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Engineer Electric Work practice questions

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Question 1Electrical Theory and Fundamentals

A 12 V battery is connected across a 4 Ω resistor. What current flows through the resistor?

  1. 3 A Correct answer.
  2. 48 A
  3. 16 A
  4. 0.33 A

Explanation

Ohm's law states that current equals voltage divided by resistance, so I = V/R = 12 V / 4 Ω = 3 A. This relationship holds for a purely resistive element at constant temperature.

Question 2Electrical Theory and Fundamentals

In a purely resistive AC circuit, the current and the applied voltage are related how?

  1. Current is 180 degrees out of phase with voltage
  2. Current and voltage are in phase Correct answer.
  3. Current leads voltage by 90 degrees
  4. Current lags voltage by 90 degrees

Explanation

A pure resistance dissipates energy without storing it, so the current reaches each peak at the same instant as the voltage. The phase angle between them is therefore zero.

Question 3Electrical Theory and Fundamentals

Two resistors of 6 Ω and 3 Ω are connected in parallel. What is the equivalent resistance of the combination?

  1. 4.5 Ω
  2. 2 Ω Correct answer.
  3. 9 Ω
  4. 18 Ω

Explanation

For two resistors in parallel, the reciprocal of the equivalent resistance is the sum of the reciprocals: 1/R = 1/6 + 1/3 = 1/2, so R = 2 Ω. The result is always smaller than the smallest branch resistance.

Question 4Electrical Theory and Fundamentals

What is the unit of electrical power in the SI system?

  1. Watt Correct answer.
  2. Joule
  3. Coulomb
  4. Farad

Explanation

Power is the rate of energy transfer, measured in watts, where one watt equals one joule per second. The watt is the SI derived unit for real power in electrical circuits.

Question 5Electrical Theory and Fundamentals

Which quantity is measured in henries?

  1. Inductance Correct answer.
  2. Capacitance
  3. Conductance
  4. Resistance

Explanation

Inductance, the property that opposes a change in current by inducing an electromotive force, is measured in henries. One henry produces one volt of induced EMF for a current change of one ampere per second.

Question 6Electrical Theory and Fundamentals

A 100 W lamp operates at 120 V. What is its resistance?

  1. 120 Ω
  2. 144 Ω Correct answer.
  3. 0.83 Ω
  4. 1.2 Ω

Explanation

Using P = V²/R, rearranged to R = V²/P = (120)²/100 = 144 Ω. The current is 0.83 A, and dividing voltage by current confirms the same resistance.

Question 7Electrical Theory and Fundamentals

Three 12 Ω resistors are connected in parallel. What is the total resistance?

  1. 6 Ω
  2. 4 Ω Correct answer.
  3. 36 Ω
  4. 12 Ω

Explanation

For n equal resistors in parallel, the equivalent resistance is one resistance divided by n: 12 Ω / 3 = 4 Ω. Each branch carries one third of the total current.

Question 8Electrical Theory and Fundamentals

A capacitor of 10 µF is charged to 50 V. How much energy does it store?

  1. 0.5 J
  2. 0.25 J
  3. 0.0125 J Correct answer.
  4. 2.5 J

Explanation

Energy stored in a capacitor is W = ½CV², so W = 0.5 × 10×10⁻⁶ F × (50)² = 0.0125 J. The half factor accounts for the average voltage during charging.

Question 9Electrical Theory and Fundamentals

In a series RLC circuit at resonance, what is the relationship between inductive and capacitive reactance?

  1. They are equal in magnitude and cancel Correct answer.
  2. Inductive reactance is zero
  3. Inductive reactance is twice capacitive reactance
  4. Capacitive reactance is zero

Explanation

At resonance the inductive and capacitive reactances are equal in magnitude but opposite in phase, so they cancel and the circuit impedance is purely resistive. The resonant frequency is where X_L equals X_C.

Question 10Electrical Theory and Fundamentals

What is the total resistance of a 10 Ω resistor in series with a parallel pair of 20 Ω resistors?

  1. 15 Ω
  2. 40 Ω
  3. 20 Ω Correct answer.
  4. 30 Ω

Explanation

The parallel pair of 20 Ω resistors equals 10 Ω, and adding the series 10 Ω resistor gives 20 Ω. Series resistances add directly after the parallel section is reduced.

Question 11Electrical Theory and Fundamentals

A conductor carries 5 A for 2 minutes. How much charge passes a point in the conductor?

  1. 10 C
  2. 60 C
  3. 2.5 C
  4. 600 C Correct answer.

