6. SYNCHRONIZER OPERATION AND MAINTENANCE
Imagine the situation where the operator starts the synchronization and your device signals the malfunction due to internal fault. The operator calls you up to fix the problem. No big deal, you have the situation under control and begin with repair. Everything looks fine except that the operator knows that the plant manager is going to ask him why the generator hasn't been generating power (dollars) when expected. So the tension increases. You have two options: to do the repair right at the moment or to proceed with back-up synchronization mode and do the repair later. I prefer the second option. Why would anybody work under pressure if it is possible to avoid it? I found it to be the best strategy in maintenance - find the problem and fix it when you are ready and not when others urge you to do it. But you need a back-up strategy.
Back-up strategy
How can you delay the synchronizer repair? Simply by activating the back-up operating modes: semi-automatic or manual mode of synchronization. You switch your device to a non-automatic operating mode which will allow you to perform the synchronization with reduced functionality. After you finish the synchronization, you make plans how to repair the synchronizer during the next generator stop. Then, you usually have several hours or days to fix it. But this time, you are prepared.
What are these back-up modes? These are special operating modes which allow you to overcome internal malfunctions by using secondary command paths or to check the validity of internal functions. I will list some of them:
Other-than-automatic modes
Semi-automatic mode is the operating mode in which automatic-mode part of the synchronizer is disabled, in case it malfunctions but you still have some supervisory functions available. In this back-up mode, the operator gives the commands locally (using push-buttons on the cubicle door) and synchro-check function of the synchronizer will just assist in circuit breaker closing (permissive action). The operator will watch the synchronizing instruments at the cubicle door, issue commands for changing the voltage and frequency and, according to his judgment, he will issue the circuit-breaker closing command. On the other hand, synchro-check will generate its closing command only when the voltage, frequency and phase differences are within preprogrammed values. The operator closing command is in series with the synchro-check approval and, if both commands are simultaneously issued, the final closing command is generated. A tip for you: take care that the operator must be aware if the synchro-check approves the differences or not. You must install some indicators which tell the operator how to guide the voltage and frequency to approach the limits of the synchro-check. Without it, the operator's judgment on “low difference” may not be aligned with the synchro-check and the final closing command will never be released. Then you may end up with the operator's "faulty semi-automatic synchronization" report.
Manual mode is the operation mode where even the synchro-check function is not available. Then, all the commands for voltage and frequency matching are performed by the operator and also the closing command but, this time, no supervision is provided. When he requests for the breaker closing, it will close. Even out the phase coincidence. Here, the operator must be very skilled to perform decent synchronization. A tip for you: This mode is your last resort, when almost everything inside the synchronizer is dead. In my opinion, you will use this mode either intentionally or in the case of serious neglecting in maintenance. Be very careful when you come to this.
In order to have functional back-up strategy, all commands in three operating modes (automatic, semi-automatic and manual) should be functionally separated by design. This means that the command circuits for automatic mode and those for semi-automatic mode have no common parts except the output terminals. The same is true for the manual mode. For example, you have one set of relays for automatic mode, the other set of relays for semi-automatic mode and the third set of relays for manual mode. Also, the push-buttons for the operator should have separate contacts for semi-automatic and for manual mode (yes, I know it’s a lot of extra equipment but at the moment you need it you will be grateful for having them). Considering the input circuits (statuses), they are essential only for automatic mode and other modes can be activated without them. The synchronizing instruments are vital for semi-automatic and manual mode but they will be used occasionally so chances are that they will not fail soon. Using this strategy in the design, you will lower the number of common elements and avoid bottlenecks.
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Test mode
Finally, I have mentioned the test-mode. In this mode, you are allowed to check the operation of the automatic mode without external ready command and with the possibility to disable or cut-off the circuit-breaker closing command output. It is used while the generator is stopped.
You will also need a special two-channel signal generator which is able to generate two voltages with variable amplitudes for both channels and variable frequency for at least one of them. To use it, your synchronizer must have the special inputs for the simulated voltages or disconnecting-terminals for voltage inputs (so you can disconnect the external voltage circuit and connect the wires from the signal generator). Finally, when you start the synchronization in this mode (it will start regardless of missing ready signal), you can variate the voltages and frequencies to check:
After you check these, you may be pretty sure that you synchronizer will operate as expected.
Preventing the problems
The operating modes shown above will give you the chance to avoid high-tension situations and delay the repair until relaxed situation appears. But, the most effective way is to prevent, to catch the problem before it gets into your face. To do it, you should track the synchronization logs during time. You should focus on features like duration of synchronization and active and reactive power values right after the breaker is closed. If any of these values deviates from acceptable levels or change over time, you can expect the problem at the horizon. For example, if the duration of the synchronization is significantly changed, it is possible that changes in the excitation or turbine governor has been made without you’ve been noticed - you should verify and change the synchronization parameters accordingly. As a second example, if the reactive power has increased (or became negative), you should check the measurement accuracy of your synchronizer - measuring errors due to aging or other factors may appear over time. If you ignore it, things may get worse and even lead to a faulty synchronization where generator inspection is mandatory. Anyway, these are just signs which will tell you what to check during the next planned generator stop.
Conclusion
This is the last article in a series dedicated to the generator synchronization. I hope I managed to transfer some of my work experience in this area and help you to improve your work. But, however you organize the synchronization process, these thing are guaranteed:
That is why you should be prepared. That is why you should keep tracks or logs of synchronization events over time. That is why you should set and treat the synchronizer as a vital device - it operates occasionally, but it may malfunction spectacularly.