Download Class-A Trip

Electrical Protection
Schemes at STPS
– An Overview
• Electrical Protection Schemes take actions
only after sensing the occurrence of the fault
and thus cannot prevent the fault. (Minor
exceptions are there.)
• Then why protections are required?
• – to limit the damage to the components
which are under fault.
• – to save the rest of the Power System.
• Basic components of the Protection Schemes:
• Current Transformer (CT) and Potential
Transformer (PT)
• Protective & Auxiliary Relays
• Circuit Breaker
• DC Power Source for operation of the Circuit
Breaker and auxiliary power for Relays
• Functioning of Protection Scheme:
Getting Inputs from CT and/or PT, Relay
determines whether there is any fault. If it
detects any fault then gives trip command to
the circuit breaker. Getting command, circuit
breaker disconnects the faulty sections from
rest of the power system.
Without leaving any portion of the Network unprotected,
different protection schemes are provided for different areas
of the Electrical Network, e.g.
•
•
•
•
•
Generator, Generator Transformer & Unit Transformers
Motors
220 kv & 132 kv feeder
220 kv Bus bar
6.6KV Bus and its systems.
Our current discussion will be based on:
• Generator, Generator Transformer & Unit Transformers
Protections.
Protection of Generator, Generator
Transformer & Unit Transformers
Various protections are classified into three major Trip Groups:
Class-A Trip: Results in simultaneous tripping of prime mover and generator.
Covers most Electrical faults.
Class B Trip:
Turbine is tripped first, followed by the generator, through LFP / RP interlock.
Trapped steam is allowed to do useful work as long as possible & when it is
confirmed that the Stop Valve is closed fully/near fully, the generator is tripped.
The closure of the Stop valve verified by the magnitude of power output (Low
forward power) being less than preset value.
Most process protections and thermal protections of the Generator, GT & UT fall in
this category.
Class C Trip:
The Turbine Generator unit is isolated from the grid by opening the GT Breaker
and Generator is allowed to continue supply to station load.
Generator Differential Protection
(87G)
• It is one of the important protections to protect generator
winding against internal faults such as phase-to-phase and
three phase-to-ground faults. This type of fault is very
serious because very large current can flow and produce
large amounts of damage to the winding if it is allowed to
persist. One set current transformers of the generator on
neutral and phase side, is exclusively used for this
protection. The differential protection cannot detect turnto-turn fault and phase to ground within one winding for
high impedance neutral grounding generator.
• Relay: GR-I and GR-II (7 UM 62 of Siemens)
• Class A tripping
• Instantaneous
Generator Stator Earth Fault
Protection
• Normally the generator stator neutral operates at
a potential close to ground. In case of ground
fault in any phase winding of the stator, the
normal low neutral voltage could rise as high as
line-to-neutral voltage (16.5/√3 KV in our case)
depending on the fault location. Although a single
ground fault will not necessarily cause immediate
damage, the presence of one increases the
probability of a second. A second fault even if
detected by differential relay, may cause serious
damage.
95% Stator Earth Fault Protection
(59NG)
• Fault is detected by measuring the voltage across the
secondary of neutral grounding transformer (NGT).
• NGT Voltage Ratio: (16.5/√3) KV / 240 V
• Relay Setting: 12 volt (5% of 240 volt)
• Relay: GR-I and GR-II (7 UM 62 of Siemens)
• Class A tripping
• Instantaneous
• By this protection maximum 95% from the line end of
the winding can be protected. Remaining 5% from
neutral end remains unprotected.
100% Stator Earth Fault Protection
(64G)
• An external 20 Hz signal is fed to the Stator
winding through secondary of the NGT. If an
earth fault occurs in the stator winding including
star point, the 20 Hz voltage drives a current
through the fault resistance. From this driving
voltage and the fault current the relay measures
the fault resistance.
• Relay: GR-I and GR-II (7 UM 62 of Siemens)
• Class A tripping
• Instantaneous
Generator Rotor Earth Fault
Protection (64 R)
• Any rotor field winding of the generator is
electrically isolated from the ground. Therefore
the existence of one ground fault in the field
winding will usually not damage the rotor.
