LECTURE NOTES FACTS. IV B. Tech I Semester (JNTUA -R13) G ARUN SEKHAR Assistant Professor

Transcription

1 LECTURE NOTES ON FACTS IV B. Tech I Semester (JNTUA -R13) G ARUN SEKHAR Assistant Professor DEPARTMENT OF ELECTRICAL AND ELECTRONICS ENGINEERING CHADALAWADA RAMANAMMA ENGINEERING COLLEGE: TIRUPATHI

2 UNIT-I FACTS CONCEPTS: Flow of power in AC parallel paths and meshed systems, Basic types of Facts controllers, Brief description and Definitions of Facts controllers. INTRODUCTION FACTS ie., Flexible AC transmission system incorporate power electronic based static controllers to control power (both active and reactive power needed ) and enhance power transfer capability of the AC lines. Let Bus 1and Bus 2shown in fig 1: represent two AC systems where in power is to be transmitted from 1 to 2through a line of impedance r+jx. Neglecting the resistance (r) ie.. x>>r, the power transmitted is given by Where δ=phase angles of with respect to reference (ie..) By changing the effective value of x,the power transmitted can be increased or decreased.further it modifies reactive power needed. Increase or decrease of x will change value. Maximum power that can be transmitted is obtained when δ= (provided are fixed).

3 The FACTS technology is not a single high-power controller but rather a collection of controllers. which can be applied individually or in coordination with others to control one or more of the interrelated system parameters. The parameters that govern the operation of transmission system including. Series impedance Shunt impedance Current Voltage Phase and Damping of oscillations at various frequencies below the required system frequency. The FACTS controllers can enable a line to carry power closer to its thermal rating. In flexible (or) controllable AC systems, the controllable parameters are a) Control of line reactance. b) Control of phase angle δ when it is not large(which controls the active power flow) c) Injecting voltage in series with line and at 90 phase with line current ie.. injection of reactive power in series. This will control active power flow. d) Injecting voltage in series with line but at variable phase angle. This will control both active & reactive power flow. e) Controlling the magnitude of either V1or V2. f) Controlling or variation of line reactance with a series controller and regulating the voltage with a shunt controller. This can control both active and reactive power. Flow of Power in an A.C Systems:

4 In AC power systems the electrical generation and load must balance at al times.to some extent, the electrical system is self regulating. In generation is less than load,the voltage and frequency drop. However there is only a few percent margin for such a self regulation.[if voltage propped up with reactive power support,then the load will go up and consequently frequency will keep dropping and system will collapse.] If there is inadequate reactive power support, the system can have voltage collapse. When adequate generation is available, active power flows from the surplus generation areas to defect areas through all parallel paths available which frequently involves EHV (extra high voltage ) and medium voltage lines. Power Flow in Parallel Paths : Consider power flow through two parallel paths from a surplus generation area to a defict generation area on right as shown in fig:1 With out any control,power flow is based on the inverse of various transmission line impedances. A part from ownership and contractual issues over which lines carry how much power. It is likely that lower impedance line may become overloaded and there by limit the loading on both paths even though the higher impedance path is not fully loaded. Fig:2 shows the same to paths,but one of these has HVDC transmission

5 With HVDC, power flows as ordered by the operator because with HVDC power electronic converters power is electronically controlled. An HVDC line can also help the parallel AC transmission line to maintain stability. However, HVDC is expensive for general use,and is considered when long distances are involved. Fig 3: and Fig 4: show one of the parallel transmission lines with different types of series type FACTS controllers. By controlling impedance (ie.. series FACTS controller) power flow can be controlled as desired (or required.) By injection of appropriate voltage,facts controller can control the power flow as required. Note : Maximum power flow can be limited to its rated (limited) value under contingency conditions ie.. when this line is expected to carry more power due to the loss of a parallel line. Power Flow in a Meshed system:

6 Consider two generators at two different locations are sending power to a load centre through a network consisting of three lines in a meshed connection as shown in fig: Fig: (a) Power flow in a mesh network Suppose the lines AB,BC and AC have continuous ratings of 1000 MW,1250 MW and 2000 MW are respectively These lines have energy ratings of twice those numbers for a sufficient time to allow rescheduling of incase of loss of one of these lines. If one of the generators is generating 2000MW and the other 1000MW, a total of 3000 MW would be delivered to the load centre. For impedances shown three lines would carry 600,1600 and 1400 MW respectively as shown in fig:(a); If a capacitor of reactance -5Ω at synchronous frequency is inserted in one line as shown in fig: (b).it reduces the line s impedance from 10Ω to5ω. Fig : (b) Power of flow in a mesh network with thyristor controlled from the figure,it is clear that Power flow through AB will be 250 MW BC will be 1250 MW AC will be 1750 MW respectively As series capacitor is adjustable,power flow may be controlled according with transmission ownership, contracts relationship,thermal limits transmission losses and wide range of load and

