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ELECTRICAL PE EXAM QUESTIONS WITH
VERIFIED ANSWERS
Inductor impedance equation - ANSWER Z = jwL How can a transformer, motor, or generator be saturated? - ANSWER The voltage/frequency ratio is altered. For example, this happens when excessive voltage is applied to the primary side or the frequency is too low. What are some sources of triplen harmonics? - ANSWER Electronics, CFL lamps, electronics dimmers, etc. What are the effects of triplen harmonics on an electrical system? - ANSWER
- Neutral over-loading
- Electrical equipment (bus bars, transformers over-heating)
- Eddy current losses (proportional to square of frequency) How are triplen harmonics mitigated? - ANSWER Detuned filter or active harmonic filter. Capacitor impedance equation - ANSWER Z = (-j)/(wC) What is conductance? - ANSWER Inverse of resistance RMS (root means square) equation - ANSWER RMS = Max/sqrt(2) Parallel resistor Equation (only 2) - ANSWER (R1R2)/(R1+R2) Convert sin(wt) to cosine form - ANSWER cos(wt-90) Is a positive reactive power inductive or capacitive? - ANSWER Inductive Is a negative reactive power inductive or capacitive? - ANSWER Capacitive If the power factor is lagging, the _______ lags the _______ - ANSWER current, voltage
If the power factor is leading, the _______ leads the _______ - ANSWER current, voltage Why is reactance required in a system? Why not have unity power factor? - ANSWER Reactance is required for transformer action in transformers and generators (inductive components consume reactive power) and to keep bus voltages at their required values. Power factor equation relating true power and apparent power - ANSWER pf = P/S Power factor equation relating the voltage and phase angles - ANSWER pf = cos(theta_V-theta_I) Equation for apparent power (S) based on real power (P) and reactive power (Q)
- ANSWER S^2 = P^2 + Q^ A balanced 3-phase system has its phases spaced exactly ______ degrees apart - ANSWER 120 Line-to-line to line-to-neutral ratio - ANSWER sqrt(3) List phase angles in positive balanced system (abc type) for phases a, b, and c - ANSWER a = 0 b = - 120 c = - 240 = 120 3 Phase Apparent Power Equation (in terms of line voltage and current) - ANSWER S = sqrt(3) * V_LL * I_L What does the subtransient reactance (X''d) represent in a generator? - ANSWER The reactance from the time a fault occurs until the time that the effect of damper windings on flux change subsides. 0 to 6 cycles. What does the transient reactance (X'd) represent in a generator? - ANSWER The reactance from the time a fault occurs until the time that the effect of field winds on flux change subsides. 6 to 30-60 cycles.
The flux direction is determined by the direction of the current around the coil using the right-hand rule. A transformer self-inductance is positive if _______ - ANSWER The current enters the coil at the positive polarity sign A transformer self-inductance is negative if _________ - ANSWER The current enters the coil at the negative polarity sign Transformers: The mutual inductance term is the same polarity as the self- inductance term if __________ - ANSWER Both currents' directions match in relation to the dotted end of the coil. Transformers: The mutual inductance term is the opposite polarity as the self- inductance term if __________ - ANSWER The currents directions are opposite in relation to the dotted end of the coil. Transformers: How to find turns ratio for individual 1-phase transformers connected to form a 3-phase transformer bank. - ANSWER Use the phase voltage. Transformers: Name rules for paralleling transformers. - ANSWER 1. Same primary and secondary voltages.
- Same tap setting.
- Same per unit impedance (within 7.5%)
- Only terminals with same polarity are connected together to avoid short circuits
- KVA may be different Name types of losses present in transformers - ANSWER 1. Copper (winding) losses - I^2*R
- Eddy current losses - Resistive heating losses in the XFMR core
- Hysteresis losses - energy to reorient domains in core during each cycle of the alternating current
- Leakage flux - Flux that escapes core and returns to original coil. What is voltage regulation? - ANSWER Percentage change in output voltage of no-load conditions vs full-load conditions.
What are the characteristics of a saturated CT? - ANSWER - Increased excitation current
- Decreased secondary current
- Larger angle between primary and secondary currents Overall: Inaccurate measurements and possible misoperation of protective equipment Typical properties of an unsaturated CT - ANSWER - Secondary current range: From 1 to 5 A
- Ratio of primary to secondary current is a nearly constant ratio that shouldn't change by load
- Phase angle between prim. and sec. current is << 1 deg
- Primary winding is frequently one turn
- Insulation separating prim. and sec. windings must withstand full line to neutral voltages during surges
Self inductance is associated to what transformer loss type? - ANSWER Leakage flux What is starting torque (locked rotor torque)? - ANSWER Torque developed with rotor held "locked" at rated voltage and frequency to simulate starting conditions. This is about 150% of full load torque. What is accelerating torque? Explain what it looks like on a torque-time chart. - ANSWER The torque right after starting: torque has a slight decrease and then gradually increases to about 200% of full load torque Pullout torque (breakdown) - ANSWER Maximum torque; the point at which any heavier motor loading would cause a fast reduction (or breakdown) in torque Full load torque - ANSWER Torque near 100% of synchronous speed (may be slightly less due to slip) What is the difference between a synchronous and inductive machine? - ANSWER The way field currents are produced. For synchronous machines,
Explain a surge arresters operation during normal operation. Explain the operation during a surge. - ANSWER Normal operation: The surge arrester has a high impedance and doesn't allow current to pass through. Surge: The surge arrester has a low impedance and allows current to pass through it and to ground. Minimum lightning side flash equation - ANSWER Distance = Height / 6 Why do systems of 69 kV and greater not require shielding and grounding for lightning protection? What method is used to protect these systems? - ANSWER They have sufficient insulation to withstand lightning strikes. Surge arresters. Describe the load when the 2-wattmeter method shows that both wattmeter power measurements are equal - ANSWER Purely resistive (2-wattmeter method) Describe the load's power factor if one wattmeter reading is 0 W. - ANSWER The power factor is 0.5. It is leading if the phase angle is positive and lagging if it is negative. (2-wattmeter method) Describe the load's power factor if one wattmeter reading is negative. - ANSWER Pf < 0.5. It is leading if the phase angle is positive and lagging if it is negative. (2-wattmeter method) Describe the load's power factor if both wattmeter readings are positive and are not equal. - ANSWER 0.5 < pf < 1.0. What are cold cranking amps? - ANSWER Number of amperes a lead-acid battery at 0 degrees Celsius can deliver for 30 seconds. What is the application of the Wheatstone Bridge? - ANSWER To measure resistance over a wide range. What does a Wheatstone Bridge consist of? How does it work? - ANSWER 2 resistors with known resistance values, a rheostat, a galvanometer, and the unknown resistance.
