When designing a transmission or distribution system, the engineer must take into account not only the power requirement of the loads, but also the fact that they consume reactive power and to the same degree key, that networks include inductive and capacitive components which themselves absorb or generate reactive power. Power is generated and consumed in a controlled manner only at specific points in the networks (ignoring losses); on the other hand, reactive power is generated and absorbed throughout the network in significant quantities that vary with the system loading and configuration. The International Electrotechnical Commissioning, IEC defines the unit for reactive power as ‘var’ (volt-ampere reactive) and uses the name “vars” for reactive power. Various tools are used for var management, and they include: shunt reactors/inductors and capacitor banks, series compensation, synchronous compensators (which is our focus of discussion in this article), static var compensators, STATCOM, among other FACTS Controllers.
All synchronous machines can give continuously variable var compensation and in industrial systems where large synchronous motors are installed, they are often used in this way to provide power factor corrections of the local load in addition to the main driving duty.
Self-driven synchronous machines which are not connected to any mechanical load referred to as synchronous compensators can be used anywhere in a system either to generate or to absorb vars on a balanced three-phase basis.
The synchronous compensator directly generates voltage at its terminals. The magnitude of this voltage is controlled by a high-speed excitation system, which is often arranged to provide control of the voltage at another point, normally at the high voltage side of the compensator transformer. When overexcited, the machine generates vars and operates in a stable condition; when under excited, it absorbs vars but its stability decreases as the excitation falls to zero. Due to this tendency towards unstable operation, the var absorption rating of a synchronous compensator is usually only 50% of its var generation rating. Most machines operate with an automatic excitation control that needs to have a swift response to assist the machine in maintaining stability through system disturbances or to follow fast reactive load changes. The control time constant is typically in the range from 5-10 cycles.
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