge
rectifier while the receiving end or inverter station consists of a similarly configured
thyristor bridge but which operates in inverter mode.
 Why used HVDC?
The major problem in the AC system is that
1 We do not have the power control.
We do not have the power control facility power control. So, the power control is not
possible here. So, this is the one of the big problem in the AC systems in a while your

10. 10
AC system if this is your control is not possible because, nowadays we are also
thinking to be provide the AC control if the AC control is possible then we can
operate our power system in a much better and in the efficient manner.[7]
1 stability problem
Stability is the one concerned if you are going more and more power transfer.[7]
2 reactive power loss
The reactive power loss is basically total of the that reactive power consumed in
the element and some of the elements they generate the reactive power. So, the
total sum of this is treated as the reactive power loss. this concept is only
happening in HVAC because this x components that is here is only is occurring in
AC and the DC only the resistance is there.
So, this reactive power loss has no impact because this does not exist at all in the
HVDC transmission system. So, this is does not this Q loss does not arise and
thereby why what happens if your line which is carrying the current of both active
and reactive components. So, here you can see if we can there is no reactive
component though line can carry both active power and then we can improve the
performance of the systems. [7]
3 skin & Ferranti effect
The Ferranti effect is nothing, but your receiving end voltage sometimes become
higher than your sending voltage. the Ferranti effect Ferranti effect also it will be
discussed Specially due to the charging or the capacitance between the line to
ground and the line to line [7]
Another here that is your skin effect skin effect is nothing, but if it is a AC system
here the current this is your complete conductor area current will try to go from
outer side due to the again So, here the current is non-uniform because the inn in
at centre current will be less outside it will be more. So, what happen the density
of the current in this over this area of the conductor is the different and that is why
the resistance effective resistance will be more and there by the loss will be more.
So, this skin effect only arise in your AC cable AC lines because AC has a some
frequency f and that is why it is having if f is 0 it is a DC and finally, it will be the
current will be the uniform and therefore, that is why always we say r AC is
always greater than your r DC means thus resistance if you‟d measure the AC
resistance will be more than your the DC resistance due to your skin and the
plasmid effect if another conductor there again there is flux linkage[7]
So, these 4 are the major reasons for people are start to thinking to go for the DC
transmission system these

11. 11
1.2 COMPRESSION THE HVAC AND THE HVDC SYSTEMS
Alternating current (AC) became very familiar for the industrial and domestic uses,
but still for the long transmission lines, AC has some limitations which has led to the
use of DC transmission in some projects. The technical detail of HVDC transmission
compare to high voltage AC (HVAC) transmission is discussed to verify HVDC
transmission for long distances. Current and voltage limits are the two important
factors of the high voltage transmission line. The AC resistance of a conductor is
higher than its DC resistance because of skin effect, and eventually loss is higher for
AC transmission
The switching surges are the serious transient over voltages for the high voltage
transmission line, in the case of AC transmission the peak values are two or three
times normal crest voltage but for DC transmission it is 1.7 times normal voltage.
HVDC transmission has less corona and radio interference than that of HVAC
transmission line [2]. The total power loss due to corona is less than 5 MW for a ±
450 kV and 895 kilometers HVDC transmission line. The long HVAC overhead lines
produce and consume the reactive power, which is a serious problem. If the
transmission line has a series inductance L and shunt capacitance C per unit of length
and operating voltage V and current I, the reactive power produced by the line is :
Qc = wCV2
and consumers reactive power
QL = wLI2
per unit length. If QC = QL
V/I = (L/C)1/2
=ZS
where Zs is surge impedance of the line. The power in the line is
PN=V=V2
/ZS
and is called natural load. So the power carried by the line depends on the operating
voltage and the surge impedance of the line. Table I shows the typical values of a
three phase overhead lines
Tab.1 Voltage rating and power capacity
Voltage
(kV)
132 230 345 500 700
Natural
load (MW)
43 130 300 830 1600

12. 12
The power flow in an AC system and the power transfer in a transmission line can be
expressed
P=(E1E2/X)sin(δ)
E1 and E2 are the two terminal voltages, δ is the phase difference of these voltages,
and X is the series reactance. Maximum power transfer occurs at δ= 90º and is
PMAX=E1 /E2
Pmax is the steady-state stability limit. For a long distance transmission system the
line has the most of the reactance and very small part is in the two terminal systems,
consisting of machines, transformers, and local lines. The inductive reactance of a
single-circuit 60 Hz overhead line with single conductor is about 0.8 Ω/mi (0.5Ω/km);
with double conductor is about 3/4 as greater. The reactance of the line is proportional
to the length of the line, and thus power per circuit of an operating voltage is limited
by steady-state stability, which is inversely proportional to length of line [1].
For the reason of stability the load angle is kept at relatively low value under normal
operating condition (about 30°) because power flow disturbances affect the load-angle
very quickly. In an uncompensated line the phase angle varies with the distance when
the line operating at natural load and puts a limit on the distance. For 30° phase angle
the distance is 258 mi at 60 Hz. The line distance can be increased using series
capacitor, whose reactance compensates a part of series inductive reactance of the
line, but the maximum part that can be compensated has not been determined yet [2].
On the other hand DC transmission has no reactance problem, no stability problem,
and hence no distance limitation.
1.3 ADVANTAGES AND INHERENT PROBLEMS
ASSOCIATED WITH HVDC
1.3.1 Advantages of HVDC
1. More power can be transmitted per conductor per circuit:
The capabilities of power transmission of an AC. link and a DC. link are different.
2. Use of Ground Return Possible:
In the case of HVDC transmission, ground return (especially submarine crossing)
may be used, as in the case of a monopolar DC. link. Also the single circuit
bipolar DC. link is more reliable, than the corresponding AC. link, as in the event
of a fault on one conductor, the other conductor can continue to operate at reduced
power with ground return. For the same length of transmission, the impedance of
the ground path is much less for DC. than for the corresponding AC. because DC.
spreads over a much larger width and depth. In fact, in the case of DC. the ground
path resistance is almost entirely dependent on the earth electrode resistance at the
two ends of the line, rather than on the line length. However it must be borne in

13. 13
mind that ground return has the following disadvantages. The ground currents
cause electrolytic corrosion of buried metals, interfere with the operation of
signaling and ships' compasses, and can cause dangerous step and touch
potentials.
3. Smaller Tower Size
The DC. insulation level for the same power transmission is likely to be lower
than the corresponding AC. level. Also the DC. line will only need two conductors
whereas three conductors (if not six to obtain the same reliability) are required for
AC. Thus both electrical and mechanical considerations dictate a smaller tower.
4. Higher Capacity available for cables
In contrast to the overhead line, in the cable breakdown occurs by puncture and
not by external flashover. Mainly due to the absence of ionic motion, the working
stress of the DC. cable insulation may be 3 to 4 times higher than under AC. Also,
the absence of continuous charging current in a DC. cable permits higher active
power transfer, especially over long lengths. (Charging current of the order of 6
A/km for 132 kV). Critical length at 132 kV ≈ 80 km for AC cable. Beyond the
critical length no power can be transmitted without series compensation in AC.
lines. Thus de-rating which is required in AC. cables, thus does not limit the
length of transmission in DC. A comparison made between DC. and AC. for the
transmission of about 1550 MVA is as follows. Six number AC. 275 kV cables, in
two groups of 3 cables in horizontal formation, require a total trench width of 5.2
m, whereas for two number DC. ±500 kV cables with the same capacity require
only a trench width of about 0.7 m.
5. No skin effect
Under AC. conditions, the current is not uniformly distributed over the cross
section of the conductor. The current density is higher in the outer region (skin
effect) and result in under utilization of the conductor cross-section. Skin effect
under conditions of smooth DC. is completely absent and hence there is a uniform
current in the conductor, and the conductor metal is better utilized.
6. Less corona and radio interference
Since corona loss increases with frequency (in fact it is known to be proportional
to f+25), for a given conductor diameter and applied voltage, there is much lower
corona loss and hence more importantly less radio interference with DC. Due to
this bundle conductors become unnecessary and hence give a substantial saving in
line costs. [Tests have also shown that bundle conductors would anyway not offer
a significant advantage for DC as the lower reactance effect so beneficial for AC
is not applicable for DC.
7. No Stability Problem
The DC. link is an asynchronous link and hence any AC. supplied through
converters or DC. generation do not have to be synchronised with the link. Hence
the length of DC. link is not governed by stability. In AC. links the phase angle
between sending end and receiving end should not exceed 30o at full-load for
transient stability (maximum theoretical steady state limit is 90o ).

