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Ø Thyristor-controlled series capacitors (TCSCs) can be used for several power system performance enhancements, namely, the improvement in system stability, the damping of power oscillations, the alleviation of sub synchronous resonance (SSR), and the prevention of voltage collapse.
Ø The effectiveness of TCSC controllers is dependent largely on their proper placement within the carefully selected control signals for achieving different functions.
Ø Although TCSCs operate in highly nonlinear power-system environments, linear-control techniques are used extensively for the design of TCSC controllers.
2. Improvement of the System – Stability Limit
Ø During the outage of a critical line in a meshed system, a large volume of power tends to flow in parallel transmission paths, which may become severely overloaded.
Ø Providing fixed-series compensation on the parallel path to augment the power-transfer capability appears to be a feasible solution, but it may increase the total system losses.
Ø Therefore, it is advantageous to install a TCSC in key transmission paths, which can adapt its series-compensation level to the instantaneous system requirements and provide a lower loss alternative to fixed-series compensation.
Ø The series compensation provided by the TCSC can be adjusted rapidly to ensure specified magnitudes of power flow along designated transmission lines.
Ø This condition is evident from the TCSC’s efficiency, that is, ability to change its power flow as a function of its capacitive-reactance setting:
Ø This change in transmitted power is further accomplished with minimal influence on the voltage of interconnecting buses, as it introduces voltage in quadrature.
Ø In contrast, the SVC improves power transfer by substantially modifying the interconnecting bus voltage, which may change the power into any connected passive loads.
Ø The freedom to locate a TCSC almost anywhere in a line is a significant advantage. Power-flow control does not necessitate the high-speed operation of power flow control devices and hence discrete control through a TSSC may also be adequate in certain situations.
Ø However, the TCSC cannot reverse the power flow in a line, unlike HVDC controllers and phase shifters.
3. Enhancement of System Damping
Ø The TCSC can be made to vary the series-compensation level dynamically in response to controller-input signals so that the resulting changes in the power flow enhance the system damping .The power modulation results in a corresponding variation in the torques of the connected synchronous generators particularly if the generators operate on constant torque and if passive bus loads are not installed.
Ø The damping control of a TCSC or any other FACTS controller should generally do the following:
1. Stabilize both post disturbance oscillations and spontaneously growing oscillations during normal operation;
2. Obviate the adverse interaction with high-frequency phenomena in power systems, such as network resonances; and
3. Preclude local instabilities within the controller bandwidth.
Ø In addition, the damping control should
1. be robust in that it imparts the desired damping over a wide range of system operating conditions, and
2. be reliable.
3.2 Principle of Damping
Ø The concept of damping enhancement by line-power modulation can be illustrated with the two-machine system depicted in Fig.
Ø The machine SM1 supplies power to the other machine, SM2, over a lossless transmission line. Let the speed and rotor angle of machine SM1 be denoted by η1 and φ1, respectively; of machine SM2, denoted by η2 and φ2, respectively.
Ø During a power swing, the machines oscillate at a relative angle
∆φ = (φ2 − φ1).
Ø If the line power is modulated by the TCSC to create an additional machine torque that is opposite in sign to the derivative of the rotor-angle deviation, the oscillations will get damped. This control strategy translates into the following actions: When the receiving end–machine speed is lower than the sending end–machine speed, that is, ∆h =(η2− η1 ) is negative, the TCSC should increase power flow in the line.
Ø In other words, while the sending-end machine accelerates, the TCSC control should attempt to draw more power from the machine, thereby reducing the kinetic energy responsible for its acceleration.
Ø On the other hand, when ∆η is positive, the TCSC must decrease the power transmission in the line.
Ø This damping control strategy is depicted in Fig. through plots of the relative machine angle ∆φ, the relative machine speed ∆η, and the incremental power variation ∆Pmod.
Ø The incremental variation of the line-power flow DP, given in megawatts (MW), with respect to DQTCSC, given in MVAR, is as follows
Ø Thus the TCSC action is based on the variation of line-current magnitude and is irrespective of its location.
