EET 3390 COURSE OVERVIEW

Transcription

1 EET 3390 COURSE OVERVIEW

2 The Power System The power network consists of several stages: 1. Power must be generated 2. Transformation (voltage must be stepped up for transmission ) 3. Transmitting power 4. Transformation (voltage must be stepped down before distribution) 5. Distribution of the power. 2

3 On-line diagram of the power system Standard voltage classes and typical operating voltages for the U.S. Power System: Low voltage (LV) Consumer 120/240,208,240, 277/480,600 (in volts) Medium voltage (MV) Distribution 2.4, 4.16, 4.8, 6.9, 12.47,13.2, 13.8, 23.0, 24.94, 34.5, 46.0, 69.0 (in kv) High voltage (HV) 115, 138, 161, 230 (in kv) Subtransmission Extra high voltage (EHV) Transmission 345, 500, 765 (in kv) 3

4 AC power versus DC power DC system: - Power delivered to the load does not fluctuate. - If the transmission line is long power is lost in the line. Case 1: Case 2: 4

5 AC system: Power is the product of voltage and current, increasing the voltage would decrease the current for a given amount of power. The line loss and voltage drop would both decrease as well. Difficult to change the voltage in a DC system. AC systems: alternate the current and voltage Power has an average value but pulsates around the average value at twice the system frequency. We can use transformer to step up the voltage for the transmission. DC system delivers constant power but to change the voltage is difficult therefore line losses are more significant. Advantages of AC systems: Voltage can be changed using transformers. AC generators are cheaper to build and maintain than DC generators The ability to transmit over longer distances with lower losses by using transformers Polyphase AC system can deliver constant power (discussed later) 5

6 Phasors Convenient way to represent voltages and currents using phasor form (work in phasor domain) In its simplest form, a phasor can be thought as a complex number that has magnitude and phase of a sinusoidal function and can be represented partially as a vector in the complex plane. IMPORTANT: The magnitude of a phasor will always be RMS value of the sinusoid it is representing. Voltage phasor magnitude Current phasor magnitude; current is lagging the voltage by 30 degrees 6

7 Impedance in AC Circuits Impedance of the inductor: Current in the inductor: Impedance of the inductor: NOTE: 1. There is a phase shift between the voltage and the current. The current is lagging the voltage by 90 degrees for the inductor. 2. The RMS of the current is the RMS of the voltage divided by (wl) Use Ohm s low in the phasor domain for the inductor by defining the inductive reactance 7

8 Impedance of the capacitor: NOTE: 1. The current is a negative sine wave, which leads the voltage (cosine wave) by 90 degrees. 2. The RMS of the current is related to the RMS of the voltage by (wc). Capacitance reactance: Impedance: Current in the capacitor: The current in the capacitor will lead the voltage by 90 degrees. example 8

9 Power in Single-Phase AC Circuits Real Power (Active Power) energy consumed by the load that is converted to heat and is not be returned to its initial stage. Real power used by the resistor: Reactive power (Inductor) Instantaneous current through the inductor varies sinusoidally. Inductor stores energy as a function of the current, thus magnetic field varies with time. As current increases the inductor stores more energy in its magnetic field and the power flows from the source to the inductor. As current decreases the inductor releases energy back to the source. The inductor does not use any power on average, there is energy moving back and forth between the source and the Inductor. This is called REACTIVE POWER. Reactive power (Capacitor) Voltage across the capacitor varies sinusoidally. Capacitor stores energy as a function of the voltage, thus capacitor s electric field varies with time. Capacitor draws energy from the source as it charges, and returns energy as it discharges. The voltage across the capacitor and the current through the inductor are 90 degrees out of phase, thus when inductor is charging the capacitor discharges. example 9

10 Complex Power Real power Reactive power Magnitude of the complex (apparent ) power Relationship between the real and apparent power. Relationship between the reactive and apparent power. Power factor Angle theta is the angle of the current with respect to the voltage. If the current lags the voltage (inductive load) then theta is negative. If the current leads the voltage (capacitive load) then theta is positive. Power factor is always positive and less than example

