Protective System Functions Quiz
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Questions and Answers

What is one of the key advantages of a feeder reactor?

  • It increases the overall power output of the system.
  • It helps to maintain bus-bar voltage during faults. (correct)
  • It absorbs all short circuit faults.
  • It completely isolates feeders from each other.

    • Feeder reactors can lead to an increased likelihood of synchronism loss in generators during fault conditions.

    False (B)

    Name one disadvantage of using feeder reactors.

    Constant power loss during normal operation.

    A feeder reactor helps isolate faults within its own __________.

    <p>feeder</p> Signup and view all the answers

    Match the following statements about feeder reactors with their respective significance:

    <p>Maintains bus-bar voltage = 1 Localized fault effects = 2 Constant power loss = 3 Required for short circuits = 4</p> Signup and view all the answers

    Which of the following is NOT a benefit of using feeder reactors in power systems?

    <p>Eliminates need for regular maintenance (B)</p> Signup and view all the answers

    The primary function of a feeder reactor is to support power generation capacity.

    <p>False (B)</p> Signup and view all the answers

    What happens to the feeder voltage when a fault occurs?

    <p>The voltage drops occur only in its reactors.</p> Signup and view all the answers

    What is the calculated short-circuit MVA based on the provided information?

    <p>66.05 MVA (D)</p> Signup and view all the answers

    The total reactance for Fault F on the HT side is calculated to be 15.14%.

    <p>True (A)</p> Signup and view all the answers

    What occurs to the currents entering the bus bar zone under normal conditions?

    <p>They are equal to the currents leaving the zone (C)</p> Signup and view all the answers

    In the differential protection scheme, a fault causes a current imbalance that activates the relay coil.

    <p>True (A)</p> Signup and view all the answers

    What is the value of XT in the calculations?

    <p>10%</p> Signup and view all the answers

    What is the primary purpose of the relay coil in a differential protection scheme?

    <p>To detect current imbalances and trip the circuit breaker</p> Signup and view all the answers

    The base kVA is set at _____ kVA.

    <p>10000</p> Signup and view all the answers

    Match the reactances with their values:

    <p>XG1 = 18% XG2 = 18% XG3 = 24% XG4 = 24% XT = 10%</p> Signup and view all the answers

    When a fault occurs within the protected zone, the __________ will trip the circuit breaker.

    <p>relay</p> Signup and view all the answers

    Match the terminology to their definitions:

    <p>Bus Bar = A conductor that connects multiple circuits in a power system Relay Coil = A component that detects fault conditions and initiates a circuit breaker operation Circuit Breaker = A device that interrupts current flow in the event of a fault Fault Condition = An abnormal current situation requiring immediate circuit intervention</p> Signup and view all the answers

    Which of the following statements is true regarding ACB and MCB?

    <p>ACBs are used for larger loads in industrial applications (B)</p> Signup and view all the answers

    In a properly functioning differential protection scheme, all currents should ideally match.

    <p>True (A)</p> Signup and view all the answers

    The sum of the currents entering the bus bar zone equals the currents __________ from it under normal conditions.

    <p>leaving</p> Signup and view all the answers

    Which device measures the current in a circuit?

    <p>CT (C)</p> Signup and view all the answers

    ACBs are less expensive than MCBs.

    <p>False (B)</p> Signup and view all the answers

    What is one major difficulty experienced in differential protection of alternators?

    <p>Unequal secondary burden on CT (D)</p> Signup and view all the answers

    What is the primary function of a current to voltage (I to V) converter?

    <p>To convert current signals into proportional voltage signals.</p> Signup and view all the answers

    Differential protection is less effective during phase-to-phase faults.

    <p>False (B)</p> Signup and view all the answers

    CTs step down the current to a low level safe to connect to the _____ .

    <p>relay</p> Signup and view all the answers

    Match the following components with their functions:

    <p>ACB = Air Circuit Breaker MCB = Miniature Circuit Breaker CT = Measures current I to V Converter = Converts current signal to voltage</p> Signup and view all the answers

    Name a remedy to overcome the difficulty of unequal length of leads in CT wire connections.

    <p>Use Biased Differential Protection</p> Signup and view all the answers

    The differential protection system sends a trip signal to the __________ after a predetermined time delay.

