Facts -Two Marks Question and Answer

UNIT-I 1. What is FACTS? FACTS: Flexible Alternating Current Transmission System IEEE Definition: Alternating current transmission systems incorporating powerelectronic based and other static controllers to enhance controllability and increase power transfer capability 2. What is the need for reactive power control in electrical power transmission lines? To make transmission networks operate within desired voltage limits, methods of making up or taking away reactive power is called reactive-power control. 3. What are the objectives of FACTS?  Rapid control of reactive power flow and voltage profile using series and shunt connected controllers.  Secure loading of lines close to their thermal limits.  Improve power transferability, transient stability and dynamic stability during fault switching etc. 4. Write the difference between FACTS & HVDC.  FACTS controllers can be retrofitted into the existing line, but not in HVDC.  Installation cost is less for FACTS compared to that of HVDC.  FACTS device provide VAR compensation 5. What are the methods of series compensation? Static synchronous series compensator (SSSC) Thyristor controlled series capacitor (TCSC) Thyristor switched series capacitor (TSSC) 6. What are the methods of shunt compensation? • Static Synchronous compensator (STATCOM) •

Static VAR Compensator (SVC)

•

Thyristor Controlled Reactor (TCR)

•

Thyristor Switched Reactor (TSR)

•

Thyristor Switched Capacitor (TSC)

•

Mechanically Switched Capacitor (MSC)

7. What are the conventional control mechanisms?  Automatic Generation Control (AGC)  Excitation Control  Transformer Tap changer Control  Phase Shifting Transformers

8. Define compensators. External devices or subsystems that control reactive power on transmission lines are known as compensators. 9. What are the objectives of line compensation?  To increase the power transmission capacity of the line  To keep the voltage profile of the line along its length within acceptable bounds to ensure the quality of supply to the connected customers as well as to minimize the line insulation costs. 10. What are the application of series compensation?  The voltage magnitude across the capacitor banks (insulation)  The fault currents at the terminals of a capacitor bank  The placement of shunt reactors in relation to the series capacitors (resonant overvoltages);  The number of capacitor banks and their location on a long line (voltage profile) 11. What is meant by degree of compensation? In an interconnected network of power lines that provides several parallel paths for power flow between two locations, it is the series compensation of a selected line that makes it the principal powr carrier. Series compensation is defined by the degree of compensation; for example, a 1-pu compensation means that the effective series reactance of a line will be zero. A practical upper limit of series compensation, on the other hand, may be as high as 0.75pu. 12. What is SVC? SVC is a shunt connected static VAR generator or absorber whose output is adjusted to control capacitive or inductive current so as to maintain or control specific parameters of the electrical power system. 13. What is STATCOM? STATCOM is a static synchronous generator operated as a shunt connected static VAR compensator (SVC) whose capacitive or inductive output current can be controlled independently of the A.C system voltage. 14. What is UPFC? UPFC is a combination of static synchronous compensator (STATCOM) and a static synchronous series compensator (SSSC) which are coupled through a D.C link, to allow

bi-directional flow of real power between series output of SSSC and shunt output of STATCOM. 15. List out the difference between UPFC and IPFC. UPFC: unified power flow controller  It is the combination of STATCOM or SSC and SSSC  It can independently and very rapidly control both real- and reactive power flows in a transmission line  It control single lines IPFC: interline power flow controller  It is the combination of two SSSC  It can independently and very rapidly control both real- and reactive power flows in a transmission line  It control two lines 16. What is an synchronous condenser? A synchronous motor running at no-load feeds positive VARs into the line under overexcited conditions and feeds negative VARs when under-excited. A machine thus running is called a synchronous condenser. 17. Compare shunt and series capacitors.