11 Three-Phase WYE Configuration Wye: Consists of three load components connected with a common point called neutral. 11

12 Three-Phase DELTA Configuration Delta: Consists of three load components connected endto-end way and has no neutral point. Phases are connected in a triangle. 12

13 Balanced and unbalanced 3-Phase systems A balanced system is one in which the line and phase currents and voltages in all three phases are equal in magnitude and separated by 120 degrees, and the impedances in all three phases are identical. An unbalanced system is one in which any of the foregoing requirements are not met. Power calculation in balanced 3-phase systems: WYE system Delta system For a single-phase system, the apparent power is the product of the phase voltage and phase current For a balanced 3-phase system, the total 3-phase apparent power is three times the power consumed by one phase 13 examples

14 The Per Unit Concept A convenient method of calculating electrical power quantities. A shorthand method of solving power system problems has been developed to eliminate many of the manipulations required in systems with more than one voltage. 14 examples

15 The Per Unit Concept (cont.) 15

16 Major Distribution Layout Classifications 1. A Radial subtransmission and distribution. The distribution lines extend from the substation to the last load with service drops to customers along the way. The major advantages of the radial layout are that it is simpler and more economical to install than other types of layouts. The major disadvantage is that any problem usually leaves a number of customers out of service until the problem is solved. A modified radial subtransmission layout is more often used in which two parallel radial subtransmission line have a provision for switching the load to the good line in the event of a failure. 16

17 2. The Loop subtransmission and distribution The loop connection is more expensive than the radial because it requires more equipment, but any point the line has service from two directions. If one is out, the customer can be fed from the other direction. Switches are placed periodically around the loop so the malfunctioning section can be repaired without removing much of the line from service. The loop layout is very reliable but expensive. 17

18 3. A combination of radial and loop Is often used to provide the most reliable service to critical customers, such as business and industrial, by a loop, and reasonably economical service to residential neighborhoods. Radial part of the system is arranged so that only a few residential customers will be out of service at one time for any fault condition. 18

19 4. Networks Networks are designed to provide very reliable service to areas with dense loads such as downtown and suburban business districts containing many multi buildings. The network consists of underground secondary lines connected at corners with transformers feeding the network every one to two blocks. The network equipment is contained in underground vaults with access through main holes in streets and alleys. Although networks can be very large, network sections are seldom larger than four square blocks. Networks are reliable but expensive. 19

20 Overhead and Underground Overhead distribution is used because the low initial cost and good reliability. The equipment for overhead distribution costs less than the equipment required for underground distribution, and it is less costly to maintain. Overhead distribution lines are more subject to storm, lightning, and wind damage than underground lines, but they are more easily and cheaply repaired when damage occurs. Underground lines are not as vulnerable to weather conditions, and they are out of sight so the neighborhood sky lines is less cluttered. Underground distribution lines require waterproof Insulation of high quality and cost. Underground lines are higher in electrical loss than overhead lines. The price of equipment for underground distribution decreasing as more underground service is installed. 20

21 1/1 Transformer When the primary winding and the secondary winding have the same amount of turns there is no change voltage, the ratio is 1/1 unity. Step-Down Transformer If there are fewer turns in the secondary winding than in the primary winding, the secondary voltage will be lower than the primary. Step Up Transformers If there are fewer turns in the primary winding than in the secondary winding, the secondary voltage will be higher than the secondary circuit. 21

22 Transformer classification by cooling: 1. Dry transformers. 2. Oil immersed transformers Dry types are air cooled, primarily by convection. Oil immersed transformers in which the windings and core are immersed in oil, are both cooied and helped in insulation by the oil. Almost all utility power and distribution transformers are oil. Dry types are used primarily where minimum cost is a factor and the transformer is supplied by the customer such as in apartment house and a building distribution systems. Because the cooling of dry transformers is by convection they are very intolerant of overloads. Dry types must be in an enclosure for safety. 22