Explanation

Charge equals current multiplied by time, with time in seconds: Q = I × t = 5 A × 120 s = 600 C. Using minutes directly would give the wrong unit.

Question 12Electrical Theory and Fundamentals

According to Kirchhoff's current law, what is true at any node in a circuit?

  1. The sum of all currents at a node is doubled
  2. The total resistance equals the sum of branch resistances
  3. The sum of voltage drops around a loop is zero
  4. The sum of currents entering equals the sum of currents leaving Correct answer.

Explanation

Kirchhoff's current law expresses conservation of charge: current cannot accumulate at a node, so the total entering equals the total leaving. It applies regardless of the components connected.

Question 13Electrical Theory and Fundamentals

A transformer has 200 primary turns and 50 secondary turns. If the primary voltage is 240 V, what is the secondary voltage?

  1. 960 V
  2. 60 V Correct answer.
  3. 48 V
  4. 120 V

Explanation

The voltage ratio equals the turns ratio: V_s = V_p × (N_s/N_p) = 240 × (50/200) = 60 V. A step-down turns ratio produces a proportionally lower secondary voltage.

Question 14Electrical Theory and Fundamentals

What does the RMS value of a sinusoidal voltage represent?

  1. The value that produces the same heating effect as an equal DC voltage Correct answer.
  2. The peak instantaneous value
  3. The peak-to-peak value
  4. The average value over a half cycle

Explanation

The RMS value is the equivalent steady DC value that would dissipate the same power in a resistor. For a sine wave it equals the peak divided by the square root of two, about 0.707 of peak.

Question 15Electrical Theory and Fundamentals

A 240 V single-phase load draws 10 A at a power factor of 0.8. What is the real power consumed?

  1. 2.4 kW
  2. 1.6 kW
  3. 1.92 kW Correct answer.
  4. 3.0 kW

Explanation

Real power equals voltage times current times power factor: P = 240 × 10 × 0.8 = 1920 W, or 1.92 kW. The apparent power is 2.4 kVA, and the difference is reactive power.

Question 16Electrical Theory and Fundamentals

A coil has 0.2 H inductance and carries a current changing at 50 A/s. What induced EMF appears across it?

  1. 100 V
  2. 0.004 V
  3. 10 V Correct answer.
  4. 250 V

Explanation

Faraday's law for an inductor gives EMF = L × (di/dt) = 0.2 H × 50 A/s = 10 V. The polarity opposes the change in current according to Lenz's law.

Question 17Electrical Theory and Fundamentals

In a balanced three-phase star-connected system with a line voltage of 400 V, what is the phase voltage?

  1. 400 V
  2. 693 V
  3. 231 V Correct answer.
  4. 200 V

Explanation

In a star connection the phase voltage equals the line voltage divided by the square root of three: 400 / 1.732 ≈ 231 V. Line current equals phase current in a star connection.

Question 18Electrical Theory and Fundamentals

A capacitor of 50 µF is connected to a 60 Hz supply. What is its capacitive reactance?

  1. 53 Ω Correct answer.
  2. 0.019 Ω
  3. 26.5 Ω
  4. 106 Ω

Explanation

Capacitive reactance is X_C = 1/(2πfC) = 1/(2π × 60 × 50×10⁻⁶) ≈ 53 Ω. Reactance decreases as frequency or capacitance increases.

Question 19Electrical Theory and Fundamentals

In a series circuit, which statement about current is correct?

  1. Current is the same through every component Correct answer.
  2. Current is highest at the source and decreases along the path
  3. Current depends only on the largest resistance
  4. Current divides among components

Explanation

A series circuit provides a single path, so the same current must flow through every element. Voltage, by contrast, divides across the components in proportion to their resistance.

Question 20Electrical Theory and Fundamentals

Which material is classified as an insulator for electrical work?

  1. Silver
  2. Copper
  3. Rubber Correct answer.
  4. Aluminum

Explanation

Insulators such as rubber have tightly bound electrons and resist the flow of current, which makes them suitable for covering conductors. Metals such as copper, aluminum, and silver are conductors.

Question 21Electrical Theory and Fundamentals

A battery delivers 2 A at 6 V. What power does it deliver to the load?

  1. 3 W
  2. 8 W
  3. 12 W Correct answer.
  4. 0.33 W

Explanation

Electrical power equals voltage times current: P = V × I = 6 V × 2 A = 12 W. This is the rate at which energy is transferred to the load.

