Generator Transformer Protections.

GENERATOR TRANSFORMER
PROTECTION
(ELECTRICAL PROTECTIONS ONLY)
- NISCHAL POPAT
(DEPUTY MANAGER - EMD)
ESSAR POWER GUJARAR LTD. JAMNAGAR

• Transformer which is connected with generator is called generator transformer.
• Working as an electrical engineer, we all know about importance of power
transformer protection.
• In this file, Following listed protection has been described.
1. Transformer biased differential protection – 87GT.
2. Overhang differential protection – 87L.
3. Backup earth fault protection. – 51NGT.
4. Overall differential protection. – 87OA.
5. Over-fluxing protection. –99GT
6. Transformer HV over current protection. – 51GT.
7. Restricted earth fault protection. – 64RGT.
(CT Details are taken here as an example, Reader can took it as reference and
can calculate for their plant.)

 Transformer Biased Differential Protection 87GT:
 Differential relay is used to give protection against internal zone faults. It should not
give trip command on outside zone fault (through fault).
 Transformer differential relay detects following type of faults.
1. Phase-phase fault (For HV & LV both winding)
2. Phase-ground fault (For HV & LV both winding)
3. Inter-turn fault in transformer
4. Core fault
5. Tank fault
 Differential relay compares corresponding currents of same phase on both side of
transformer.
 In normal condition differential current should be zero, but there are some conditions
arise where some amount of current difference arises for same phase and it will
measure by protection relay.

 Those conditions are discussed as under.
 Magnetic inrush current of transformer leads to malfunction of relay.
Due to 3 conditions magnetic inrush current will rises.
1. During energising transformer: At starting while energising transformer,
magnetic inrush current picks up-to 8 to 30 times of full load current. It can
lead to malfunction of relay.
And also while charging transformer primary winding will carry no load
current and this condition leads to flow of differential current into relay, so this
condition should be avoided.
2. Voltage recovery following external fault: This condition rises when fault
occur on nearby feeder, at this condition terminal voltage of transformer will be
zero.
When fault get clear and faulty section gets isolated at that time voltage will
rise to some high value and it will lead to induce high magnetic inrush
current.
At this instant protection relay should not generate trip command.

3. Sympathetic inrush due to a parallel transformer being energized:
Sometimes due paralleling a transformer, high voltage pick comes to other
transformer, which further induce high magnetic inrush current and further it
can give tripping.
 If character of fault got changed immediately at this instant also relay can give false
tripping, so this condition also should be avoided. (Ex. Fault character change from P-
G fault to P-P-G fault.)
 While tap changing process 20-30% of differential current flow through differential
relay on base of particular load, so at this instant also relay should not give tripping
command.
 CT ratio error of HV winding and LV winding will induce differential current in healthy
condition, because we know it is impossible to match CT ratio of both side without its
error.
(Currently ratio correction function is provided by digital numerical relay to ensure
correct operation.)

 Vector group correction need to be done to compensate phase shift between two
windings.
 Zero sequence current filters should be provided to eliminate zero sequence current,
because in delta-star connected transformer zero sequence current flows on one side
of transformer without flowing in other side (Flows in neutral), so this condition leads
to flow differential current.
(Currently zero sequence filter function is provided by digital numerical relay.)
 To avoid such conditions percentage biased differential protection is used which
avoids such conditions using slope feature.

2 Protection CT’s are used in differential protection for each phase. (SLD Shown
further for reference)
Differential Protection SLD

Percentage biased differential scheme & graph is shown in figure.

 As shown in graph, one offset is provided to avoid no load current of transformer.
 The current on X axis is called restraining current or biasing current (IB) which is
average current of primary and secondary current of transformer.
 And current on Y axis is called differential current, which is resultant difference of
primary and secondary current. (ID).
 The differential relay will generate high bit for trip command if differential current will
be more than set percentage biasing current.
 Consider that differential relay will operate if ID will be greater than A% of IB.
 ID > %A*IB,
 Therefore ID/IB > %A
 Here ID/IB = Slope,
 Hence differential relay will give trip command if slope value will be greater than
set % value in relay. (For graph calculation)

Ex. Condition 1. (Normal condition)
 Consider that current measure by both CT of same phase is 2A.
 Hence Biasing current will be (2+2) / 2 = 2A.
 Differential current will be 2-2 = 0A.
 Hence relay will not give trip command.
 In graph we can see that value of differential current increases with increase in value
of biasing current.

