Electrical PE Exam Completed with Correct Answers

Electrical PE Exam Completed with Correct Answers Electrical PE Exam Completed with Correct Answers 1. Inductor impedance equation Z = jwL 2. How can a transformer, motor, or generator be saturated? 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. 3. What are some sources of triplen harmonics? Electronics, CFL lamps, electronics dimmers, etc. 4. What are the effects of triplen harmonics on an electrical system? 1. Neutral over-loading 2. Electrical equipment (bus bars, transformers over-heating) 3. Eddy current losses (proportional to square of frequency) 5. How are triplen harmonics mitigated? Detuned filter or active harmonic filter. 6. Capacitor impedance equation Z = (-j)/(w*C) 7. What is conductance? Inverse of resistance 8. RMS (root means square) equation RMS = Max/sqrt(2) 9. Parallel resistor Equation (only 2) (R1*R2)/(R1+R2) 10. Convert sin(wt) to cosine form cos(wt-90) 11. Is a positive reactive power inductive or capacitive? Inductive 12. Is a negative reactive power inductive or capacitive? Capacitive 13. If the power factor is lagging, the _______ lags the _______ current, voltage 14. If the power factor is leading, the _______ leads the _______ current, voltage 15. Why is reactance required in a system? Why not have unity power factor? Reactance is required for transformer action in transformers and generators (inductive components consume reactive power) and to keep bus voltages at their required values. 16. Power factor equation relating true power and apparent power pf = P/S 17. Power factor equation relating the voltage and phase angles pf = cos(theta_V-theta_I) 18. Equation for apparent power (S) based on real power (P) and reactive power (Q) S^2 = P^2 + Q^2 19. A balanced 3-phase system has its phases spaced exactly ______ degrees apart 120 20. Line-to-line to line-to-neutral ratio sqrt(3) 21. List phase angles in positive balanced system (abc type) for phases a, b, and c a = 0 b = -120 c = -240 = 120 22. 3 Phase Apparent Power Equation (in terms of line voltage and current) S = sqrt(3) V_LL I_L 23. What does the subtransient reactance (X''d) represent in a generator? 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. 24. What does the transient reactance (X'd) represent in a generator? 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. 25. What does the synchronous reactance (Xd) represent in a generator? The reactance present during steady-state conditions. This is usually higher than 1 pu. 26. New pu impedance equation Z_pu_new = Z_pu_old (S_base_new/S_base_old) (V_base_old/V_base_new)^2 27. Advantages of HVDC over HVAC 1. For transmission lengths of 600 km (370 mi), the system is less expensive 2. No skin effect in DC systems therefore conductors may be smaller 3. Bridging asynchronous electrical systems 4. DC transmission system is isolated with transformers therefore short-circuit fault current is smaller. 28. In a Y-connection, the line voltage _______ (leads/lags/equals) the phase voltage by ______ degrees Leads, 30 29. In a Y-connection, the line current _______ (leads/lags/equals) the phase current by ______ degrees Equals 30. In a delta connection, the line voltage _______ (leads/lags/equals) the phase voltage by _______ degrees Equals 31. In a delta connection, the line current _______ (leads/lags/equals) the phase current by ______ degrees Leads, 30 32. Why is HVDC less expensive than HVAC for transmission lengths over 600 km (370 mi)? The lack of skin effect in DC systems means that the conductors can be smaller. DC systems generally require less towers and a smaller right-of-way for the towers. 33. What is charging current in a transmission line? The current created by the capacitive effect between the line conductors. 34. What do the polarity dots on a transformer indicate? How is the direction of the flux determined? The direction that the windings are wrapped. The dot on top indicates the windings are wrapped around the core on the top. The flux direction is determined by the direction of the current around the coil using the right-hand rule. 35. A transformer self-inductance is positive if _______ The current enters the coil at the positive polarity sign 36. A transformer self-inductance is negative if _________ The current enters the coil at the negative polarity sign 37. Transformers: The mutual inductance term is the same polarity as the self-inductance term if __________ Both currents' directions match in relation to the dotted end of the coil. 38. Transformers: The mutual inductance term is the opposite polarity as the self-inductance term if __________ The currents directions are opposite in relation to the dotted end of the coil. 39. Transformers: How to find turns ratio for individual 1-phase transformers connected to form a 3-phase transformer bank. Use the phase voltage. 40. Transformers: Name rules for paralleling transformers. 1. Same primary and secondary voltages. 2. Same tap setting. 