However the presence of two or more ground
faults in the winding will cause magnetic and
thermal imbalance plus localized heating and
damage to the rotor metallic parts. The rotor
earth fault may be caused due to insulation
failure of winding or inter-turn fault followed by
localized heat.
.
• A DC voltage is applied between rotor shaft
(Earthed) and neutral of the Exciter Armature.
Armature winding of Exciter is connected to the
Generator Field (Rotor) through Rotating Diode. If
an earth fault occurs in Generator Rotor, the
applied voltage drives a fault current. From this
driving voltage and the fault current the relay
measures the fault resistance. Depending on
value of the fault resistance, relay gives signal for
Alarm or Trip.
• Relay: GR-I and GR-II (7 UM 62 of Siemens)
• Class A tripping
Generator Stator Inter Turn Fault
Protection (87 GI)
• Generator Stator Inter Turn Fault occurs due to failure of inter turn
insulation resulting in reduction of line to neutral voltage in the
affected phase. PT secondary voltage of the generator is fed to a
star-open delta transformer. In normal condition (Balanced
generator terminal voltage) open delta voltage will be zero but in
case of inter turn fault a voltage will be appeared there. Sensing this
voltage the relay will operate if the voltage be greater than preset
value (corresponding to single turn inter turn fault).
• Open delta voltage also appears In case of stator earth fault and
thus NGT secondary voltage is used as a correction factor for inter
turn fault protection.
• Relay: 7SJ62 of Siemens
• Class A tripping
GENERATOR UNDER EXCITATION OR
LOSS OF EXCITATION (40G)
• This protection is applied to generators to detect
reduction or loss of excitation to the field windings.
This condition of the generator may lead to heating in
turbo alternators.
• When the synchronous machine with excitation, is
connected to the grid, it generates reactive power
along with active power to the grid and the rotor speed
is same as that of grid frequency. Loss of field or loss of
excitation results in loss of synchronism between rotor
flux & stator flux. The synchronous machine operates
as an induction machine at higher speed and draws
reactive power from the grid.
• This will result in the flow of slip
frequency currents in the rotor body
as well as severe torque oscillations
in the rotor shaft. As the rotor is not
designed to sustain such currents or
to withstand the high alternating
torques which results in rotor
overheating, coupling slippage and
even rotor failure.
• A loss of excitation normally indicates a
problem with the excitation system.
Sometimes it may be due to inadvertent
tripping of field breaker, open or short
circuit of field winding or loss of source
to the exciter. If the generator is not
disconnected immediately when it loses
excitation wide spread instability may
very quickly develop and major system
shutdown may occur.
Generator unbalance load protection
or Negative sequence Current
Protection (46G)
• This is to protect the generator from sustained
unbalanced load. When the machine delivering
the equal currents in three phases, no unbalance
or negative phase sequence current is produced
as the vector sum of these currents is zero, when
the generator is supplying an unbalanced load to
a system, a negative phase sequence current is
imposed on the generator. The system unbalance
may be due to opening of lines, breaker failures
or system faults.
• The negative sequence current in the stator
winding creates a magnetic flux wave in the air
gap which rotates in opposite direction to that of
rotor synchronous speed. This flux induces
currents in the rotor body, wedges, retaining
rings at twice the line frequency. Heating occurs
in these areas and the resulting temperatures
depend upon the level and duration of the
unbalanced currents. The protective relay
extracts the negative sequence component of the
stator current and the relay characteristic is an
inverse current time operation. The setting is
matched with withstand characteristic of the
generator (provided by the manufacturer).
GENERATOR OVER EXCITATION
PROTECTION (99GT)
• Per unit voltage divided by per unit frequency
commonly called Volts/Hertz (V/F) is a
measurable quantity that is proportional to
flux in the generator or step-up transformer
cores. Moderate over fluxing (105-110%)
increases core loss resulting in increase of core
temperatures due to hysterics & eddy currents
loss. Long term operation at elevated
temperatures can shorten the life of the stator
insulation.
• Severe over fluxing can breakdown interlaminar insulation followed by rapid local core
melting. Over fluxing normally can be caused
by over excitation during Off-line condition,
and load rejection or AVR mal-functioning
during On-line condition.