Ex. Condition 2. (Through fault condition)
 Consider that, and first protection CT current is 3A & second protection CT of same
phase got saturated due to through fault heavy current.
 Hence in this case biasing current will be (3+0) /2 = 1.5
 Differential current will be 3-0 = 3
 Here the condition arise where
 DIFFRENTIAL CURRENT > BIAS CURRENT
 But in this condition also relay should not give trip command, so how in now days 3
slope function relay is used (As shown in graph) which eliminate this type of
condition and avoid malfunction tripping.

Differential relay settings are as under. (APPLICABLE FOR MICOM P643 Only)
 Is1 – Minimum differential threshold ( Setting – 0.2 pu)
 K1 – First slope setting of low set differential protection. ( Setting – 20 %)
 Is2 – Bias threshold current. ( Setting – 1 pu)
 K2 – Second slope setting of low set differential protection. ( Setting – 80 %)
 T Diff LS – Low set differential time delay.
 Is HS1&HS2 – High set differential element 1 & 2. 1 settle in simple mode and 2
settle in advance mode. ( Setting for advance mode – Is HS 1: 9 pu, Is Hs 2:
12 pu )
 Zero sequence filter on HV or LV side. – To eliminate zero sequence current.
( HV side enable & LV side disable)

Case-1:
 Is1/K1 ˂ I bias ˂ Is2.
 I Diff = K1*I bias.
Case-2:
 I bias ˃ Is2.
 I Diff = K1* Is2 + K2 (I bias – Is2)
 Protection CT for HV side – Located at GT HV Line bushing .
 Protection CT for LV side –Located at Generator bus duct (Phase side).

Overhang differential protection 87 L:
 It is unit type protection, which covers large portion of HV side of generator
transformer.
 It is also known as HV differential OR Line differential protection.
 It covers area of transformer HV winding to switchyard HV current transformer.
 Generally it provides protection against earth fault on HV side of generator
transformer.
 For protection purpose alarm and tripping command both are taken from it.
 Protection related theory and setting calculations are same as per biased differential
protection.

Overhang Differential Protection SLD

Overhang Differential relay settings are as under. (APPLICABLE FOR MICOM
P643 Only)
 Is1 – 0.2 pu
 K1 – 20 %
 Is2 – 1 pu
 K2 – 80 %
 Is HS1 – 9 pu
 Is HS2 –12 pu
 Zero sequence filter on HV and LV side – HV side disable & LV side disable

Backup earth fault protection 51NGT:
 Backup OR standby earth fault protection operates for inside zone & outside zone
both fault condition.
 This type of protection is either DMT type or IDMT type.
 This relay is connected on load (HV) side of generator transformer, which provides
local back up protection.
 If transformers LV side protection got failed then stand-by or backup protection will
act and clear fault, and will give trip command to both primary and secondary CB.
 In 51N type protection relay is connected to neutral return path means at residual
path.
 The residual current will be derived from three phase current input.
 This type of relay will respond to unbalance phase current, which is caused by
ground fault or load unbalance current.
 In 51G type protection relay is connected to ground sensor. This relay will respond
to ground fault only.

Earth fault relay setting is as under.
IN ˃ 1 –First stage tripping setting in A = 0.1A
Time delay – In Second = 1 Second.
As current in earth will goes beyond this setting, after completion of given time delay
relay will give trip command.
Backup earth fault protection SLD

Overall differential protection 87OA
 Overall differential protection is required in power plant. In overall differential
protection no. of biased input are required as per scheme shown in further slide.
 It is fed from CT’s of the GT-LV side, GT-HV side, and LV side of UAT&SAT.
 Protection related theory and setting calculations are same as per biased differential
protection.