3. Same per unit impedance (within 7.5%) 4. Only terminals with same polarity are connected together to avoid short circuits 5. KVA may be different 41. Name types of losses present in transformers 1. Copper (winding) losses - I^2*R 2. Eddy current losses - Resistive heating losses in the XFMR core 3. Hysteresis losses - energy to reorient domains in core during each cycle of the alternating current 4. Leakage flux - Flux that escapes core and returns to original coil. 42. What is voltage regulation? Percentage change in output voltage of no-load conditions vs full-load conditions. 43. What are the characteristics of a saturated CT? - Increased excitation current - Decreased secondary current - Larger angle between primary and secondary currents Overall: Inaccurate measurements and possible misoperation of protective equipment 44. Typical properties of an unsaturated CT - 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 - 45. Self inductance is associated to what transformer loss type? Leakage flux 46. What is starting torque (locked rotor torque)? Torque developed with rotor held "locked" at rated voltage and frequency to simulate starting conditions. This is about 150% of full load torque. 47. What is accelerating torque? Explain what it looks like on a torque-time chart. The torque right after starting: torque has a slight decrease and then gradually increases to about 200% of full load torque 48. Pullout torque (breakdown) Maximum torque; the point at which any heavier motor loading would cause a fast reduction (or breakdown) in torque 49. Full load torque Torque near 100% of synchronous speed (may be slightly less due to slip) 50. What is the difference between a synchronous and inductive machine? The way field currents are produced. For synchronous machines, this is produced at the rotor via a separate source or by being a permanent magnet. For inductive machines, the field current is created by transforming the stator current. 51. What are field windings? Windings that produce the main magnetic field on a machine 52. What are armature windings? Windings where the main voltage is induced 53. How can synchronous motors improve power system efficiency? Synchronous motors can adjust the power system power factor to run nearer unity power factor and reduce transmission line losses. 54. What should you do if the anticipated current to be measured in an ammeter exceeds the rating? Place a shunt resistance 55. What is the SIL of a transmission line? Surge Impedance Level 56. What is the BIL of a transmission line? Basic Lightning Impulse Insulation Level - Insulation level specified in terms of the crest value of the standard lightning impulse. 57. Name the 3 coordinated design methods to protect equipment against lightning strikes. Shielding, grounding, surge arresters. 58. Name the 3 coordinated design methods to protect equipment against switching transients. Switching resistors, breaker timing, and surge capacitors. 59. How does the BIL of a line affect the installation of surge arrestors? A higher BIL allows for greater spacing of arrestors. 60. What are the 2 methods for lightning protection on utility lines? 1. Overhead static line connected to a ground of less than 25 ohms 2. Surge arrestors 61. Explain a surge arresters operation during normal operation. Explain the operation during a surge. 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. 62. Minimum lightning side flash equation Distance = Height / 6 63. Why do systems of 69 kV and greater not require shielding and grounding for lightning protection? What method is used to protect these systems? They have sufficient insulation to withstand lightning strikes. Surge arresters. 64. Describe the load when the 2-wattmeter method shows that both wattmeter power measurements are equal Purely resistive 65. (2-wattmeter method) Describe the load's power factor if one wattmeter reading is 0 W. The power factor is 0.5. It is leading if the phase angle is positive and lagging if it is negative. 66. (2-wattmeter method) Describe the load's power factor if one wattmeter reading is negative. Pf 0.5. It is leading if the phase angle is positive and lagging if it is negative. 67. (2-wattmeter method) Describe the load's power factor if both wattmeter readings are positive and are not equal. 0.5 pf 1.0. 68. What are cold cranking amps? Number of amperes a lead-acid battery at 0 degrees Celsius can deliver for 30 seconds. 69. What is the application of the Wheatstone Bridge? To measure resistance over a wide range. 70. What does a Wheatstone Bridge consist of? How does it work? 2 resistors with known resistance values, a rheostat, a galvanometer, and the unknown resistance. 71. When is real power transferred over a transmission line? When there is a phase angle difference between the sending and receiving voltages. 72. When is reactive power transferred over a transmission line? When there is a magnitude difference between the sending and receiving voltage and/or a phase angle difference between the two. 73. What is a bushing? 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. 74. Tap setting equation Tap Setting = Minimum trip / CT ratio 75. Series RLC equivalent impedance equation. Z_eq = sqrt(R^2 + (X_L-X_C)^2) 76. Equation for capacitor's charge and voltage relationship Q = C x V 77. In a power grid, how do you raise voltage and frequency? Shed some of the load. 6-10% for every 1 Hz 78. In a power grid, how do we increase voltage at the source? Increase reactive power? 