When is real power transferred over a transmission line? - ANSWER When there is a phase angle difference between the sending and receiving voltages. When is reactive power transferred over a transmission line? - ANSWER When there is a magnitude difference between the sending and receiving voltage and/or a phase angle difference between the two. What is a bushing? - ANSWER A hollow electrical insulator that allows a conductor to pass through a conducting barrier (such as transformer or circuit breaker) without making contact with it. Tap setting equation - ANSWER Tap Setting = Minimum trip / CT ratio Series RLC equivalent impedance equation. - ANSWER Z_eq = sqrt(R^2 + (X_L-X_C)^2) Equation for capacitor's charge and voltage relationship - ANSWER Q = C x V In a power grid, how do you raise voltage and frequency? - ANSWER Shed some of the load. 6-10% for every 1 Hz In a power grid, how do we increase voltage at the source? - ANSWER Increase reactive power? Inductive loads ______ VARs to/from the generator. - ANSWER Receive Capacitive loads ______ VARs to/from the generator. - ANSWER Send Voltage regulation equation for generator - ANSWER VR = (Ea-Va)/(Va), all magnitudes Name the advantages and disadvantage (3 each) of a main and transfer bus. - ANSWER Advantages: Low cost and less land required, line protection remains intact during breaker maintenance, can be expanded in the future. Disadvantages: Breaker or bus failure causes an outage for the whole substation and load, an additional circuit breaker is required for a bus tie, relaying can be complex due to breaker substations.
Lighting: CCR - ANSWER Ceiling Cavity Ratio Lighting: FCR - ANSWER Floor Cavity Ratio Lighting: RRC - ANSWER Reflected Radiation Coefficient Lighting: LC - ANSWER Luminance Coefficient Lighting: RPM - ANSWER Room Position Multiplier Lighting: CBU - ANSWER Coefficient of Beam Utilization In a parallel generator set, how do you control the reactive load sharing between the generators? - ANSWER Adjust the voltage Arc-flash let-through energy equation - ANSWER I^2 * t In a parallel generator set, how do you control the real power load sharing between the generators? - ANSWER Adjust the frequency What is the use of a slack bus in a power flow study? What bus is usually the slack bus? - ANSWER It is a reference bus for the power flow studies where no variables have to be solved for. It is typically the bus with the largest generator capability. What's the first step in solving a non-annual compounding problem? - ANSWER Determine the effective annual interest Geometric mean radius equation - ANSWER GMR = (Dab x Dac x Dbc)^(1/3) Foot candle to lumen equation - ANSWER Fc = lumen/(ft^2) Pick-up setting equation - ANSWER Pick-up setting = current / pick-up The number of poles in an electric machine is always ________ - ANSWER Even
What is the applied voltage to a 3-phase battery charger with respect to the input 3 - phase voltage? - ANSWER Sqrt(2) * input 3-phase voltage Transformer no-load losses equal what? - ANSWER Core losses Transformer copper losses equation - ANSWER I^2 * R Transformer core losses are the sum of what two losses? - ANSWER Eddy current losses and hysteresis losses Rating (in kVA) equation when using a single-phase transformer as an autotransformer - ANSWER Primary voltage * (current through winding 1 + current through winding 2) Why do DC loads cause the greatest reduction in the life of a relay compared to AC loads? - ANSWER DC flows in one direction only and supports sustained arcing much better than AC. DC also tends to remove material from one contact to the other more quickly. Voltage regulation equation - ANSWER [magn(Vnl)-magn(Vfl)]/[magn(Vfl)] If a transformer and a system have the same X/R ratio, what does that mean? - ANSWER They have the same base Fault duty equation - ANSWER Fault duty = 1 / (reactance) Line to line voltage to line current equation (Y-connection) - ANSWER I_L = V_LL / sqrt(3) Describe the zero-sequence components - ANSWER Three phasors with equal magnitude and pointed in same direction. Describe the positive-sequence components - ANSWER 3 phasors with equal magnitudes, 120 degree displacement, and positive sequence. Describe the negative-sequence components - ANSWER 3 phasors with equal magnitudes, 120 degree displacement, and negative sequence