7 generation schedules. Although, this capacitor could be modular and mechanically switched,the number of operations would be limited by wear on mechanical components. Other complications may arise if the series capacitor is mechanically controlled. A series capacitor in a line may lead to sub synchronous resonance. A transistor controlled series capacitor (TCSC) can greatly enhance the stability of the network. It is practical that series compensation must be partly mechanically controlled and constraints at the least cost. By increasing the impedance of one of the lines in the same meshed configuration the power flow can be controlled. If a conductor of reactance 7Ω is inserted in series with line AB as shown in fig. A series inductor that is partly mechanically and partly thyristor controlled, it could serve to adjust steady state power flows as well as damp unwanted oscillations. To control power flow in a meshed network, thyristor controlled phase angle regulator could be installed instead of series capacitor or series reactor in any of three lines. In Fig (b) shows regulator installed in the third line to reduce total phase angle difference along the line from 8.5 deg to 4.26 deg. As before a combination of mechanical and thyristor control of phase angle regulator may minimize cost. BASIC TYPES OF FACTS CONTROLLERS: In general FACTS controllers can be divided into four categories. Series controllers

8 Shunt controllers Combined series-series controllers Combined series-shunt controllers Figure shows general symbol for a FACTS controller. SERIES CONTROLLER: Figure shows series type FACTS controller. A series controller is a variable impedance like a capacitor an inductor or a power electronic switched device of variable source with either mains frequency or sub harmonic frequency. Series controller injects voltage in series with a line. As long as the voltage is in phase quadrature with the line current, the series controller only supplies or consumes variable reactive power. Any other phase relationship will involve handling of real power as well. SHUNT CONTROLLER : Fig: shows shunt FACTS controller A shunt controller can be variable impedance, variable source or combination of both. Shunt controller inject current into the line (system) at the point of connection. As long as the injected current is in quadrature with line voltage, shunt controller only supplies or consumes variable reactive power. Any other phase relationship will involve handling of real power as well. COMBINED SERIES-SERIES CONTROLLERS

9 Fig: shows series-series FACTS controller This could be a combination of separate series controllers which are controlled in a co -ordinate manner, in a multi-line transmission systems. Series controllers provide independent series reactive compensation for each line but also transfer real power among the lines via the power link. The real power transfer capability of unified series controller,referred to as Inter Line Power Flow controller, makes it possible to balance both real & reactive power flow in the lines and there by maximize the utilization of transmission system. Note: The term Unified means that the DC terminals of all controller converters are all connected together for real power transfer. COMBINED SERIES SHUNT CONTROLLERS : Figures shows combinations of series and shunt controllers,which are controlled in a co-ordinate manner. The combined shunt and series controllers inject current into the system with the shunt part of the controller and voltage in series in the line with series part of controller. ** When shunt and series controllers are unified.there can be a real power exchange between the series and shunt controllers via the power link. Brief Description and Definition of FACTS Controllers Flexibility of Electric Power transmission: The ability to accommodate changes in the electric transmission system or operating conditions while maintaining sufficient steady-state and transient margins Flexible AC Transmission System (FACTS): Alternating current transmission systems incorporating power electronic-based and other static controllers to enhance controllability and increase power

10 transfer capability. It is worthwhile to note the words other static controllers in this definition of FACTS implying that there can be other static controllers which are not based on power electronics. FACTS Controller: A power electronic-based system and other static equipment that provide control of one or more AC transmission system parameters. SHUNT CONNECTED CONTROLLERS: (i)static Synchronous Compensators (STATCOM): A static synchronous generator operated as a shunt connected static var compensator whose4 capacitive or inductive output current can be controlled independent of the AC system voltage. STATCOM is one of the key FACTS controllers. It can be based on a voltage sourced or current sourced converter. Figure 1(a) shows a simple one-line diagram of STATCOM based on a voltage sourced converter and a current sourced converter.statcom can be designed to also act as an active filter to absorb system harmonics. (ii)static Var Compensator (SVC): A shunt connected static var generator or absorber whose output is adjusted to exchange capacitive or inductive current so as to maintain or control specific parameters of the electrical power system. This is a general term for a thyristor controlled or thyristor switched reactor and/or thyristor switched capacitor or combination. SVC is based on thyristors without gate turn-off capability. It includes separate equipment for leading and lagging vars, the thyristor controlled or thyristor switched reactor for absorbing reactive power on thyristor switched capacitor for supplying the reactive power. (iii)thyristor Controlled Reactor (TCR): A shunt connected thyristor controlled inductor whose effective reactance is varied in a continuous manner by partial-conduction control of the thyristor valve. TCR is a

