Table of Contents
Static Var Compensator
Static Var Compensator (SVC) a first-generation FACTS controller is taken up for study. It is a variable impedance device where the current through a reactor is controlled using back-to-back connected thyristor valves. Applying thyristor valve technology to SVC is an offshoot of the developments in HVDC technology. The major difference is that thyristor valves used in this are rated for lower voltages as it is connected to an EHV line through a step-down transformer or connected to the tertiary winding of a power transformer.
The application for transmission line compensators commenced in the late seventies :
- Increase power transfer in long lines
- Improve stability with fast-acting voltage regulation
- Damp low-frequency oscillations due to swing (rotor) modes.
- Damp subsynchronous frequency oscillations due to torsional modes.
- Control dynamic overvoltages.
An SVC has no inertia compared to synchronous condensers and can respond extremely fast (2-3 cycles). This enables the fast control of reactive power in the control range.
Analysis of SVC
The location of SVC is important in determining its effectiveness. Ideally it should be located at the electrical center of the system or midpoint of a transmission line. For example consider a symmetric lossless transmission line with SVC connected at the midpoint. Without SVC, the voltage at the midpoint is given by ,
Vmo = (V cos δ/2) / (cos θ/2)
where θ = βl is the electrical length of the line, l is the length of the line and β is the phase constant given by
β = ω√lc = 2πf√lc
where l and c are the line’s positive sequence inductance and capacitance per unit length and f is the operating frequency .
Configuration of SVC
The Configuration is divided into two types :
- Fixed Capacitor- Thyristor Controlled Reactor (FC-TCR)
- Thyristor Switched Capacitor- Thyristor Controlled Reactor (TSCTCR).
The second type is more flexible than the first one requires a smaller rating of the reactor and consequently generates less harmonics.
The TCR and TSC are connected on the secondary side of a step-down transformer . Tuned and high-pass filters are also connected in parallel which provides capacitive reactive power at fundamental frequency. The voltage signal is taken from the high-voltage SVC bus using a potential transformer .
The TSC is switched in using two thyristor switches (connected back to back) at the instant in a cycle when the voltage across the valve is minimum and positive. This results in minimum switching transients. In a steady state, TSC does not generate any harmonics . To switch off a TSC, the gate pulses are blocked and the thyristors turn off when the current through them falls below the holding currents.
It is to be noted that several pairs of thyristors are connected in series as the voltage rating of a thyristor is not adequate for the voltage level required. However, the voltage ratings of valves for an SVC are much less than the voltage ratings of an HVDC valve as a step-down transformer is used in the case of SVC. To limit di / dt in a TSC it is necessary to provide a small reactor in series with the capacitor.
Modelling of SVC
It is necessary to perform transient simulation for which SVC is modeled in detail including the switching of the thyristor valves in TCR and TSC. The transient network is modeled by differential equations rather than algebraic equations. However, for a stability study, it is not necessary to consider the switching of valves and assume that SVC generates only fundamental current. In addition, the network transients are neglected and represented by algebrai equations of the type :
[Y ] V = I
With these simplifications, SVC can be modeled as a variable susceptance which is the output of the control system. If SMC is a part of the SVC controller, it should be included in the model. However, the susceptance regulator, gain supervisor and protective functions can be neglected in the model.
Applications of SVC
The major application of SVC is for rapid voltage regulation and control of dynamic (temporary) overvoltages caused by load throw-off, faults or other transient disturbances. The dynamic reactive control at the load bus increases power transfer and can solve the problem of voltage instability
(collapse) caused by contingency conditions .
It is to be noted that steady-state voltage regulation can be achieved by mechanically switched capacitors and reactors (MSC and MSR). However, fast voltage regulation is required to prevent instability under transient conditions. Thus, generally, aanSVC is operated with minimum reactive power output under normal conditions. This is achieved by the Susceptance Regulator described earlier which ensures that full dynamic range is available for control under contingency conditions.
The fast controllability provided by the thyristor switches can be also utilized to improve system stability (both transient and small signal). The use of auxiliary damping controllers can help dampen low frequency, interarea power oscillations that can appear at stressed operating conditions (involving high loading of tie lines).
The location of SVC is an important issue. If the objective is to compensate for a long transmission line, the SVC is to be located at the midpoint of the line (if a single SVC is to be used). For very long lines, multiSVCs SVC at regular intervals can be applied. For example, if two SVCs are to be used, one is located at a distance d / 3 from the sending end while the other is located at a distance, d / 3 from the receiving end (d is the length of the line) .
When SVCs are applied to improve the power transfer in a transmission network, the location can be determined by the sensitivity of voltage at the critical buses concerning the reactive power injection (∆Vi/∆Qj ).
In general, it can be stated that a bus with a low short circuit level can be a candidate bus. Incidentally, a synchronous condenser can raise the fault level while providing controllable reactive power. It is to be noted that a SVC does not raise the fault level which can be a blessing as the requirement of the fault current interruption capability of circuit breakers does not go up. On the other hand, for reactive power control at HVDC converter stations with low Short Circuit Ratios (SCR), the synchronous condenser improves voltage regulation and system performance.
However , an SVC has several advantages over SC (synchronous condenser) namely ,
- Faster response under transient conditions
- There are no moving parts, hence requires less maintenance
- There are no problems of loss of synchronism
- As mentioned earlier, it does not contribute to short-circuit currents.
The drawbacks of an SVC are mainly related to the injection of current harmonics. These can be minimized by segmented TCR (with several segments/modules of TCR connected in parallel) and operating all except one module in the TSR (Thyristor Switched Reactor) mode .
Frequently Asked Questions (FAQs)
What is the principle of SVC?
This system stabilizes voltage and improves power efficiency by quickly adjusting reactive power, keeping voltage steady and reducing fluctuations.
What is SVC used for?
The SVC automatically adjusts power to stabilize voltage and improve efficiency, either on power lines or near large factories.
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