14. 14
The phase angle change at the natural load of a line is thus 0.6o per 10 km. The
maximum permissible length without compensation ≈ 30/0.06 = 500 km With
compensation, this length can be doubled to 1000 km
8. Asynchronous interconnection possible
With AC. links, interconnections between power systems must be synchronous.
Thus different frequency systems cannot be interconnected. Such systems can be
easily interconnected through HVDC links. For different frequency
interconnections both convertors can be confined to the same station. In addition,
different power authorities may need to maintain different tolerances on their
supplies, even though nominally of the same frequency. This option is not
available with AC. With DC. there is no such problem.
9. Lower short circuit fault levels
When an AC. transmission system is extended, the fault level of the whole system
goes up, sometimes necessitating the expensive replacement of circuit breakers
with those of higher fault levels. This problem can be overcome with HVDC as it
does not contribute current to the AC. short circuit beyond its rated current. In fact
it is possible to operate a DC. link in "parallel" with an AC. link to limit the fault
level on an expansion. In the event of a fault on the DC line, after a momentary
transient due to the discharge of the line capacitance, the current is limited by
automatic grid control. Also the DC. line does not draw excessive current from the
AC. system.
10. Tie line power is easily controlled
In the case of an AC. tie line, the power cannot be easily controlled between the
two systems. With DC. tie lines, the control is easily accomplished through grid
control. In fact even the reversal of the power flow is just as easy.
1.3.2 Inherent problems associated with HVDC
1. Expensive convertors
Expensive Convertor Stations are required at each end of a DC. transmission link,
whereas only transformer stations are required in an AC. link.
2. Reactive power requirement
Convertors require much reactive power, both in rectification as well as in
inversion. At each convertor the reactive power consumed may be as much at 50%
of the active power rating of the DC. link. The reactive power requirement is
partly supplied by the filter capacitance, and partly by synchronous or static
capacitors that need to be installed for the purpose.
3. Generation of harmonics
Convertors generate a lot of harmonics both on the DC. side and on the AC. side.
Filters are used on the AC. side to reduce the amount of harmonics transferred to
the AC. system. On the DC. system, smoothing reactors are used. These
components add to the cost of the convertor.
4. Difficulty of circuit breaking

15. 15
Due to the absence of a natural current zero with DC., circuit breaking is difficult.
This is not a major problem in single HVDC link systems, as circuit breaking can
be accomplished by a very rapid absorbing of the energy back into the AC.
system. (The blocking action of thyristors is faster than the operation of
mechanical circuit breakers). However the lack of HVDC circuit breakers
hampers multi-terminal operation.
5. Difficulty of voltage transformation
Power is generally used at low voltage, but for reasons of efficiency must be
transmitted at high voltage. The absence of the equivalent of DC. transformers
makes it necessary for voltage transformation to carried out on the AC. side of the
system and prevents a purely DC. system being used.
6. Difficulty of high power generation
Due to the problems of commutation with DC. machines, voltage, speed and size
are limited. Thus comparatively lower power can be generated with DC.
7. Absence of overload capacity
Convertors have very little overload capacity unlike transformers
1.4 ECONOMIC COMPARISON
The HVDC system has a lower line cost per unit length as compared to an equally
reliable AC. system due to the lesser number of conductors and smaller tower size.
However, the DC. system needs two expensive convertor stations which may cost
around two to three times the corresponding AC. transformer stations. Thus HVDC
transmission is not generally economical for short distances, unless other factors
dictate otherwise. Economic considerations call for a certain minimum transmission
distance (break-even distance) before HVDC can be considered competitive purely on
cost. Estimates for the break even distance of overhead lines are around 500 km with
a wide variation about this value depending on the magnitude of power transfer and
the range of costs of lines and equipment. The breakeven distances are reducing with
the progress made in the development of converting devices.
Figure 1 shows the comparative costs of DC. links and AC. links with distance,
assuming a cost variation of ± 5% for the AC. link and a variation of ± 10% for the

16. 16
DC. link. For cables, the break-even distance is much smaller than for overhead lines
and is of the order of 25 km for submarine cables and 50 km for underground cables.
1.5 COMPONENTS OF AN HVDC TRANSMISSION
SYSTEM
The essential components in a HVDC transmission system are 6/12/24 pulse
converters, converter transformer with suitable ratio and tap changing, filters at both
DC and AC side, smoothening reactor in DC side, shunt capacitors and DC
transmission lines.
Fig A typical HVDC transmission schemes
1.5.1 Converter Unit or Convertors
HVDC transmission requires a converter at each end of the line. The sending end
converter acts as a rectifier which converts AC power to DC power and the receiving
end converter acts as an inverter which converts DC power to AC power.
This unit usually consists of two three phase converter which are connected in series
to form a 12 pulse converter. The converter consists of 12 thyristor valves and these
valves can be packaged as single valve or double valve or quadrivalve arrangements.
Due to the evaluation of power electronic devices, the thyristor valves have been
replaced by high power handling devices such as gate turn-off thyristors (GTOs),
IGBTs and light triggered thyristors.
The valves are cooled by air, water or oil and these are designed based on modular
concept where each module consists of a series connected thyristor levels.
Firing signals for the valves are generated in the converter controller and are
transmitted to each thyristor in the valve through a fibre optic light guide system. The
light signals further converted into electrical signals using gate drive amplifiers with
pulse transformers. The valves are protected using snubber circuits, gapless surge
arrestors, and protective firing circuits

17. 17
Now a days, there are more than 92 HVDC projects worldwide transmitting more than
75GW of power employing two distinct technologies as follows
Line-Commutated Converter- This is also called as Current-Source Converters
(CSCs). Thyristors are used in this converter technology. This technology is well
established for high power, typically around 1000MW, with the largest project being
the Itaipu system in Brazil at 6300MW power level
Forced-Commutated Converters – This is also called as Voltage-Source Converters
(VSCs). In this technology, gate turn-off thyristors (GTOs) or in most industrial cases
insulated gate bipolar transistors (IGBTs) are used. It is well established technology
for medium power levels thus far, with the largest size project being the latest one
named Estlink at 350MWlevel (Table 2) .CSC-HVDC systems represent mature
technology today (i.e.,also referred to as “classic” HVDC) and recently, there have
been a number of significant advances .
1.5.2 Converter Transformers
The transformers used before the rectification of AC in HVDC system are called as
converter transformers. The different configurations of the converter transformer
include three phase- two winding, single phase- three winding and single phase- two
winding transformers.
The valve side windings of transformers are connected in star and delta with
ungrounded neutral and the AC supply side windings are connected in parallel with
grounded neutral.The design of the control transformer is somewhat different from the
one used in AC systems. These are designed to withstand DC voltage stresses and
increased eddy current losses due to harmonic currents.
The content of harmonics in a converter transformer is much higher compared to
conventional transformer which causes additional leakage flux and it results to the
formation of local hotspots in windings. To avoid these hotspots, suitable magnetic
shunts and effective cooling arrangements are required.
1.5.3 Filters
Due to the repetitive firing of thyristors, harmonics are generated in the HVDC
system. These harmonics are transmitted to the AC network and led to the overheating
of the equipment and also interference with the communication system.
In order to reduce the harmonics, filters and filtering techniques are used. Types of
filters include