79. Inductive loads ______ VARs to/from the generator. Receive 80. Capacitive loads ______ VARs to/from the generator. Send 81. Voltage regulation equation for generator VR = (Ea-Va)/(Va), all magnitudes 82. Name the advantages and disadvantage (3 each) of a main and transfer bus. 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. 83. What map is useful for determining annual thunderstorm activity? Isokeraunic map 84. When using differential relaying why are the CTs connected differently in a 3-phase Y-Delta transformer? To compensate for 30 degree phase shift. 85. What distributed line parameter is normally significant and is usually ignored in calculation? Shunt resistance 86. What happens to zero-sequence currents in a delta connection? They flow around the delta and therefore don't affect the line currents. 87. Equation for magnitude of impedance Sqrt(R^2 + X^2) 88. How does the nameplate speed of an induction motor compare to the synchronous speed? It is slightly under. 89. The SI unit for luminous intensity Candela 90. The SI unit for illumination Foot-candle 91. SI unit for luminous flux Lumen 92. Lighting: LLF Light loss factor 93. Lighting: LLD Lamp lumen depreciation factor 94. Lighting: LDD Luminaire dirt depreciation factor 95. Lighting: II Initial Illumination 96. Lighting: MMI Minimum Maintained Illumination 97. Lighting: CU Coefficient of Utilization 98. Lighting: RCR Room Cavity Ratio 99. Lighting: CCR Ceiling Cavity Ratio 100. Lighting: FCR Floor Cavity Ratio 101. Lighting: RRC Reflected Radiation Coefficient 102. Lighting: LC Luminance Coefficient 103. Lighting: RPM Room Position Multiplier 104. Lighting: CBU Coefficient of Beam Utilization 105. In a parallel generator set, how do you control the reactive load sharing between the generators? Adjust the voltage 106. Arc-flash let-through energy equation I^2 * t 107. In a parallel generator set, how do you control the real power load sharing between the generators? Adjust the frequency 108. What is the use of a slack bus in a power flow study? What bus is usually the slack bus? 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. 109. What's the first step in solving a non-annual compounding problem? Determine the effective annual interest 110. Geometric mean radius equation GMR = (Dab x Dac x Dbc)^(1/3) 111. Foot candle to lumen equation Fc = lumen/(ft^2) 112. Pick-up setting equation Pick-up setting = current / pick-up 113. The number of poles in an electric machine is always ________ Even 114. What is the applied voltage to a 3-phase battery charger with respect to the input 3-phase voltage? Sqrt(2) * input 3-phase voltage 115. Transformer no-load losses equal what? Core losses 116. Transformer copper losses equation I^2 * R 117. Transformer core losses are the sum of what two losses? Eddy current losses and hysteresis losses 118. Rating (in kVA) equation when using a single-phase transformer as an autotransformer Primary voltage * (current through winding 1 + current through winding 2) 119. Why do DC loads cause the greatest reduction in the life of a relay compared to AC loads? 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. 120. Voltage regulation equation [magn(Vnl)-magn(Vfl)]/[magn(Vfl)] 121. If a transformer and a system have the same X/R ratio, what does that mean? They have the same base 122. Fault duty equation Fault duty = 1 / (reactance) 123. Line to line voltage to line current equation (Y-connection) I_L = V_LL / sqrt(3) 124. Describe the zero-sequence components Three phasors with equal magnitude and pointed in same direction. 125. Describe the positive-sequence components 3 phasors with equal magnitudes, 120 degree displacement, and positive sequence. 126. Describe the negative-sequence components 3 phasors with equal magnitudes, 120 degree displacement, and negative sequence 127. What is the zero-sequence network impedance for a balanced Y-load? Z_Y + 3Zn 128. Which sequence network includes the voltage/current sources? Positive sequence 129. Describe the equivalent sequence network for a single line-to-ground fault (SLG) All sequence impedance and fault voltage in series. 3*Z_F included when fault isn't bolted. 130. Describe the equivalent sequence network for a line-to-line fault (LL) Fault voltage in series with positive and negative sequence impedance and ZF. 131. Zones of protection are assigned for: Generators, transformers, buses, transmission/distribution lines, motors 132. What happens in a zone of protection when a fault occurs? All circuit breakers in that zone open to isolate the fault. 133. What are located in the areas where zones of protection overlap? Circuit breakers 134. What is a directional relay? What is a good application of this type of relay? A relay designed to operate for fault currents in only one direction. Two source systems. 135. Impedance relays respond to what ratio? Why is this useful for fault protection? Voltage-to-current ratio. During a 3-phase fault, current increases while bus voltages close to the fault decrease. 136. Differential relays are commonly used to protect: Generators, buses, and transformers from internal faults 137. When working with single phase voltage drop over a cable remember _________ To multiply the impedance by 2 to also include the return impedance.