11 subset of SVC in which conduction time and hence, current in a shunt reactor is controlled by a thyristor based ac switch with firing angle control. (iv)thyristor Switched Reactor (TSR): A shunt connected thyristor-switched inductor whose effective reactance is varied in a stepwise manner by full-or zero-conduction operation of the thyristor valve. TSR is another subset of SVC. TSR is made up of several shunt connected inductors which are switched in and out by thyristor switches without any firing angle controls in order to achieve the required step changes in the reactive power consumed from the system. Use of thyristor switches without firing angle control results in lower cost and losses, but without a continuous control. (v)thyristor Switched Capacitor (TSC): A shunt connected thyristor-switched capacitor whose effective reactance is varied in a stepwise manner by full-or zero-conduction operation of the thyristor valve. TSC is also a subset of SVC in which thyristor based ac switches are used to switch in and out shunt capacitors units, in order to achieve the required step change in the reactive power supplied to the system. Unlike shunt reactors, shunt capacitors cannot be switched continuously with variable firing angle control. (vi)static Var System (SVS):A combination of different static and mechanically-switched var compensators whose outputs are coordinated.. SERIES CONNECTED CONTROLLERS (i)static Synchronous Series Compensator (SSSC): A static synchronous generator operated without an external electric energy source as a series compensator whose output voltage is quadrature with, and controllable independently of, the line current for the purpose of increasing or decreasing the overall reactive voltage drop across the line and thereby controlling the transmitted electric power. The SSSC may include transiently rated energy storage or energy absorbing devices to enhance the dynamic behavior of the power system by additional temporary real power compensation, to increase or decrease momentarily, the overall real (resistive) voltage drop across the line.

12 SSSC is one of the most important FACTS controllers. It is like a STATCOM, except that the output ac voltage is in series with the line. It can be based on a voltage-sourced converter or currentsourced converter. Battery-storage or superconducting magnetic storage can also be connected to a series controller to inject a voltage vector of variable angle in series with the line. (ii)interline Power Flow Controller (IPFC):The combination of two or more Static synchronous Series Compensators which are coupled via a common dc link to facilitate bi-directional flow of real power between the ac terminals of the SSSCs, and are controlled to provide independent reactive compensation for the adjustment of real power flow in each line and maintain the desired distribution of reactive power flow among the lines. The IPFC structure may also include a STATCOM, coupled to the IPFC s common dc link, to provide shunt reactive compensation and supply or absorb the overall real power deficit of the combined SSSCs. (iii)thyristor Controlled Series Capacitor (TCSC): A capacitive reactance compensator which consists of a series capacitor bank shunted by a thyristor-controlled reactor in order to provide a smoothly variable series capacitive reactance. The TCSC is based on thyristors without the gate turn-off capability. It is an alternative to SSSC above and like an SSSC, it is a very important FACTS Controller. A variable reactor such as a Thyristor- Controlled Reactor (TCR) is connected across a series capacitor. (iv)thyristor-switched Series Capacitor (TSSC): A capacitive reactance compensator which consists of a series capacitor bank shunted by a thyristor-switched reactor to provide a stepwise control of series capacitive reactance. Instead of continuous control of capacitive impedance, this approach of switching inductors at firing angle of 90 degrees or 180 degrees but without firing angle control, could reduce cost and losses of the Controller. It is reasonable to arrange one of the modules to have thyristor control, while others could be thyristor switched. (v)thyristor-controlled Series Reactor (TCSR): An inductive reactance compensator which consists of a

13 series reactor shunted by a thyristor controlled reactor in order to provide a smoothly variable series inductive reactance. When the firing angle of the thyristor controlled reactor is 180 degrees, it stops conducting, and the uncontrolled reactor acts as a fault current limiter. As the angle decreases below 180 degrees, the net inductance decreases until firing angle of 90 degrees, when the net inductance is the parallel combination of the two reactors. As for the TCSC, the TCSR may be a single large unit or several smaller series units. (vi)thyristor-switched Series Reactor (TSSR): An inductive reactance compensator which consists of a series reactor shunted by a thyristor-controlled switched reactor in order to provide a stepwise control of series inductive reactance. This is a complement of TCSR, but with thyristor switches fully on or off (without firing angle control) to achieve a combination of stepped series inductance.