23 Voltage and Current For the ideal transformer, all the flux is confined to the iron core and thus links the primary and secondary. Because the losses are zero in the ideal transformer, the apparent power in and out of the transformer must be the same: Turns ratio Ratio of the currents is inverse of the voltage ratio or the inverse of the turns ratio. It makes sense: if we raise the voltage level to a load with a step-up transformer, then the secondary current drawn by the load would have to be less than the primary current, science the apparent power is constant For step-down transformer, the primary side has more turns than secondary, therefore a >1; For step-up transformer, the primary side has fewer turns than secondary, therefore a <1; 23

24 Impedance Due to the fact that the transformer changes the voltage and current levels in opposite directions, it also changes the apparent impedance as seen from the two sides of the transformer. Ohm s law applied at the load: Recollect: When we move an impedance from the secondary to the primary side of the transformer we multiply by the turns ratio squared. When moving the impedance from the primary to the secondary, we divide it by the turns ratio squared. The Reflected (referred) impedance (the impedance looking into the primary side of the transformer) This process is called referring the impedance to the side we move it, and allows us to use transformers to match impedances between a source and a load 24

25 Exciting Current In real live we deal with real transformers which require current in the primary winding to establish the flux in the core. The current that establishes the flux is called the exiting current. Magnitude of the exciting current is usually about 1%-5% of the rated current of the primary for power transformers but may be much higher for small transformers. According to Faraday s law if we apply a sinusoidal voltage to the transformer, then the flux will also be sinusoidal, but due to the non-linearity of B-H curve for the iron curve, the current will not be sinusoidal even if the flux is sinusoidal and the current will be out of phase with flux. 25

26 Equivalent T-circuit Third order circuit. It takes third order differential equation to solve it. 26

27 We can refer the impedances of the secondary to the primary side (or vise versa) yielding the equivalent circuits All resistances and reactances have been referred to the primary side All resistances and reactances have been referred to the secondary side 27

28 Series Equivalent Circuit Note: In large-scale system studies, even cantilever model becomes too complex, so one final simplification is made. We completely neglect the magnetizing branch of the transformer model. Only combined winding resistance and leakage reactance are included, resulting in the first order model (takes first order differential equation to solve it). Series equivalent circuit. Science there are no shunt elements, the primary and secondary currents are equal to each other. 28

29 Determining Circuit Parameters For developed model to be useful, there must be a way to determine the values of the model parameters. We use two test to determine this parameters: Short-circuit test One side of the transformer is shorted, and voltage is applied on the other side until rated current flows in the winding. The applied voltage, winding current, and input power are measured. Technique: the low-voltage side of the transformer is shorted and voltage is applied to the high-voltage side, because it only takes about 4%-7% of rated voltage to cause rated current to flow in the winding. The measurements are used to calculate value of Req and jxeq. Input impedance to the transformer is the primary winding in series with the parallel combination of the secondary winding and the exciting branch: The core branch elements are much larger that the winding impedance: 29

30 To conduct the short-circuit test: - Measure the voltage applied to the transformer high side Vsc - Measure the short-circuit current in the high-side winding Isc - Measure the power into the transformer Psc. - Having these parameters we calculate the magnitude of input impedance: Once we have values for the equivalent winding resistance and reactance we can apportion them to the two sides by assuming the windings have equal resistance and reactance when referred to the same side: 30

31 Open-circuit test High-voltage side is opened and rated voltage is applied to the low-voltage side (primary side). The low-voltage side is used to avoid high-voltage measurements. With no load current, only exciting current Io flows and because the impedance of the primary side is small, the voltage across the magnetizing branch is approximately equal to the applied voltage. Procedure: - Measure the voltage applied to the transformer low side Voc, the open-circuit current in the low-side winding Ioc and the power into the transformer during open circuit test Poc. - Calculate resistance and reactance. 31

32 The input impedance during open-circuit test is the primary winding in series with the exciting branch: Now we have the voltage applied to and the power dissipated, so we can calculate: Because the magnetizing reactance is in parallel with reactive power to find reactance: we first need to find the 32

33 Transformer Efficiency Efficiency is defined as: Input is the output plus losses: LOSSES Copper losses (the energy dissipated in the resistance of the windings ) Core losses (hysteresis and eddy current losses in ferromagnetic core of the transformer) 33