18. 18
1 AC filters
These are made with passive components and they provide low impedance and
shunt paths for AC harmonic currents. Tuned as well as damped filter
arrangements are generally used in HVDC system.
2 DC filters
Similar to AC filters, these are also used for filtering the harmonics. Filters used at
DC end, usually smaller and less expensive than filters used in AC side. The
modern DC filters are of active type in which passive part is reduced to a
minimum.
Specially designed DC filters are used in HVDC transmission lines in order to
reduce the disturbances caused in telecommunication systems due to harmonics.
3 High frequency filters
These are provided to suppress the high frequency currents and are connected
between converter transformer and the station AC bus. Sometimes these are
connected between DC filter and DC line and also on the neutral side.
1.5.4 Shunt Capacitors Or Reactive Compensation
Due to the delay in the firing angle of the converter station, reactive volt-amperes are
generated in the process of conversion. Since the DC system does not require or
generate any reactive power, this must be suitably compensated by using shunt
capacitors connecting at both ends of the system.
1.5.5 Smoothening Reactor
It is a large series reactor, which is used on DC side to smooth the DC current as well
as for protection purpose. It regulates the DC current to a fixed value by opposing
sudden change of the input current from the converter. It can be connected on the line
side, neutral side or at an intermediate location.
1.5.6 Transmission Medium Or Lines Or Cables Overhead lines act as a
most frequent transmission medium for bulk power transmission over land. Two
conductors with different polarity are used in HVDC systems to transfer the power
from sending end to receiving end.
The size of the conductors required in DC transmission is small for the same power
handling capacity to that of AC transmission. Due to the absence of frequency, there
is no skin effect in the conductors.
High voltage DC cables are used in case of submarine transmission. Most of such
cables are of an oil filled type. Its insulation consists of paper tapes impregnated with
high viscosity oil.

19. 19
1.5.7 DC and AC Switchgear
The switchgear equipment provides the protection to the entire HVDC system from
various electrical faults and also gives the metering indication. The switchgear
equipment‟s include isolator switches, lightening arrestors, DC breakers, AC
breakers, etc
1.6 TYPES OF HVDC SYSTEMS OR CONFIGURATION
There are mainly three types of HVDC links and are discussed below.
1.6.1 MONOPOLAR LINK
In this DC system, sending end and receiving end converters are connected by a
single conductor (or line) with positive or negative polarity. Mostly negative polarity
is preferred on overhead lines due to lesser radio interference.
It uses ground or sea water as a return path. Sometimes a metallic return is also used.
It is to be noted that earth offers less resistance to DC as compared with AC. The
figure below shows a monoploar link.
Fig.2: Mono polar HVDC system
This monopolar as its name the pole is 1 means, one conductor and the ground is used
as the return, you can see here, in this is your we are having only one conductor and
the ground is used here as a the current is flowing through the ground normally this
polarity here is used as a negative polarity, rather than positive polarity. It is possible
to have positive it is possible to have negative, but the negative is preferred due to
negative polarity, we will have the less corona loss compared to the positive polarity
and that is, why this is operating at the negative polarity.
The major problem here is, that, if there is any problem in this link, either in the
converter or in the DC system, you have to stop the power flow, and completely it
will be the power, cannot flow from either end. So, this is a monopolar means, you are

20. 20
having only one pole means, only one wire pole means, wire and the ground is used as
a return. Another problem here, that during here, the current which is flowing through
the ground there may be so many pipes; so, many other devices; inside the ground,
that will be huge corrosion due to the DC current flowing there. So, this is normally is
not practiced in the system but conceptually it is possible because sometimes we will
see the type two and type three they operate as a monopolar
1.6.2 BIPOLAR LINK
This is the most commonly used configuration of HVDC system. It uses two
conductors; one is a positive conductor or pole and the other negative conductor of the
same magnitude (typically of ± 650V).
Each terminal has two sets of converters of identical ratings connected in series on
DC side. The neutral points (junction between the converters) are grounded at one or
both ends and hence the poles operate independently.
Normally, both poles are operated at same current and hence there is no ground
current flowing under these conditions.
Fig.3: Bi polar HVDC System
In the event of a fault in one conductor, the other conductor with ground return can
supply half the rated load and thus increase the reliability of the system. The bipolar
link has two independent circuits and it can be operated as a monopolar link in an
emergency situation.
The HVDC link is your bipolar as it is name there is a two poles means; two poles
here is that is, a two wires here. One is go; another is a return conductor; in the bipolar
here you will see, 1 conductor is on the positive polarity, and another is on your
negative polarity, to make this you can see here, the in the normal operation the
current will not flow through the ground, because current will flow here through here
through this converters and this is your positive and another will be your negative or
vice versa, it can be positive this can be negative again based on the operation of the

21. 21
converter. So, there are the two conductors means, two poles are there one operate at
positive polarity and another operates at the negative polarity. Here advantage of this
system, if there is any problem in any side of pole, if there is a problem here either in
the converters, here any of the converter here or in the line, we can just open this
converters and we can use half of the link here and the power can flow so, half of
power we can maintain.
So, whatever right, now, we are having from rihand to dadri it is operating at your
plus minus 500 kilovolt and this shows the plus minus means 1 is operating at plus
another is operating at negative. So, this sign is normally used in HVDC links this
shows that you are having a bipolar operation. The plus responds one and here and
negative here this so, the this is plus 500 this is minus 500 means voltage difference
between these two it is 1000 kilovolt so, the advantage of this that even as I said here
even though one pole is down we can provide half of the power that is, flowing from
one end to another end and that is, very very advantageous.
1.6.3 HOMOPOLAR LINK
This link has two or more conductors with the same polarity, usually of negative and
they are operating with ground return. If fault takes place in one conductor, the
converter equipment can be connected to healthy pole and it can supply more than
50% of the rated power by overloading at the expense of increased line loss.
This is not possible in case of bipolar link where graded insulation is used for
negative and positive poles. This system is preferred when continuous ground currents
are inevitable.The advantage of the system is that less corona loss and radio
interference due to the negative polarity on the lines. However the large earth return
current is the major disadvantage.
Fig.4: Homo Polar HVDC System
Another is your homopolar in homopolar the difference between the bipolar here
again, we are having the two poles but the poles are of same nature means, both are
operating at the negative again due to the less corona loss because they are operating

22. 22
at high voltage is the 500 kilovolt so, huge corona loss will be there and thereby what
we try to do we try to reduce the corona loss by going for the both negative poles. But
major problem here that this, is the current is flowing, here this current is flowing and
the ground is used as the return path. Once you are using ground return again that
creates lot of problem to the system, may be the some voltage is induced sometimes
corrosion is there; sometimes even though some your the ground rods are broken; that
again that creates problem. So, this is also not very common the advantage of here is
again, if there is some problem in one of the pole here you can still use half of the
pole and you can provide half of the power that can flow from one end to another end.
So, this is a major difference between the bipolar and homopolar so, normally in
homopolar it is simply written it is minus or simple minus it is sufficient for this
because if homopolar it will be not it will be in the negative both pole, it cannot be
positive because more loss and we can operate the converters successfully in the
negative voltage operation as well. So, we can broadly we can classify in the three
category conceptually it is possible but this is more popular and half of this is a
possibility suppose one fail here you are going in the monopolar operation.
So, that is, why I explained the monopolar but to reduce the corona loss et cetera here
we can go for the monopolar and we can use the ground as a return path. In both cases
here the double of power is flowing from this monopolar because this is a voltage the
current is rating of the pole is same we are going to have here the voltage here
multiplied by2. So, twice of the power is flowing here but no doubt we have the two
monopolar here one monopolar here another here also we are having the two
monopolar so, the power step up of the single monopolar. So, these three category of
HVDC links are basically practically possible but the bipolar is the better option and it
is existing in India and it is planned several in India we are going to have recently
plus minus 800 k v DC system and so, many locations are planned. So, that we can go
for the higher and higher voltage again the concept appear going for higher voltage
means you can transmit more power. Because the current which is flowing here in
these lines are limited again the conductor size, if you are going for more conductor
size to reduce the losses, then it will be very bulky, then you will have to go for bulky
towers, and so many other problems should occur.

