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ElectricPowerSystems
| Question | Answer |
|---|---|
| What are the three major components of a power system? | Generation systems - generation stations like power plants, Transmission lines - way of transporting power long distance, and distribution systems - step down transformers and such to directly give consumers power |
| What are examples of AC and DC power generation? | AC generation, used in three phase power applications like synchronous generators, used in power plants. DC generators used in like solar power, batteries, fuel cells, etc |
| How do we connect DC to an AC grid? | We used power electronic converters/inverters to convert from DC to AC |
| There are generation system categories, there are energy based on conversion mechanism, and based on energy resources. What are some examples of each | Based on conversion: thermal (fossil fuels, nuclear, biomass, solar thermal), non-thermal (hydro, wind, solar). Based On Energy: fossil fuels, nuclear, hydroelectric, wind, solar, bioenergy(biomass, biogas, municipal waste) |
| What do generation systems do? | Convert chemical, nuclear, mechanical and solar energy to electrical energy. They do this through a prime mover(steam, gas, wind, hydraulic) that rotates the shaft of the generator, creating magnetic field and inducing voltage |
| How do solar power generation systems work to make power? | They convert light into energy and communicate with the grid using DC to AC converters |
| What is the most common type of generation system in Canada? | Canada has alot of hydro resources, so it makes up 60% of our power |
| Why do (this is my opinion: fossil fuel lobbyists say that) we need to still use fossil fuels? | Renewable are intermittent (wind and solar) depending on time of day and weather, fossil fuel and nuclear respond fast during emergencies, and until energy storage can be done widely, a mixed portfolio is 'necessary' |
| What is the point of power transmission lines? | To deliver electric powerto distribution systems and connect different power systems over provinces or countries |
| What are some power transmission equipment? | Transformers (step up, step down, voltage regulators, phase shifters to control power flow, and grounding transformers), Transmission Lines and Cables (circuit breakers/isolaters, shunt & series reactors and capacitors, lightening arresters, inverters..) |
| Why can't we always build generation systems near loads? | Noise, pollution. expensive land cost, safety hazards |
| Why do we interconnect power systems? | To share resources, reduce generation reserves, lower investment costs(in new plants presumably), and increase reliability |
| What the wire difference between AC and DC transmission? | AC transmission requires three lines, and goes from AC to AC. DC transmission starts from an AC source, is converted to DC using an AC to DC converter, transmitted only using two lines, and converted back to AC at the end using a DC to AC converter. |
| What are some things about AC transmission? | IN AC: step up and down transformers are used to change voltage, requires synchronization and has stability issues over long distance, and is cheaper than DC transmission in distances less than 500km |
| What are some things in DC transmission? | AC and DC conversion at beginning and end, and suitable for long distance transmission and submarine cables, but more expensive |
| Why are transmission systems high voltage? | Lower transmission losses, lower line voltage drop, higher transmission capacity, cheaper, reduced residential wahala (right of way requirements) |
| Can we use transformers to change voltage levels in DC? | No because transformers need three phase power for magnetic field, so AC. Not DC. Cannot induce voltage in a secondary winding. |
| Why is it more efficient to use DC transmission over long distance and submarine transmission? | In short distance, AC transmission is too costly. So don't use it long distance. Reasons to use DC in such application though is lower current requirements for power transmission, less restive losses because lower current, and no reactive power loss |
| What do step up transformers do? | Increase voltages for long distance power transfer in power systems. Generally just increases voltage. Primary side is LV and secondary side is HV. turn ratio N1/N2 is less than 1. |
| What do step down transformers do? | Reduce voltage for local use in power systems. Generally just decrease voltage. Primary side is HV, secondary side is LV. turn ratio is usually greater than 1. |
| What is a load in a power system? | Any device that draws electric power and converts it into a useful form. Ex. Appliances, offices, factories, city infrastructure |
| What is a three phase system? How is power supplied, and coils positioned? | A three phase system is a system made by combining three single phase systems. In a balance three phase system, power is supplied by a three phase generator. Coils are position 120 deg apart, and exposed to a magnetic field |
