Phase Checking a Generator after Replacing Instrument Transformers

This article will explain proper procedures for ensuring a synchronous AC generating unit is in phase with the system when the generator breaker is first closed after work on protection system instrument transformers or after a unit rewind.

By John Hunter

Many hydropower plants have units that are old enough that the electrical protection instrument transformers (both current transformers or CTs and potential transformers or PTs) need to be replaced. (Although I am unaware of any standards for when you replace this equipment, in our systems, the design criteria indicates about 30 years. However, if the equipment is maintained properly and tested periodically, the useful life can be longer.) This work requires special procedures to ensure that a wiring or equipment error during installation doesn’t cause damage to the unit resulting from closing into the system with the generator out of phase. Much of this discussion should also be applied to a stator rewind test procedure.

When and why are these procedures necessary?

Relay technicians and system protection engineers should already have established procedures to check out the new equipment prior to and as a part of installation and testing: CT and PT ratio confirmation, high-voltage testing, saturation testing on CTs, etc. This should also include confirming the wiring from the instrument transformers into the control system, metering and protection relays, and into the unit excitation system.

If a wiring or installation error occurs on a CT, the unit will most likely trip on differential immediately after the generator breaker is closed for the first time. In this event, the system protection engineer will easily be able to determine the CT wiring problem based on the vectors observed at the time of the trip. This assumes that most plants now have microprocessor-based generator and transformer protection relays installed. (If the plant is still equipped with the old electromechanical protection relays, an upgrade would be beneficial. The new relays are excellent tools in evaluating the magnitude and duration of fault currents occurring during a unit electrical trip.) The system protection engineer will also easily be able to determine if the trip was due to a wiring error or a genuine fault.

Although PTs provide voltage values to the control system and protection relays, the function of concern here is their contribution to the synchronizing process. If the plant operators are simply changing out the PTs with no additional wiring, the risk is minimized. However, if they are rewiring back to the control room and/or installing new protection relaying or a new control system, the chance of a wiring or configuration error becomes greater. The rewire can also affect the accuracy of the synchroscope and lights. Hence, an error in the PT circuits can result in closing a unit out of phase and cause significant damage to the unit. The process needed to ensure against this is outlined in this article.

What procedures do you need to follow?

First, it would be of value to review how the PTs work during the synchronizing process as it relates to the synchronizing lights. In Figure 1, a PT is coming off of the generator ahead of the generator breaker, and another is coming off of the bus, with the secondary of each transformer wired through two lights to the other transformer. Assuming the unit is at speed with an applied field and the synchroscope is rotating slowly in the fast direction, the synchroscope lights will be alternately getting lighter and then darker and then lighter again. If at a single moment in time, the generator is 180 degrees out of phase with the system, that means (sine waves at the bottom) that when the generator PT is at +120 volts, the bus PT would be at -120 volts. Hence, there would be 240 volts across the lights and the lights would be at their brightest. The synchroscope would indicate at the 6 o’clock position. If the unit is in phase with the system, both PTs would be putting out +120 volts, and the voltage across the lights would be zero, the lights would both be out and the synchroscope would indicate at the 12 o’clock position.

Figure 1 resembles what would be found on a single-line diagram of a plant and would include many additional circuits. By looking at a three-line diagram, it can be inferred that the plant probably has three PTs on the generator bus and three on the running bus: one on each for each phase. If, for instance, three analog voltmeters were correctly wired each to an individual phase between that phase incoming and running PTs – AØ to AØ, BØ to BØ and CØ to CØ – all three voltmeters would raise in voltage together as the unit went more out of phase and all three would approach 0 volts as the plant went in phase. This can be a valuable test if the plant operator is looking for phase rotational confirmation and/or wiring errors.

Furthermore, what the synchroscope and lights are indicating can be compared to these three meters to ensure they are showing the same thing. Additionally, if the synchronizer is automatic, it is useful to monitor its output contact, which will be in series with the generator breaker close coil, to see if it is in sync with these meters. Any control system contact in the breaker close circuit can also be monitored to make sure it is working properly. All this can be done with the generator breaker close circuits defeated and/or the breaker racked out.

The relay technicians and system protection engineers also have a device that will indicate phase rotation off the PTs. Both the generator and bus PT circuits should be checked to confirm they have the same rotation.

Another device that can be used in conjunction with the above is a high-voltage phasing tester at the generator breaker. Basically, these consist of two hot sticks with a cable connecting them and a small meter on one of the hot sticks. The distribution field electricians probably have these for use on a distribution system breaker where they wish to ensure that two circuits they are tying together are phased correctly. In that application, they will place one stick on one side of the breaker and the other stick on the other side for each phase. If the meter reads 0 for all three phases, the distribution circuit phasing is correct. When used in the above application where one side (the generator) is moving in relation to the bus, the meter on the hot sticks will move in sync with the synchroscope, lights and three analog meters as described above and can be used for each phase at the generator breaker to confirm everything is wired correctly. The substation electricians will need to have the meter movement explained to them because they probably have never seen it used in this application.

If something is not proper while using the above procedure, or in the case of a fresh generator rewind, one method that is beneficial to assist in determining precisely where the problem is located includes the following:

1. De-energize the bus and disconnect any auxiliary transformers and surge protection and lightning arrestors. Disconnect the bus at the main transformer bus side.

2. Rack out any generator surge protection and lightening arrestors.

3. Un-terminate the generator neutral.

4. Remove any shorts and grounds, just leaving static grounds on BØ and CØ.

5. Complete any temporary control and protection system modifications necessary to close the generator breaker, and close it.

6. Have the test group apply a test source and energize AØ to about one-third of rated voltage. Confirm that this voltage appears on AØ correctly throughout the entire control, protection and metering system.

7. Disconnect the test source and move the statics to ground AØ and CØ and apply the test voltage to BØ, confirming the voltage appears properly on all devices.

8. Complete the above test for CØ with AØ and BØ statics applied.

9. Return all circuits and terminations to normal.

10. For a pumped storage facility with a phase reversing switch on the generator bus (the same generator breaker is used for pump and generate), the above may need to be performed with the switch in generate and again in pump, depending on where the switch is in relation to the generator PTs, generator breaker and bus PTs.

The above procedures should help isolate any problems in wiring. If it is a fresh generator rewind and the above procedure indicates the generator is reverse phased, any two of the generator leads can be swapped to get the phase rotation correct.

When completing this kind of work at a hydroelectric plant, there is no such thing as being too cautious. Using simple procedures and the correct equipment can simplify the process and go a long way toward ensuring a successful end to an outage.

John Hunter is a distributed control system (DCS) administrator for the hydro generation division at the Salt River Project in Arizona.

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