    <p>circuit breaker</p> Signup and view all the answers

    What does the CT do in a microprocessor-based over current protection system?

    <p>It measures the high current in a circuit. (C)</p> Signup and view all the answers

    The block diagram of a microprocessor-based overcurrent protection includes a CT and an I to V converter.

    <p>True (A)</p> Signup and view all the answers

    What kind of relay should be used for differential protection to avoid errors?

    <p>Biased differential relay (C)</p> Signup and view all the answers

    Grounding the neutral reduces errors in differential protection.

    <p>True (A)</p> Signup and view all the answers

    Name one reason why CTs are necessary in overcurrent protection systems.

    <p>They reduce high circuit current to a safe level for relays.</p> Signup and view all the answers

    List one advantage of using a vacuum circuit breaker.

    <p>High insulation resistance</p> Signup and view all the answers

    Match the difficulties experienced in differential protection with their remedies:

    <p>Sensitivity to current ratios = Use Biased Differential Protection Unequal secondary burden on CT = Use Biased Differential Protection Unequal length of leads in CT connections = Use Biased Differential Protection Ungrounded neutral = Ground the neutral</p> Signup and view all the answers

    Which of the following is NOT a fault related to stator winding/coils?

    <p>Failed bearings (C)</p> Signup and view all the answers

    Phase to ground fault is a type of electrical fault.

    <p>True (A)</p> Signup and view all the answers

    Name one mechanical fault that can affect the operation of machinery.

    <p>Damaged bearings</p> Signup and view all the answers

    A fault characterized by excessive heat due to prolonged use is known as ________.

    <p>overloading</p> Signup and view all the answers

    Match the following types of faults with their descriptions:

    <p>Inter-turn faults = Short circuits between turns in a winding Earth faults = Current leakage to the ground Single phasing = Loss of one phase in a three-phase system Stalling = Failure to start or operate due to load</p> Signup and view all the answers

    What is a symptom of damaged core stampings?

    <p>Abnormal operation (D)</p> Signup and view all the answers

    Supply under-voltage can lead to a rotor jam.

    <p>False (B)</p> Signup and view all the answers

    What is one cause of phase to phase faults?

    <p>Electrical insulation failure</p> Signup and view all the answers

    Flashcards

    Open circuit in stator winding

    A break in the electrical connections of the stator winding.

    Loose rotor bars

    Rotor bars that are not securely connected.

    Damaged core stampings

    Problems with the core of the motor, like the metal parts.

    Unbalanced rotor

    An uneven rotor, likely causing problems.

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    Damaged bearings

    Faulty components supporting the rotor.

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    Phase faults

    Problems in the electrical phases.

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    Inter-turn faults

    Faults within the windings of a coil.

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    Differential protection scheme

    A protection method that detects faults in specific parts of a circuit.

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    Importance of Feeder Reactor

    Feeder reactors prevent voltage drops in bus-bars during feeder faults, preventing generator synchronism loss and isolating fault effects.

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    Feeder Reactor Fault Effect

    Faults on a feeder only affect that feeder's voltage, not the bus-bar voltage, keeping other feeders unaffected.

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    Feeder Reactor Fault Prevention

    Feeder reactors minimize voltage drops in bus-bars during feeder faults, thus contributing to system stability during faults.

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    Feeder Reactor's Role in Protection

    Feeder reactors are essential in electrical power systems for protecting against short circuits on feeders.

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    Feeder Reactor Disadvantage

    Feeder reactors cause constant power loss and voltage drop during normal operation.

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    Another Feeder Reactor Disadvantage

    Voltage drops in the reactors happen even during regular operation, which can affect the entire electrical system.

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    Feeder Reactor Fault Localization

    Faults on a feeder are confined to that feeder's reactor, preventing their effects from propagating to the entire system.

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    Why are Feeder Reactors needed?

    Feeder reactors are most crucial in power systems due to their frequent use in feeder circuit protection.

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    What is differential protection?

    A system that compares currents entering and leaving a protected zone. If they don't match, a fault is detected and the circuit breaker trips.

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    Bus bar protection

    A type of differential protection scheme that protects the bus bar, the main distribution point of electrical power.

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    Normal bus bar conditions

    Currents entering the bus bar are equal to those leaving it. No current flows through the relay coil.