Shunt capacitors Series Capacitors 1. The function of shunt capacitor 1. Sereis capacitors are connected in applied as the single unit or in series with the line and are used to groups of units is to supply reduce inductive reactance lagging KVAR to the system at the between supply point and load. point where they are connected. 2. It is mainly used for power factor 2. It is mainly used to compensate the correction at the load terminal of effect of series reactance. low voltage. 3. If the load VAR requirement is 3. If the load VAR requirement is small, shunt capacitor are of high small, series capacitors are of use. small use. 4. If the total line reactance is high, 4. If the total line reactance is high, shunt capacitors are not effective. series capacitors are very effective and stabilty is improved. 18. What is IPFC? The combination of two or more Static Synchronous Series Compensators which are coupled via a common de 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 ofreactive power flow among the lines. The IPFC structuremay also include a STATCOM, coupled to the IFFC's common de link, to provide shunt reactive compensation and supply or absorb the overall realpower deficit of the combined SSSCs. 19. What are the constraint in UPFC? The UPFC operates with constraints on the following variables  the series-injected voltage magnitude;  the line current through series converter;  the shunt-converter current;  the minimum line-side voltage of the UPFC;  the maximum line-side voltage of the UPFC; and  the real-power transfer between the series converter and the shunt converter. 20. What are the salient features of UPFC?  Voltage control  Transient stability  Damping power oscillation  Reactive power compensation  Power flow control  SSR mitigation 21. Draw the single line diagram of UPFC.

22. Draw the single line diagram of IPFC.

23. List out the difference between SVC and STATCOM STATCOM(static synchronous compensator)

SVC(static VAR compensator)

 It uses gate turn-off thyristors.

 It uses conventional thyristors.

 It is based on voltage source converter.

 It is a voltage regulator and variable susceptance controller.

 It has short time overload capability

 It cannot have short time overload capability.

 It operates both inductive and capacitive regions.

 It operates mostly in capacitive region.

 Better performance during transients.

 Slow performance during transients.

 It reduces system harmonics.

 It generates system harmonics.

 Even with very weak A.C system, it maintains stable voltage.

 Even with very weak A.C system, its operation is difficult.

24. Explain the terms- load compensation and system compensation. Load Compensation It is possible to compensate for the reactive current Ix of the load by adding a parallel capacitive load so that Ic −Ix. • Doing so causes the effective power factor of the combination to become unity. • The absence of Ix eliminates the voltage drop ∆V1, bringing Vr closer in magnitude to Vs; this condition is called load compensation. •

• Actually, by charging extra for supplying the reactive power, a power utility company makes it advantageous for customers to use load compensation on their premises. • Loads compensated to the unity power factor reduce the line drop but do not eliminate it; they still experience a drop of ∆V2 from

System Compensation • To regulate the receiving-end voltage at the rated value, a power utility may install a reactive-power compensator as shown in Fig. 2 • This compensator draws a reactive current to overcome both components of the voltage drop ∆V1 and ∆V2 as a consequence of the load current Il through the line reactance Xl. • To compensate for ∆V2, an additional capacitive current, ∆Ic, over and above Ic that compensates for Ix, is drawn by the compensator. • When the receiving-end voltage, Vr, equals the sending-end voltage, Vs. • Such compensators are employed by power utilities to ensure the quality of supply to their customers.

25. List out the various reactive power compensators.  STATCOM(Static Synchronous compensator)  SSSC(Static synchronous series compensator)  UPFC(Unified power flow controller)  IPFC(Interline power flow controller)  UPQC(Unified power Quality conditioner)  IPQC(Interline power quality conditioners) 26.What are the advantages of FACTS controller?

 Improved steady state system performance  Improvements in system transient or dynamic stability.  Reduced financial costs and /or environmental impacts.  FACTS controller require minimal maintenance  Increase the system security.

 Reduces power system oscillations. STATIC VAR COMPENSATOR (SVC) AND APPLICATIONS 1. State the advantages of the slope in the SVC dynamic characteristic  substantially reduces the reactive-power rating of the SVC for achieving nearly the same control objectives;  prevents the SVC from reaching its reactive-power limits too frequently;and  facilitates the sharing of reactive power among multiple compensators operating in

parallel. 2. What is sub-synchronous resonance?

It is defined by the IEEE as an electric power condition where the electric network exchanges energy with a turbine generator at one or more of the natural frequencies of the combined system below the synchronous frequency of the system.

3. What is voltage instability?

Voltage instability is caused by the inadequacy of the power system to supply the reactive-power demand of certain loads, such as induction motors. A drop in the load voltage leads to an increased demand for reactive power that, if not met by the power system, leads to a further decline in the bus voltage. This decline eventually leads to a progressive yet rapid decline of voltage at that location, which may have a cascading effect on neighboring regions that causes a system voltage collapse.