Ø Typically, the change in line-power transfer caused by the introduction of the full TCSC is in the range of 1–2, corresponding to an angular difference (d) of 308–408 across the line.
Ø The influence of any bus load on the torque/ power control of the synchronous generator is derived for the case of a resistive load and completely inductive generator impedance.
Ø The ratio of change in generator power to the ratio of change in the power injected from the line into the generator bus is expressed as
Ø The effect of all practical passive loads is generally moderate, and the sign of generator power is not changed. In the absence of any bus load,
∆Pm = ∆P.
Ø The controlled-to-fixed ratio of capacitive reactance in most applications is in the 0.05– 0.2 range, the exact value determined by the requirements of the specific application.
3.3 Bang – Bang Control
Ø Bang-bang control is a discrete control form in which the thyristors are either fully switched on (α = 900) or fully switched off (α = 1800).
Ø Thus the TCSC alternates between a fixed inductor and a fixed capacitor, respectively, and it is advantageous that such control is used not only for minimizing first swings but for damping any subsequent swings as well.
Ø Bang-bang control is employed in face of large disturbances to improve the transient stability.
3.4 Auxiliary Signals for TCSC Modulation
Ø The supplementary signals that could be employed for modulating TCSC impedance are listed in the text that follows:
3.4.1 Local Signals
Ø These signals constitute the following:
1. The line current,
2. The real-power flow,
3. The bus voltage, and
4. The local bus frequency.
3.4.2 Remote Signals
Ø These signals constitute the following:
1. The rotor-angle/ speed deviation of a remote generator,
2. The rotor-angle/ speed (frequency) difference across the system, and
3. The real-power flow on adjacent lines.
Ø The angular difference between remote voltages can be synthesized by using local voltages at the two terminals of the TCSC and through the line current. Alternatively, a recent approach may be adopted wherein the phase angles of remote areas can be measured directly by using synchronized phasor measurement units.
Ø Adjacent-line real-power flow can be measured remotely and transmitted to the TCSC control system through telecommunication.
Ø Despite telecommunication delays, this signal can be used satisfactorily and economically for line power scheduling, which itself is a slow control.
Selection of Input Signals
Ø It is a desirable feature that the TCSC controller input signals can extend as far as possible without sensitivity to the TCSC output. This feature ensures that the control signals represent mainly the system conditions for which the TCSC is expected to improve.
Ø Local bus frequency is seen to be less responsive to system power swings as compared to the synthesized-voltage frequency,although both line current and bus voltage are also shown to be fairly effective.
4. Voltage – Collapse Prevention
Ø Voltage-collapse problems are a serious concern for power-system engineers and planners.
Ø Voltage collapse is mathematically indicated when the system Jacobian becomes singular.
Ø The collapse points are indicative of the maximum load ability of the transmission lines or the available transfer capability (ATC).
Ø The TCSCs can significantly enhance the load ability of transmission networks, thus obviating voltage collapse at existing power-transfer levels.
Ø The TCSC reduces the effective line reactance, thereby increasing the power flow; it generates reactive power with increasing through-current, thus exercising a beneficial influence on the neighboring bus voltage.
Ø The system faces voltage collapse or a maximum loading point corresponding to a 2120-MW increase in the net load.
Ø If a TCSC is installed to provide 50% compensation of the line experiencing the highest increase in power at the point of collapse, the maximum load ability will be enhanced to 3534 MW.
Ø The influence of the TCSC on the voltage profile of a critical bus is illustrated in Fig.
Ø A performance factor, fp, is proposed in that indicates the maximum increase in load ability, λ0, for a given percent of line compensation:
Ø This index can be gainfully employed to obtain the best location of the TCSC in a system.
Ø The enhancement of system loading and variation of the performance factor with TCSC compensation are depicted in Fig.
Ø It is suggested that TCSC reactance-modulation schemes based on line current or line power, or on the angular difference across lines, may prove unsuccessful for voltage-stability enhancement. The reason is that these controls constrain any variation in the corresponding variables that may be necessary with changing loads, thereby limiting any power-flow enhancement on the line.
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