Load Flow Over-Voltage Scenario

[ThePunisher]

Hi all,

We are performing load flow simulations in ETAP and have couple of transformers whose primary taps are proposed to be set at -2.5% and -5%. These taps are proposed to maintain the bus voltages within normal operating limits. The load models are based on calculated running demand and are reasonable with diversity factors applied.

1) WE did an over-voltage check using a "no motor running (motors OFF, static loads ON)" scenario with the upstream system operating at normal switching configuration. The bus voltage results are within normal operating limits.

2) However, a concern was raised by a member of our team to consider "Motor and Static Load OFF" which would result to a scenario were all our switchgears are at NO LOAD conditions. This resulted to bus nominal voltages exceeding the normal operating limits but still below the extreme operating limits (>104% but <8% nominal).

I am under the impression that item 2) above is a bit over-conservative as when say, at black start, majority static loads will be switched-on and the worst case over-voltage operating scenario is such that of item 1) instead. Furthmore, since the resulting over-voltage at item 2) is below the extreme operating limit...no emergency tap adjustment is required and we can say that the realistic probability of having majority of the static loads operational will place the system under the item 1) scenario.

I wish to see what are the experts' opinion here as my goal is not to base my conclusions on operating conditions that are not realistic. Our applicaiton is an oil and gas facility in Canada and we use CSA Std. CAN3-C235-83 (re-affirmed 2010).

 

[crshears]

...with the upstream system operating at normal switching configuration.

If your facility is supplied at 28 kV or 44 kV, one pertinent factor to consider is what percentage your facility is of the total load on the feeder / transformer station; the presence [or, more precisely in your case, the absence] of the facility's load may well have an effect on the local utility's load drop compensation scheme such that the underload tapchangers at the TS will tap down and reduce the supply voltage, bringing it up as the facility's load increases.

This can be tricky to quantify, but the characteristic is quite common. Take note that the parameters a utility typically uses to program the logic of the voltage regulation scheme it harnesses at any given station may include [a] the average length of the feeders supplied from the TS, the amount of real power supplied, and [c] how inductive the supplied loads are.

It's just my opinion, but in your case, I'd go with Door #1.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]