CPRI-Power quality report

NTPC ELECTRIC SUPPLY COMPANY Ltd. 1

Abstract— Power Quality mainly deals with supply voltage magnitude disturbances (short term) and
waveform distortion of supply voltage and currents. Power quality can only be maintained with
combined effort of utilities and the consumers.
Utilities have to maintain quality supply even under increased renewable generation and grid
disturbances. Similarly, consumers have to prevent the electrical disturbances and distortions from
spreading into the distribution system.
EN 50160 defines the quality of power and the max. acceptable levels at the consumer’s supply
terminals. Also, IEEE 519-1992 stipulates max. acceptable levels of harmonic distortions.
This paper discusses about the spectrum of power quality, causes of power quality problems,
solutions using various power electronic technologies suitable for distribution systems. Also, this
paper presents case studies conducted with measurements taken at three industries.
Index Terms— Power Quality (PQ), THD, D-SVC, D-STATCOM, Active filters, DVR.
I. INTRODUCTION
ower Quality has gained tremendous concern in distribution utilities as well as consumers, and is also
mandated by international standards like EN 50160 and IEEE 519.
Generally, the power quality disturbances are caused by industries like Automobile, Cement Steel/
foundries, Pulp processing, Printing press etc. Also, wave form distortions are generally caused, as identified
by IEEE 519:1992 standard are power converters, arc furnaces, static VAR compensator, inverters of
dispersed generation, electronic phase control of power, switched mode power supplies and Pulse wide
modulated drives.
The poor power quality in turn increases the losses in the system as well as technical losses in the electrical
product/ equipment itself. The electrical disturbances & distortions caused by one consumer/ industry are
not only pollutes the power supply of other equipment of his own, but also pollutes the power supply of
neighboring consumers. And all the distortions are transmitted back to the source through distribution
transformers, distribution & transmission network, thereby polluting the entire system.
M. Nageswara Rao has been working with NTPC Electric Supply Company Ltd (wholly owned subsidiary of NTPC), Noida, India as Manager (Engg.). (Mobile:
+91-9650992103; e-mail: nageswar_nescl@ntpceoc.co.in ).
Application of Power Electronics
in electricity distribution system for improvement in power quality
M. Nageswara Rao
P

Poor power quality affects Utilities with Frequent failures of equipment, Reduced life time of equipment,
Reduced safety levels of installations, Increased carbon footprint, Increased kWh losses in network
components like DTs and cables etc., Reduced system capacity, Nuisance tripping of safety devices,
Vibration and audible noise in electrical machines like motors, transformers etc., Large neutral currents.
Similarly, poor power quality affects industrial consumers with Production loss, Non-compliance with utility
regulations, DG hunting , Frequent failures of equipment, Reduced life time of equipment, Vibration and
audible noise in electrical machines like motors, transformers etc., Low p.f. and hence penalty. Commercial/
Residential consumers also get affected with Increased kWh consumption and billing charges, Low p.f. and
hence penalty, Reduced life time of equipment etc.
II. POWER QUALITY
Power quality is defined by
a) Magnitude variations in fundamental voltage of power supply, and
b) Waveform distortion of fundamental voltage and current of power supply.
The term Power Quality is rather nebulous and may be associated with reliability by electric utilities.
Power Quality refers to those characteristics of power supply that enable the equipment to work properly.
Reliability refers to the non-availability of electricity supply to consumers because of sustained interruptions.
The common power quality issues are
a) Transients
b) Short-duration variations
a. Voltage sag
b. Voltage swell
c. Momentary interruptions
c) Long-duration variations
a. Interruption, sustained
b. Under voltages
c. Over voltages
d) Voltage unbalance
e) Waveform distortions
a. Harmonics, Inter-harmonics
b. Notching
c. Noise
f) Voltage fluctuations
g) Power frequency variations
IEEE 1159-1995 stipulates typical characteristics like time duration and voltage magnitude variations of
the above power quality distortions, as tabulated below.
Instantaneous
(0.5–30 cycles)
Momentary
(30 cycles–3 s)
Temporary
(3 s–1 min)
Sustained
(>1 min)
Others
Voltage sag 0.1–0.9 pu 0.1–0.9 pu 0.1–0.9 pu -- --

