The growing capabilities of power electronic components have developed the use of FACTS devices. The higher power levels have been made available in converters for highest voltages. The overall starting points are network elements influencing the impedance or the reactive power of a part of the power system. For FACTS devices, it has two terms to be explained. One of the terms is dynamic, which is used to express the fast controllability of FACTS devices which is provided by the power electronics. This is one of the main differentiating factors from the conventional devices. Another one is static, which is used to perform the dynamic controllability because it has no moving parts like mechanical switches. This article discusses about compensators used in power electronics, that includes Static Synchronous Series compensator (SSSC), TSSC, TSSR, TCSC and TCSR.
Compensators in Power Electronics
The Static Synchronous Series compensators in power electronics mainly involve in series and shunt devices. Let us discuss about these types of devices
Series devices have developed from mechanically or fixed switched compensations to the voltage source converter based devices. The series devices are given below
- Static Synchronous Series Compensator (SSSC)
- Thyristor-Switched Series Capacitor (TSSC)
- Thyristor-Switched Series Reactor (TSSR)
- Thyristor-Controlled Series Capacitor (TCSC)
- Thyristor-Controlled Series Reactor (TCSR)
The most used facts devices are voltage source converter called STATCOM or SVC. These shunt devices are operating as reactive power compensators. These shunt devices are used in transmission and distribution systems. The shunt devices are
- Static synchronous compensator (STATCOM)
- Static VAR compensator (SVC), this SVC includes
- Thyristor switched reactor(TSR)
- Thyristor switched capacitor(TSC)
- Thyristor controlled reactor(TCR)
- Mechanically switched capacitor(MSC)
Static Synchronous Compensator (STATCOM)
A STATCOM is a controlled reactive power source. The characteristic of STATCOM is similar to the characteristics of synchronous condenser, but as an electronic device, it has no inertia and it is better than synchronous condenser because it has lower operating cost, better dynamics, a lower investment cost and low maintenance costs. A STATCOM is built with a turn-off capability like GTO Thyristors or IGCT or with more number of IGBTS.
The static line has a certain steepness between the current limitations determining the control characteristic for the voltage. In the distributed energy sector, it is common for the usage of voltage source converters for grid interconnection. The next step in STATCOM development is the combination with energy storages on the DC-side. The performance for balanced network operation and power quality can be improved much more with the combination of active power and reactive power.
Operating Principles of STATCOM
The STATCOM is connected to the power system at a PCC (point of connection), through step-up coupling transformers, where the voltage-quality problem is the main concern. The PCC is also known as the terminal and its terminal voltage is ‘Vt’. All required currents and voltages are measured and fed into the controller, which is compared with the commands. To drive the power converter main semiconductor, switches of the accordingly to either decrease the voltage or increase the voltage, the controller has to perform feedback control and outputs a set of firing angles (switching signals). It provides voltage support by absorbing or generating reactive power at the point of common coupling without the use of large external capacitor or reactors banks.
The charged capacitor provides DC voltage (Vdc) to the converter, which provides a set of controllable three-phase output voltages when V in synchronism with the AC system. The synchronism of three-phase output voltage has to be performed by an external controller with the transmission line voltage. The amount of desired voltage across STATCOM, which is the voltage reference, Vref is set manually in the controller. The matching of voltages is done by varying the amplitude of output voltage V, which is done by firing angle set by the controller. The controller sets Vm equivalent to Vref. The reactive power exchange between the converter and AC system can also be controlled. This reactive power exchange is the reactive current injected by the STATCOM, which is current from the capacitor produced by absorbing real power from the AC system.
SVC (Static VAR Compensator)
Under different system conditions, a fast operating SVC can endlessly provide the reactive power which is necessary to control dynamic voltage oscillations and thereby get better the power system distribution and the stability of transmission. Electrical loads both absorb and generate reactive power. Since the transmitted load varies significantly from hour to hour, the reactive power balance in a grid varies slowly. At the intense voltage collapse, the result can be improper voltage depression or even voltage amplitude variations.
Applications of the SVC Systems in Transmission Systems
- To increase active power transfer capacity and transient stability margin
- To achieve effective voltage control
- To damp power oscillations
Installing a SVC in the network can increase transfer capability and reduce losses. In addition an SVC can mitigate active power oscillations by using voltage amplitude modulation. SVC installations consist of a number of building blocks. The most important is the thyristor valve. High voltage AC capacitors and air core reactors are the reactive power elements used together with the thyristor valves. The step up connection of this equipment is achieved through a power transformer. Most common topologies of SVC are TCR/TSC/FC or TCR/FC. The main advantage of using these branches is to reduce the losses. To increase the total amount of reactive power support, the mechanical switched banks include both on LV side and the HV side of SVC transformer.
TSR and TCR are both composed of the shunt connected reactor, which is controlled by two parallel reverse connected thyristors. TSR is controlled without any firing angle control which results in a step change in reactance. TCR is controlled by firing angle input, which operates in a continuous operation. The operational mode of TSC is similar to the TSR. Due to the characteristic of the capacitor, the reactance can either be fully disconnected or fully connected. With different combinations of TSC and fixed capacitors, TSR/TCR, an SVC can meet various requirements to supply/absorb reactive power from the transmission line or to the transmission line.
Principle of SVC
The reactors may be variably switched into the circuit by means of a phase angle modulation switched by the thyristors. And it offers continuously variable MVAR injection to the electrical n/w. The voltage control is provided by the capacitors. More smoother control and flexibility can be provided by the thyristor to control the capacitor switching. The thyristor controlled reactor is used to provide smooth control.
