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Terminal voltage decay test on shutdown and tripping of TG-701/702

Objectives 1. To check the health and functionality of shutdown system and field suppression winding

circuit on Generator 701/702.

2. To re-visit the incident of 2004 when there was a three phase short circuit and ground fault on the main circuit breaker of TG-702 due to inadvertent closing of its earth isolator switch at Substation No.1 when TG-702 was running at FSNL.

3. To determine the shortcoming in the control logic of TG 701/702 system leading to above incident and carry out measures to avoid its recurrence.

Background

In 2004, there was a severe short circuit and ground fault on the Circuit Breaker of TG-702 due to inadvertent closing of its earth isolator switch at Substation # 1 when TG-702 was at FSNL. There was a significant damage occurred to TG-702 breaker and its adjacent panels on MS-1 in Substation # 1.As a result TG-702 and its bus bar B remained off-line under repair for about a month.

Data required for above analysis

Following data is required to assess above incident.

a) Generator field voltage and generator terminal voltage decay rates in case of normal shut down of turbine and generator.

b) Generator field voltage and generator terminal voltage decay rates in case of turbine and generator tripping on actuation of lock out relay 86G.

The difference between normal shutdown and tripping of generator on severe fault is the activation of field suppression circuit through the lockout relay 86G/HR. It injects DC current in the suppression winding of the saturable current transformer (SCT) and suppresses the excitation voltage and brings down the generator terminal voltage in minimum time. When actuated, the field suppression windings remain in circuit till the drop out of 14HM (2% speed) and thereafter they are isolated by 41S.

TG-701 & 702 shutdowns were planned on Nov 19 & Dec 31, 2014 respectively. Availing these opportunities, the turbines were tripped manually on generator differential protection 87G1 to observe generator terminal voltage and field voltage decay rates.

On Jan 1, 2015 TG-702 was shutdown at FSNL from Master Control OFF switch to observe Generator terminal voltage and field voltage decay rate during normal shutdown

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Sequence of Activities

1. Load was reduced to 2 MW on Gas Turbine as per SOP.

2. Scope Meter was connected across field voltmeter, located on GCP to record the trend of generator field voltage (This logging done for TG-702 only).

3. Power Quality Analyzer was connected to "Incoming Voltmeter", located on the GCP front panel door to log the trend of generator terminal voltage.

4. DC Clamp On meter was clamped to wire no. 875 on terminal No. 53 of TBC15 in GAC panel to measure the Field Suppression Current.

5. AC Clamp meter was clamped to outgoing cable from GAC panel to skid fan motors MK 706 A/B to measure the decrease in current rate to these motors (for TG-702 only).

6. A video camera was used to record the metering on generator control panel front door. Video Links at the end of report.

7. Tripping signal was given by actuating the Differential protection Relay 87 G-1 which in turn actuated the Generator lock out relay 86G.

8. Actuation of 86G tripped the turbine and generator simultaneously. Generator Field Suppression windings were energized with actuation of lockout Relay 86G and DC amperes were recorded.

9. The change in 6.3KV terminal voltage & 250 Volts field voltage was observed and recorded (Field Voltage recorded only for TG-702).

Observations

TG-701 Tripping data on November 19, 2014, TG tripped on Generator Lockout Relay 86G

1. The TG-701 generator terminal voltage decayed exponentially with time; while the field voltage reduced in step from 100 volts to 10 volts in 3 seconds.

2. After 5 seconds, the generator terminal voltage reduced to 4100 volts and in 40 seconds to 78 volts (Reference: Graph No. 1)

3. The current in suppression winding was recorded as 12.38 Amps DC .Which reduced to 0 when Speed Relay 14HM dropped out @ 2% speed.

TG-702 Tripping data on December 31, 2014 TG tripped on Generator Lockout Relay 86G

1. TG-702 generator terminal voltage decayed exponentially with time at much reduced rate compared to TG-701. Field voltage reduced from 107 volts to 7 volts in 26 seconds. (Reference: Graph 2)

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2. After 5 seconds, the generator terminal voltage reduced to 5500 volts and in 210 seconds to 48 volts. 190 seconds more time as compared to TG-701.

3. The current in suppression winding was recorded as 9.55 Amps DC (3 Amps lesser than TG-701). This current reduced to 0 when 14HM (Speed Relay) dropped out @ 2% speed in 2min 36 seconds.

4. The current drawn by the radiator cooling fan motors MK 706 A/B was 78 amperes before tripping of turbine. It decreased gradually to 23 Amp in 1 min 30 seconds and maintained this value for about 40 sec. Later it gradually decreased to zero in 2 min 36 seconds.

TG-702 Normal Shutdown from FSNL from Master Control OFF switch on January 01, 2015

1) The TG-702 generator terminal voltage decayed exponentially with time while the field voltage reduced in step from 81 to 11 volts in 94 seconds. (Field Voltage reference: Graph 2)

2) After 5 seconds, the generator terminal voltage reduced to 5.9kV and in 133 seconds to 48 volts. (Reference: Graph no. 1)

3) In normal shutdown of TG-702 the terminal voltage of generator reduced to 48 volts in 133 seconds as compared to 210 seconds in fault condition. This anomaly to be investigated.

SWR and CWR Resistors

CWR 1, CWR 2

CWR 3, SWR 3B

SWR 3A, SWR 2B

SWR 2A, SWR 1B

SWR 1A, SWR 4

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Conclusion: The voltage decay rate for TG-702 on tripping of gas turbine at 86G/HR is too long as compared to TG-701 (200 vs 50 seconds). This indicates a fault in the circuit and is under investigation. Following additional measurements were taken:

SN Description TG 701 TG 702 Remarks

1 Suppression winding (SW)

parameters

a Resistance of Suppression circuit

between E4-E5 * 12.4

Between E5 and E4 terminals in GCP. 41S was

operated manually to open its NC contact

(Ref: Figure No.1)

b Suppression winding of SCT

between terminals 7 and 8 * 1.4 Ea.

Measurement Terminals: 7 and 8 terminals on

each SCT. (This measurement on spare SCT is

found to be 1.7 ohms).

c SWR1A, SWR1B, SWR2A, SWR2B,

SWR3A and SWR3B * 4.5 Ea. As per specifications.

d SWR4 * 4.5 As per specifications.

e SWR5 * 3.5 As per specifications.

2 Control Winding (CW)

parameters

a Load on Generator 11.6MW 11.3MW

b Current through Control winding 2.2 Amp DC 1.24 Amp DC On wire no. 868 TBC-15 in GAC Panel by

clamp meter.

c Voltage across control winding 53 VDC 60.6 VDC Measurement terminal: 868 &869 in GAC

d AC volts across E4-E5 22.8 Volts 24.6 Volts Across wire 872-875 @

TG-701 9.3MW TG-702 8MW

*To be measured on shutdown of TG-701.

Future Checks:

1. The data of generator terminal voltage drop rate in case of turbine tripping is taken for the very first time since commissioning of TG-701/702.

2. The normal shutdown voltage decay rate for TG-701 will be recorded in the next planned shutdown for comparison.

3. Fuse rating of FU-2, (10 Ampere) and the reason for prolonged voltage decay rate for TG-702 is being taken up with the GE.

4. Resistance across suppression winding of TG701 shall be measured in next shutdown.

Video Link: \\GM-ELCT1-KASHF1\share\VIDEO LINK

Prepared By: Reviewed By: Muhammad Kashif Idrees/ Basharat Tahir Abdul Rauf/Faisal Naeem

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Graph No.1

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Graph No.2