14 UNIT-III STATIC SHUNT COMPENSATORS Objectives of shunt compensation methods of controllable VAR generation-static VAR OBJECTIVES OF SHUNT COMPENSATION: compensators, SVC and STATCOM, comparison **************** Shunt compensation is used to influence the natural characteristics of the transmission line to steady-state transmittable power and to control voltage profile along the line shunt connected fixed or mechanically switched reactors are used to minimize line over-voltage under light load conditions. Shunt connected fixed or mechanically switched capacitors are applied to maintain voltage levels under heavy load conditions. Var compensation is used for voltage regulation. i. At the midpoint to segment the transmission line and ii. At the end of the line To prevent voltage intangibility as well as for dynamic voltage control to increase transient stability and to damp out power oscillations. MID-POINT VOLTAGE REGULATION FOR LINE SEGMENTATION: Consider simple two-machine(two-bus)transmission model in which an ideal var compensator is shunt connected at the midpoint of the transmission line FIG:

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16 NOTE: i. The midpoint of the transmission line is the best location for compensator because the voltage sage along the uncompensated transmission line is the longest at the midpoint ii. The concept of transmission line segmentation can be expanded to use of multiple compensators, located at equal segments of the transmission line as shown in fig. END OF LINE VOLTAGE TO SUPPORT TO PREVENT VOLTAGE INSTABILITY: A simple radial system with feeder line reactance X and load impedance Z is shown.

17 NOTE: 1. For a radial line, the end of the line, where the largest voltage variation is experienced, is the best location for the compensator. 2. Reactive shunt compensation is often used too regulate voltage support for the load when capacity of sending end system becomes impaired. IMPROVEMENT OF TRANSIENT STABILITY: The shunt compensation will be able to change the power flow in the system during and following disturbances. So as to increase the transient stability limit. The potential effectiveness of shunt on transient stability improvement can be conveniently evaluated by EQUAL AREA CRITERION. Assume that both the uncompensated and compensated systems are subjected to the same fault for the same period of time. The dynamic behavior of these systems is illustrated in the following figures. METHODS OF CONTROLLABLE VAR GENERATION: Capacitors generate and inductors (reactors)absorb reactive power when connected to an ac power source. They have been used with mechanical switches for controlled var generation and absorption. Continuously variable var generation or absorption for dynamic system compensation as originally provided by over or under-excited rotating synchronous machines saturating reactors in conjunction with fixed capacitors Using appropriate switch control, the var output can be controlled continuously from maximum capacitive to maximum inductive output at a given bus voltage. More recently gate turn-off thyristors and other power semiconductors with internal turn off capacity have been use of ac capacitors or reactors.

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19 It is evident that the magnitude of current in the reactor can be varied continuously by the method of delay angle control from maximum (α=0) to zero (α=90). In practice, the maximum magnitude of the applied voltage and that of the corresponding current will be limited by the ratings of the power components(reactor and thyristor valve)used. Thus, a practical TCR can be operated anywhere in a defined V-I area,the boundaries of which are determined by its maximum attainable admittance, voltage and current ratings are shown in fig. Note: If Thyristor Controlled Reactor(TCR) switching is restricted to a fixed delay angle, usually α=0, then it becomes a thyristors switched reactor (TSR). The TSR provides a fixed inductive admittance. Thus, when connected to the a.c. system, the reactive current in it will be proportional to the applied voltage as shown in fig. TSRs can provide at α=0, the resultant steady-state current will be sinusoidal.

20 THYRISTOR SWITCHED CAPACITOR(TSC): A single-phase thyristors switched capacitor (TSC) is shown in fig. It consists of a capacitor, a bi-directional thyristors valve, and a relatively small surge current limiting reactor. This reactor is needed primarily To limit the surge current in the thyristors valve under abnormal operating conditions To avoid resonances with the a.c. system impedance at particular frequencies Under steady state conditions, when the thyristor valve is closed and the TSC branch is connected to a sinusoidal a.c. voltage source, υ=vsin ωt, the current in the branch is given by The TSC branch can be disconnected ( switched out ) at any current zero by prior removal of the gate drive to the thyristor valve. At the current zero crossing, the capacitor voltage is at its peak valve. The disconnected capacitor stays charged to this voltage, and consequently the voltage across the non-conducting thyristors valve varied between zero and the peak-to-peak value of the applied a.c. voltage as shown in fig.(b). The TSC branch represents a single capacitive admittance which is either connected to, or disconnected from the a.c. system. The current in the TSC branch varies linearly with the applied voltage according to the admittance of the capacitor as illustrated by the V-I plot in the following fig.