23. 23
1.7 COMPARISOR OF HVDC LINK AND HVAC LINK
S. No. Characteristics HVDC Link HVAC Link Criterion For
Preference
1 Power transfer
ability
High ,limited by
temperature rise
Lower, limited
by power angle
and the
reactance
HVDC Link
for higher
power
2 Control of power
flow
Fast accurate and
bi-directional
Slow and
difficult
HVDC is
preferred
3 Frequency
disturbance
Reduced Communicated
between the
system
HVDC is
better
4 System support Excellent .power
flow is quickly
modulated for
damping oscillation
Poor
oscillations
continue for
long time
HVDC is
preferred
5 Transient
performance
Excellent Poor
6 Fault levels Remains unchanged
after
interconnection
Get add after
the
interconnection
HVDC is
better
7 Power swing Damped quickly Continues for
long time
HVDC is
better
8 Frequency
conversion
Possible Not Possible HVDC is
preferred
9 Cascade tripping
of ac system
Avoided Likely HVDC is
preferred
10 Spinning reserve
of ac network
Reduced Not much
reduced
HVDC is
preferred
11 interconnection Asynchronous synchronous HVDC is
preferred
12 Transient stability
limit
Very high, limited,
by thermal capacity
of the equipment
Less than half
thermal limit
of line
conductor
HVDC is
preferred

24. 24
Chapter 2
FAULT IN HVDC SYSTEM
2.1 FAULTS IN HVDC SYSTEM
High voltage transmission system has more capacity and certain characteristics.
Hence it is used for long distance transmission systems. Various types of fault occur
on HVDC system such as DC faults, AC faults and converter stations faults. In the
HVDC transmission system, DC line is the one of the component which has high
failure probability. The study of faults in HVDC system is necessary because the DC
line faults will induce a significant effect on operation of HVDC transmission system.
There are two types of fault occurs on DC link of HVDC system such as DC Line to
Ground fault and DC Line to Line fault. These DC faults on the HVDC system are
most dominant and frequent faults. In HVDC system, an AC fault also occurs such as
symmetrical faults and unsymmetrical faults i.e., Line to Line fault, Line to Ground
fault and Three Phase Short Circuit fault. Some faults occur on converters station at
rectifier or inverter side of HVDC system which has great impact on system stability.
Fire-through, misfire and short circuit across the inverter station, flashover and three
phase short circuit in the AC system are considered in converter station faults.
2.1.1 DC LINE FAULTS
Faults on DC transmission line are generally caused by external mechanical stress,
lightning strikes and pollution. In HVDC transmission system, Line to Ground fault
and Line to Line fault are common types of faults. These faults are permanent and for
which a lengthy repair is needed. After detecting the cable faults in DC transmission
line, the converter should be stopped immediately. These faults are likely to be
temporary which required fault restoration after the fault clearance.[3]
1 DC LINE - TO - LINE FAULT
The DC Line to Line faults are usually caused by insulation failure between the two
DC conductors. The DC Line to Line faults is a rare accident. When line to line fault
occurs in DC transmission line, the capacitor will be discharged rapidly.
Simultaneously the AC system will be three phases short-circuited through fault point.
When fault occurs in DC side, the IGBTs can be blocked for self-protection during
faults, leaving reverse diodes exposed to over current [1]. The fault demands that both
converters should be blocked [4]. The equivalent circuit of DC Line-to- Line fault as
shown in figure 5. The DC short-circuit fault can be divided to three stages are as
follows.

25. 25
Fig.5: An equivalent circuit of DC Line-to-Line fault
a. Capacitor Discharge Stage
When a DC Line to Line fault occurs, a loop circuit without source is formed. After
the fault occurs, the system, firstly experiences the capacitor discharge stage. In this
stage, the capacitor voltage drops to zero
b. Diode Freewheel Stage
At the instant, when the DC-Line voltage drops to zero, the cable inductance and
freewheel diodes will form a loop circuit. Initially, the IGBT is blocked for self-
protection, and there is a high initial over-current though the diodes, which may make
huge damage to the diodes. Then the DC current and diodes current will decrease
rapidly [8].
c. Capacitor Recharging Stage
In Capacitor Recharging Stage, the DC link capacitor, cable inductance and AC side
form a forced response and the capacitor will be charged. During this stage, the DC
voltage increases [3]
2.1.2 DC LINE TO GROUND FAULT
The DC Line to Ground fault is caused by insulation failure between DC conductor
and ground. In overhead HVDC transmission system, the DC Line to Ground fault is
temporary which is caused by lightning strikes and pollution. For underground HVDC
transmission system, the DC line to ground fault is the most frequent fault. The
equivalent circuit of DC Line to ground fault as shown in figure 6. This fault will
produce ground point besides the mid-point of DC-link capacitor and the neutral-
ground link of transformer [8]. This fault can be divided to three stages are as follows.

26. 26
Fig.6: An equivalent circuit of DC Line to Ground Fault
a. DC Side Capacitor Discharge Stage
When a DC Line to Ground faults occurs, a discharge circuit is formed among the
fault pole capacitor and fault impedance through the fault line. After the fault occurs,
the system experiences the DC side capacitor discharge stage
b. Grid-Side Current Feeding Stage
When DC Line to Ground faults occurs; the DC side capacitor discharging due to this
the DC voltage drops constantly. When the DC voltage drops to below any grid phase
voltage, then the system will experience the grid side current feeding stage
c. Voltage Recovery Stage
The fault pole capacitor voltage drops and non-fault pole capacitor voltage rising with
the capacitor discharging. The DC voltage gradually restores, so the system enters the
voltage recovery stage [3]
B. AC LINE FAULTS
On rectifier side of HVDC system, when a temporary fault occurs on the AC side, the
DC transmission system may suffer a power loss. On inverter side of HVDC system,
when a fault occurs on the AC system, the commutation failure can occur and may
interrupt power flow .Line to Ground Fault: The single line to ground fault occurs on
the primary side of the converter transformer at rectifier Line to Line Fault: The line
to line fault occurs on the primary side of the converter transformer at rectifier .Three
phase to ground fault: A three phase fault is one of the most severe fault in the HVDC
system as compared to the other two faults [3]

