Matching Voltage vs. Step

[smilleree]

Recently we have had some discussion on whether it is better to match voltage output or regulation step when tying and transferring load between substations.

It is my opinion that we should match voltage step. This is due to the voltage at the tie point being dependant on conductor properties, distance, and load. Without measuring the voltage at the tie point, there is no guarantee of the voltage at the tie with either method. Essentially by matching voltage and not step, we would be connecting two transformers with different voltage outputs; the only reason that they appear to be the same is because of loading.

Due to the distance (impedance) between substations, circulating current does not appear to be an issue. We do have some instances of two transformers in one substation. On occasion, it is necessary to parallel these transformers. During this situation we match the step of the regulation, due to circulating current.

I am confused as to why this would not be the best solution in all cases. My main worry would be a younger lineman getting comfortable with matching voltage between two different substations and doing the same in the afore mentioned instance. Any input would be greatly appreciated.

 

[wareagle]

In my experience, we just made the regulator non-auto and made the tie. These transformers were all 115/12.46 kv. There was one incidence when a sub was fed from a 230/115 kv
was tied to a sub fed from a 115 kv generator and there was a reverse power flow occurred and caused some problems.

 

[apowerengr]

Think about what problems you are trying to avoid. If you simply match step and you have mis-matched loads, differing transformer sizes and/or impedances or different supply voltages (transmission line source voltage at remote substations) your tie point voltage could be off significantly. If you match secondary voltage at the substation busses regardless of vreg tap position, your tie point voltage is just a function of the feeder voltage drop which will typically be 5% or less.

In most cases, just disable the vregulation at the bus which will be losing load and let the bus which will be picking up load run on auto. That way when the load transfers the unloaded bus voltage will rise somewhat and will stay until you return the vregs to auto however the bus which increases load will be able to automatically respond to the increased voltage drop associated with the increased load through the automatic operation of the vregs. You don't want to create a low voltage condition during switching and have a very remote chance of causing a high-voltage condition.