An interruptible load management method with considering voltage stability in a restructured power system: an OPF approach

Abstract

A common standpoint shared by nearly all restructured electricity market is designation of a system operator that is responsible for reliable real time control and secure operation of the transmission system. This enables efficient operation of competitive electricity and ancillary services markets. Reactive power and voltage support are among those services should be provided efficiently for preventing occurrence of voltage instability in a power system. Common experiences of power system operation under restructuring environment in different countries, reveal that Independent System Operator (ISO) should pay attention to recognition of power stability margin more precisely than before. In this new era all rivals compete for higher profit margin in trading of electricity as much as possible. This strategy might endanger some parts of power system. Therefore it is vital for ISO to procure different kinds of ancillary services necessary for harness of power system instability. Generally voltage magnitudes at load buses are regulated with injection or absorption of determined amounts of reactive power at voltage control buses where reactive power can be provided by special devices such as generators, fixed capacitors, static VAR compensator and etc. Besides procurement ISO will attempt to use reactive power resources to keep away from voltage instability points such as saddle node Bifurcation (SNB) or Limit Induced Bifurcation (LIB) points as far as possible [1]. Prominent characteristic of reactive power strongly requests that it should be provided locally rather than transferring from long distances. In these areas where reactive power encounters with shortage in resources ISO can take advantage of interruptible loads to overcome the problems. Interruptible loads can be exploited for different purposes, for instance increasing active / reactive power reserves, promoting stability conditions, congestion management and etc in a deregulated power system. The benefits of using interruptible loads are debated in [2–4] with all the specifics. [5] Demonstrates the advantage of interruptible loads to increasing active power reserve while [6] investigates the profits gained at using them for improving of introduced static voltage stability index. In this paper appose to [6] we use voltage stability margin index as a main constraints of an OPF based structure proposed for managing interruptible loads in a restructured power system. Other constraints of this optimisation problem consist of limits pertain to magnitudes of bus voltages, magnitudes of transmission line currents and constraints related to resource's limits. The model is then implemented by introducing a new OPF method through Particle Swarm Optimisation algorithm. Case study on the IEEE 14 bus test system is reported to illustrate the proposed method.