4. Draw the dynamic characteristic of SVC

The voltage–current characteristic of the SVC

the voltage–reactive-power characteristic of the SVC. 5. What is synchronizing torque? The synchronizing torque ensures that the rotor angles of different generators do not drift away following a large disturbance. (In other words, the synchronizing torque binds the different generators into synchronism, assuring transient stability.) In addition, the magnitude of the synchronizing torque determines the frequency of oscillation.

6. What is damping torque? Damping torque influences the decay time of oscillations. Even if a power system is stable, the oscillations may be sustained for a long period without adequate damping torque.

7. Draw the power angle curves for transient-stability margins in the SMIB system. a)

Uncompensated system

b)

SVC compensated system

Power-angle curves depicting transient-stability margins in the SMIB system: (a) the uncompensated system and (b) the SVC-compensated system.

8. Draw the effect of the system short circuit level on the SVC response time.

Effect of the system short-circuit level on the SVC response time

9. What are the factors of PSDC? PSDC: Power Swing Damping constant SVC PSDC The effectiveness of the SVC auxiliary PSDC is dependent mainly on the following three factors  Controllability  Observability  Inner-loop sensitivity

10. Write down the three forms of SSR.  Induction generator effect (involving the electrical system alone).  Torsional interaction (involving both the electrical and mechanical systems)  Transient torque (involving both electrical and mechanical systems but initiated by severe disturbances, such as fault) 11.What is meant by TSR? Thyristor-switched reactor (TSR) A shunt-connected, thyristorswitched inductor whose effective reactance is varied in a stepwise manner by full- or zeroconduction operation of the thyristor valve.

12.What is meant by TCR? Thyristor-controlled reactor (TCR) A shunt-connected, thyristorcontrolled inductor whose effective reactance is varied in a continuous manner by partialconduction control of the thyristor valve. 13. Explain the Rating of the SVC. Rating of the SVC The inductive rating of the SVC is chosen to be that which can reduce the likely highest temporary overvoltage following a disturbance (such as permanent dc block) to within acceptable limits. The fixed capacitors and filters are assumed to be connected at the inverter bus in this evaluation. The capacitive rating of the SVC must be such that it can completely fulfill the reactive-power demand of the inverter terminal at the rated power and voltage, thereby ensuring a unity power factor at the ac bus. Additional capacitance reactive power may also be provided for improved handling of undervoltage situations. 14. Write down the expression for synchronizing torque coefficient of the uncompensated system.

THYRISTOR CONTROLLED SERIES CAPACITOR AND APPLICATION 1. Draw the single line diagram of TCSC.

2. Draw the TCSC constant impedance controller.

A TCSC constant-impedance controller.

3. Draw the TCSC constant current controller model.

A TCSC constant-current (CC) controller model.

1. Draw the TCSC control characteristics i)

CC control

ii)

CA control

The TCSC control characteristics: (a) CC control and (b) CA control 4. What are the different modes of operation of TCSC?  Bypassed-Thyristor Mode  Blocked-Thyristor Mode  Partially Conducting Thyristor, or Vernier, Mode 5. What is meant by thyristor switched series capacitor? Thyristor-switched series capacitor (TSSC) A capacitive reactance compensator which consists of a series capacitor bank shunted by a thyristorswitched reactor to provide a stepwise control of series capacitive reactance. 6. What are the advantages of TCSC?  Rapid, continuous control of the transmission-line series-compensation level.  Dynamic control of power flow in selected transmission lines within the network to enable optimal power-flow conditions and prevent the loop flow of power.  Damping of the power swings from local and inter-area oscillations  Reduction of the short-circuit current. During events of high short-circuit current, the TCSC can switch from the controllable-capacitance to the controllableinductance mode, thereby restricting the short-circuit currents. 8.What is meant by thyristor controlled series capacitor? 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 smoothly variable series capacitive reactance. 7. Draw the single line diagram of TSSC.

8. Draw the impedance vs delay angle characteristic of TCSC.

9. What are the losses in the TCSC? the series-capacitor losses; the reactor-conduction loss; the thyristor-conduction loss; and the switching losses from heating the snubber resistor when voltage step occurs across the snubber capacitor, as well as from heating the thyristor during the recovery process. 10. Draw the reactance capability curve of a multimodule TCSC.    

A simplified reactance-capability curve of a multimodule TCSC.