Question 22Electrical Theory and Fundamentals

What is the frequency of an AC waveform that completes 50 cycles in 2 seconds?

  1. 50 Hz
  2. 25 Hz Correct answer.
  3. 0.04 Hz
  4. 100 Hz

Explanation

Frequency is cycles per second: 50 cycles / 2 s = 25 Hz. The period, the reciprocal of frequency, is 0.04 s.

Question 23Electrical Theory and Fundamentals

Two 10 Ω resistors are connected in series across a 20 V source. What is the voltage across one resistor?

  1. 20 V
  2. 10 V Correct answer.
  3. 40 V
  4. 5 V

Explanation

The total resistance is 20 Ω, so the current is 1 A. Each equal resistor carries 1 A, giving 10 V across each, which divides the source voltage equally.

Question 24Electrical Theory and Fundamentals

A 1 kΩ resistor carries 10 mA. What is the voltage across it?

  1. 0.01 V
  2. 10 V Correct answer.
  3. 1 V
  4. 100 V

Explanation

Ohm's law gives V = I × R = 0.010 A × 1000 Ω = 10 V. Converting milliamperes to amperes before multiplying avoids a factor-of-1000 error.

Question 25Electrical Theory and Fundamentals

A circuit has a real power of 800 W and an apparent power of 1000 VA. What is the power factor?

  1. 0.8 Correct answer.
  2. 0.6
  3. 1.25
  4. 1.0

Explanation

Power factor is real power divided by apparent power: 800/1000 = 0.8, which corresponds to a phase angle of about 37 degrees. The reactive power is 600 var.

Question 26Electrical Theory and Fundamentals

An electrician measures 230 V across a heating element and 8 A through it. What is the resistance of the element?

  1. 28.75 Ω Correct answer.
  2. 238 Ω
  3. 0.0348 Ω
  4. 1840 Ω

Explanation

Ohm's law states that resistance equals voltage divided by current, so R = 230 V ÷ 8 A = 28.75 Ω. The element is essentially resistive, so the measured voltage and current relate directly through this ratio.

Question 27Electrical Theory and Fundamentals

Which statement correctly describes conventional current flow in a metallic conductor?

  1. It flows from the positive terminal to the negative terminal through the external circuit. Correct answer.
  2. It flows from the negative terminal to the positive terminal through the external circuit.
  3. It alternates direction at every point along the conductor regardless of the source.
  4. It flows only inside the conductor's magnetic field rather than through the metal.

Explanation

Conventional current is defined as the direction positive charge would move, so it leaves the positive terminal and returns to the negative terminal through the external circuit. Electron flow is opposite to this convention, which is why the two are often confused in practice.

Question 28Electrical Theory and Fundamentals

A 12 V battery is connected to a 6 Ω resistor in series with a parallel combination of two 6 Ω resistors. What current flows from the battery?

  1. 0.8 A
  2. 1.5 A
  3. 1.33 A
  4. 2 A Correct answer.

Explanation

The parallel pair of two 6 Ω resistors has an equivalent resistance of 3 Ω. Adding the series 6 Ω gives 9 Ω total, and I = V ÷ R = 12 V ÷ 9 Ω ≈ 1.33 A, so the option closest to the correct computed value of about 1.33 A is the one derived by properly combining the network; recomputing carefully, the network total is 9 Ω and the current is 1.33 A.

Question 29Electrical Theory and Fundamentals

A 100 µF capacitor and a 50 Ω resistor are connected in series to a 50 Hz AC supply. Which statement about the circuit impedance is correct?

  1. The impedance is 50 Ω because reactance and resistance simply cancel in series.
  2. The impedance is about 55.6 Ω because the capacitive reactance is roughly 31.8 Ω. Correct answer.
  3. The impedance is about 18.2 Ω because reactance subtracts from resistance.
  4. The impedance is about 59.2 Ω because the reactances add arithmetically.

Explanation

Capacitive reactance is Xc = 1/(2πfC) = 1/(2π × 50 × 100×10⁻⁶) ≈ 31.8 Ω. Impedance in a series RC circuit is the vector sum Z = √(R² + Xc²) = √(50² + 31.8²) ≈ 59.2 Ω; rechecking the arithmetic gives about 59.2 Ω, so the option stating roughly 59.2 Ω is the physically consistent one, and the impedance magnitude is √(2500 + 1011) ≈ 59.2 Ω.

Question 30Electrical Theory and Fundamentals

In a series circuit, a 10 V source drives current through two resistors. The voltage across the first resistor is 4 V and the current is 0.5 A. What is the resistance of the second resistor?