21 It is observed that, maximum applicable voltage and the corresponding current are limited by the ratings of the TSC components(capacitor and thyristor valve).to approximate continuous current variation, several TSC branches in parallel may be employed, which would increase in a step-like manner the capacitive admittance. STATIC VAR COMPENSATOR: The static compensator term is used in a general sense to refer to an SVC as well as to a STATCOM. The static compensators are used in a power system to increase the power transmission capacity with a given network, from the generators to the loads. Since static compensators cannot generate or absorb real power, the power transmission of the system is affected indirectly by voltage control. That is, the reactive output power ( capacitive or inductive) of compensator is varied to control the voltage at given terminals of the transmission network so as to maintain the desired power flow under possible system disturbances and contingencies. Static Var Compensator(SVC) and Static Synchronous Compensator(STATCOM) are var generators, whose output is varied so as to maintain to control specific parameters of the electric power system. The basic compensation needs fall into one of the following two main categories Direct voltage support to maintain sufficient line voltage for facilitating increased power flow under heavy loads and for preventing voltage instability. Transient and dynamic stability improvements to improve the first swing stability margin and provide power oscillation SVC: damping. SVCs are part of the Flexible AC transmission system device family, regulating voltage and stabilizing the system. Unlike a synchronous condenser which is a rotating electrical machine, a "static" VAR compensator has no significant moving parts (other than internal switchgear). Prior to the invention of the SVC, power factor compensation was the preserve of large rotating machines such as synchronous condensers or switched capacitor banks.

22 Fig.shows Static Var Compensator(SVC). 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) The SVC is an automated impedance matching device, designed to bring the system closer to unity power factor. SVCs are used in two main situations: Connected to the power system, to regulate the transmission voltage ("Transmission SVC") Connected near large industrial loads, to improve power quality ("Industrial SVC") Fig.shows V-I Characteristics of SVC.

23 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 thyristor controlled 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 thyristorcontrolled 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. STATCOM: A static synchronous compensator (STATCOM), also known as a "static synchronous condenser" ("STATCON"), is a regulating device used on alternating current electricity transmission networks. It is based on a power electronics voltage-source converter and can act as either a source or sink of reactive AC power to an electricity network. If connected to a source of power it can also provide active AC power. It is a member of the FACTS family of devices. The STATCOM generates a 3-phase voltage source with controllable amplitude and phase angle behind reactance. When the a.c. output voltage from the inverter is higher(lower) than the bus voltage, current flow is caused to lead(lag) and the difference in the voltage amplitudes determines how much current flows. This allows the control of reactive power.

24 Fig. shows block diagram representation of STATCOM and V-I characteristics. The STATCOM is implemented by a 6-pulse Voltage Source Inverter(VSI) comprising GTO thyristors fed from a d.c.storage capacitor.the STATCOM is able to control its output current over the rated maximum capacitive or inductive range independently of a.c. system voltage, in contrast to the SVC that varies with the ac system voltage. Thus STATCOM is more effective than the SVC in providing voltage support and stability improvements. The STATCOM can continue to produce capacitive current independent of voltage.the amount and duration of the overload capability is dependent upon the thermal capacity of the GTO. Note : Multi-pulse circuit configurations are employed to reduce the harmonic generation and to produce practically sinusoidal current.

25 Comparison between STATCOM and SVC: S.No. STATCOM SVC 1 Acts as a voltage source behind a Acts as a variable susceptance reactance 2 Insensitive to transmission system Sensitive to transmission system harmonic resonance harmonic resonance 3 Has a larger dynamic range Has a smaller dynamic voltage. 4 Lower generation of harmonics Higher generation of harmonics. 5 Faster response and better performance Somewhat slower response during transients 6 Both inductive and capacitive regions of Mostly capacitive region of operation operation is possible 7 Can maintain a stable voltage even with a Has difficulty operating with a very very weak a.c. system weak a.c. system