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    Fault in a bus bar

    Currents entering the bus bar are not equal to those leaving it. This difference in current flows through the relay coil.

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    How does relay coil react to a fault?

    The current difference through the relay coil triggers the opening of the circuit breaker, isolating the fault.

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    What is the goal of bus bar protection?

    To quickly isolate faults in the bus bar to prevent damage and maintain system integrity.

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    ACB

    Air Circuit Breaker, a device used to interrupt a circuit in the event of a fault, using air as an insulating medium.

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    MCB

    Miniature Circuit Breaker, a small device used to protect electrical circuits from overload and short circuits.

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    ACB vs. MCB

    ACB (Air Circuit Breaker) is more expensive than MCB (Miniature Circuit Breaker).

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    Microprocessor Based Over Current Protection

    A system using a microprocessor to detect and respond to overcurrents in a circuit.

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    CT (Current Transformer) Function

    CTs reduce high circuit currents to a safe level for the relay to handle.

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    I to V Converter Function

    Converts current signals from the CT into voltage signals for the microprocessor.

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    Microprocessor in Protection

    The microprocessor processes the information from the I to V converter and determines the appropriate response to overcurrents.

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    Relay Function in Protection

    The relay receives the processed information from the microprocessor and activates the circuit breaker to interrupt the current.

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    Circuit Breaker Response

    The circuit breaker is activated by the relay to interrupt the flow of current in the circuit.

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    Microprocessor-Based Protection Advantages

    Offers precise overcurrent detection, fast tripping times, and adaptability to changing conditions.

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    Differential Protection

    A method that compares currents entering and leaving a protected zone (like an alternator). Any discrepancy indicates a fault.

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    Neutral Grounding Impact

    If an alternator's neutral point is not grounded, errors in differential protection can occur due to potential imbalances in the system.

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    CT Ratio Error

    When current transformers (CTs) in a differential protection scheme have different ratios, it can cause inaccurate fault detection.

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    Biased Differential Protection

    A method that accounts for minor variations in the CT ratios and wire lengths, improving accuracy and reducing false trips.

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    Unequal Wire Lengths

    Differences in the lengths of CT wires can create imbalances in current readings, affecting the differential protection scheme's reliability.

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    Vacuum Circuit Breaker

    A type of switch that uses a vacuum to interrupt an electrical circuit, offering a reliable and rapid method for interrupting high currents.

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    Vacuum Circuit Breaker Construction

    Key elements include a vacuum chamber, contacts, spring mechanism, and an operating mechanism that enables the switch to open and close.

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    Vacuum Circuit Breaker Advantages

    Offers high interrupting capacity, fast operation, low maintenance, and minimal noise and fire hazards due to vacuum interruption.

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    Study Notes

    Important Instructions to Examiners

    • Answers should be assessed using key words, not a strict word-for-word match to the model answer.
    • Examiners should evaluate the candidate's understanding, allowing for variations in wording.
    • Grammatical and spelling errors should not be heavily penalised.
    • When assessing figures, examiners should credit the principal components.
    • Figures used by the candidate may vary from the model answer, so equivalent figures should be given credit.
    • Some questions may be assessed using judgment based on the candidate's understanding.
    • For programming languages, assess answers based on equivalent concepts.
    • Bilingual answers (English and Marathi) are acceptable and should be evaluated as per the policy.

    Protective System Functions

    • Protects the power system by quickly isolating faulty components.
    • Provides reliable and continuous power supply to consumers.
    • Isolate faulty sections of the system without impacting healthy sections.
    • Minimises the likelihood of major faults.
    • Ensures maximum safety from generating stations to consumers.

    HRC Fuse Applications

    • Protects circuits from short circuits in high-voltage switchgear.
    • Provides backup protection for circuit breakers.
    • Protects electrical devices like motors and transformers.
    • Protects motor stators.
    • Provides protection to high-selectivity radial and ring networks.
    • Used in semiconductor device protection circuits.

    RCBO and MPCB Functions

    • RCBO: Protects against short circuits, over-currents, and electrical shock (leakage currents), and current imbalances.
    • MPCB: Protects against short circuits, line-to-ground faults, and line-to-line faults, and motor overload.