Relay: GR-I and GR-II (7 UM 62 of Siemens)
• Class A tripping
GENERATOR OVER VOLTAGE
PROTECTION (59G)
• Generator voltage is at present value under
normal operating conditions as selected by
operator in AVR. If it parts from preset value,
may be due to AVR mal-functioning or a
system disturbance. Severe over voltage can
cause over fluxing and winding insulation
failure.
• Two stage operation with definite time delay –
Alarm & trip.
Generator Stand By Earth Fault
Protection (51NG)
• Relay gets input from 150/1 A CT of NGT
secondary circuit.
• In case of stator ground fault, voltage induced
in secondary of the NGT drives a current
through the NGR (0.3359 ohm). Sensing this
current, Relay operates.
• Class A tripping with normal inverse
characteristics.
Generator Backup Impedance
Protection (21G)
• The Backup Impedance Protection is
a definite time graded protection, for
a short circuit phase fault in
Generator, Generator Transformer,
Bus duct or as back up of uncleared
external network fault.
• Two stage operations are there.
• Stage I covers up to the GT and gives Class A
tripping with 0.1 sec delay.
• Stage II covers up to the longest 220 kv feeder
and gives Class C tripping with 0.6 sec time
delay. Time delay is allowed to clear the
feeder fault through tripping of the feeder
breaker itself.
• From output of the CT & PT, Relay measures
the impedance and if it comes within the
protected zone, the relay operates. Relay
characteristic is polygon shaped.
Low Forward Power Protection (37G)
and
Reverse Power Protection (32G)
• Due to stoppage of steam supply, motoring of the
TG can occur. It will not affect the generator but
the turbine blades. To avoid such situations Low
Forward power Protection (37 G) and Reverse
Power Protection (32G) are used.
• Low Forward power (0.5%):
• with Turbine trip – delay 2 sec – Class B trip
• without Turbine trip – delay 10 sec – Class A trip
• Reverse power (-0.5%):
• with Turbine trip – delay 1 sec – Class B trip
• without Turbine trip – delay 9 sec – Class A trip
GENERATOR ‘DEAD MACHINE’
PROTECTION (50GDM)
• Employed for protection against accidental
energisation of Generator unit under S/D.
• Appearance of current before normal terminal
voltage is established, within the preset time.
• Employed for protection against accidental
closing of Circuit Breaker.
• Class A protection.
Under Frequency (81 UG) and Over
Frequency (81OG) Protection
• Under frequency may occur due to sudden outage of
huge amount of Power Input to the Network and Over
Frequency due to outage of a massive load of the
Network. These are associated with System
Disturbance. Protection is required to prevent
abnormal vibration of the turbine.
• Under frequency – bellow 48.5 Hz – 3sec delay – Alarm
- below 47.5 Hz – 0.5 sec delay – Class C trip
• Over frequency – above 51.0 Hz – 1sec delay – Alarm
-above 53.0 Hz – 0.5 sec delay – Class C trip
Generator Out Of Step Protection
(78G)
• To protect the Generator going out of
synchronism or stability due to heavy external
faults, insufficient excitation etc.
• The relay measures the rate of change of
impedance seen by the Generator.
• The relay has a polygon impedance
characteristics defined by the impedance of
the Generator, GT and associated network.
• Out Of Step is detected if
• the no. of swing of the measured impedance
through the impedance characteristics in a
predetermined period exceeds the preset
value.
• positive sequence current is greater than
preset value.
• negative sequence current is less than preset
value. (as it is a symmetrical fault)
Overall Differential Protection (87 OA)
• Inputs to the Relay are from secondary of four
CTs:
• Generator neutral side CT
• GT 220 KV side CT
• UAT- A 16.5 KV side CT
• UAT- B 16.5 KV side CT
• Protect the Generator, GT, 16.5 KV Bus Duct from
phase to phase fault, three phase-ground fault
etc.
•
•
•
•
•
•
•
•
•
•
The Relay is of Biased Differential characteristics.