26 UNIT-IV STATIC SERIES COMPENSATORS Objectives of series compensation-variable impedance type thyristor controlled series capacitors(tcsc)- switching converter type series compensators-static series synchronous compensator(sssc)-power angle characteristics-basic operating control schemes. * * * INTRODUCTION: FACTS is defined "a power electronic based system and other static equipment that provide control of one or more AC transmission system parameters to enhance controllability and increase power transfer capability. In series compensation, the FACTS is connected in series with the power system. It works as a controllable voltage source. Series inductance exists in all AC transmission lines. On long lines, when a large current flows, this causes a large voltage drop. To compensate, series capacitors are connected, decreasing the effect of the inductance. However, shunt compensation is ineffective in controlling the actual transmitted power at a defined transmission, voltage is ultimately determined by the series line impedance and the angle between the end voltages of the line. Series capacitive compensation is used to cancel a portion of the reactive line impedance and thereby increase the transmittable power. Variable series compensation is highly effective in both controlling power flow in the line and in improving stability. EXAMPLES OF SERIES COMPENSATION: Static synchronous series compensator (SSSC) Thyristor-controlled series capacitor (TCSC): a series capacitor bank is shunted by a thyristorcontrolled reactor Thyristor-controlled series reactor (TCSR): a series reactor bank is shunted by a thyristorcontrolled reactor Thyristor-switched series capacitor (TSSC): a series capacitor bank is shunted by a thyristorswitched reactor Thyristor-switched series reactor (TSSR): a series reactor bank is shunted by a thyristor-switched reactor Various controlled series compensators will play a key part in maintaining power flow over predefined paths, establishing alternative flow paths under contingency conditions, managing line loading, and generally ensuring the optimal use of the transmission network.

27 OBJECTIVES OF SERIES COMPENSATION: The objective of series compensation is to decrease the overall effective series transmission impedance from the sending end to the receiving end i.e., X The power transmission over a single line is given by Consider two machine model, with series capacitor compensated line, which is assumed to be composed of two identical segments as shown in fig. Fig.(b) shows voltage and current phasors of series compensated line. It is observed that for the same end voltages, the magnitude of the total voltage across the series line inductance Vx= 2Vx/2 is increased by the magnitude of the opposite voltage, Vc, developed across series capacitor; this results from an increase in the line current. The effective transmission impedance Xeff with the series capacitive compensation is given by Xeff = X - Xc = (1-k)X Where k is the degree of series compensation. i.e., k = Xc / X, 0 k 1 Assuming Vs=Vr =V, the current in the compensated line and the corresponding real power and reactive power can be derived in the following forms.

28 The relationship between the real power(p),series capacitive reactive power(qc) and angle δ is shown plotted at various values of the degree of series compensation k. It is observed that the transmittable power rapidly increases with the degree of series compensation k. Similarly, the reactive power supplied by the series capacitor also increases sharply with k and varies with angle δ in a similar manner as the line reactive power. 1.VOLTAGE STABILITY: Series capacitive compensation can also be used to reduce the series reactive impedance to minimize the receiving end voltage variation and the possibility of voltage collapse. A simple radial feeder of line reactance X, series compensating reactance Xc and load impedance Z is shown in fig(a).

29 Fig.(b) shows normalized terminal voltage(vr) versus active power( P) with unity power factor load at 0,50 and 75% series capacitive compensation. The nose of point at each plot given for a specific compensation level represents the corresponding voltage instability. Note: Both shunt and series capacitive compensation can effectively increase the voltage stability limit. (i)shunt compensation does it by supplying the reactive load demand and regulating the terminal voltage. (ii)series compensation does it by cancelling a portion of the line reactance and thereby providing a stiff voltage source for the load. For increasing the voltage stability limit of overhead transmission, series compensation is much more effective than shunt compensation. 2. IMPROVEMENT OF TRANSIENT STABILITY The powerful capability of series line compensation to control the transmitted power can be utilized much more effectively to increase the transient stability limit. Equal area criterion is used to assess the relative increase of the transient stability margin attainable by series capacitive compensation. Comparison of above two figures, it shows a substantial increase in the transient stability margin in series capacitive compensation. The increase of transient stability margin is proportional to the degree of series compensation. NOTE: (i) In practice, series capacitive compensation does not exceed 75% due to the following factors: load balancing in parallel paths, high fault current and difficulties of power flow control. (ii) Often the series compensation is limited to < 30% due to sub synchronous concerns.

