The document summarizes troubleshooting done for a thyristor open circuit alarm on a power station unit. Actions taken included monitoring the exciter PLC and collecting raw data. Findings showed the alarm was occurring when either converter was in use, currents were balanced, and signals from two analogue input modules were lower. It was recommended to check the functionality of the DCCTs with a process calibrator.

1. KARIBA SOUTH POWER STATION
UNIT 2 THYRISTOR OPEN CIRCUIT
TROUBLESHOOTING
23 JANUARY 2019

2. Presentation Road Map
1. Introduction
2. Actions carried out
3. Findings
4. Recommendations
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PRESENTATION ON THYRISTOR OPEN CIRCUIT FAILURE ALARM JAN
2019

3. Introduction
Unit 2 Exciter experienced a thyristor
open circuit alarm on 21/01/2019
Troubleshooting was carried out on
22/01/2019
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PRESENTATION ON THYRISTOR OPEN CIRCUIT FAILURE ALARM JAN
2019

4. Actions carried out
 The Exciter PLC was connected and
monitored using the Exciter Laptop.
 Raw data was collected from the AI
modules.
 Actual signal VDC values were measured
using a multi-meter.
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PRESENTATION ON THYRISTOR OPEN CIRCUIT FAILURE ALARM JAN
2019

5. Actions carried out
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PRESENTATION ON THYRISTOR OPEN CIRCUIT FAILURE ALARM JAN
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6. Actions carried out
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PRESENTATION ON THYRISTOR OPEN CIRCUIT FAILURE ALARM JAN
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7. Actions carried out
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PRESENTATION ON THYRISTOR OPEN CIRCUIT FAILURE ALARM JAN
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8. Actions carried out
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PRESENTATION ON THYRISTOR OPEN CIRCUIT FAILURE ALARM JAN
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9. Actions carried out
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PRESENTATION ON THYRISTOR OPEN CIRCUIT FAILURE ALARM JAN
2019

10. Actions carried out
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PRESENTATION ON THYRISTOR OPEN CIRCUIT FAILURE ALARM JAN
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11. Findings
 Thyristor Open Circuit Alarm was observed
when either Convertor 1 or Convertor 2
were in service
 The currents on the ammeters were
balanced
 This showed the problem was neither in
the thyristors nor in the thyristor firing
card but was in the monitoring circuit.
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2019

12. Findings
 After measuring, signals from AI10 and AI14 were
relatively lower compared to the rest of the signals.
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2019
Signal VDC
AI Module HE693 ADC405 (AI9-AI12)
AI9 0.843
AI10 0.325
AI11 0.426
AI Module HE693 ADC405 (AI13-AI16)
AI13 0.84
AI14 0.337
AI15 0.707

13. Findings
 It was discovered that both Analogue Input
modules were not faulty since they were
showing a true reflection of signals from
the DCCTs.
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2019

14. Recommendations
There is need to connect the process
calibrator on the outputs of the DCCTs
to measure their current output and
check for their correct functionality.
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2019

15. THANK
YOU!
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