| What are some advantages of three phase over single phase? | Three phase machines have 1.5 times output rating then single phase. In single phase, output pulses, but three phase output is constant. Single phase is not self starting, three phase is. Three phase needs less copper and aluminum |
| What are some disadvantages of three phase compared to single phase? | Unbalanced loading can cause voltage imbalance, three phase systems are more complex and often need symmetrical components, and the phase sequence must be maintained very carefully. |
| What are the 2 main methods of generating three phase power? | Rotating magnetic field method - stator coil is stationary, magnetic field rotates in stator, inducing three phase voltage in coil. Rotating coil - magnetic field is stationary, coil rotates in field, inducing three phase voltage |
| In three phase voltage, how many degrees are each of the coils separated by? | 120 deg |
| If we are not using a delta or wye connection, how many conductors do we need for a three phase connection and why? | Each phase coil has two terminals, which require two conductors. So for a whole three phase connection we would need 6 conductors, which is costly. |
| What is a benefits of a wye connection? | They offer a neutral point that can be grounded easily |
| What are some benefits of a delta connection? | The connection provides higher voltage across each individual winding, increasing the speed of the motor, and for a given load power, supply voltage can be reduced for a delta connected load |
| What exactly is the definition of power? | The instantaneous transfer of energy, expressed in watts. |
| What is energy, mathematically (think calc)? | The integration of power over time, expressed in units of joules. Or in other words, work. |
| What is real/active/average power? | The component of power that does real work (ex. heating your home, charging devices, etc.) It is the instantaneous power delivered to the resistive part of impedance. |
| What is imaginary/reactive power? | Instantaneous power delivered to the inductive or capacitive part of impedance. It is a pulsating power that is exchanged back and forth between the inductive and capactive components of a circuit and its source. |
| What is a key difference between reactive and real power, in regards to how they do work? | Reactive power does NOT perform any mechanical work. Real power DOES perform mechanical work. |
| What is apparent power? | Total electrical power available, even if it doesn't perform tasks. It is the magnitude of complex power. |
| What are the units for active power, reactive power, complex power and apparent power respectively? | Watts, VAR, VA, VA. |
| What is power factor? | arccos(V/I) in an AC circuit, ratio of resistance/impedance, or ratio of (real power)/(active power). It is very useful in high power applications. |
| For resistive, inductive, and capactive loads, state whether, power factor is unity, lagging or leading. | Resistive loads have a unity power factor. Inductive loads have a lagging power factor. Capacitive loads have a leading power factor. |
| What are the draw backs of poor power factor. Ideally, what do we want our power factor to be? | Ideally we want a power factor of unity. Drawbacks include the utility needs to supply more current for the same load, power loss increases significantly, and voltage drops across the line causing low voltage conditions. |
| What is power factor correction? | Using additional components to try and get your power factor close to unity. A capacitor in parallel with a lagging load can do this 'cancelling out' the inductive load, or a capacitor in series can be used, but since this alters voltage, it is done less. |
| How do capacitors do power factor correction? | They supply leading reactive current, offsetting the lagging current of inductive loads. |
| How can synchronous machines do power factor correction? | By operating in over excited mode. |
| What are inductive and capacitive compensation, and their effects on voltage? | Inductive compensation refers to the fact that inductors absorb reactive power, which in turn decreases voltage. Capacitive compensation refers to the fact that capacitors inject reactive power, which in turn increases voltage. |
| What are the impacts of injecting or absorbing active power? | It changes bus voltage, and frequency. The change in voltage is dependent on the ratio of (source reactance a.k.a X)/(resistance a.k.a R) |
| Describe the voltage changes in systems with an X/R ratio less than 2, an X/R ratio from 3 to 5, and X/R ratio greater than 10 | The first one causes a high change in voltage in a system. The second one causes a moderate voltage change in a system, and the third causes a very very small voltage change in a system. |
| What are the changes in voltages due to X/R ratio in transmission lines, medium voltage systems, and low voltage systems? | Transmission lines have high ratios, so use reactive power to control voltage. Medium voltage systems, moderate ratios so use active and reactive power for voltage control, low voltage systems, low ratio, its best to use active power for voltage control |