27. 27
C. CONVERTER FAULTS
Some common types of faults that occur at converter station they are as follows:
1. FAULTS DUE TO MALFUNCTIONS OF VALVES AND CONTROLLERS
a. Arc backs
Arc backs is of random nature and non-self-clearing fault. This is a major fault which
is produced in mercury arc valve. When valves get fail to block the reverse direction
then arc back or arc fire occurs. This result in the temporary destruction of the
rectifying property of the valve due to conduction the reverse direction. This fault
results in severe stresses on transformer windings as the incidence of arc back or arc
fire is common
b. Arc through
This is a fault likely to occur at inverter station. At the inverter station, when valve
voltages are positive most of the time then this fault occurs. A malfunction in the
arrival of thegate pulse generator can fire a valve which is not supposed to conduct,
but is forward biased. In the arc through fault, the firing delay angle of the faulted
valve is reduced from its normal value to smaller value or zero
c. Misfire
When the required gate pulse is missing or the incoming valve is unable to fire, then
such type of fault occurs. The modern converter stations have properties of duplicate
converter controls, monitoring and protective firing of valves due to which the
probability of the occurrence of misfire is very small. The misfire can occur either
rectifier side or inverter side but the effect of misfire is more severe in the latter case.
This is due to the fact that in inverters, persistent misfire leads to the average bridge
voltage going to zero, while an AC voltage is injected into the link. This result in
large current and voltage oscillations in the DC link as it presents a lightly damped
oscillatory circuit viewed from the converter. The DC current may even extinguish
and result in large over voltages across the valves
d. Quenching or current extinction
When the current through valve falls below the holding current then quenching
occurs in the valve. This can arise at low value of the bridge currents when any
transient can lead to current extinction. The current extinction can result in over
voltages across the valve due to current chopping in an oscillatory circuit formed by
the smoothing reactor and the DC line capacitance. In the short pulse firing method,
the current extinction fault is more severe. However, in modern converter stations, the
return pulses coming from thyristors levels to the valve group control, indicate the
build up of voltage across the thyristors and initiate fresh firing pulses when the valve

28. 28
is supposed to be conducting. It may happen that a number of firing pulses may be
generated during a cycle when the current link is low [3]
2. COMMUTATION FAILURES
This type of fault occurs in thyristors and thyristors required a definite turn-off time
so there is a need to maintain a minimum value of extinction angle defined by,
Y = 180 – a-μ
Where, μ is the overlap angle which is a function of the commutation voltage and the
DC current. The overlap angle increases when the voltage or current increases or both
increases simultaneously. This gives rise commutation failure [3]
2.1.3 SHORT CIRCUITS IN A CONVERTER STATION
The valves are kept in valve hall with air conditioning; hence the probability of short
circuits in a converter station becomes very low. Short circuit across the bridge
mainly occurs due to bushing flashover which results in producing large peak currents
in the valve that are conducting. Short Circuit Current from HVDCand Configuration
of HVDC Converter
HVDC can be distinguished into two types by converter: Current source converters
(CSC) and voltage source converters (VSC). CSCs are also called linecommutated
converters (LCC), because they rely on a synchronous voltage for proper operation.
The thyristor is the key element of a CSC, which allows very high transmission
powers. CSC-HVDC systems with voltages of ±800 kV and powers of more than
8000 MW are possible
VSCs are referred as self-commutated converters because of the ability to operate
independently from the power grid. The AC/DC-conversion is accomplished by
insulated gate bipolar transistors (IGBTs) with turn-on and turn-off capability.
Because of the high complexity of the control system, there are only a few
realizations by far now. In this paper, UHVDC based on LCC is considered. The
Equivalent circuit of a six-pulse bridge at a short circuit on the DC side is shown as
Figure 1. The six-pulse bridge is the basic element of a CSC-HVDC, which is shown
in Figure 1 at a short circuit on the DC side. The largest short circuit current of a
single bridge can be expected, if a line-to-line short circuit occurs directly at the
terminal (RDC = 0, LDC = 0). For this particular case the DC short circuit current can
be derived according to the loops I and II shown in Figure 1. In this instant the diodes
D1, D3 and D2 are conducting.

29. 29
Chapter 3
FAULT CURRENT’ EFFECT IN POWER SYSTEM
3.1 INTRODUCTION
When a fault occurs in transmission system, a „fault current‟ (also known as „short
circuit current‟) arises. A „fault current‟ is a flow of massive current through an
electric circuit his high level „fault current‟ can largely damage the equipment
insulation system, lead to power surges that damage equipment that is powered by the
current, or possibly charge the devices so that when they are touched, an electric
shock is administered. Depending on the nature of the fault current, that shock can be
sufficient to cause death.
In case of asymmetric fault‟ in HVAC power transmission system, the high level
„fault current‟ can largely affect the transmission line, sending end bus section and
also the power generation unit. Generators are frequently subjected to high level „fault
current‟ . Faults in particular subject the generator to stress beyond its design limits
and cause high temperature increase, amplify and distort air gap torques, and create
unbalanced flux densities. Even more stressful as a consequence of faults are sudden
loss of load, fault clearance and reclosing. Mechanically, the abnormal forces that are
generated excite the rotor and as a result, amplify the shaft‟s normal mode of
oscillation.
As the „fault current ‟on the power system is cleared by circuit breakers, hence the
magnitude of „fault current‟ determines the types, settings and the size of the circuit
breakers. The lower the „fault current‟, the size and cost of circuit breaker is reduced.
In turn, the flexibility of operation of circuit breaker is increased. [1]
2.2 SIMULATION
3.2.1 HVAC TRANSMISSION SYSTEM :
The generation of electric power takes place in a power plant. Then the voltage level
of the power is raised by the transformer before the power is transmitted. Electric
power is proportional to the product of voltage and current this is the reason why
power transmission voltage levels are used in order to minimize power transmission
losses.
Fig 1.1 represents a three-phase HVAC transmission system simulation model,
transmitting 1680 MW (50 Hz, 500 kV, 0.8 lagging) power from a power plant
consisting of six 350 MVA generators to a sub-station through a 300 km transmission
line. To increase the transmission capacity and power quality, each line is series
compensated by capacitors representing 40% of the line reactance and is shunt

30. 30
compensated by a 330 Mvar shunt reactance. Shunt capacitive compensation is widely
employed to reduce the active and reactive power losses and to ensure satisfactory
voltage levels during excessive reactive loading conditions. Series compensation
reduces the transfer reactance between buses to which the line is connected, increases
the maximum power that can be transmitted, and reduces the effective reactive power
losses[14].Sub-station receives 1645 MW power at the very final stage of this HVAC
power system simulation model.
3.2.2 HVDC Transmission System :
Technical feasibility has been already proved for HVDC power transmission system
with the development of Power electronics devices [15]. These devices make the
efficient conversion from AC to DC and thus are the main component of any HVDC
power transmission system.
Fig 1.2 represents the simulation model of a high-voltage direct current (HVDC)
transmission system using 12-pulse Thyristor converters, transmitting 1680 MW (50
Hz, 500 kV, 0.8 lagging) power from a power plant consisting of six 350 MVA
generators to a sub-station through a 300 km transmission line. The AC systems are
represented by damped L-R equivalents with an angle of 80 degrees at fundamental
frequency and at the third harmonic. The rectifier and the inverter are 12-pulse
converters using two Universal Bridge blocks connected in series. The converters are
interconnected through a 300-km line and 0.9 H smoothing reactors. The Smoothing
reactor provides the prevention of the intermittent current, Limits the fault current,
Prevents resonance in the DC circuits
The tap position is rather at a fixed position determined by a multiplication factor
applied to the primary nominal voltage of the converter transformers (0.90 on the
rectifier side; 0.96 on the inverter side). Sub-station receives1645 MW power which is
same as that of HVAC powertransmission system.
Fig 1.1 and Fig 1.2 shows the simulation model of both HVAC and HVDC power
transmission system sequentially. For better analysis and observation of fault current
effects, the parameters for both simulation were kept same.
2.3 SIMULTION OUTPUT
Condition 01: Single line to Ground Fault
Fig 2.1 shows a very high „fault current‟ in HVAC transmission system (37000 A)
compared to „fault current‟ in HVDC transmission system (29000 A) due to „Single
line to ground‟ fault at receiving end (Fig 2.2).

31. 31
Fig 2.1: Fault current (HVAC system).