The electronically controlled Thyristors, like different semiconductors, produce heat and de- ionized water is commonly used to cool them. Chopping reactive load into the circuit in this manner adds undesirable odd-order harmonics and so banks of high-power filters are usually offered to smooth the waveform. Since the filters themselves are capacitive, they also export MVARs to the power system.
In.practical, there are more complex arrangements voltage regulation is required. Voltage regulation is offered by means of a closed-loop controller. Here, the supervisory control of a remote and the set point of the manual adjustment of the voltage are also common.
Thyristor Controlled Reactor (TCR)
TCR is a shunt connected static VAR generator or absorber. It comprises of a fixed reactor with bidirectional thyristor switches in series. The impedance of the device changed in a constant manner by varying the conduction angles of thyristors. The o/p of the device is regulated to swap either inductive or capacitive current. It keeps and controls the parameters of the power system. It is a choice to STATCOM in terms of cost.
Thyristor Switched Capacitor (TSC)
TSC includes a shunt connected capacitor which is connected in series to bidirectional thyristor switches. The reactance or impedance of this device is changed in a stepwise procedure by controlling the thyristors either in a zero or full conduction operation. This controller presents no transients, no harmonics, and low losses.
Thyristor Switched Reactor (TSR)
TSR is a special case of a TCR where the controlling of the current phase is not exercised, instead of that the reactor is switched in such a way that thyristors are either fully ON/OFF as in case of TSC. The main advantage of TSR over TCR is that there is no harmonic current generation. Also, this controller uses thyristors without firing control, lower cost and losses. The controlling of a reactive compensation in electric power system uses the above stated types of SVC in different patterns, such as grouping of TCR & TSC with filter circuit and TCR with filter circuit is shown in the below figure.
Static Synchronous Series Compensator (SSSC)
SSSC a series version of STATCOM and it is an advanced kind of control series compensation. It produces the output voltage in quadrature with the line current such that the overall reactive voltage drop across the line is increased or decreased. Although it is like a STATCOM, the output voltage is in series with the line and hence it controls the voltage across the line, so its impendence. It has a capability to induce both inductive and capacitive voltage in series with the line and hence the power control.
The VSC (Voltage Sourced Converter) based series compensators – SSSC was proposed by Gyugyi in 1989. The single line diagram of a two machine system with SSSC is shown in Figure 3.10. The SSSC injects a compensating voltage in series with the 41 line irrespective of the line current. From the phasor diagram, it can be stated that at a given line current, the voltage injected by the SSSC forces the opposite polarity voltage across the series line reactance. It works by increasing the voltage across the transmission line and thus increases the corresponding line current and transmitted power.
Following figure is a Simplified diagram of series compensation with the phasor diagram. The compensating reactance is defined to be negative when the SSSC is operated in an inductive mode and positive when operated in capacitive mode. The voltage source converter can be controlled in such a way that the output voltage can either lead or lag the line current by 90o. During normal capacitive compensation, the output voltage lags the line current by 90o.
The SSSC can increase or decrease the power flow to the same degree in either direction simply by changing the polarity of the injected AC voltage. The reversed (180o) phase shifted voltage adds directly to the reactive voltage drop of the line. The reactive line impedance appears as if it were increased. If the amplitude of the reversed polarity voltage is large enough, the power flow will be reversed. The transmitted power verses transmitted phase angle relationship is shown in Equation (3.1) and the transmitted power verses transmitted angle as a function of the degree of series compensation is shown in Figure 3.11.
P=V2/X sin delta + V/X Vq cos delta/2
Thyristor Controlled Series Capacitor (TCSC)
TCSC is a capacitive reactance compensator. It comprises of a series capacitor bank which connected in parallel with a thyristor controlled reactor that provides a smooth variable series capacitive reactance.
The total impedance of the system can be mixed by changing the conduction angle of the thyristors and therefore the circuit becomes either capacitive or inductive. If the impedance of the total circuit is inductive, the fault current is incomplete by this controller. A simple model of TCSC is shown in the below figure.
Thyristor Switched Series Capacitor (TSSC)
Similar to thyristor controlled switched capacitor, it is also a capacitive reactance compensator including a thyristor switched reactor in parallel with a series capacitor. It gives the stepwise control of series capacitive reactance. As an alternative of controlling in a continuous manner, it toggles the reactor such that the thyristors are fired up at 900 & 1800. This controller can be realized without firing angle control to decrease the cost & losses.
Thyristor Controlled Series Reactor (TCSR)
TCSR is an inductive reactance compensator which consists of a series reactor in parallel with thyristor switched reactor. This controller provides a smooth variable inductive reactance. When the firing angle of the thyristor is 1800, then the reactor ends conducting and hence only the uncontrolled reactor is in series with the line that works as a fault current limiter. If the firing angle is below 1800, the net inductance reduces, thus voltage is controlled in the network.
Thyristor Switched Series Reactor (TSSR)
Similar to TCSR, TSSR is also an inductive reactance compensator but, it offers the stepwise control. In order to attain stepped series inductance, this controller switches thyristors to fully ON or fully OFF in.
Thus, this is all about different types of compensator used in power electronics namely Static Synchronous Series Compensator (SSSC), Thyristor-Switched Series Capacitor (TSSC), Thyristor-Switched Series Reactor (TSSR), Thyristor-Controlled Series Capacitor (TCSC) and Thyristor-Controlled Series Reactor (TCSR). We hope that you have got a better understanding of this concept. Furthermore, any doubts regarding this topic or electrical and electronics projects. Please give your valuable suggestions by commenting in the comments section below.