| How is active power used to control voltage and frequency? | Active power is very good at controlling voltage in systems with low to moderate X/R ratios. It is very effective for frequency control. |
| How is reactive power used to control voltage. | Its very effective for controlling voltage in systems with high or moderate X/R ratios. |
| Are synchronous generators able to act as both a motor and a generator? | Yes. |
| What are the main components of a synchronous generator? | Rotor - rotating part holding the field windings. Stator - stationary part with three phase windings that AC voltage is induced in. |
| What are ways to start a synchronous generator? | Excitation - using DC current on field winding to make rotate mag field. Rotation - using prime mover to reach synch speed, make rotate mag field. Induction - rotate mag field makes 3 Φ voltage in stator windings, supply power to loads. |
| Describe a salient pole generator to me. | A hydro generator, driven by hydraulic turbine, with a large num of poles, and low speed. Its short with large diameter. |
| Describe a cylindrical/round rotor to me. | A turbo generator, driven by steam/gas turbines, has 2 or 4 poles, and operates at high speed. Its long with smaller diameters. |
| With induced armature voltage, can voltage be controlled by adjusting the field current? | Yes |
| To maintain generator stability, what should the electric frequency of the synchronous generator be synchronized with? | The grid frequency. |
| For a generator model under no load conditions (armature current = 0), voltage is equal to? | Internal voltage, Ea. |
| Is field current changed between full load and no load conditions in a generator? | No |
| For lagging, unity, and leading power factor, what are the signs of the voltage regulation. | Lagging - positive voltage regulation. Unity - small positive voltage regulation. Leading - negative voltage regulation |
| Explain infinite bus. | When a synch generator connects to a grid, grid is so large, that how the generator acts doesnt change the grid. So from the generators eyes, the grid is an 'infinite bus,' an ideal voltage source with const voltage magnitude, freq and phase angle. |
| What is the typical reference phase angle for infinite bus? | 0 deg |
| The phase angle between Ea and V is shown by symbol and represents what? | 𝛿, representing power angle |
| In three phase what does a positive and negative real power value mean, and what does a positive and negative reactive power value mean? | P - positive - generator operation. P - negative - motor operation. Q - positive - delivering reactive power. Q - negative - receiving reactive power. |
| For infinite bus, (also assume everything im about to say is in magnitude signs) E>V means what, and E<V means what? | E>V means an over excited generator, and the generator supplies active power to the bus. E<V means the generator is under excited, and the generator takes power from the bus. Generally generator adjusts E to control reactive power. |
| Whats are two ways of making your transformers 3 phase? | Either have three separate single phase transformers, or one transformer with three sets of primary and secondary windings each on individual limbs of a iron core. The windings can each be independently connected in delta or wye configurations. |
| What are advantages of Y-Y connections in transformers? | Neutral connection, no phase shift between primary and secondary side, decreased insulation costs because neutral. |
| What is a disadvantage of a Y-Y connection in a transformer. | Third harmonic, and unbalanced mode, whatever that means. |
| What are the advantages if a Y-delta connection in transformers? | Suitable for step down transformers. Y side provides a neutral connection. |
| What is the disadvantages of a Y-delta connection in transformers? (It is the same for delta-Y connections too) | There is a 30 deg phase shift between the primary side and secondary side. |
| What are the advantages of a delta-Y connection? | Suitable for a step up transformer, Y-side provides neutral connection. |
| What are the advantages of a delta-delta connection? | Provides a path for the third harmonic current (🤷🏾♀️), and no phase shift between primary and secondary side. |
| What are the disadvantages of a delta-delta connection? | No neutral connection, and windings must be able to withstand full line to line voltage |
| In delta-Y and Y-delta connections, in transformers, the line voltage on the ___ voltage side corresponds to the line voltage on the ___ voltage side. | Higher, Lower. |
| In a single phase transformer, turn ratio is voltage ratio. Does this stand in three phase transformers? | Not always. |
| Describe the changes that must be made from line voltage/current to phase voltage/current in delta and wye line current and voltages. | Line Voltage -> Delta-> Ephase=Eline. Line Voltage-> Y-> Eline=sqrt(3)*Ephase. Line current->delta->Iline=sqrt(3)*Iphase. Line current->Y-> Iline=Iphase |