32. 32
Fig 2.2: Fault current (HVDC system).
The „Fault current‟ effects can be observed from the following figures. Fig 3.1 shows that, due to
„Single line to ground‟ fault, sending end voltage and current waveforms of HVAC system are
no longer in proper position but highly distorted even if the fault is occurred at the receiving end
of the total power system. The phase angles between the phases are highly disturbed. For the
same fault condition Sending end voltage and current waveforms (Fig: 3.2) of HVDC
transmission system are in approximate proper position. The current waveforms (Fig 3.2) look
like distorted because of 12 pulse converter but doesn‟t affect the overall power transmission
system.
Fig 3.1: Sending end bus (HVAC system).
In case of HVAC transmission system, a fault in receiving end section can affect the total power
system, even also the generator section. Fig 5.1 depicts the distorted generator bus section of
HVAC transmission system for „Single line to ground‟ fault at receiving end. This distortion can
cause a massive destruction in the overall HVAC power system. But, from Fig 5.2, there is no
effect found at generator bus section in HVDC transmission system for the same fault condition.

33. 33
Fig 3.2: Sending end bus (HVDC system).
Fig 4.1 and Fig 4.2 describe the receiving end voltage and current waveforms for the both HVAC
and HVDC transmission system. A fault in HVDC transmission system only affects the
corresponding section. As the „Single line to ground‟ fault was occurs at the receiving end, the
faulty phase is grounded; Voltage and current waveforms are affected for the both HVAC and
HVDC transmission system. Specifically, only this section of HVDC system is affected.
Fig 4.1: Receiving end bus (HVAC system).

34. 34
Fig 4.1: Receiving end bus (HVAC system).
end section can affect the total power system, even also the generator section. Fig 5.1 depicts the
distorted generator bus section of HVAC transmission system for „Single line to ground‟ fault at
receiving end. This distortion can cause a massive destruction in the overall HVAC power
system. But, from Fig 5.2, there is no effect found at generator bus section in HVDC
transmission system for the same fault condition.
Fig 5.1 : Generator bus (HVAC system).

35. 35
Fig 5.2 : Generator bus (HVDC system)
Condition 02: Line to line Fault
„Line to line‟ Fault is another type of asymmetrical fault. Fig 6.1 shows a very high fault current
(almost 34000A) due to „Line to line‟ Fault at receiving end of HVAC transmission system,
while fault current is much lower ( 20000A) in case of HVDC transmission system under same
fault condition shown in Fig 6.2.
Fig 6.1: Fault Current (HVAC system)
Fig 6.2 : Fault Current (HVDC system)

36. 36
Condition 03: Fault in Transmission line
In case of HVAC transmission system, „Fault in transmission line‟ is similar to that of „Single
line to ground fault‟, but the previous „Single line to ground‟ fault occurred in receiving end
section while „fault in transmission line‟occurred in transmission line section. And, in case of
HVDC transmission system, „fault in transmission line‟ occurs in DC transmission line section.
Fig 7.1 : Fault Current (HVAC system)
Fig 7.2 : Fault Current (HVDC system)
It is illustrated from the Fig 7.1 and & Fig 7.2 that, „fault current‟ in HVAC transmission system
for „fault in transmission line‟ is too high (12000 A) compared to HVDC transmission system
(700 A).[1]
2.4 RESULT ANALYIS
Fault current comparison for different types of fault for both HVAC and HVDC transmission
system can be illustrated from the following table (Table 01) –

37. 37
Table -01: Fault Current Comparison
Fault HVAC
( Peak value )
HVDC
( Peak value )
Single line to
ground fault
37000 A 29000 A
Line to line fault 34000 A 20000 A
Transmission
line fault
12000A 700 A

38. 38
Chapter 4
AC FAULT ANALYSIS
4.1 INTRODUCTION
The behaviour of 2-level VSC-HVDC & 12-pulse VSC-HVDC transmission systems under the
AC faults like L-G; L-L & LLL faults are analyzed. Simulation for 2-level VSC–HVDC is
carried out for fault on AC side. AC faults like L-G, L-L, and LLL fault are created and
variations in current magnitude are observed.
4.2 SIMULATION
The single line to ground fault occurs on the primary side
of the converter transformer at rectifier end. Fault occurs on phase A at 0.08s and last for
0.01sec, the current magnitude of phase A increases and voltage across it decreases rapidly to
zero. When fault isolates at 0.09sec the waveform comes to normal value as shown in fig 5(a)
- line (LL) fault: The double line fault occurs on the primary side of the converter
transformer at rectifier. Fault occurs on phase A & phase B at 0.08s and lasts for 0.01sec, the two
phase currents are same in magnitude and system becomes unbalanced as shown in fig.5 (b)
A three phase is the most severe fault compared to the
other two faults. Fault occurs on 3-phases at 0.08sec, the fault current magnitude is found to be
more compare to other two faults as shown in the fig.5(c) [2]

39. 39
Figure 5 Simulation results for 2-level VSC-HVDC System (a) fault current during, (b) Fault current
during L-L, (c) fault current during LLL
4.3Mathematical Analysis
To validate the result of 2-level VSC–HVDC under the AC fault with the simulated result the
mathematical analysis has done, the single line block diagram under such condition is shown in
fig.6
Figure 6 Diagram indicating AC fault location
The mathematical analysis of same system during L-G, L-L & LLL faults has been attempted;
the fault current magnitudes during L-G, L-L & LLL fault are calculated [2].
The magnitude of AC fault current during L-G fault is given by

40. 40
The magnitude of AC fault current during L-L fault is given by
The magnitude of AC fault current during L-L-L fault is given by
It is observed that the magnitude of fault current is calculated mathematically is found to be
nearly equal with the simulated result.[2]
Table II Fault current magnitude
Type of fault Simulated Result Mathematical calculated
Result
L-G 4×10 6A 3.24×10 6A
L-L 4.9×10 6A 4.54×10 6A
LLL 6×10 6A 5.4×10 6A
Simulation of 12-pulse VSC-HVDC under AC faults L-G, L-L, LLL faults has been carried out
& corresponding fault current magnitude are shown in fig.7 (a) (b)

41. 41
Figure 7 Simulation results for 12-pulse VSC-HVDC System (a) fault current during L-G, (b) Fault
current during L-L, (c) fault current during LLL

42. 42
Chapter 5
PROTECTION SCHEMES IN HVDC SYSTEM
5.1 HVDC GRID PROTECTION OBJECTIVES AND REQUIREMENTS
This Section describes the HVDC grid protection objectives and the requirements they impose
on the protection system.
5.1.1. Objectives of HVDC Grid Protection
Similar to AC grids, the HVDC grid will be affected by a variety of faults. Therefore, a fault
clearing strategy is needed to minimize the negative effect of these faults on the system aspects
such as stability and reliability. The objectives of the HVDC grid protection are similar to those
of AC protection 11:
1. Ensure human safety
An essential task of the protection system is to ensure human safety by fast isolation of the faults
in the related equipment such as DC transmission lines, buses or DC substation converter
transformers. A fault that persists for a longer time not only damages HVDC grid system
equipment but also poses hazards to the surrounding areas due to creation of electro-magnetic
fields.
2. Minimize fault impact on the grid
First, DC side faults disturb the normal operation of the HVDC grid. To ensure the reliability of
the HVDC grid, the protection system should minimize the section of the HVDC grid that is
disconnected after a fault occurs. The extent of a disturbance after a fault in the HVDC grid
depends on the speed of fault current interruption and the protective algorithm. The slower the
fault is cleared, the further the fault has propagated in the HVDC grid. Second, as a HVDC grid
will be used for bulk transport of power, outage of a large part of the HVDC grid might cause
problems to the underlying AC system. Hence, the HVDC grid protection must also limit the
impact of a DC side fault on the AC grid.
3. Minimize stress to components
Besides the protection of the grid, damage to components should be avoided as this can shorten
their lifetime. Therefore, in case of faults, the HVDC grid protection system must minimize the
stress to its components.

