Switchgear and Protection

a) Discuss why backup protection is necessary in a power system and explain the overlapping of protective zones with neat sketch. [4] b) A current transformer is connected in a $400\text{ V}$ system and has a single turn on its primary and $180$ turns on its secondary. An ammeter with $0.15\Omega$ internal resistance is connected to the CT. The ammeter is required to give a full-scale deflection when the primary current is $900\text{ A}$. Calculate, [4] i) The maximum secondary current ii) Secondary voltage at CT terminals when ammeter is removed c) Explain the cut-off characteristics of a fuse.

a) List down the different applications of contactor. With the help of necessary schematic circuit explain how star-delta starting of motor can be achieved with the help of contactors. [6] b) Calculate the inductance and kVA rating of Peterson coil of a $230\text{ kV}$, $3$-phase, $50\text{ Hz}$, $200\text{ km}$ overhead transmission system having capacitance to earth of $0.01\mu \text{F/km}$ per phase. [5] c) With the help of neat sketch, explain the types of isolators and state the major differences between isolator and circuit breaker.

a) Why contactors are preferred to control a motor instead of a manual switch? Explain with a neat sketch. [4] b) Explain, with a neat Single-Line Diagram (SLD), how the isolator, circuit breaker, and earthing switch operate in a coordinated manner to ensure the safe operation and maintenance of a substation.

a) A 50 Hz 3 phase alternator with grounded neutral has an inductance of $1.6\text{ mH}$ per phase and is connected to the bus-bars through a circuit breaker. The capacitance to earth of the circuit between the alternator and the circuit breaker is $0.0032\mu F$ per phase. Due to a short on the bus-bars the breaker opens when the rms value of the current is $8000\text{ A}$. Determine the following (i) frequency of the oscillation (ii) active recovery voltage (iii) time for maximum RRRV (iv) maximum RRRV [6] b) Explain the construction and working of $S{F}_6$ circuit breaker. Also enumerate the dielectric properties and arc quenching characteristic of $S{F}_6$ circuit breaker with their limitations. [6] c) What are the main parts of equipment which are used for the testing of a circuit breaker in a laboratory type testing station?

a) A 50 Hz 3 phase alternator with grounded neutral has an inductance of $1.6\text{ mH}$ per phase and is connected to the bus-bars through a circuit breaker. The capacitance to earth of the circuit between the alternator and the circuit breaker is $0.0032\mu F$ per phase. Due to a short on the bus-bars the breaker opens when the rms value of the current is $8000\text{ A}$. Determine the following (i) frequency of the oscillation (ii) active recovery voltage (iii) time for maximum RRRV (iv) maximum RRRV [6] b) Explain the construction and working of $S{F}_6$ circuit breaker. Also enumerate the dielectric properties and arc quenching characteristic of $S{F}_6$ circuit breaker with their limitations. [6] c) What are the main parts of equipment which are used for the testing of a circuit breaker in a laboratory type testing station?

(a) The neutral point of a three-phase 20 MVA, 11 kV alternator is earthed through a resistance of $5\Omega ,the$ relay is set to operate when there is $a^n$ out of balance current of $1.5\text{A}$. The CTs hence a ratio $1000/5$. What percentage of winding is protected against $a^n$ earth fault and what should be the minimum value of earthing resistance to protect 90% of the winding? [6] (b) Explain the operating principle and types of Current Transformers (CTs). Discuss the significance of CT ratios, burden, and polarity in power system protection.

