Performance Analysis of SVC for Damping Oscillations in Two Area
System
Shivam
1Puneet Chawla
22
Associate Professor
1,2
Department of
Electrical Engineering
1,2
PEC University of Technology Chandigarh, India
Abstract— Development of electric power transmission facilities is constrained despite the fact that bulk power transfers and use of transmission systems by consumers are increasing rapidly. Due to this fact power system stability is becoming the major issue these days. Due to vast and complex power system variation in the load occurs with respect to time which results in rotor angle instability which causes the oscillations in the power system Power system oscillation is one of the major problems in power system operation. If not damped, these oscillations can grow and decrease transmission capacity of the lines which may cause interruption in energy supply. Traditionally, these oscillations have been damped by power system stabilizers (PSS). Recently, FACTS devices such as static synchronous compensator (SVC) equipped with a power oscillation damper (POD) have been also efficiently used for damping oscillation. The function of SVC is to regulate voltage at its terminal by changing the amount of reactive power exchanges with the power system and power oscillation damping is also provided by this device.
Key words: Power System Oscillations, SVC, Power Oscillator Damper, Power System Stabilizer
I. INTRODUCTION
Power system is mainly concerned with the production of electrical power and its transmission from the sending end to the receiving end as per consumer requirements incurring minimum amount of losses. The power at the consumer end is often subjected to changes due to the variation of load or due to disturbances induced within the length of transmission line. For this reason the term power system stability is of utmost importance in this field. Power system stability is also referred to as synchronous stability and is defined as the ability of the system to return to synchronism after having undergone some disturbance due to switching on and off of load or due to line disturbance. Conversely, instability means a condition denoting loss of synchronism or falling out of step. Power System stability may be voltage, frequency and rotor angle or small signal stability [1].
Rotor angle stability refers to the ability of synchronous machines of an interconnected power system to remain in synchronism after being subjected to a disturbance. It depends on the ability to maintain/restore equilibrium between electromagnetic torque and mechanical torque of each synchronous machine in the system, The rotor angle stability problem involves the study of the electromechanical oscillations inherent in power systems. Small-disturbance rotor angle stability problem is usually associated with insufficient damping of oscillations. The aperiodic instability problem has been largely eliminated by use of continuously acting generator voltage regulators; however, this problem can still occur when generators
operate with constant excitation when subjected to the actions of excitation limiters (field current limiters) [1,2]. Small-disturbance rotor angle stability problems may be either local or global in nature. Local problems involve a small part of the power system, and are usually associated with rotor angle oscillations of a single power plant against the rest of the power system. Such oscillations are called local plant mode oscillations. Stability (damping) of these oscillations depends on the strength of the transmission system as seen by them power plant, generator excitation control systems and plant output. Power system stabilizers (PSSs) are one of the most common controls used to damp out power system oscillations in local area usually in the range between 1to 2 Hz [2].
Global problems are caused by interactions among large groups of generators and have widespread effects. They involve oscillations of a group of generators in one area swinging against a group of generators in another area. Such oscillations are called interarea mode oscillations. Their characteristics are very complex and significantly differ from those of local plant mode oscillations. Load characteristics, in particular, have a major effect on the stability of interarea mode oscillations. These are damped through FACTS devices like SVC etc.
FACTS technologies offer competitive solutions to today’s power systems in terms of increased power flow transfer ability, enhancing constant control over the voltage profile, minimizing losses, improving damping capacity, etc. The FACTS devices are known to improve both the transient as well as dynamic performance of a power system. The efficiency of FACTS device like SVC is to increase transmission capacity, damping of low frequency oscillations, improve transient stability and controlling the terminal voltage. From the viewpoint of power system dynamic stability, it is essential for SVC equipped with a supplementary damping controller in order to mitigate the power system oscillations [8].
II. STATIC VAR COMPENSATOR
Connected to the power system, to regulate the transmission voltage ("Transmission SVC") Connected near large industrial loads, to improve
[image:2.595.53.282.68.296.2]power quality ("Industrial SVC").
Fig. 1: One-line diagram of a typical SVC configuration; here employing a thyristor controlled reactor, a thyristor switched capacitor, a harmonic filter, a mechanically switched capacitor and a mechanically switched reactor.
Typically, an SVC comprises one or more banks of fixed or switched shunt capacitors or reactors, of which at least one bank is switched by thyristors. Elements which may be used to make an SVC typically include:
Thyristor controlled reactor (TCR), where the reactor may be air- or iron-cored
Thyristor switched capacitor (TSC) Harmonic filter(s)
Mechanically switched capacitors or reactors (switched by a circuit breaker)
[image:2.595.315.533.418.561.2]In transmission applications, the SVC is used to regulate the grid voltage. If the power system's reactive load is capacitive (leading), the SVC will use reactors to consume VARs from the system, lowering the system voltage. Under inductive (lagging) conditions, the capacitor banks are automatically switched in, thus providing a higher system voltage. By connecting the thyristor-controlled reactor, which is continuously variable, along with a capacitor bank step, the net result is continuously variable leading or lagging power. In industrial applications, SVCs are typically placed near high and rapidly varying loads, such as arc furnaces, where they can smooth flicker voltage [10].