    Directional Relay Need

    • Essential for power systems, to accurately identify the direction of fault transmission lines.
    • Enables accurate fault location and faster response in transmission or distribution lines.

    Alternator Protection Schemes

    • Mertz Price protection
    • Differential protection
    • Percentage differential protection
    • Over-current protection
    • Earth-fault protection
    • Inter-turn fault protection
    • Protection against unwanted unbalanced loading
    • Overheating protection
    • Balanced earth fault protection
    • Protection against loss of excitation
    • Protection against excessive revolutions per minute (RPM)
    • Protection against short circuits

    Busbar Faults

    • Support insulator failure leading to earth faults.
    • Circuit breaker failure to interrupt fault current.
    • Foreign object contact with busbars
    • Flash-over across support insulators due to overvoltage
    • Heavily polluted insulators, causing flash-over
    • Earthquake/mechanical damage
    • Operating/maintenance errors

    Feeder Reactor Importance

    • Fault on a feeder doesn't severely affect the voltage of other feeders or the synchronism of generators.
    • Localizes the effect of a feeder fault, limiting its impact.
    • Short circuits are frequently located on feeders, making protection a necessity.
    • Maintains synchronous operations by minimizing voltage drop during faults.

    Feeder Reactor Disadvantages

    • Constant power loss and voltage drop in normal operation.
    • Increased feeder reactor size needed as the number of generators increases to keep fault current within breaker ratings.
    • Requirement for additional reactors per feeder.
    • Offers no protection for busbar faults.

    Arc Voltage, Restriking Voltage, Recovery Voltage and RRRV.

    • Arc voltage: Transient voltage across circuit breaker contacts during arcing period.
    • Restriking voltage: Transient voltage appearing at or near current zero during arcing, across breaker contacts.
    • Recovery voltage: Normal power frequency voltage appearing after arc extinction at breaker, across the contacts.
    • RRRV (Rate of Rise of Restriking Voltage): Slope of the steepest tangent to the restriking voltage curve, measured in volts per microsecond.

    3Φ 66kV/33kV Transformer Protection

    • Use of current transformers (CTs) on the low-voltage side, followed by calculation to determine the CT ratio on the high-voltage side (HV).

    3Φ Induction Motor Protection

    • External protection: using devices like fuses, circuit breakers, and short circuit relays.
    • Built-in motor protection: using thermostats or thermistors for thermal overload protection

    Transformer Protection

    • Restricted earth fault protection: This protection scheme is used to identify and protect faults (e.g. from the winding) that are close to the transformer's neutral point.
    • This method limits the protection zone, preventing unnecessary tripping of the circuit breakers, which occurs when a fault happens well away from the transformer neutral point. (in the secondary)

    Power System Faults

    • Open-circuit faults (single-phase, two-phase, three-phase)
    • Short-circuit faults (symmetrical, asymmetrical including single line to ground, double line to ground, three phase to ground)

    Distance Relay

    • Impedance or distance relays work on the measured impedance between the relay and the fault point.
    • Relays in the power system are used to measure the impedance between the relay location and the fault point (Z = V/I),.

    Reverse Power Protection of 3Φ Alternator

    • Mechanism in which the alternator acts as a motor during faults or abnormal situations.
    • Use of directional relays to detect the reversed power flow, triggering protective measures to prevent damage.

    Single Phasing Fault

    • Causes a significant decrease in the voltage at the motor terminal connections.

    Differential Protection Scheme for Bus Bars

    • Used to protect the bus bar system from faults.
    • Under normal operations, the currents flowing into the load and the currents flowing out are equal and opposite, cancelling out when measured.
    • A fault within the protected zone causes an imbalance between the entering and exiting currents, triggering the relay and circuit breaker.

    ACB and MCB Comparison

    • ACB: Uses air as the arc quenching medium, designed for high current ratings, more expensive.
    • MCB: Uses magnetic blow out, typically used for lower current ratings, more economical.

    Microprocessor-based Overcurrent Protection

    • Key components include current transformers (CTs), current-to-voltage converters, rectifiers, multiplexers, Analog to Digital Converters (ADC), and a microprocessor kit.
    • The microprocessor processes the digital signal to determine fault duration and magnitude.
    • The tripping mechanism is initiated if the fault current exceeds the threshold values, or preset levels

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