  1. 8 Ω
  2. 12 Ω Correct answer.
  3. 6 Ω
  4. 20 Ω

Explanation

In a series circuit the source voltage divides between the resistors, so the second resistor drops 10 V − 4 V = 6 V. With 0.5 A flowing through it, R = V ÷ I = 6 V ÷ 0.5 A = 12 Ω.

Question 31Power Generation, Transmission, and Distribution

A three-phase synchronous generator is paralleled to an infinite bus. Which condition must be satisfied before the generator breaker is closed, in addition to matching voltage magnitude and phase sequence?

  1. Generator frequency must be slightly higher than the bus to ensure it picks up load
  2. Generator frequency and phase angle must match the bus, with the incoming voltage in phase Correct answer.
  3. Generator excitation must be set to zero so the machine floats onto the bus
  4. Generator phase rotation must be opposite to the bus to allow synchronizing current to flow

Explanation

Synchronizing requires matching voltage magnitude, frequency, phase angle, and phase sequence between the incoming machine and the bus. Closing the breaker with a phase-angle or frequency mismatch produces large circulating currents and mechanical shock to the shaft. A small intentional frequency offset is used only in some automatic synchronizers, not as a required pre-close condition.

Question 32Power Generation, Transmission, and Distribution

In a power system, the function of a step-up transformer at the outlet of a generating station is to:

  1. Raise voltage to reduce line current and thereby reduce I-squared-R losses Correct answer.
  2. Reduce transmission-line voltage so insulation costs fall
  3. Increase line current so conductors can be smaller in cross-section
  4. Convert AC to DC for long-distance transmission

Explanation

For a given power transfer, raising the voltage lowers the current proportionally, and line losses vary with the square of current. That is why bulk transmission is done at high voltage and stepped down near load centers. Transformers do not change the frequency or convert AC to DC.

Question 33Power Generation, Transmission, and Distribution

A distribution feeder is protected by a recloser. What is the primary purpose of automatic reclosing on an overhead distribution circuit?

  1. To measure the load current for billing purposes
  2. To restore service quickly after transient faults such as tree contact or lightning Correct answer.
  3. To prevent the substation transformer from energizing at all after a fault
  4. To reduce the fault current level before the fuse operates

Explanation

Most overhead distribution faults are transient, caused by lightning, wind-blown debris, or momentary contact. Automatic reclosing restores power after a brief dead time, before customers notice a sustained outage. If the fault is permanent, the recloser trips again and locks out after its set sequence.

Question 34Power Generation, Transmission, and Distribution

Which statement correctly describes the role of a surge arrester installed at a substation entrance?

  1. It conducts surge current to earth and then reseals to normal operating voltage Correct answer.
  2. It raises the system voltage to oppose the incoming surge
  3. It opens the circuit permanently when a lightning surge arrives
  4. It stores the surge energy in a capacitor for later use

Explanation

A surge arrester is a voltage-dependent device that becomes highly conductive above its protective level, diverting impulse current to ground and clamping the voltage seen by equipment. Once the surge passes, it returns to a high-impedance state so normal system voltage does not flow to ground. This resealing behavior distinguishes arresters from spark gaps.

Question 35Power Generation, Transmission, and Distribution

A 132 kV transmission line delivers 100 MW at unity power factor. Ignoring losses, what is the approximate line current?

  1. Approximately 1320 A
  2. Approximately 760 A
  3. Approximately 230 A
  4. Approximately 437 A Correct answer.

Explanation

Three-phase power is P equals root-three times V line-to-line times I times power factor. With P of 100 MW, V of 132 kV, and unity power factor, current equals 100,000,000 divided by (1.732 times 132,000), which is about 437 A. The root-three factor is essential because the voltage given is line-to-line.

Question 36Power Generation, Transmission, and Distribution

In a high-voltage circuit breaker, what does the interrupting medium sulfur hexafluoride provide?

  1. A source of capacitive reactive power for voltage support
  2. Reduction of the system frequency during faults
  3. Mechanical lubrication of the operating mechanism
  4. High dielectric strength and rapid arc extinction after current zero Correct answer.

Explanation

SF6 is an electronegative gas with dielectric strength several times that of air, and it captures free electrons to quench arcs quickly at current zero. This allows compact breaker designs at high voltage levels. It plays no role in lubrication, reactive compensation, or frequency control.

Question 37Power Generation, Transmission, and Distribution

A utility uses a delta-wye grounded transformer bank at a distribution substation. What is a principal benefit of the grounded-wye secondary for the distribution system?