| Delta-wye connections change voltage levels and grounding characteristics, but what remains the same? | Total three phase power. |
| What is a Single Line Diagram? | A simplified per phase representation of balanced three phase power that shows system topology rather then circuit representation. |
| Why are single phase per-unit systems useful? | Single phase per phase analysis is useful when dealing with simple systems, but as the number of transformers increases, it gets difficult to compute. Per unit normalizes quantities - simplifying analysis. |
| What must you do after finding per unit quantities? | Convert them back from per unit quantities to actual quantities. |
| Per unit impedance is the same on both sides if the... | voltage bases on both sides are related by turns ratio of the transformer. |
| In per unit analysis, a transformer becomes... | an impedance |
| Remember for per unit analysis of three phase transformers: | Divide system in sections, ensure power base same in all zones. Ensure base voltage on both sides the transformer have same ratio as line to line. Ensure per unit imped of 3 ph transformer is same on both sides. Change imped of equip to base values. |
| Per unit system advantages | Simpler to use than quantities like volts, amps, and resistance. Manufacturers supply data in pu on equip ratings, its widely used in transmission line calculations. |
| How do you convert a per unit value back to an actual value? | Multiply by per unit base |
| What are the differences between single phase per unit and three phase per unit? | Single phase per unit uses a single phase base, 3 phase per unit uses 3 power bases, and line to line voltage bases |
| What are the purposes of single line diagrams? | Visualizing structure of power systems, including generators, transformers, buses, circuit breakers and loads concisely. |
| What is an impedance diagram? | A diagram in which each power system is represented by its equivalent impedance, often expressed in per unit quantities. |
| What components do impedance diagrams include? | Generators, transformers, transmission lines, and loads, using their respective impedance values |
| How are impedance diagrams used in analysis? | They are used in load flow and short circuit studies to analyse the effects of impedance on voltage, current, and power distribution. |
| What is load flow analysis? | A technique used to determine steady state operating conditions of an electrical power systems, and how power flows through transmission lines, transformers and other components while maintaining voltage levels in acceptable limits |
| What is a reactance diagram? | A 'version' of the impedance diagram where only reactance is represented |
| When are reactance diagrams used? | In symmetrical fault analysis where resistance is negligible compared to reactance. It enables faster fault calculation in power systems. |
| What are some assumptions we make for reactance diagrams? | Transformer magnetizing current << full load current, resistancs << reactances, passive loads do not affect total fault currents and are ignored. Induction motors are ignored after a few cycles. |
| What are some advantages of overhead lines? | Lower installation and maintenance costs, faults are easier to locate, high power transfer capability |
| What are some disadvantages of overhead lines? | Performance is affected by temp, wind, rain, thunder, lightning. Less safety in densely populated areas |
| How are underground cables built? | They are one or more conductors insulated with a material and protected with an outer covering. It is below ground |
| What are underground cables mostly made of, and the cost of underground cables | Stranded copper, and they are about 8 times more costlier then overhead lines. At highest voltage levels they are 20 times more costlier |
| What are the advantages of underground cables? | Less public opposition, less storm damage, fewer faults, small voltage drops |
| What are the disadvantages of underground cables? | High installation cost, insulation and thermal issues at high voltages, and capacitance problems in AC systems - thus not suitable for long distances |
| what are the four fundamental electrical parameters that define electrical behaviour under transmission lines? | Line resistance(series) R, line inductance(series) L, line conductance(shunt) G, line capacitance(shunt) C - all specified usually as per unit. |
| What does line conductance/G represent | leakage current at insulators of overhead lines, and insulation of cables |
| What type of losses do resistance cost | Copper losses. Higher transmission voltages are utilized for reducing line losses |
| What does line resistance depend on? | Conductor material - copper, aluminum, etc, conductor geometry - length and cross section area, operating temperature, transmission frequency |
Created by:
EngineeringGirl