a) Two relays $R_1$ and $R_2$ are connected in two sections of a feeder is shown in figure. Fault current level at 11 kV side is $3500\text{A}$, CT ratio at 11 kV side is $200:1$ and 132kV side is $100:1$. If $R_1$ set on 125% and $R_2$ set on 150% plug setting. Determine the operating time for both the relays when time grading margin of $0.75\text{second}$ is given and TMS for relay $R_1$ is $0.25$. Make use of the following characteristics. [5] IDMT Characteristics

b) A transmission line is to be designed and protected using distance protection schemes. (i) Draw a neat single-line diagram of the transmission line and illustrate the division of the protection system into appropriate distance protection zones, clearly indicating the reach and coverage of each zone. (ii) Explain the concept of zone-wise protection in distance relays, and justify the need for multiple zones to ensure selectivity and reliability. (iii) Describe the working principle of carrier-aided protection and explain how it improves the performance of distance protection in transmission lines. [2+2+1] c) A three phase, 11/132 kV, delta star connected power transformer is protected by differential protection. The CTs on the LV side have a current ratio of 250/1. What must be the current ratio of the CTs on the HV side?

a) A $11\text{ kV}$, $100\text{ MVA}$ generator is grounded through a resistance of $6\Omega$. The CTs have a ratio of $1000:5$. The relay is set to operate when there is an out of balance current of $1\text{ A}$. What percentage of generator winding will be protected by percentage differential scheme of protection? [6] b) In the given figure below, the fault current is $2000\text{ A}$ and the CTs are of ratio $=200:1$. The plug setting of Relay $R_1$ is $100\%$ and the plug setting of Relay $R_2$ is $125\%$. The time grading scheme has a discriminative time margin of $0.5\text{s}$ between the relays and the time multiplier setting (TMS) of $R_1$ is $0.2$. [8] Determine the time of operation of the two relays assuming that both relays have the characteristics as shown in the following table. Also determine the TMS of $R_2$.

c) What is an impedance relay? Explain its operating principle and characteristic along with torque equation.

a) Explain the working of static differential relay with the proper block diagram and circuit diagram. [8] b) Discuss the advantages of digital relays. Describe the basic functional blocks of a digital relay.

Also explain the main components of a static relay with the help of block diagram.

Calculate the inductance and kVA rating of Peterson coil of a $230\text{ kV}$, $3$-phase, $50\text{ Hz}$, $200\text{ km}$ overhead transmission system having capacitance to earth of $0.01\mu \text{F/km}$ per phase.

A 50 Hz 3 phase alternator with grounded neutral has an inductance of $1.6\text{ mH}$ per phase and is connected to the bus-bars through a circuit breaker. The capacitance to earth of the circuit between the alternator and the circuit breaker is $0.0032\mu F$ per phase. Due to a short on the bus-bars the breaker opens when the rms value of the current is $8000\text{ A}$. Determine the following (i) frequency of the oscillation (ii) active recovery voltage (iii) time for maximum RRRV (iv) maximum RRRV

Describe the basic functional blocks of a digital relay.

Explain, with a neat Single-Line Diagram (SLD), how the isolator, circuit breaker, and earthing switch operate in a coordinated manner to ensure the safe operation and maintenance of a substation.

What are the main parts of equipment which are used for the testing of a circuit breaker in a laboratory type testing station?

Define the concept of zones of protection within a power system. Using a clearly labeled Single-Line Diagram (SLD), illustrate how protection zones are made to overlap and distinguish between primary and backup protection. Finally, provide a technical justification for why a "blind spot" occurs if these zones fail to overlap.

(a) For a 220 kV system, the reactance and capacitance up to the location of the circuit breaker is $3\Omega$ and $0.035\mu \text{F},respectively$. Calculate the following: (i) The frequency of transient oscillation (ii) The maximum value of restriking voltage across the contacts of the circuit breaker (iii) The maximum value of RRRV. [5] (b) Explain the phenomenon of arc in a circuit breaker and justify its necessity. With the help of a neat waveform, define and explain the following terms: arc voltage, re-striking voltage, and recovery voltage. [4] (c) What tests are included under routine test of circuit breaker and how are these tests done?

(a) Explain the functional operation of numerical relays with a suitable block diagram. Discuss how signals are processed from input acquisition to final tripping action. [3+2] (b) Compare differential and directional types of static relays in terms of their operating principles, characteristics, and applications in power system protection.