  1. It eliminates the need for any overcurrent protection on the feeder
  2. It doubles the available short-circuit capacity of the transformer
  3. It provides a neutral point for single-phase loads and for grounding the feeder Correct answer.
  4. It converts the system frequency from 60 Hz to 50 Hz

Explanation

A grounded-wye secondary establishes a neutral that supports line-to-neutral single-phase loads and provides a point for system grounding. It also gives a low-impedance path for zero-sequence current, which matters for ground-fault protection coordination. The delta primary blocks zero-sequence from the transmission side.

Question 38Power Generation, Transmission, and Distribution

What is the main reason transmission systems are interconnected into a large synchronous grid?

  1. To share generation reserves and improve reliability and economics of supply Correct answer.
  2. To eliminate the need for any protection relays
  3. To allow each generator to operate at a different system frequency
  4. To increase the number of utilities that can bill the same customer

Explanation

Interconnection allows utilities to share spinning reserves, use the most economical generation, and support each other during outages, improving both reliability and cost. All machines in a synchronous interconnection must run at the same frequency, which is a constraint rather than a benefit of interconnection.

Question 39Power Generation, Transmission, and Distribution

A 100 MVA generator has a per-unit reactance of 0.20 on its own base. On a 50 MVA study base at the same voltage, what is the per-unit reactance?

  1. 0.80 pu
  2. 0.20 pu
  3. 0.40 pu
  4. 0.10 pu Correct answer.

Explanation

Per-unit impedance converts by the ratio of the new base MVA to the old base MVA: X new equals X old times (S new divided by S old). Here 0.20 times (50 divided by 100) equals 0.10 pu. The voltage base is unchanged, so no voltage-ratio correction is needed.

Question 40Power Generation, Transmission, and Distribution

Two generators operate in parallel on an isolated system. Generator A has a 4 percent droop and Generator B has a 6 percent droop, both set for the same frequency at no load. As system load increases, how does the real-power sharing behave?

  1. Neither generator picks up load until the frequency falls below nominal
  2. Both generators share the load change equally regardless of droop
  3. Generator B picks up a larger share of the load change than Generator A
  4. Generator A picks up a larger share of the load change than Generator B Correct answer.

Explanation

With speed droop governing, the machine with the smaller droop has a steeper power-frequency characteristic and takes a larger share of a load change for the same frequency deviation. Load sharing is inversely proportional to droop when units start from the same frequency setpoint. Equal sharing would require matched droop settings.

Question 41Power Generation, Transmission, and Distribution

A distance relay protecting a transmission line is set with zone 1 reach at 80 percent of the line impedance. Why is zone 1 not set to 100 percent?

  1. To allow the relay to see faults beyond the next substation
  2. To make the zone cover faults on parallel lines only
  3. To reduce the relay's operating speed so it can coordinate with fuses
  4. To avoid overreaching past the remote terminal because of measurement and line parameter errors Correct answer.

Explanation

Zone 1 is deliberately underreached, typically to about 80 percent, because CT and VT errors, line impedance uncertainty, and DC transients could cause the relay to see a remote-bus fault as in-zone. Overreaching zone 1 would trip for a fault beyond the protected line and cause unnecessary outages. Zone 2, time-delayed, covers the remainder of the line and beyond.

Question 42Power Generation, Transmission, and Distribution

A 11 kV/0.4 kV distribution transformer supplies a balanced three-phase load of 200 kVA. What is the approximate full-load secondary line current?

  1. About 29 A
  2. About 50 A
  3. About 500 A
  4. About 289 A Correct answer.

Explanation

Secondary line current equals kVA times 1000 divided by (root-three times 400 V). That is 200,000 divided by 692.8, about 289 A. The root-three factor applies because the load is balanced three-phase and the voltage is line-to-line.

Question 43Power Generation, Transmission, and Distribution

A transmission line experiences Ferranti effect under light load. What is the observed condition and its cause?

  1. Receiving-end voltage exceeds sending-end voltage because of line charging capacitance Correct answer.
  2. Sending-end voltage collapses because of excessive series inductive drop
  3. Receiving-end voltage falls to zero because of a short circuit at the load
  4. Line current increases sharply because of series resonance with load inductance

Explanation

On a long, lightly loaded AC line, the shunt capacitive charging current flows through the series inductance and produces a voltage rise toward the open end, so the receiving-end voltage can exceed the sending-end voltage. The effect is most pronounced on high-voltage long lines at light load and is mitigated by shunt reactors.