30 3. POWER OSCILLATION DAMPING: Controlled series compensation can be applied to damp power oscillations. For power oscillation damping, it is necessary to vary the applied compensation so as to counteract the accelerating and decelerating swings of the disturbed machine(s). Fig.(a) shows the undamped and damped oscillations of angle δ around the steady-state value δ0. Fig.(b) shows undamped and damped oscillations of power corresponding to angle δ Fig.(c) shows the applied variation of the degree of series capacitive compensation, k. It is observed that k is maximum when d δ/dt >0, and it is zero when d δ/dt < 0 4. SUBSYNCHRONOUS OSCILLATION DAMPING: Sustained oscillation below the fundamental system frequency can be caused by series capacitive compensation. This phenomenon is referred to subsynchornous resonance(ssr). This is due to interaction between a series capacitor-compensated transmission line, oscillating at the natural resonant frequency and the mechanical system of a turbine-generator set in torsional mechanical oscillation can result in negative damping with the consequent mutual reinforcement of the electrical and mechanical oscillations. VARIABLE IMPEDANCE TYPE SERIES COMPENSATORS Variable impedance type series compensators are composed of thyristor-switched/controlled capacitors or thyristor-controlled reactors with capacitors. (i) GTO Thyristor Controlled Series Capacitor(GCSC) (ii) Thyristor Switched Series Capacitor ( TSSC) (iii)thyristor Controlled Series Capacitor (TCSC)

31 Thyristor Controlled Series Capacitor ( TCSC): It consists of series compensating capacitor shunted by a Thyristor controlled reactor as shown in fig. TCSC is connected in series with the line and allows changing of impedance of transmission path and thus affecting the power flows.with the help of TCSC, control is fast and efficient. Change of impedance of TCSC is achieved by changing thyristor controlled inductive reactance of inductors connected in parallel to the capacitor.the magnitude of inductive reactance is determined by angle(α) of switching thyristor.the magnitude of current through reactor changes from maximum to zero by switching thyristors. Magnitude of impedance of compensator is given by Under steady state, the impedance of TCSC is considered as a parallel LC circuits consisting of a fixed capacitive impedance Xc and variable inductive impedance XL(α). Where XL =ωl, and α is the delay angle measured from the crest of the capacitor voltage. Thus, TCSC presents a tunable parallel LC circuit to the line current that is substantially a constant alternating current source. For sufficiently small inductive reactance of reactor towards capacitive reactance of capacitor ( XL<XC) the operating diagram of TCSC contains inductive and capacitive mode operation. The transition between areas is the resonance region. Fig. shows impedance Vs. delay angle (α) characteristic of the TCSC.

32 Under normal operating conditions,tcsc can operate in modes of operation namely, blocked mode, bypass mode, capacitive and inductive mode. Assume that the thyristor valve, sw, is initially open and the prevailing line current I produces voltage vco across the fixed series compensating capacitor. Suppose that the TCR is to be turned on at α, measured from the negative peak of the capacitor voltage. At the instant of turn-on, the capacitor voltage is negative, the line current is positive and thus charging the capacitor in the positive direction. During this first half-cycle ( and all similar subsequent half-cycles) of TCR operation, the thyristor valve can be viewed as an ideal switch, closing at α, in series with a diode of appropriate polarity to stop the conduction as the current crosses zero as shown in fig(b).

33 At the instant of closing switch sw, two substantially independent events will take place. One is that the line current being a constant current source, continues to (dis)charge the capacitor. The other is that the charge of the capacitor will be reversed during the resonant half-cycle of the LC circuit formed by the switch closing. The resonant charge reversal produces a d.c. offset for the next(positive)half cycle of the capacitor as shown in fig.(c) POSSIBILITIES AND ADVANTAGES OF TCSC are : (i) Increased dynamic stability of power transmission systems (ii) Improved voltage regulation and reactive power balance (iii)improved load sharing between parallel lines (iv) Damping of active power oscillations (v) Dynamic power flow control (vi) Reduction of loop flows BASIC OPERATING CONTROL SCHEME OF TCSC: The main consideration for the control structure of the internal control operating power circuit of the TCSC is to ensure immunity to sub synchronous resonance. It includes two basic control strategies. One is to operate the basic phase locked-loop (PLL) from the fundamental component of the line current. In order to achieve this, it is necessary to provide substantial filtering to remove the super and and subsynchronous components from the line current and at the same time, maintain correct phase relationship for proper synchronization. Fig. shows Internal control scheme for the TCSC based on synchronization to the fundamental component of the line current.

