A static var compensator is a parallel static device that generates or absorbs reactive power. Its output is controlled to maintain or control the specified parameters of the power system. The SVC includes a power generator and absorber, as well as the corresponding controller, which can provide capacitive or inductive current to the power system. The static var system is a combination of static and mechanical switching compensators that work together. The reactive power compensation system refers to the combination of static var compensators and rotating var compensators that work together.
The typical voltage-current characteristics of a SVC static var compensator indicate that within its capacity range, a static var compensator is absorbing or generating reactive power to maintain a constant voltage. This characteristic is equivalent to having an ideal voltage source Uref and the system voltage U in phase at the junction point, or a voltage-controlled reactance at the same junction point. Thus, the role of the static var compensator is similar to that of a synchronous phase shifter, but without inertia and with a much faster response.
If the short-circuit capacity of the power system is low or the transmission lines are long, the voltage will be severely affected by load changes. Under heavy load, the voltage will drop significantly or even cause system collapse. Voltage drop can trigger low-voltage protection actions, resulting in power outages for many loads. However, under light load, the system capacitance is overcompensated and the synchronous generator is over-excited for a short time, causing transformer saturation and excessive harmonics, which may lead to ferromagnetic resonance with capacitor banks, transmission lines, etc. This can cause surge arrester operation and damage, overheating of capacitors and motors due to harmonics, and damage to user equipment. If a static var compensator is installed on the system, within its rated capacity range, it will maintain the load voltage within the designed limit. If the capacity of the SVC is infinite, it will maintain the bus voltage at a constant level under any load.
Asymmetric or single-phase loads will cause voltage imbalance, leading to equipment overload and additional losses for rotating machinery. By installing appropriate static var compensators, the following objectives can be achieved: load and voltage balancing and power factor correction.
A static var compensator is a compensator that can accomplish different functions when connected to the power grid. By utilizing its economic, fast, and continuous reactive power control capabilities, it is more effective than traditional system methods. It can maintain system voltage, balance three-phase loads, increase the transmission capacity of existing transmission systems, and enhance transient stability limits. In addition, static var compensators also play a role in damping subsynchronous resonance and reducing transient overvoltage.