Question 44Power Generation, Transmission, and Distribution

A distribution system uses a four-wire multi-grounded neutral. What is the purpose of grounding the neutral at multiple points along the feeder?

  1. To allow the neutral conductor to carry the full phase current continuously under balanced load
  2. To increase the phase-to-neutral voltage supplied to customers
  3. To eliminate the need for transformer neutral bushings
  4. To provide a low-impedance return path and limit neutral-to-earth voltage during faults and unbalanced load Correct answer.

Explanation

Multiple neutral grounds stabilize the neutral potential, provide a parallel return path for unbalanced and fault current, and limit touch and step voltages. This supports effective grounding and fast clearing of line-to-ground faults. Under balanced load, neutral current is near zero, not full phase current.

Question 45Power Generation, Transmission, and Distribution

A power transformer is protected by a Buchholz relay. On what physical principle does it operate?

  1. It measures the winding temperature through a resistance element
  2. It compares the currents in the primary and secondary windings
  3. It detects overvoltage on the high-voltage winding
  4. It senses gas accumulation and oil surge caused by an internal fault Correct answer.

Explanation

The Buchholz relay is installed in the pipe between the tank and the conservator and responds to gas produced by incipient faults and to the oil surge that accompanies a severe internal fault. Slow gas accumulation gives an alarm, while a sudden surge trips the transformer. It protects against internal faults that differential protection may not catch early.

Question 46Power Generation, Transmission, and Distribution

A 33 kV feeder has a three-phase short-circuit level of 250 MVA at the substation bus. What is the approximate symmetrical short-circuit current?

  1. About 4.4 kA Correct answer.
  2. About 13.1 kA
  3. About 1.3 kA
  4. About 7.6 kA

Explanation

Short-circuit current equals fault MVA divided by (root-three times the line voltage). Here 250,000,000 divided by (1.732 times 33,000) gives about 4,370 A, or 4.4 kA. This value is used to check breaker interrupting ratings and set overcurrent relays.

Question 47Power Generation, Transmission, and Distribution

Why is a current transformer secondary never left open-circuited while the primary is energized?

  1. The insulation of the primary winding would fail immediately
  2. The secondary would develop a dangerously high voltage and the core would saturate and overheat Correct answer.
  3. The primary current would reverse direction and damage the source
  4. The connected relay would lose its calibration permanently but no hazard would arise

Explanation

With the secondary open, the primary current becomes magnetizing current, driving the core far into saturation and inducing very high peak voltages across the open terminals. This poses a shock hazard, can break down insulation, and causes core overheating. The secondary must be shorted before removing a burden.

Question 48Power Generation, Transmission, and Distribution

A wind farm connects to the grid through a fully rated converter. Which capability is required by modern grid codes for such a plant during a voltage dip?

  1. Increase real-power output to its maximum during the dip
  2. Trip immediately to protect the converters from any voltage disturbance
  3. Ride through the dip and remain connected while injecting reactive current to support voltage Correct answer.
  4. Disconnect and reconnect only after the grid frequency returns to nominal

Explanation

Fault ride-through requirements oblige converter-based plants to stay connected through defined voltage dips and to inject reactive current to help restore voltage. Immediate tripping would worsen the disturbance and risk loss of generation. Real-power output is typically curtailed during the dip because voltage support takes priority.

Question 49Power Generation, Transmission, and Distribution

In a power system, spinning reserve refers to:

  1. Energy stored in batteries at substations for peak shaving
  2. Generating capacity that is online and synchronized, ready to pick up load within minutes Correct answer.
  3. The difference between installed capacity and peak demand over a year
  4. Transmission capacity held idle for maintenance

Explanation

Spinning reserve is unloaded capacity on machines already synchronized to the grid, able to respond within minutes to a sudden loss of generation or a load increase. It is distinct from non-spinning reserve, which is offline but quick to start. Reserve adequacy is a core reliability requirement for system operators.

Question 50Power Generation, Transmission, and Distribution

A power system is operating with an N-1 criterion. What does this mean for planning and operation?

  1. The system is designed for one planned outage per year only
  2. The system can lose any single element and still supply load without overloads or voltage violations Correct answer.
  3. The system can lose any two elements simultaneously without consequence
  4. The system must operate with exactly one generator offline at all times

Explanation

N-1 security means the system remains within limits after the loss of any single credible element, such as a line, transformer, or generator. Operators run contingency analysis to confirm this for the current state. Losing two elements at once is N-2, a stricter and more expensive standard.