This set of FACTS Multiple Choice Questions & Answers (MCQs) focuses on “Special Purpose Controllers – NGH-SSR Damping Scheme”.
1. NGH-SSR stands for ________
a) Narain G. Hingorani-SSR
b) Negative gain high impedance-SSR
c) Normal gain high impedance-SSR
d) Nominal gain high impedance-SSR
View Answer
Explanation: Actually the part ‘NGR’ in NGR-SSR stands for the initials of the inventor named Narain G. Hingorani. NGR-SSR denotes to the sub-synchronous resonance damping scheme invented by Narain G. Hingorani. It finds applications in executing an efficient and reliable FACTS Controller.
2. NGH-SSR Damping Scheme is characterized by a ________ capacitor impedance control scheme.
a) high power series
b) low power series
c) low power shunt
d) high power shunt
View Answer
Explanation: NGH-SSR Damping Scheme is characterized by a low power series capacitor impedance control scheme. It revealed that there is no limit to series capacitor compensation with an active Controller.
3. NGH-SSR Damping Scheme is ________ employed for series compensation.
a) detrimental to series capacitors
b) not of much use to series capacitors
c) a boon to series capacitors
d) a boon to series resistors
View Answer
Explanation: NGH-SSR Damping Scheme is a boon to series capacitors employed for series compensation. Series resistors are NOT employed for series compensation while series capacitors are employed for series compensation of long and medium ac transmission lines. This NGH-SSR Damping scheme mitigates the technical issue of protection of capacitors during line faults.
4. NGH-SSR Damping Scheme is employed to minimize shaft twisting.
a) True
b) False
View Answer
Explanation: NGH-SSR Damping Scheme is employed to minimize shaft twisting. Actually this scheme decreases sub-synchronous electrical torque. Thus it diminishes the mechanical torque and the resultant shaft twisting.
5. NGH-SSR Damping Scheme suppresses the ________ of the series capacitor.
a) ac offset
b) neither dc offset nor ac offset
c) either dc offset or ac offset
d) dc offset
View Answer
Explanation: NGH-SSR Damping Scheme suppresses the dc offset of the series capacitor. DC offset of a series capacitor comes into picture due to faults. DC offset also arises during fault clearing, reclosing, etc.
6. NGH-SSR Damping Scheme helps to reduce capacitor cost.
a) True
b) False
View Answer
Explanation: NGH-SSR Damping Scheme helps to reduce capacitor cost. This is because the scheme reduces the rate of discharge current and removes oscillations associated with capacitor discharge during bypass with the thyristor bypass switch. It also reduces capacitor stresses.
7. NGH-SSR Damping Scheme is basically NOT a passive scheme.
a) True
b) False
View Answer
Explanation: NGH-SSR Damping Scheme is basically a passive scheme. This is because the scheme employs no feedback unlike other methods of controlling/eliminating SSR. It fundamentally acts to radically lessen the contribution of the electrical system attributed as its gain in the resulting electromechanical oscillatory system.
8. NGH-SSR Damping Scheme for one phase employs ________ with an ac thyristor switch connected across the capacitor.
a) an inductance in series
b) a resistance in series
c) an impedance in shunt
d) an impedance in series
View Answer
Explanation: NGH-SSR Damping Scheme for one phase employs an impedance in series with an ac thyristor switch connected across the capacitor. This impedance may consist of an inductance or a resistance. It may even be a combination of an inductance or a resistance.
9. The best impedance in NGH-SSR Damping Scheme is ________
a) a combination of a small-value resistor and a small-value resistor
b) a combination of a large-value resistor and a large-value resistor
c) a combination of a large-value resistor and a small-value resistor
d) a combination of a small-value resistor and a large-value resistor
View Answer
Explanation: The best impedance in NGH-SSR Damping Scheme is a combination of a small-value resistor and a small-value resistor. The underlying reason is that the impedance must be as small as possible in order to enable discharging of the capacitor with the thyristor bypass switch. However, the size of resistor employed, is essentially limited by the peak transient current ability of the switch.
10. The proposition behind NGH-SSR Damping Scheme is that the unbalanced charge in the series capacitor interchanges with the system inductance to produce oscillations.
a) True
b) False
View Answer
Explanation: The proposition behind the NGH scheme is that the unbalanced charge in the series capacitor interchanges with the system inductance to produce oscillations. In case this unbalanced charge is removed from the series capacitor, the system would be effectively detuned. The size of inductor is limited by the di/dt limit of the thyristors.
11. In NGH-SSR Damping Scheme the series resistor has to be designed to limit the rate of current rise and peak capacitor discharge current.
a) True
b) False
View Answer
Explanation: In NGH-SSR Damping Scheme the series resistor has to be designed to limit the rate of current rise and peak capacitor discharge current. The resistor only consumes power and does not supply power. Also the inductance is accordingly to be designed for getting the desired benefits.
12. In case of transient torque problem, the thyristors in NGH-SSR Damping Scheme DO NOT conduct at all during steady state.
a) True
b) False
View Answer
Explanation: In case of transient torque problem, the thyristors in NGH-SSR Damping Scheme DO NOT conduct at all during steady state. This is because the presence of only transient torque problem means there is no steady-state resonance and the thyristors are not required to conduct at all during steady state and small changes. However, the damping scheme will be effective during large changes from steady state.
13. In case of likelihood of presence of steady-state resonance problem, the thyristors in NGH-SSR Damping Scheme DO NOT conduct at all during steady state.
a) True
b) False
View Answer
Explanation: In case of likelihood of presence of steady-state resonance problem, the thyristors in NGH-SSR Damping Scheme MUST conduct during steady state. By doing so, it can provide detuning effect in case any steady build up of oscillations is detected. In this course there will be continuous power loss but it is of least importance with respect to thyristor rating.
14. In NGH-SSR Damping Scheme the thyristor switch is designed to be controlled when zero crossing of the capacitor voltage is detected.
a) True
b) False
View Answer
Explanation: In NGH-SSR Damping Scheme the thyristor switch is designed to be controlled when zero crossing of the capacitor voltage is detected. At each capacitor voltage zero crossing of each half-cycle, a new timing count starts. This is done to discharge the capacitor for the time such that its half-cycle voltage exceeds the set period.
15. The operation of the NGH-SSR Damping Scheme is dependent on specific signal from the power generator.
a) True
b) False
View Answer
Explanation: The operation of the NGH-SSR Damping Scheme is NOT dependent on specific signal from the power generator. The NGH scheme operates independently for each phase and does not need any signal from the power generator. Also it does not call for specific detection of any particular sub-synchronous frequency or presence of any particular dc bias voltage.
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