43. 43
5.2.2 HVDC Grid Protection Requirements
The objectives of protection for a grid can be translated into a philosophy for the protection
system, meeting following requirements 2:
• Reliability: Reliability of the protection system includes dependability and security.
Dependability implies the correct action of the protection system against faults that need action.
Security implies the non-action of the protection system in cases when the protection system
must not act [4]. Especially when considering large protection zones, in which large sections are
isolated in case of a fault, security is important.
• Speed: The protection system must act fast to avoid damage to equipment, limit the fault
current within the maximum interruptible current and limit the impact of the disturbance on the
network. In the DC grid, time constraints are extremely stringent, typically in the order of
milliseconds.
• Sensitivity: Every faulty situation must be detected and cleared.
• Selectivity: The protection system must define zones of protection which are separated by
selectivity criteria in the fault detection. Only the zone that contains the fault should be isolated.
• Robustness: The protection system must be able to operate even in degraded situations.
Duplication of protection systems to provide redundancy can aid to robustness.
• Stability: After fault clearance, the system must reach stable operation within an acceptable
time period.
5.2 FAULT CLEARING STRATEGIES FOR HVDC GRIDS
Based on the above discussion on HVDC grid protection objectives, requirements and
constraints, several fault clearing strategies can be defined. Fig. 3 shows possible fault clearing
strategies in the HVDC grid. Below, a definition of these fault clearing strategies for HVDC
grids is given. Note that only primary protection is treated in the discussion, whereas backup
protection should be provided in case of failure of any part of the equipment involved in fault
clearing.
5.1. Strategy (a): “line protection”
For this strategy, fault currents are interrupted by breakers at the end of the faulted line. To
minimize the impact of a disturbance on the grid, a fault must be cleared before any converter in
the HVDC grid blocks. Therefore, fast DC breakers at the end of every transmission line are
needed in combination with non-unit protection. The time before a converter blocks is limited to
several milliseconds (Fig. 2). To extend the time before the converter blocks, additional fault
current limiting might be needed.
5.2. Strategy (b): “line+ protection”

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For strategy (b), blocking of converter IGBTs of the converters at the buses terminating the
faulted line is allowed. If the converter is of fault blocking type, this diminishes the current that
the breakers must interrupt. The time constraint for fault clearing is relaxed compared to strategy
(a). However, the fault must be cleared timely to avoid collapse of the full HVDC grid, limiting
the time constraint to a few tens of milliseconds.
5.3. Strategy (c): “open grid protection”
All breakers and converters at a bus adjacent to the faulted line co-operate to interrupt the DC
fault current. After fault current interruption, the breakers in the healthy lines reclose to restore
normal operation of the grid. For this strategy, stability of the HVDC grid is also the limiting
factor for interruption of the fault current. The protection system must now open and reclose
breakers within this limited timeframe. Therefore, constraints on the protective relaying
algorithm are more stringent than for the previous strategy.
5.4. Strategy (d): “grid-splitting protection”
In this strategy, the DC grid is split up in different protection zones. The strategy consists of two
stages; first the faulted zone is swiftly isolated from the healthy part of the grid. Second, the
faulted line is isolated within the faulted zone. The first part of strategy d requires fast protection
and fast breakers. The time constraint is here imposed by the converter IGBTs of the healthy part
of the grid. For the second part of strategy (d), the time constraint is either imposed by the
converter diode surge withstand capability or ac system stability, which allows slower fault
clearing.
5.5. Strategy (e): “low-speed HVDC grid protection”
The entire HVDC grid is affected in case of faults. A first option is to clear the fault by actions at
all converters in the network. This strategy is limited to small DC grids, where the impact of
switching of the HVDC grid on the AC grid is tolerable. A second option is to limit the fault
current in feed of the converters, which enables the use of slower protective algorithms and
breakers. For this option, the HVDC grid is not completely switched off, which enables faster
fault recovery. The time constraint for this option is determined by the AC network
constraints.[8]
5.3 METHODS OF DC LINE PROTECTION
As previously mentioned the most commonly used methods in the protection of HVDC lines is
differential protection, travelling wave protection, dc voltage derivative and voltage level
protection. This section will briefly discuss each of these methods of protection illustrating some
of the advantages, disadvantages and possible gaps in the various methods.

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1 Voltage Derivative Protection
When a dc line fault occurs, the travelling waves initiated by the fault cause the dc voltage and
line current to decrease and increase at a certain rate respectively. This method makes use of the
travelling wave concept and is normally used as a main dc line protection. The dc voltages and
currents are measured and the derivatives dv/dt and di/dt are then calculated. The sign of di/dt is
used to indicate whether the fault is located on the line or in the DC yard. The weighted sum of
the derivatives is than calculated using equation and compared to a set threshold If this threshold
is exceeded the protection will operate.
E=K1 dv/dt+K2 dv/dt
where KI and K2, are the assigned weights and E is the weighted sum of the derivatives.
The detection method is very fast and provides fault detection with 2-3m [2]. In a bipolar dc line
each pole will require this type of detection. Determining the settings required involves detailed
network studies in order to ensure that the protection only operates for dc line faults and is stable
for all other disturbances. A major disadvantage of this method is that dv/dt is dependent on the
fault loop impedance and therefore high impedance faults and faults close to the inverter on long
lines are difficult to detect using this method.
2 Travelling wave protection
This method like the name says is also based on travelling wave theory and is normally used as a
main dc line protection. In this method the instantaneous voltages and currents are continuously
sampled. On detection of the wave front, the difference between two samples is measured. If this
is greater than the threshold the protection is initiated and a series of different sample
measurements will start to determine if the wave has sufficient amplitude for a specific time. If
all these measurements are greater than the set thresholds, a line fault is detected. la a bipolar
HVDC line the faulted pole is determined by the polarity of the ground mode wave. Fig. 1,
shows an example of these different measurements.
The strength of the algorithm is that both current and voltage contribute to the detection.
However the longer the line, the more the waves are damped and difficulties in detecting the
waves may arise for extremely long lines or relevantly high impedance faults.[6]
3 Current differential protection
The differential line protection is generally used as backup protection and basically involves the
measuring and comparing of the currents at both line ends. This information is relayed to the
converter stations via a telecommunication infrastructure. The difference between the two
measured currents is known as the differential current and if it exceeds a set threshold for a
predefined time the protection will operate. This is a relatively simple method of fault detection
and if correctly setup provides good reliability and protection coverage. Fig. 2, shows that the
differential protection can also be equipped for multi-terminal operation quite easily.

