Subtransmission voltage regulation

[Mbrooke]

For those utilities that run sub transmission (IEEE or IEC), do you employ onboard tap changers for transmission to sub transmission transformers?

For example when stepping 132kv to 33kv is voltage regulation applied or only reserved at 33kv to 11kv substations?

 

[juleselec]

I work at a transmission utility and we have on-load tap changers on transformers at both transmission to sub-transmission (e.g. 330/132kV) and sub-transmission to distribution (e.g. 132/33kV) levels. There are also switched shunts and (very occasionally) SVCs for regulating sub-transmission voltages. Some shunts have automatic voltage regulation, others are manual or on time control (i.e. they switch on/off at fixed times during the day).

 

[ausphil]

We run both those voltage ratings that you have used as examples, and have automatic on-load tapchangers on both of those voltage rating transformers.

We also then run manually set off-load tapchangers for our 11 kV to 415 V transformers.

If you were trying to run without on-load tapchangers on one of those two voltage rating transformers, you would probably need a very wide tapping range, which may not give you the fine control for the customer downstream.

Ausphil

 

[stevenal]

I would propose that the goal is not just to provide proper voltage to the end use customer, but to provide proper voltage to the equipment along the way. ANSI C84.1 has max and mins for medium voltages and maximums for high on up.

 

[Mbrooke]

In these designs, are the taps set to provide 1.05 PU voltage at each buss or is there a sequencing scheme (where the 33kv is boosted on the 33kv side to maintain the 11kv buss and then the 11kv transformer is raised to keep 1.00pu at the 11kv buss)? I can elaborate further if need be.

 

[krisys]

For the transmission to sub-transmission level transformers, the requirement of On Load Tap Changer (to regulate the voltage) will be dictated by the results of voltage stability study. Please see the below figure.

<img src="

http://i57.tinypic.com/29yf987.jpg"

border="0" alt="Image and video hosting by TinyPic">

This family of curves will be produced by maintaining the sending end voltage constant, by increasing the load. The receiving end voltage will be calculated for each loading. These curves are also referred as maximum system loadability curves.

You may notice that at some lower voltage, the loadability (power transfer capability) suddenly start decreasing. This is the point where the system will start collapsing. So based on the system characteristics, the available steady state stability margin shall be evaluated. If there is no sufficient margin available from the steady state loading (refer to the dotted line in the figure, which is the steady state stability margin line for various load power factors).

The OLTCs may help in improving the steady state stability margins, by increasing the receiving end voltage.

 

[stevenal]

I know of no sequencing schemes. Just need to coordinate the time delays so the LTCs don't fight each other.

 

[Mbrooke]

@Krisys

Thank you, this is just what I was looking for!

Are these stability limits based on any particular line length? And they assume that thermal limits will not be reached first before voltage collapse?

In terms of voltage collapse, would you happen to know why beyond a certain loading point the system voltage begins to sharply drop rather than being linear? This part in particular has me confused. Would increasing the sending end buss voltage under peak load increase system load-ability?

 

[krisys]

The attached curve is a typical one. But basically, it is drawn from the equation, which is derived from the basic power transfer equation as below:

P = ({V1*V2)/XL} * sinδ

 

[Mbrooke]

Thanks! (I assume brackets area tad off?)



In any case, what might be behind the equation's sudden voltage drop onset?

 

[MatthewDB]

"In any case, what might be behind the equation's sudden voltage drop onset? "

When a transformer taps up, it increases the voltage at the secondary, but the lower ratio causes an increase in the primary current. The voltage at the primary drops because of the higher current. At sufficiently high loading, the drop in the primary voltage negates the tap change and the output voltage drops instead of increasing. Once that happens that regulator and everything down stream runs to full positive tap and the voltage collapses.