Relay settings are as follows:
Pick up: 0.2 I/In
Slope I: 0.25
Base point I: 0.0 I/In
Slope II: 0.5
Base point I: 2.5 I/In
2nd Harmonic Restraining: 15%
5th Harmonic Restraining: 30%
High Set: 7.5 I/In
Relay:7UT635 of Siemens
Class A tripping
GT HV side Restricted Earth Fault
(64 HGT)
•
•
•
•
For protection of HV winding of GT.
HV winding is star connected.
Neutral and three phase Bushing CTs are used.
Operates in Differential principle with
Stabilising resistance.
• Setting: 0.2 A
• CTR: 800/1
• Calculation for Stabilising Resistance:
• GT full load current 315000/1.732/220= 826.68 A
• Max fault current = 826.68/0.145 = 5701 A
(%Impedance 14.5)
• Secy. Fault current = 5701/800 = 7.13 A
• Stabilising voltage
= 1.3 x If x (Rct+Rrelay+2 x R lead)
= 1.3 X 7.13 X (4 +0+2 x 1.1)
= 57.44 volt
• Stabilising Resistance = 57.44/0.2 = 287 ohm (relay
setting 0.2 A)
• Relay:7UT635 OF Siemens
• Class A tripping
GT Over current Protection (51 GT)
•
•
•
•
•
•
Relay type: 7SJ61 of Siemens
Pick up : 0.9 A (CTR 1000/1)
Time setting 0.25 sec
Normal Inverse characteristics
High set : 8 A (Inst)
Class A tripping.
UAT Differential protection (87UAT)
and 6.6 kv side REF (64R)
•
•
•
•
•
•
•
•
•
•
•
Biased Differential Protection
Pick up:
0.2 I/In
Slope I:
0.25
Base point I: 0.0 I/In
Slope II: 0.5
Base point I: 2.5 I/In
2nd Harmonic Restraining: 15%
5th Harmonic Restraining: 30%
High Set: 12.0 I/In
I-REF : 0.2 A
Stabilising resistance : 1000 ohm.
Relay:7UT613 of Siemens
Class A tripping
UAT Over current Protection
(50/51 UAT)
•
•
•
•
•
•
Relay type: 7SJ61 of Siemens
Pick up : 5 A (CTR 800/5)
Time setting 0.6 sec
Normal Inverse characteristics
High set : 60 A (Inst)
Class A tripping.
GT /UAT PROTECTIONS
(Mechanical)
• Gas operated Buchholtz relay protection
• Operates when gases produced by Electrical
discharge/arcing inside the transformer travel from
Tank to Conservator.
• Alarm element operates by the slow collection of
gas in the gas trap of the relay while the gases are
escaping from the transformer tank to the
conservator. The collected gases displaces oil
downward and the float switch operates.
• Sudden or rapid rush of gases and oil to the
conservator, for rapid arcing or a severe fault pushes
a flap in the flow path, to operate the trip element.
• Also trips for very low oil level in Transformer, in
case of tank leakage, which is sensed by fall in Oil
level in conservator.
• Pressure Relief Device (Diaphragm):
• Operates for excessive pressure rise and / or rate
of rise of pressure.
• Its purpose is to Avoids rupture of tank, in case of
an internal fault/flashover leading to Oil spillage
and spread of fire.
• Oil surge protection:
• Normally provided for ‘OLTC’ Chamber
• It is similar to Buchholtz relay, but with only oil
surge element (no gas collection chamber and
float)
• OIL TEMPERATURE (ALARM & TRIP):
• Set as per manufacturer’s recommendation
• Operates for condition like sustained over load or loss of cooling which
cause abnormal Oil temperature rise which can reduce the life of the
transformer insulation.
• WINDING TEMPERATURE (ALARM & TRIP):
• Top oil temperature corrected for I2R heating by proportional current in
replica resistance, otherwise functionally identical to OTI system above.
• ‘Mercury contact’ switches mounted on the temperature indicating dial
Gauge gives alarm
• In case, the Unit operator ignores or neglects Alarm, trip contact operates
and initiates ‘Unit’ tripping.
• OIL LEVEL (MAGNETIC OIL GAUGE) ALARM:
• MOG alarm operates on Conservator oil level ‘low’ condition, before
Buchholtz’s relay trip occurs, to give early warning;
• NOTE: Regular noting of OTI and WTI should be done