Power quality is one of the major concerns for both electrical power systems and end electronic equipment nowadays. The widespread use of non-linear loads, such as rectified inputs( without PFC), electronic ballasts, thyristor-based converters and motor drivers,  welders, home appliances, etc,  cause AC power line current distortion and voltage waveform disturbances.  The polluted AC power can cause malfunction or even shutdown of some sensitive electronic equipment, such as data centers, communication systems, radar systems, semiconductor manufacturing equipment, etc.  A large number of inductive or capacitive linear loads,  such as large number of induction motors fed by AC grid power directly, can add reactive power to the power systems and cause AC current out of phase of the AC voltage, which adds power loss and capacity burdens to the systems.

Conventional solutions are to add passive power system filters and reactive power compensators. The bulk filters and compensators occupy significant percentage of power transmission station area, as what the following photo shows.  The compensators can only be added or removed in steps, which has a slow response and could cause some disturbance to power system itself.

With the advance of power semiconductor technology and digital control, power line current can be monitored in real time, and reactive power and harmonic current can be compensated optimally and seamlessly. The reactive power compensators, the so-called Static Var Generators (SVG) and active harmonic filters（AHF） both are inverter-based systems.  SVGs inject a line-frequency current with the desired amplitude into power lines. To be able to output high reactive power,  the SVGs are commonly connected in parallel, namely tired to the power lines directly, as shown below,

3-Level Static Var Generator Circuit and Parallel Grid Connection

AHFs are essentially an inverter, which is able to inject inverse currents to cancel out the undesired harmonics. Compared with SVGs, AHFs output relatively smaller currents, but in higher harmonic orders.  To be able to “filter” out up to 40-order harmonics, the AHF inverters would need to switch much higher switching frequencies.  For 50Hz line frequency, 40-order harmonic is 2kHz, which requires a PWM frequency be over 40kHz in order to effectively smooth out the 2kHz harmonic current component, where SiC MOSFETs are the current only valid option to operate at power grid-level voltage and switch over 20kHz.  AHFs can be connected a power line in parallel or series.

2-Level Active Harmonic Current Filter Circuit and Series Grid Connection

From the block diagrams above, it can be seen that SVG and AHF both sense load line currents and use an inverter to generate desired currents to compensate reactive power and harmonic currents. To fully utilize the equipment hardware and reduce system cost, a SVG and an AHF can be combined in one design. The equipment of both SVG and AHF functionalities is called Power Quality Compensator (PQC).  

[1]  P. M. Balasubramaniam , S. U. Prabha “Power Quality Issues, Solutions and Standards: A Technology Review,” Journal of Applied Science and Engineering, Vol. 18, No. 4, pp. 371380 (2015)