Voltage swell 1.1–1.8 pu 1.1–1.4 pu 1.1–1.2 pu -- --
Interruptions -- <0.1 pu <0.1 pu 0.0 pu --
Under voltages -- -- -- 0.8 pu --
Overvoltage -- -- -- 1.1–1.2 pu --
Waveform
distortion
(Harmonics)
-- -- -- Steady state
Voltage fluctuations -- -- -- Intermittent
0.1–7%
Power frequency
variations
-- -- -- <10 s
Various international standards have been evolved to maintain power quality in electrical systems. They
are listed as follows:
i) IEEE 519-1992, Recommended Practices and Requirements for Harmonic Control in Electric
Power Systems established limits on harmonic currents and voltages at the point of common
coupling (PCC), or point of metering. This standard stipulates max. acceptable levels of Total
Harmonic distortions of voltage and Total demand distortion of currents of supplies as follows:
Application Class THDV % (max.)
Special System 3%
General System 5%
Dedicated System 10%
.
where,
Isc: Maximum short-circuit current at the Point of Common Coupling (PCC).
IL: Maximum demand load current (fundamental) at the PCC.
ii) IEC 61000-3-2 and IEC 61000-3-4: These standards specify limits for harmonic current
emissions applicable to electrical and electronic equipment, and intended to be connected to
public low-voltage distribution systems.
iii)IEEE Standard 1159-1995, Recommended Practice for Monitoring Electric Power Quality
iv) IEEE Standard 1250-1995, Guide for Service to Equipment Sensitive to Momentary Voltage
Disturbances

III. POWER ELECTRONIC SOLUTIONS
Conventional solutions like APFC panels, Voltage boosters, Static Balancer Transformer etc. have poor
dynamic response and are limited in improving power quality. On the other hand, with improvement in
Power Electronic systems and microcontroller development, various solutions have been evolved for power
quality in electrical distribution systems.
The Power electronic solutions for power quality improvement can generally be categorized in two Shunt
controllers and series controllers.
i) Shunt controllers:
a. Distribution Static VAR Compensators (D-SVC)
b. Distribution Static Synchronous Compensators (D-STATCOM) (or) Active -Filters
ii) Series Controllers
a. Dynamic Voltage Restorer (DVR)
Shunt controllers protect the utility electrical system from the unfavorable impact of customer loads. They
are recommended mainly for mitigation of the causes of disturbances, and not their effects in distanced
nodes of a power-electronics system. Series controllers are preferred in case when reduction of disturbances
effects is required, that leads to protection of sensitive loads from the deterioration in the supply-side
voltage.
IV. DISTRIBUTION STATIC VAR COMPENSATORS (D-SVC)
The Static VAR Compensators have been widely used by utilities since the mid 1970s in the world. SVC
provides reactive power, load balancing, power factor improvement, and also helps in reducing voltage
variations and associated light flicker due to arc furnace loads.
SVC is based on conventional capacitors and inductors combined with thyristor switching facilities. TCR
(thyristor controlled reactor) is connected to either Fixed Capacitor banks (FC) or Thyristor Switched
Capacitor banks (TSC) through a step-down transformer to the system, as shown in the figure below.

The rating of the reactor is chosen larger than the rating of the capacitor by an amount to provide the
maximum lagging VARS that have to be absorbed from the system. By changing the firing angle of the
thyristor controlling the reactor from 90° to 180°, the reactive power can be varied over the entire range
from maximum lagging VARS to leading VARS that can be absorbed from the system by this compensator.
It is common to use wye–delta transformers with SVCs because the delta windings provide a path to
circulate zero-sequence components of the fundamental and other harmonic currents.
The major disadvantages of SVC are the significant harmonics that will be generated because of the partial
conduction of the large reactor under normal sinusoidal steady-state operating condition when the SVC is
absorbing zero MVAR. These harmonics can either be reduced by using delta winding in the transformer or
by using TSC instead of FC banks. Further losses are high due to the circulating current between the reactor
and capacitor banks. These SVCs do not have a short-time overload capability because the reactors are
usually of the air-core type.
V. DISTRIBUTION STATIC SYNCHRONOUS COMPENSATORS (D-STATCOM)
(OR) ACTIVE -FILTERS
D-STATCOM and Active filters are synonymously called. Development & design wise, it is called as D-
STATCOM and the same is called as Active filter in the industrial market. D-STATCOM is the most
important controller for distribution networks. It has been widely used since the 1990s.
D-STATCOM helps in precisely regulate the system voltage, Improve voltage profile, Reduce voltage
harmonics, Reduce transient voltage disturbances, and Load compensation.
The main advantage of D-STATCOM is its significant short-term transient overload capabilities, that helps
in reducing the size of the compensation system needed to handle transient events. The short-term overload
capability is up to 325% for periods of 1 to 3 seconds, which allows applications such as wind farms and
utility voltage stabilization to optimize the system’s cost and performance. The other major advantage is its
lesser power handling requirement.
Due to lesser power handling requirement, D-STATCOM is built with PWM converters (at higher
switching frequencies) with IGBTs as against Thyristors used in STATCOM (FACTS controllers) for
transmission systems.
The principle of operation of D-STATCOM is explained with following equivalent circuit of a power
system with a DSTATCOM. DSTATCOM generates a variable voltage, Vd, that is very nearly in phase with
the source voltage, Vs. The inductance in this simplified circuit, L, consists of the inductance of the coupling
transformer and filter. The voltage across the inductance, VL, equals Vs-Vd and is small in per-unit terms of
the order of 5-20%.
i) If Vs > Vd, VL is in phase with Vs and current IL lags Vs by 90°; DSTATCOM, acting as a
generator, produces leading (inductive) reactive current.
ii) If Vs < Vd, VL is antiphase with Vs and current IL leads Vs by 90°; DSTATCOM produces lagging
(capacitive) reactive current.