Overall differential protection SLD

Overall Differential relay settings are as under. (APPLICABLE FOR MICOM P645
Only)
 Is1 – 0.2 pu
 K1 – 20 %
 Is2 – 1 pu
 K2 – 80 %
 Is HS1 – 9 pu
 Is HS2 –12 pu
 Zero sequence filter GT Side – Enable.
 Zero sequence filter Gen Side – Disable.
 Zero sequence filter UAT LV Side – Enable.
 Zero sequence filter SAT LV Side – Enable.

Over fluxing protection 99GT
 Over fluxing is generally occurring on transformer which is directly connected to
generator. This condition may rises while starting or stopping generator.
 In normal condition voltage should be gradually increases with frequency.
 (V/F ratio should be within limit)
 Over fluxing condition occurs due to overvoltage or under frequency condition due
to this V/F ratio increases and which further increase magnetizing current.
 This magnetizing current further generates greater amount of heat and it can fail the
insulation.
Now Φ = KE/F,
 Means Magnetic flux density ∝ voltage/frequency

 Magnetic flux density is depends on two factor
1. Current flowing in primary winding, and this current depends on voltage applied
across primary winding. If higher amount of voltage is applied on transformer
winding, it can damage winding and insulation of transformer
This condition rise if load on generator suddenly falls, due to this, terminal voltage
of generator will rise to very high value and same will impact on generator
transformer.
2. Impedance is depends on frequency. And so why if nominal voltage with lower
amount of frequency is applied on transformer, due to low inductive reactance
higher amount of current will flow through winding. And which can damage
transformer.
This condition rise if generator rpm falls down.

 Over fluxing can be avoided by using over fluxing protection relay. It will require 3
phase VT input and relay will continuously monitor V/F ratio. If V/F ratio will exceed
from its setting, relay will give trip command.
 Morden technology is equipped with V/HZ limiters, which limit voltage accordance
with frequency.

Over fluxing relay settings are as under. (APPLICABLE FOR MICOM P643 Only)
 V/HZ alarm setting = 2.31
 V/HZ alarm time delay = 10 Second
 V/HZ ˃ 1 first stage tripping setting = 2.31 (IDMT Type)
 V/HZ ˃ 2 Second stage tripping setting = 2.75
 Time delay – In Second = 4 Second.
 V/HZ ˃ 3 Third stage tripping setting = 3.08
 Time delay – In Second = 1 Second.
 V/HZ ˃ 4 fourth stage tripping setting = 3.3
 Time delay – In Second = 0.4 Second.
 As abnormal condition occurs and V/HZ ratio goes beyond this setting, after
completion of given time delay relay will give trip command.
 (VT Secondary input – 110V, Frequency – 50Hz is considered in settings)

GT HV Over current protection 51GT
 Transformer cannot run on overload for long time because it may reduce transformer
life and also it can damage insulation of transformer, that’s why over current
protection on transformer is provided on load side of transformer.
 Over current relay cannot distinguish between in-zone and out-zone fault.
 Over current relay may be DMT or IDMT type which detects following type of
abnormal condition in transformer.
 Over load of transformer
 Phase-phase fault (For HV winding)
 Over current setting is taken at 120% of transformer FLC.

Restricted earth fault 64RGT:
 In power transformer REF protection is used to detect internal earth faults of
transformer.
 Restricted earth fault means it will respond to only restricted zone earth fault, it
should not give trip command for outside zone faults.
 In this type of protection all 3-phase CT’s connected together and common
terminals connected to NCT (Neutral CT).
 It is also differential type protection which compares un-balance current of phase
with neutral current.
 However question may arise that, differential protection can also detect such type
of fault, but differential relay has certain minimum value of pickup for fault current,
(Generally 20%) fault current below this value cannot detected by differential relay.
Such types of fault like Winding-core fault, 1-phase to ground type fault etc. That’s
what a reason to use a one additional protection for transformer in name of REF.
 REF protection is cheaper than differential protection.
 It is zone type protection which will give trip if earth fault occur inside zone and it
will avoid outside zone earth fault.
 Generally REF protection is taken as instantaneous protection, no time delay is
provided.

Relay setting is as under:
 IN ˃ 1 –First stage tripping setting in A = 0.1A
 As current in earth will goes beyond this setting, after completion of given time
delay relay will give trip command.

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