Power Control by Reactive Power of Shunt Reactor

[timm33333]

I am looking for the logic for the control of power system using the reactive power provided by 230kV shunt reactor. For example if the voltage is not within +/- 5%, then the taps of shunt reactor would be changed to bring the voltage within acceptable limits. Likewise if the current at the sending (or receiving) end of the cable exceeds the cable ampacity, then the taps of the shunt reactor would be changed until the current comes down to the acceptable limits.

I thought this information would be available in IEEE C37.015, but it is not available there. Is there some standard (or document) which tells that when the power system is controlled by the reactive power supplied by the shunt reactor, then what are the acceptable limits (of voltage, current, power factor, frequency) which must be maintained? Thanks.

 

[Amit Bajpayee]

shunt reactors are an important component of power systems that help to maintain the voltage levels and prevent voltage instability. The consumption of reactive power by the shunt reactor can be controlled by varying its excitation current, which is usually done automatically by the system's control and protection equipment.

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[timm33333]

We are controlling the reactive power by changing the taps of shunt reactor (not by varying excitation current of shunt reactor). My question is that what are the acceptable limits of voltage/current/power-factor/frequency (in percentages) which should be maintained while we vary the reactive power by changing the taps of the shunt reactor?

 

[RRaghunath]

timm33333,
Typically voltage of EHV/HV transmission system is allowed to vary between -10% and 10% of rated. So, the regulator setting of +/-5% seems appropriate.

 

[bacon4life]

The control logic is very much dependent upon your specific grid. In my area, an actual voltage of less than 230 kV would indicate dire problems with the grid. Reactors would be switched off several percent above 230 kV and all the capacitor would be online before voltage dropped as low as 230 kV.

In my region the shunt reactors tend to be very large units with no taps. Routine switching of the reactors tends to controlled centrally by either a dispatcher or by regional-wide automation. Relying only on local voltage measurements would not result in coordinated behavior between nearby substations. I assume the central control would hold it much tighter than 5% under many circumstances. In addition to the central control, there may also be backup relay that can switch the reactor for very large voltage deviations. Such voltage deviations could be caused by the loss of transmission line.

Any automatic switching of reactive power devices within a region needs to be coordinated to avoid voltage oscillations. Capacitor banks usually need to be de-energized for 5 minutes before they can be re-energized (This is due to DC voltage getting trapped on the capacitor that is slowly bled off into the discharge resistors). During voltage oscillations, if you switch off the capacitors before turning on the reactors, the capacitors will not be available to provide voltage support for a subsequent low voltage period.

Also, the actual logic used for controlling the reactor is likely to be only slightly related to how the reactor is simulated inside of power flow software. Typically power flow software makes many simplifying assumptions in order to approximate typically dispatcher behavior.

 

[davidbeach]

And in the region bacon is referring to, sometimes that centrally controlled voltage management is to simply take out the really big capacitors, aka the 500kV lines. Many nights will see one or more 500kV lines out for "voltage control".

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[crshears]

My utility does that, sometimes for weeks on end, during the shoulder seasons of spring and fall when there is less load on the grid to eat all that reactive power.

CR

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