The general arrangement of DSTATCOM (in VSI topology) is shown in figure below. The VSI converter
is connected to the feeder via a reactor Lf and has a voltage source (Capacitor CD) on the dc side.
There are two modes of D-STATCOM operation: load compensation in current control mode and voltage
regulation in voltage control mode.
In the load-compensation mode, D-STATCOM is controlled in current mode. In this current control
mode, the feeder currents are made proportion to the fundamental, positive component of terminal voltage.
The control system of D-STATCOM has to generate reference currents, and compensating harmonic,
unbalance and fundamental reactive components of non-linear load supply currents. The block diagram of a
control system for load compensating D-STATCOM is given in the Figure below.

In Voltage Regulation mode, the operation of D-STATCOM is consistent with D-SVC, discussed in
previous section. It is realized by compensating reactive power (i.e. by injecting or absorbing reactive
power). The advantage of D-STATCOM over D-SVC is also V-I characteristics and dynamics, but this
controller is more expensive. D-STATCOM in voltage regulation mode, requires higher compensating
power than for load compensation. The block diagram of a voltage-regulating D-STATCOM is presented in
Figure below.
VI. DYNAMIC VOLTAGE RESTORER (DVR)
DVR is a series power electronic controller, protects sensitive loads from all supply-side disturbances
other than outages. They are connected in series to the feeder between supply and load. They operate as
synchronous voltage source and inject voltage into the feeder in phase with supply voltage and with required
waveform to mitigate supply side disturbances, and thereby maintaining quality power at the load side. A
typical location and operation principle of DVR is shown in figure below.

DVRs can be divided into two groups with and without energy storage (ES). ES devices like batteries,
capacitors or flywheels are used to store and deliver energy during disturbances. In cases of DVR without
internal ES, the energy is taken from the supply grid during disturbances.
VII. POWER QUALITY MEASUREMENTS
Power Quality Measurements were carried out at following locations..
i) NTPC, Noida EOC Substation
ii)M/s Goldwyn Ltd., NSEZ, Noida- An LED manufacturing unit
iii) M/s Karna Apparels (P) Ltd. , NSEZ, Noida- A garment factory
KRYKARD/ AMPROBE Power Analyzers have been used for conducting measurements on Low voltage
side of the incomers at above locations.
CASE STUDY-1: NTPC EOC Substation
Measurements were carried out on the low voltage side of incomer 9R and the results of the measurement
are tabulated as follows:
R Y B N R Y B R Y B R Y B PF1 PF2 PF3
INCOMER OF9R(27july) #1 307 360 348 53 5 6 6 240 240 240 1.3 1.4 1.4 0.86 0.84 0.81
#2 270 308 327 15 15 14 255 256 254 1.9 2.1 1.8 0.82 0.82 0.74
#3 91 105 86 71 59 68 257 256 257 3.2 3.3 3.1 0.52 0.75 0.59
INCOMER OF9R(28july) #4 202 229 254 44 13 13 13 247 248 246 2.1 2.1 2.2 0.88 0.86 0.78
CASEContents
Arms iTHD% Vrms vTHD% PF
The phase currents variations are recorded and are as follows:
100.0
150.0
200.0
250.0
300.0
350.0
400.0
A
3:29:30.000 PM
7/27/2012
12:40:58.000 PM
7/28/2012
4 h/Div
21:11:28 (h:min:s)
The current THD% of the three phases is also recorded and is as follows:
R Phase Y Phase B Phase N Phase Critical Points
Legend

