5. SYNCHRONIZATION START AND STOP

Starting Conditions

As I have mentioned in previous articles, synchronization is the last step in the process of generator starting. As usually the case, everybody wants it to last as short as possible. If only it could last for just a second. Of course, it is not achievable but you should take all the steps to make sure it doesn’t last longer than necessary. So, what you should take care of?

First of all, you must be sure that all the equipment involved in process is at the right position and ready for the command. It means that:

• all the earthing disconnectors are open,
• the generator is excited,
• the circuit breaker subject to closing command is open and energized on both sides (all passing disconnectors and circuit breakers between the generator and the grid are closed except the one you want to close),
• synchronization voltages are present at the input of the synchronizer,
• the excitation systems is ready to accept the voltage up/down commands,
• the turbine governor is ready to accept the frequency up/down commands,
• the circuit breaker is ready to accept the closing command.

All these conditions are essential for start and they may be separated in three groups:

• those which can be known before the start,
• those that can be known only at the moment the synchronization starts,
• those that can not be known.

Conditions that can be known before the start are statuses of switching equipment, status of the excitation system and status of the circuit breaker input circuit (ready to accept the closing command). It is preferred that a common signal is formed to indicate that all these conditions are met so the operator can easily understand if the synchronization is ready or not. This signal may be generated in DCS/SCADA system or in the synchronizer itself (using any kind of digital logic controller, for example PLC).

Whether all the measuring voltages are present at the synchronizer input or not will be found out only when the synchronization start. If any of the voltages required is not present, the synchronization must stop due to an error. Of course, you can check these voltages directly on synchronizer's terminals before the start but it would be just a waist of time and nobody is willing to do so.

Finally, what can not be known in advance is whether the next-stage equipment will accept commands from the synchronizer or not. For example, it is quite possible that the wire transferring the "frequency up" command is interrupted so the turbine governor will respond to commands only partially and the closing command conditions may never be reached. The same is true for the excitation and the circuit breaker commands. Or, your equipment may be accidentally fixed at local mode so no remote commands will be accepted.

To cope with loss of measurements, the synchronizer must be capable of generating a unambiguous indication that some measuring voltage is missing. For that, the synchronizer must offer some parameters to program the minimum and maximum levels of voltages and frequencies which it will accept as regular. When voltages are out of this range, the synchronizer will stop the process with clear indication which measurement is out of range or missing. This will speed up the diagnostics and help synchronization to finish quickly. If you are in a hurry, this can certainly help.

Finally, how does the synchronizer figure out that the exciter or turbine governor does not respond to the commands? The synchronizer could measure the step change achieved for the command issued and compare it to expected value (derived from parameters). However, it turns out that this algorithm can sometimes falsely alert and stop the synchronization just because the parameters are not correctly set. In my opinion, this algorithm should be used very carefully or even left inactive. What remains is a synchronization timeout feature which undoubtedly states that something is wrong in conducting of commands. A tip for you: install visual indicators of voltage and frequency commands going out of the synchronizer. It might save you some time by quickly understanding if commands are really generated or they get lost somewhere in its path to the next-stage equipment.

Considering the circuit breaker response, another helpful feature is the counter that increments each time the closing command is issued. If the circuit breaker closes and the synchronization is done, this counter will receive the value of one. If it reaches some other value set as a threshold, for example three, it means that multiple commands have been sent to the circuit-breaker but it has not closed. Stopping the synchronization with indication of faulty closing path clearly states where is the problem.

The Duration

You have started the synchronization with all conditions met and correct parameters but, how can you be sure it will last as short as possible? It absolutely depends on the initial value of voltage and frequency difference. If you have set the initial values of generator voltage and frequency to be very close but slightly larger than the grid values, chances are that your synchronizer will immediately detect that voltage and frequency differences are within limits, it will cease the "up" and "down" commands and only wait for the phase coincidence. This way, you will save some time on voltage and frequency change. However, the operator must be skilled enough to understand how to set the initial voltage and frequency to achieve the shortest synchronization possible. But, no pain no gain.

Finishing

The last act in the process is stopping the synchronizer. It should be stopped after the circuit breaker is closed but who is the one responsible for generation of the stop command? Naturally, you expect it to be the one who started it. However, it may take few seconds until the operator is certain that the circuit breaker is closed and reach the stop button. Meanwhile, the synchronizer sees both voltages and frequencies equal and tries to speed up the generator by sending frequency-up commands. Since the generator is on-line, the turbine governor might respond to the commands by increasing the generator power without operator being aware of it. Besides, some synchronizers (those treating equal frequencies as regular closing condition) might issue another closing command which may interfere with opening commands as stated in previous article. Obviously, the best solution is that the synchronization is stopped automatically when the circuict breaker is closed. This should be done independently of the operator, using control logic in DCS/SCADA or within the synchronizer itself.

Misleading warnings

Why I insist on starting conditions and unambiguous warnings? For example, imagine the generator is not excited and the synchronizer doesn't supervise the exciter status before the start. You start the synchronization and, as a result, your synchronizer reports missing generator voltage. You check the voltages at the measuring input and confirms the zero level. You conclude that a fuse in measurement path is faulty and start looking for the failed part somewhere in cabling zone. After a while, you'll end up confused because everything looks ok but the synchronization won’t start. However, if you had an indicator saying the synchronization is not ready due to excitation missing, you would have solved the problem in seconds.