Fig. 2 V-I Characteristic of SVC
Fig.2 shows the steady-state and dynamic voltage-current characteristics of the SVC. In the active control
range, current/ susceptance and reactive power is varied to regulate voltage according to a slope (droop) characteristic. The slope value depends on the desired voltage regulation, the desired sharing of reactive power production between various sources and other needs of the system. The slope is typically 1-5%.At the capacitive limit, the SVC becomes a shunt capacitor. At the inductive limit, the SVC becomes a shunt reactor (the current or reactive power may also be limited.. SVC parameters have to be determined according to the compensation requirements. With as the capacity of the SVC and the bus voltage (voltage at the bus where SVC is to be connected) .
Most power flow programs do not include a specific static var compensator model. SVCs are often modeled as a conventional PV (generator) bus with reactive power limits. This results in large errors if the SVC is on limit, operating as a capacitor or reactor. If low voltage is the main concern, the SVC can be modeled as a TCR-FC type of SVC (PV bus with shunt capacitor). With a conventional power flow program, a SVC with susceptance regulator can be represented by a PQ (load) bus with voltage constraints. Often the SVC slope setting must be represented to properly simulate system performance. Coordination and interaction with other WCs and slower-acting voltage control equipment are studied. Power flow simulation is the primary tool for determining appropriate slope settings. Representation of the slope is essential in the case of weak systems. Because SVCs are sited at critical locations in the network, and because of the regulation of transmission (high side) voltage, representation of SVC slope is usually more important than representation of generator AVRslope.
Fig. 3(a): SVC models with slope representation using conventional power flow busses. (b) includes the SVC
transformer.
III. POWER SYSTEM STABILIZER
[image:2.595.72.258.617.742.2]frequency dependent gain and phase characteristics. Therefore the PSS transfer function should have appropriate phase compensation circuits to compensate for the phase lag between the exciter input and electrical torque. In the ideal case, with the phase characteristic being an exact inverse of the exciter and generator phase characteristics to be compensated, the PSS would result in a pure damping torque at all oscillating frequencies.
Fig. 4 PSS model PSS model consists of three main blocks:
Gain: It should be computed at the frequency of oscillations and should be enough to make damping coefficient positive.
Phase compensation: It provides appropriate phase lead to compensate for the phase lag between the exciter input and generator electrical torque. In practice, two or more first order blocks may be used to achieve the phase lead.
Washout filter: It is a high pass filter which blocks the interference of PSS during regular function of exciter during steady state operation. Its time constant is large. It allows PSS to act only during small frequency oscillations.
[image:3.595.56.276.157.227.2]IV. FACTS POD
Fig. 5 FACTS Controller
The structure of shunt FACTS POD controller is similar to PSS controllers. In this study, the output denotes the controlled variable of the shunt FACTS device.
[image:3.595.54.292.235.496.2]V. RESULTS
Fig. 6 Two area system with FACTS device
Figure 6 shows the two area system with FACTS POD controller enabled in it. The oscillations produced in the system because of the fault occurred in the line which will be damped with the help of FACTS POD controller
[image:3.595.308.545.289.511.2]Fig. 7 System performance during healthy condition Figure 7 shows the system i.e. voltage and line power in MW during healthy condition. The voltage and power characteristic remain constant throughout the simulation time.
Fig. 8 Machine performance during healthy condition Figure 8 shows machine characteristics before fault conditions. Alternator angle(δ), terminal voltage and angular speed(w) remains constant i.e. a straight line before fault takes place in the system.
[image:3.595.58.276.542.662.2]Now when single line to ground fault occurred in the system for some seconds.
[image:3.595.311.549.580.759.2]Figure 9 shows the system characteristic i.e. positive sequence voltages and active power during single line to ground fault. The single line to fault occurred in the system during to 5 to 5.06 seconds and oscillations occur in the system which was damped with the help of SVC POD controller.
Fig. 10 Machine performance during single to ground fault Figure 10 shows the machine characteristics during single to ground fault occurred in the system. Alternator angle(δ), terminal voltage and angular speed(w) oscillate for some time and the oscillations are damped with the help of SVC POD controller.
VI. CONCLUSION
In this paper, SVC POD controller used for damping the power system oscillations of low frequency range which were produced during disturbance or any fault took place in the system. In this paper, single line to ground fault is used to demonstrate the desired result.
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