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Tranmission nominal voltage range 3

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Mbrooke

Electrical
Nov 12, 2012
2,546
Typically how much do transmission voltages 115kv and over vary? Or to be candid what would you think about a distribution substation without voltage regulation?

I have a situation where a substation voltage regulator needs to be bypassed for an extended period of time, relying entirely on the ratio of the 30/40/50MVA power transformer itself.

PSS/E is giving me steady state variances between 118kv and 114kv, and worst case N-1-1 contingencies of 111kv. Not as bad as I assumed.

I'm thinking of placing the primary no load taps at 112.125kv over from 115kv covering me well enough. Thoughts welcome.
 
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0.95 to 1.05 PU normally. You can swing out further depending on the requirements by your Planning Coordinator. Under contingency, you usually have a wider band.


To look at the planning requirements, google "oasis TPL-001". You can see the planning criteria a whole bunch of utilities use.


Here is AEP's TPL-001. They break it down by what region they are in.


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If you can't explain it to a six year old, you don't understand it yourself.
 
See the table below with typical voltage limits. Notice that the allow lower voltage limits may be above or below 95% depending upon the ISO and post N-1 contingency. It suggested also to check the compliance the Operating Limit required by NERC.
I hope this help
System_Voltage_Limits_blpxbw.jpg


 
Big help- thanks to both HamburgerHelper and Cuky2000.

I take it 500kv autos have onbaord tap changers? Or simply supplying a large amount of reactive power? I ask because of the 110% upper limit instead of 105%.
 
Looking at NERC guidance won't tell you much about actual operating practices in your specific region. Your transmission provider should be able to provide SCADA recordings. As this seems to be an emergency measure, it may not be feasible to keep end use customer within the ANSI range for all N-1 transmission contingencies.

We have a number of substation that do not have LTCs, so we regulate the 115kV voltage very tightly to maintain adequate voltage at the distribution level. Also, it is common for emergency mobile transformers to not have an LTC. Regulating at the distribution buses becomes challenging whenever there is a load imbalance between substations. Having a consistently loaded transformer allows you allocate more of the voltage drop margin to the transmission system voltage variation.

The 500 kV system tends to swing quite a bit more than lower voltage systems. Although the 500 kV transformers near me have LTCs, the 500 kV voltage is more typically managed by switching shunt reactors/capacitors than by making tap changes.

 
I'm curious about those substations that do not have LTCs- is there a specific reasoning behind doing so?
 
Some utility use regulated transformer in distribution feeders instead in a centralized LTD or provide voltage support with shunt cap banks.

Perhaps some of the reasons not to use LTC are: reliability, maintenability, and cost.


Transf_Component_Failure_pmxrxj.jpg

Source: AC Substation Equipment Failure Report, NERC ACSETF
 
One utility I worked at had LTCs but never used them because they had problems with them always tapping while fighting each other. It was probably more an issue of somebody not setting the controls properly than the LTCs themselves. They just went with a winter and summer fixed tap and switched in and took out cap banks to regulate the voltage.

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If you can't explain it to a six year old, you don't understand it yourself.
 
In India (we follow IEC) we have maximum system voltage 110% of nominal voltage up to 220 kV and above it is 105%. 33-36 kV, 66-72.5 kV, 132-145 kV,220-245 kV, 400-420 kV, 765-800 kV, 1150-1200 kV. Transformer rated voltages are nominal voltages.
India has a huge population of 315 &500 MVA 400/220 KV 3 phase auto transformers with line end OLTC for varying 400 KV by +- 10 % in 17 steps. In reality these tap changers are never operated as its operation will not affect the strong interconnected 420 kV grid voltage. But the line end OLTC (at 220 kV potential) is a maintenance liability and any deterioration of oil in diverter switch can cause line end voltage breakdown (low impedance fault) and the grid will collapse inside the small oil volume leading to high energy release , explosion and fire. So in my opinion avoiding on -load tap changers in such interconnecting EHV transformers( cost savings up to 20 %) is a good strategy and voltage regulation can be done in down side step down, two winding transformers with neutral end tap changers.
 
@HamburgerHelper: Sounds like multiple tap changers in series. Key IMO is only having them at one location, preferably the final stage of transformation- ie 69kv to 12.47kv.


@PRC- fully agree.
 
The global trend of transformer failures is 1/3 in windings +others 1/3 in OLTC and 1/3 in bushings. In Europe OLTC failures are more than bushing failures. In India bushing failures are much more than OLTC failures.
 
Thank you for sharing this :)

Ideally I wish I could eliminate all automatic tap changers- but the only thing stopping me is that even with a +-5% transmission system, long distribution circuits amplify it.

(Typically under peak load the bus is raised to 105% to account for far end drop- maximum deviation of 105% at the start of feeder 96% of nominal at the end, with a 90% allowance for distribution contingency switching. 107% raise if really needed.)

I guess I need to start a thread on controlling transmission voltage.

 
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