Am I correct on how voltage control works on a step voltage regulators?

[chao_david]

Case: 19.92 kV Line to ground voltage with a step voltage regulator

If the Set Voltage setting is 120 V, and the reading on the display screen is 120 V, then the line is currently at 19.92 kV. Correct?

Getting the ratio 19920/120 = 166. Does this mean that every 1 V deviation translates to 166 V deviation on the line?

 

[cranky108]

With most regulators I have seen for 19.92kV line top ground, it would be hard to get exactly 1 V deviation on the low side. I believe most taps are 5/8 %. But in general that sounds correct.

 

[jghrist]

You are correct if you are referring to the load side of the regulator. The source side is higher or lower depending on the tap. The voltage is reading the secondary of a 166:1 PT.

 

[bacon4life]

You have the right idea, assuming everything about the regulator is designed for operation at 19.92 kV.

Some manufactures have a single regulator design that can be used over a wide range of voltages. For example the Eaton VR-32 can operate at nominal voltages from 10 kV to 23 kV using combination of internal PT taps and a ratio correction transformer. Figure 9 shows the various PT ratios that regulator may be configured for. If your specific regulator uses a some PT ratio besides 166:1, you may need to set the controlling just above or just below 120 V to achieve 19.92 kV. Having different ratios is somewhat common for substation LTC applications.

 

[cranky108]

Substation transformer LTC's use the station PT, so the ratio is likely to be different. The PT's in a station will be optimized for other equipment, such as relaying, and metering.
Free advice, don't use the station power transformer as a sensing potential source for an LTC. Seen it done.

A few companies will use station voltage regulators, and not a transformer LTC. This is a choice. Most voltage regulators I have seen are on a line on a platform (in the middle of nowhere).

Another piece of advice, get a 911 address for each of these, in case of an emergency. Out of several of these, one will need a call in location for a fire or something else in it's life.

 

[bacon4life]

I had weird case where the station PT was on the line side of the substation breaker. Unfortunately, this meant that when distribution circuit was transferred to another station, the PT was measuring the end-of-line voltage from the adjust station instead of the local bus voltage. As a temporary measure until the PT's could be relocated, I ended up switching the LTC to use the station service transformer voltage.

 

[cranky108]

And how did the station service voltage work? Did the LTC move when the station building hear, or AC came on?

 

[bacon4life]

In my case there was very little load on the station service transformer, so the voltage drop through the station service transformer was fairly constant. Since the ratio accuracy of a station service transformer is not specified, we did test to make sure that ratio was close enough to work.

 

[Power Engineer_BH]

If you have 13.09 kV on the low-side of the power transformer should you use a step regulator PT ratio of 60 to get a 126 V Base or a ratio of 63 to get 120 V base. The local utilities buy 13.09 kV transformers for 12.47 kV designed distribution circuit. Why not just buy a 12.47 kV transformer?