46. 46
Fig. 2. Differential current calculation for parallel multi-terminal HVDC
system
The disadvantage of this method is that because of the line lengths used in HVDC system
(especially in cable systems), errors are introduced at each line end. These errors are due to the
charging and discharging currents caused by any voltage variations. This limits the sensitivity
settings that can be applied in order to avoid malfunctions and hence the ability of the protection
to detect high impedance faults is reduced. A proposed solution to this problem bas been
presented and results verified using simulators [3]. The paper basically presents the design for a
circuit that will remove the capacitive currents him the current measurements at each end. This
allows the use of more sensitive settings, but increases the requirements on the
telecommunication infrastructure.
Since the information regarding line currents needs to be transmitted to the various converter
stations, the signal propagation time greatly influences the response time of this method. A major
disadvantage of this method is that the reliability of this type of protection is directly related to
the reliability of the telecommunication infrastructure chosen.
4 DC voltage level protection
DC voltage level protection is used to respond to voltage depressions over a large time interval to
detect high impedance faults or faults close to the inverter terminal. This method provides good
protection coverage and is normally used as a backup to voltage derivative or travelling wave
protection. The level and time delays are selected such that normal switching operations or
voltage transients not initiated by dc line faults do not cause protection operation.
In the advertent of main protection failure the response of this device to close up faults may be
undesired. A possible solution would be to setup the protection with multiple levels, with deeper
depressions having a much shorter response time. On newer systems this should not be a
problem as it will just involve slight changes to the algorithm, but obviously with the older
system which used electronic circuits this may not be possible. The reliability of this method is
not influenced by telecommunication infrastructure, and with multiple detection levels, it will in
most cases, provide adequate backup protection for the HVDC transmission line.[6]
5 Possible Methods Of Improving The Protective Reach Of Main Dc Line Protection

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1 Fault Detection Method Using Telecommunications
Most HVDC system should have communication between converter stations for relaying control
signals. Irrespective of the reliability of the telecommunications, if it is available, it should be
used to enhance the protection system. Consider Fig. 4 above. With GPS technology, the
difference between Ta, and Tb can be obtained by noting the time of arrival of the wave fronts
and transferring this information to both ends were the difference can be calculated. The distance
to the fault can quite accurately be calculated by using this information together with the length
of the line (e) in the equations below:
where DA and DB are the distance to the fault from station A and B respectively
We can now setup a distance relay for a HVDC system. Let assume station A is the rectifier. We
can then define D AB as the distance to the fault from A as seen by B (inverter station). This can
be simply calculated using .
A detection method as given in (8) can be used i.e. if DAB and DA are within a specified tolerance
(E) and DA is less than a distance setting specified (Ds) the protection will be released. However,
this protection is going to be initiated for normal switching operations. We therefore need to
implement a protection starter which could be based on voltage level, voltage derivative,
polarity, etc. This as well as the impact of current control for very long lines on this protection
system will be investigated and discussed in future presentations.
2 Local fault detection methods
With reference to the Below diagram in Fig. 4, if the time at which the first reflected wave from
the behind the relay (ea) and from the fault (er3) pass through the relaying point are recorded.
The distance to the fault is then given by (9) below.
To determine this time from the voltage and current waveforms measured at the relaying point,
let us define two relaying signals a(t) which is the value of the forward travelling wave as it
leaves the relaying point heading in to transmission line and b(t) which is the value of the
backward travelling wave as it arrives at the relaying point from the transmission line. Equations
(10) and (11) derived from (3) and (4) above are used to calculate the instantaneous values of
forward and backward travelling waves from the physically measured voltage
and current at the relaying point.

48. 48
Now if we consider the fault in Fig. 3. The forward travelling wave will be reflected at the fault
and return to the relaying point after the elapse of twice the travel time (2X/V). Assuming a
lossless line, then
where K, is the reflection co-efficient as defined in (5) with Zt in this case being the fault
resistance. The equation is valid for t>0 and assumes that the fault occurred at time t =x/v
The basic relationship in (12) together with (10) and (1 1) defining the wave signals, constitutes
the line equations on which protection schemes are based [4]. The basic relation states that in
case of a fault the two wave signals a(t) and b(t) are exactly congruent except for a constant time
shift, (2x/v), proportional to the fault distance and a constant scaling factor, &, related to the fault
resistance [4].
So far we have assumed that the current backward travelling wave is due to reflection of the
previous forward travelling wave. This is not always the case especially when there is significant
fault impedance. Therefore, a correlation method is required to identify the returning wave from
the fault. Possible correlation methods for ac systems are discussed in [4], [5] and [6].
From the above explanations, once the reflected wave as been correlated, the time shift Y"
between a(t) and b(t) can be measured. The distance to fault can then be calculated using (9).
This distance can be compared to some reference distance setting and if the measured distance to
fault is less than the threshold setting the protection can be initiated. Note that there must &t be a
change in a(t) before measurements are taken and time shifts calculated. The actual methods on
how to extract the time shift from the measured signals as well as the correlation methods will
not be discussed here as this is not the purpose of this paper. Possible methods of extraction for
ac systems are discussed in[6].
This method of protection will detect the fault only after at least 3t, where T is travel time from
the fault to the relaying point. Therefore depending on the length of the dc transmission line and
the speed of the control HVDC control system, the effect of the control system will probably
have to be taken into account when implementing this type of protection system.

49. 49
CONCLUSION
IN this we analyses the behavior of HVDC systems under DC pole-ground fault & AC fault It Is
observed that the occurrence of the DC pole to ground faults leads to substantial over current in
the system which may lead to damage of the converter valve.
As the „Single line to ground‟ fault was occurs at the receiving end, the faulty phase is grounded;
Voltage and current waveforms are affected for the both HVAC and HVDC transmission system.
Specifically, only this section of HVDC system is affected. „Line to line‟ Fault is another type of
asymmetrical fault. Is a very high fault current (almost 34000A) due to „Line to line‟ Fault at
receiving end of HVAC transmission system, while fault current is much lower ( 20000A) in
case of HVDC transmission system under same fault condition fault current‟ in HVAC
transmission system for „fault in transmission line‟ is too high (12000 A) compared to HVDC
transmission system (700 A). Its observe that fault current in HVDC transmission is low than
HVAC
Long distances are technically unreachable by HVAC line without intermediate reactive
compensations. The frequency and the intermediate reactive components cause stability
problems in AC line. On the other hand HVDC transmission does not have the stability problem
because of absence of the frequency, and thus, no distance limitation. The cost per unit length of
a HVDC line lower than that of HVAC line of the same power capability and comparable
reliability, but the cost of the terminal equipment of a HVDC line is much higher than that of the
HVAC line. The breakeven distance of overhead lines between AC and DC line is range from
500 km (310 miles) to 800 km (497 miles). The HVDC has less effect on the human and the
natural environment in general, which makes the HVDC friendlier to environment

50. 50
REFERENCES
[1] Md. Mizanur Rahman1
*, Md. Fazle Rabbi2
, Md. Khurshedul Islam2
and F. M. Mahafugur
Rahman1
, “HVDC over HVAC Power Transmission System: Fault Current Analysis and
Effect Comparison” (ICEEICT) 2014
[2] Mujib J. Pathan , Dr. V. A Kulkarni , “Fault Analysis Of HVDC Transmission Systems”
(IJEET) Volume 7, Issue 3, May–June, 2016 IAEME Publication
[3] Ashwini K. Khairnar, Dr. P. J. Shah, “Study of Various Types of Faults in HVDC
Transmission System”, International Conference on Global Trends in Signal Processing,
Information Computing and Communication ICSPICC (Technically Sponsored by IEEE
Bombay Section), Proceeding, 2016
[4] .Manish Kumar, Manjeet and Pooja Khatri, “Study of Faults On HVDC Transmission
Lines”, Golden Research Thoughts, 3(8), 2014, 1-11.
[5] New Comparison of HVDC and HVAC Transmission system, Vahid Behravesh. Nahid
Abbaspour International Journal of Engineering Innovation & Research Volume 1, Issue
3, ISSN : 2277 – 5668
[6] HVDC Line Protection for the Proposed HVDC Systems. D. Naidoo and N.M. Ijumba,
Member, Leee 2004 lntematlonal Conference on Power System Technology -
POWERCON 2004 Singapore, 21-24 November 2004
[7] High Voltage DC Transmission , Prof. S. N. Singh ,Department of Electrical Engineering
Indian institute of Technology, Kanpur ,NPTEL Note and lecture
[8] Classification of Fault Clearing Strategies for HVDC Grids, by W. Leterme*, D. Van
Hertem* published LUND by CIGRE 2015
[9] http://circuitglobe.com used for image and basic information about HVDC link

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