11. Draw the X-I capability characteristic of a multimodule TCSC .

The X-I capability characteristic of a multimodule TCSC indicating time dependent overload limits.

12. What are the condition to apply the placement of TCSC?  The TCSCs should be located in lines that experience limiting power oscillations.  The swing of voltages on each side of the TCSC must be within acceptable limits; otherwise, multiple sites may be necessary.  The control action of the TCSC in one transmission path should not cause undue power swings in a parallel path. If it does, then variable series compensation may become necessary in the parallel path.  Sometimes, it may be advisable to distribute the control action among multiple TCSCs rather than confining the control action to one large-rating TCSC. Doing so ensures some system reliability if one of the TCSCs should fail.

13.Draw the VI capability curves for single module TCSC.

The V-I capability characteristics for a single-module TCSC.

14. Draw the X-I capability characteristic of a multimodule TCSC .

The X-I capability characteristic for a single-module TCSC.

15. What are the needs for Variable series compensation?  enhanced base-power flow and loadability of the series-compensated line;  additional losses in the compensated line from the enhanced power flow; And  increased responsiveness of power flow in the series-compensated line from the outage of other lines in the system.

EMERGING FACTS CONTROLLER 1. Draw the single line diagram of STATCOM.

2. Draw the phasor diagram of UPFC for simultaneous control of voltage, impedance and angle.

A phasor diagram illustrating the simultaneous regulation of the terminal voltage, line impedance, and phase angle by appropriate series-voltage injection.

3. Draw the VI characteristic of STATCOM.

The V-I characteristic of the STATCOM.

4.

Write the capabilities of Different FACTS controller.

5. Draw the steady state characteristic of STATCOM.

The steady-state characteristics of a STATCOM.

6. Draw the IEEE first benchmark system with a STATCOM for SSR damping studies.

The IEEE First Benchmark System with a STATCOM for SSR damping studies.

7. Draw the equivalent circuit of STATCOM.

8. How to improve the power system performance using STATCOM? The dynamic voltage control in transmission and distribution systems;  the power-oscillation damping in power-transmission systems;  the transient stability;  the voltage flicker control; and  the control of not only reactive power but also (if needed) active power

 in the connected line, requiring a dc energy source. COORDINATION OF FACTS CONTROLLER 1. What are the methods of controller interactions?  Steady-State Interactions  Electromechanical-Oscillation Interactions  Control or Small-Signal Oscillations  Subsynchronous Resonance (SSR) Interactions  High-Frequency Interactions 2. What are the basic procedure for controller design?

3. List the frequency ranges for different control interactions. • • • • •

0 Hz for steady-state interactions 0–3/ 5 Hz for electromechanical oscillations 2–15 Hz for small-signal or control oscillations 10–50/ 60 Hz for subsynchronous resonance (SSR) interactions >15 Hz for electromagnetic transients, high-frequency resonance or harmonic resonance interactions, and network-resonance interactions 4. What are the causes for SSR interactions? Subsynchronous oscillations may be caused by the interaction between the generator torsional system and the series-compensated-transmission lines, the HVDC converter controls, the generator excitation controls, or even the SVCs. These oscillations, usually in the frequency range of 10–50/ 60 Hz, can potentially damage generator shafts. Subsynchronous damping controls have been designed for individual SVCs and HVDC links. In power systems with multiple FACTS controllers together with HVDC converters, a coordinated control can be more effective in curbing these torsional oscillations. 5. What is the need for coordination of FACTS controllers? Controller interactions can occur in the following combinations: 1.

Multiple FACTS controllers of a similar kind.

2. Multiple FACTS controllers of a dissimilar kind.

3. Multiple FACTS controllers and HVDC converter controllers. Because of the many combinations that are possible, an urgent need arises for power systems to have the controls of their various dynamic devices coordinated. The term coordinated implies that the controllers have been tuned simultaneously to effect an overall positive improvement of the control scheme 6. What is Bang-Bang control? Bang-bang control is a discrete control form in which the thyristors are either fully switched on (α= 90o) or fully switched off (α=180o). 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. Bang-bang control is employed in face of large disturbances to improve the transient stability

7. List the local signals & Remote Signals. Local Signals These signals constitute the following:

1. the line current, 2. the real-power flow, 3. the bus voltage, and 4. the local bus frequency. Remote Signals 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.