5.000
10.00
15.00
20.00
25.00
30.00
35.00
40.00
45.00
50.00
55.00
60.00
65.00
70.00
75.00
80.00
85.00
%
3:29:30.000 PM
7/27/2012
12:40:58.000 PM
7/28/2012
4 h/Div
21:11:28 (h:min:s)
Following observations are inferred from above measurements and recordings that
i) % Unbalance of phase currents is very high, of the order of 15%.
ii)Loads are highly non-linear and pulsating.
iii) % Current THD increases to abnormal values upto 75%.
CASE STUDY-2: M/s Goldwyn Ltd., NSEZ, Noida
Measurement was carried out on incomer LT cables housed inside the LT distribution panel. These LT
cables are run from the 11/0.433kV Distribution Transformer and are 2 runs of single core type for R-ph, Y-
ph, B-ph and Neutral. These 2 runs of cables are terminated on common LT busbars inside the panel.
Due to limitation in clamp-on CT diameter, measurement was conducted in 2 stages. One set of data
recorded on one set of cables and the other set of data on the 2nd set of cables inside the panels. Neutral
current was calculated by the Instrument, based on the three phase current measurements.
Measurements on 1st set of cable (R1,Y1,B1) and 2nd set of cable (R2,Y2,B2) are as follows:

Note: -ve sign in currents & power is due to clamp of meter CTs in reverse direction, hence they should be
considered +ve for power drawal from UPPCL.
CT current setting in the meter was 10A whereas the clamp on CT used with the instrument was of 1000A
rating, hence multiplying factor was derived as 100 (1000A/10A). (M.F. for current readings = 100.).
Basic scope / File: 43.DAT
-2
-2
-2
-1
-1
-1
-1
-1
0
0
0
0
0
1
1
1
1
1
2
2
2
U1 I 1 U2 I 2 U3 I 3
i) % Unbalance of phase currents is very high.
iii) % Current THD is very high.
CASE STUDY-3: M/s Karna Apparel, NSEZ, Noida
Measurement was carried out on incomer LT cables of both LT feeders separately.
i) 1st LT feeder is that connected on 250kVA distribution transformer with sanctioned load of
175kVA.
ii)2nd LT feeder is that connected on common 1000kVA distribution transformer (common to 4
neighboring industries). Sanction load for M/s Karna from this transformer is 89kVA.
The measurements on 1st & 2nd LT feeder are carried out separately, and the meter recordings are as
follows:

The scope of waveforms of voltage and currents of all phases is as follows:
Basic scope / File: 01_01_01.DAT
-400
-360
-320
-280
-240
-200
-160
-120
-80
-40
0
40
80
120
160
200
240
280
320
360
400
U1 I1 U2 I2 U3 I3
i) % Unbalance of phase currents is very high.
iii) % Current THD is very high.
VIII. CONCLUSION
It is generally observed from the case studies conducted that the current distortions are very high at consumers’ PCC, and the
loads are high unbalanced. Also, due to increased use of computers, switch mode power supplies and controlled supplies, lot of
harmonics are being injected into the system.
It is high time for distribution utilities as well as consumers to install power electronic controllers to mitigate power quality
problems and to restrict the disturbances from spreading into the system.
ACKNOWLEDGMENT
Author expresses deep gratitude to NESCL (NTPC) for the extended support and motivating to present this paper in the
forum. Also, author is indebted to M/s L&T, M/s ABB, M/s P2Power Solutions for their valuable support extended which
helped in preparation of the report.
REFERENCES
[1] IEEE 519:1992 “IEEE Recommended Practices and Requirements for Harmonic Control in Electrical Power Systems”.
[2] Power Quality: Mitigation Technologies in a Distributed Environment – By Antonio Morento-Munoz (Ed.)
M. Nageswara Rao (S’11) received B.E.(EEE) from Andhra University, Visakhapatnam in and then joined NTPC in 2001. After training, he is posted to NTPC
Electric Supply Company Ltd (wholly owned subsidiary of NTPC) in Engineering dept, Noida. Also, the author received M.Tech (Power electronics & Electrical
Machine Drives) from IIT-Delhi in 2011. The author is currently working as Manager (Engg.) and deals with load flow studies of power system networks, designing
of Transmission & Distribution networks and Substations upto 220kV. The author also deals with BOQ finalization, Cost estimate preparations, Tender document
preparations etc. The main interests of author are renewable power generation like solar & wind power generation technologies, Active filters, Smart Grid technologies
etc.

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