MITIGATION OF VOLTAGE STABILITY PROBLEMS

The following methods can be used to mitigate voltage stability problems.

MUST-RUN GENERATION: Operate uneconomic generators to change power flows or provide voltage support during emergencies or when new lines or transformers are delayed.

SERIES CAPACITORS: Use series capacitors to effectively shorten long lines, thus decreasing the net reactive loss. In addition, the line can deliver more reactive power from a strong system at one end to one experiencing a reactive shortage at the other end.

SHUNT CAPACITORS: Though the heavy use of shunt capacitors can be part of the voltage stability problem, sometimes additional capacitors can also solve the problem by freeing “spinning reactive reserve” in generators. In general, most of the required reactive power should be supplied locally, with generators supplying primarily active power.

STATIC VAR COMPENSATORS (SVC): SVCs, the modern counterpart to the synchronous condenser, are effective in controlling voltage and preventing voltage collapse, but have very definite limitations that must be recognized. Voltage collapse is likely in systems heavily dependent on SVCs when a disturbance exceeding planning criteria takes SVCs to ceiling.

OPERATE AT HIGHER VOLTAGES: Operating at higher voltage may not increase reactive reserves, but does decrease reactive demand. As such, it can help keep generators away from reactive power limits, and thus help operators maintain control of voltage. The comparison of receiving end Q–V curves for two sending end voltages shows the value of higher voltages.

UNDER-VOLTAGE LOAD SHEDDING: A small load reduction, even 5 to 10%, can make the difference between collapse and survival. Manual load shedding is used today for this purpose (some utilities use distribution voltage reduction via SCADA), though it may be too slow to be effective in the case of a severe reactive shortage. Inverse-time under-voltage relays are not widely used, but can be very effective. In a radial load situation, load shedding should be based on primary side voltage. In a steady-state stability problem, the load shed in the receiving system will be most effective even though voltages may be lowest near the electrical center (though shedding load in the vicinity of the lowest voltage may be more easily accomplished, and will be helpful).

LOWER POWER FACTOR GENERATORS: Where new generation is close enough to reactive-short areas or areas that may occasionally demand large reactive reserves, a .80 or .85 power factor generator may sometimes be appropriate. However, shunt capacitors with a higher power factor generator having reactive overload capability, may be more flexible and economic.

USE GENERATOR REACTIVE OVERLOAD CAPABILITY: Generators should be used as effectively as possible. Overload capability of generators and exciters may be used to delay voltage collapse until operators can change dispatch or curtail load when reactive overloads are modest. To be most useful, reactive overload capability must be defined in advance, operators trained in its use, and protective devices set so as not to prevent its use.

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