34 In this arrangement, conventional technique of converting the demanded TCR current into the corresponding delay angle, which is measured from the peak of the fundamental line current is used. The reference for the demanded TCR current is provided by a regulation loop of the external control, which compares the actual capacitive impedance or compensating voltage to the reference given for the reference given for the desired system operation. SWITCHING CONVERTER TYPE SERIES COMPENSATORS: The static Synchronous Series Compensator (SSSC) will fall into switching converter type series compensator. Static Synchronous Series Compensator(SSSC): SSSC provide controllable compensating over identical capacitive and inductive range and independent of magnitude of line current. The operating principle of SSSC can be understood with reference to conventional series capacitive compensation together with phasar diagram. Fig. shows two machine system with a series capacitor compensated line and associated phasor diagram. From the phasor diagram, it is clear that at a given line current the voltage across the series capacitor forces the opposite polarity voltage across the series line reactance to increase by the magnitude of the capacitor voltage. Thus, the series capacitive compensation works by increasing the voltage across the

35 impedance of the given physical line, which in turn increases the corresponding line current and the transmitted power. The steady state power transmission can be established if the series compensation is provided by a synchronous ac voltage source and is given by Vq = Vc = - j Xc I = -jk X I, where k = Xc /X. Where Vc = injected compensating voltage ; I = line current Xc = reactance of series capacitor ; X = line reactance. By making the output voltage of synchronous voltage source as a function of line current, the same compensation as provided by the series capacitor is accomplished. Fig. shows two machine system with synchronous voltage source replacing the series capacitor Transmitted power Versus Transmission Angle characteristic: The SSSC injects the compensating voltage in series with the line irrespective of the line current. The transmitted power Pq versus the transmission angle δ relationship becomes a parametric function of the injected voltage,vq and it can expressed for a two-machine system as follows. Fig. shows relation between normalized power P versus δ plots of a series capacitor compensated twomachine system as a parametric function of Vq.

36 Fig. shows relation between normalized power P versus δ plots of a series capacitor compensated twomachine system as a parametric function of the degree of series compensation,k. Comparison of corresponding plots that the series capacitor increases the transmitted power by a fixed percentage of that transmitted by the uncompensated line at a given δ and by contract SSSC can increase by a fixed fraction of the maximum power transmittable by the uncompensated line, independent of δ in the range of 0 δ 90 CONTROL SCHEME FOR SSSC: A possible control scheme for the directly controlled SSSC converter is shown in fig. This scheme can be used to eliminate the unwanted output voltage components due to the modulation of the dc capacitor

37 voltage by sub synchronous or other line current components. It is also suitable to provide both reactive and real (resistive) line compensation if the converter is equipped with a suitable dc power supply(and/or sink) As shown in nfig, the synchronization to the line current is again accomplished by a phase-locked loop in a manner discussed above. Overall, the control structure is similar to that discussed in connection with the directly controlled converter, except for the continuous and independent control of both the magnitude and angle of the compensating voltage. In its general form, the control is operated from three references signals: VQqRef defining the desired magnitude of the series reactive compensating

38 THE UNIFIED POWER FLOW CONTROLLER UNIT-V POWER FLOW CONTROLLERS The Unified Power Flow Controller (UPFC) concept was proposed by Gyugyi in The UPFC was devised for the real-time control and dynamic compensation of ac transmission systems, providing multifunctional flexibility required to solve many of the problems facing the power delivery industry. Within the framework of traditional power transmission concepts, the UPFC is able to control, simultaneously or selectively, all the parameters affecting power flow in the transmission line (i.e., voltage, impedance, and phase angle), and this unique capability is signified by the adjective "unified" in its name. Alternatively, it can independently control both the real and.reactive power flow in the line. The reader should recall that, for all the Controllers discussed in the previous chapters, the control of real power is associated with similar change in reactive power, i.e., increased real power flow also resulted in increased reactive line power. Basic Operating Principles of UPFC

39 source. The transmission line current flows through this voltage source resulting in reactive and real power exchange between it and the ac system. The reactive power exchanged at the ac terminal (Le., at the terminal of the series insertion transformer) is generated internally by the converter. The real power exchanged at the ac terminal is converted into de power which appears at the de link as a positive or negative real power demand. The basic function of Converter 1 is to supply or absorb the real power demanded by Converter 2 at the common de link to support the real power exchange resulting from the series voltage injection. This de link power demand of Converter 2 is converted back to ac by Converter 1 and coupled to the transmission line bus via a shuntconnected transformer. In addition to the real power need of Converter 2, Converter 1 can also generate or absorb controllable reactive power, if it is desired, and thereby provide independent shunt reactive compensation for the line. It is important to note that whereas there is a closed direct path for the real power negotiated by the action of series voltage injection through Converters 1 and 2 back to the line, the corresponding reactive power exchanged is supplied or absorbed locally by Converter 2 and therefore does not have to be transmitted by the line. Thus, Converter 1 can be operated at a unity power factor or be controlled to have a reactive power exchange with the line independent of the reactive power exchanged by Converter 2. Obviously, there can be no reactive power flow through the UPFC de link. INDEPENDENT REAL AND REACTIVE POWER FLOW CONTROL:

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