The SVG / STATCOM is a voltage source converter (VSC) using insulated gate bipolar transistors (IGBTs) or insulated gate commutated thyristors (IGCTs) to achieve reactive power compensation.
STATCOM Main Components
The key element of this technology is the standard basic Power Module (see Figure 2.1). These Power Modules are connected in series to obtain a high AC voltage with extremely low harmonic distortion, and very low dv/dt. This also allows use of a low switching frequency that reduces power losses.
The Power Module is also designed to guarantee high maintainability due to its special electrical plugs so it is very easy to install or remove.
The converters are designed to be installed within a specially designed container.
The converters are cooled by forced air cooling. The basic semiconductor devices are IGBT.
STATCOM Starting System
The STATCOM starting device is composed of an arrester, connecting reactor, charging resistance and bypass switch (circuit breaker). The main role of the connecting reactor is to limit the instantaneous current when STATCOM is first energised.
The charging resistance is used to restrict the inrush current. Once the DC capacitors are fully charged, the charging resistor by-pass switch is closed and STATCOM can start normal operation.
Disconnectors and Earthing Switches
The switches are adequately sized to carry the maximum steady-stage and overload currents including fault, inrush, harmonic currents and over-voltage.
The Earthing switch shall be closed during device maintenance and repair for safety reasons.
The disconnectors and earthing switches are positioned to enable maintenance work to be carried out in complete safety throughout the entire unit.
STATCOM Control Strategy Design
STATCOM control strategy is categorized into primary control strategy and secondary control strategy. Primary control strategy offers device reactive power control, constant system reactive power control, power factor control, and constant system voltage control. The secondary control strategy offers low voltage ride through control, conditioned restarting control, pulse locking control etc.
Constant System Reactive Power Control
This control strategy means STATCOM can compensate all system reactive power dynamically to make system reactive power to be effectively 0, or compensate part of system reactive power dynamically to keep the system reactive power to a constant value.
According to the system current detected by STATCOM and STATCOM output current, Load current can be calculated. The reactive component can be calculated through PARK transformation of load current.
Power Factor Control
Power factor control can make system power factor reach to the reference value which is usually supplied by customers. STATCOM completes the task of controlling system power factor by outputting corresponding reactive power detected by using the calculation method of power factor control.
Iq_ref is the reference value of reactive power, Cos_ref is the reference value of power factor, Ip_load is load active power, Iq_load is load reactive power, Coff_cos is a reference value coefficient which can be calculated by the equation as below.
Constant System Voltage Control
STATCOM generates reactive power fast with the slope in transient process to maintain stability of the power grid.
STATCOM receives the output from a closed-loop regulator as the reactive current (or reactive power) reference that the compensator should generate. A PI controller is used to ensure voltage regulation in both steady state and transient state.
When the voltage drops and the compensator U-I characteristics decreases, STATCOM can adjust the amplitude and phase of the AC voltage of its convertor to provide the required reactive current. This is limited by the rated current of STATCOM. When the voltage reference changes, the U-I characteristics will move upwards or downwards accordingly.
The Voltage Regulator compares the measured control variable with the reference value, and then puts it in the transfer function of the controller. The controller then calculates the reactive current according to STATCOM U-I characteristics and generates (absorbs) the same amount of current for compensation through the STATCOM closed-loop controller.
The slope of the U-I characteristics curve known as difference adjustment rate, is defined as the ratio of voltage amplitude increment to current amplitude increment in the linear control area of the compensator.
The slope can also be defined as the ratio of voltage change to voltage rating when STATCOM generates maximum reactive power. The slope is usually kept in between 0% to 10% and is typically between 3% to 5%. In practice, current feedback is used to calculate the slope in order to get control diagram of the voltage regulator:
Device Reactive Power Control
STATCOM can be operated in open-loop control mode, which means operators can control STATCOM to output any required reactive power value in the range of rated capacity. In this way, we can detect the device’s control precision or test the device’s basic characteristic.
Device reactive power control only outputs fixed reactive power without considering the system reactive power situation. Under special working conditions, device reactive power control is very useful and can be operated by receiving a reference value from the user’s SCADA/PLC system or will also accept manual input.