34 Losses in the transformer Winding resistance Current flowing through the windings causes resistive heating of the conductors. Eddy currents Ferromagnetic materials are also good conductors, and a solid core made from such a material also constitutes a single short-circuited turn throughout its entire length. Induced eddy currents therefore circulate within the core in a plane normal to the flux, and are responsible for resistive heating of the core material. Hysteresis losses Each time the magnetic field is reversed, a small amount of energy is lost to hysteresis within the magnetic core, the amount being dependant on the particular core material. Mechanical losses The alternating magnetic field causes fluctuating electromagnetic forces between the primary and secondary windings, that induce vibrations. 34

35 Stray losses Not all the magnetic field produced by the primary is intercepted by the secondary. A portion of the leakage flux may induce eddy currents within nearby conductive objects, such as the transformer's support structure, and be converted to heat. Cooling system Large power transformers may be equipped with cooling fans, oil pumps or watercooled heat exchangers designed to remove heat. The power used to operate the cooling system is typically considered part of the losses of the transformer. 35

36 Voltage Regulation The transformer windings have impedance so there will be a voltage drop across them that changes with current. The secondary voltage will vary as the load changes. Voltage regulation is a measure of the change in secondary voltage from no-load to full-load and is usually expressed as a percentage of the full-load voltage. If there were no load on the transformer, the current would be zero and the referred secondary voltage would be equal to the primary voltage. The no-load voltage (referred to the primary) of the transformer is the primary voltage. As the load increases to full load, current flows in the windings of the transformer and there is a voltage drop across the transformer, and the referred value of the secondary voltage is no longer equal to the primary voltage. Voltage regulation: 36

37 Methods for Voltage Regulation Capacitors for voltage regulation Switch shunt capacitors used across the line to increase the voltage by reducing the inductive VARs drawn as in power factor correction. Shunt capacitors are only used for lagging load power factors and their main goal is to correct for load power factor, and their only current is from the VARs. Switched capacitor banks are expansive because they must have sensing equipment to monitor the line voltage and control equipment activate the proper switching. All capacitor banks require protection: fuses, circuit breakers etc. Series capacitors are connected in series with the line and carry full line current. The capacitive reactance of the series capacitance is used to cancel the inductive reactance of the line to reduce the voltage drop along the line. 37

38 Series capacitors operation a : line equivalent circuit b : phasor diagram with lagging power factor load Figure 4.35 page 143 c : the capacitive reactance cancels a portion of the line inductive reactance causing the receiving voltage to rise Series capacitors can be switched or fixed and are protected the same way as are shunt capacitors 38

39 Three-phase transformer connections There are four major three-phase transformer connections : 1. WYE-WYE 2. Delta-Delta 3. WYE-Delta 4. Delta-WYE Three-phase transformers are less expensive than 3-single-phase transformers because less total core material is needed for the three-phase transformer and the packaging cost is reduced. Additionally they take up less space, are lighter, require less on site external wiring for installation, and more efficient than three single-phase transformers. Single phase transformer has one voltage ratio which agrees with the turns ratio. For 3-phase transformer: 1. Bank Ratio = BR = the ratio of line-to-line voltages. 2. Phase Ratio = PR = the ratio of the voltages in the coils of the transformer and thus agrees with the turns ratio. 39

40 The WYE-WYE connection has significant third harmonic content on the secondary lines (unless the neutral point is grounded) There is no phase shift between the primary and secondary of a Y- Y connected transformer. The Delta-Delta connection has no harmonic problem and no phase shift from primary to secondary. The only disadvantage with respect to a WYE connection is that the delta insulation class must be for the line to line instead of line to neutral voltage. 40

41 There is a 30 degrees phase shift in both connections. United States industry convention is to connect the secondary so it lags the high voltage primary by 30 degrees. Figure 4.16 page 120 When possible the Y is connected to the high voltage side because the insulation requirements are lower (recall that Y phase voltage is 1/sqrt(3) that of the line voltage). The Y may be necessary on the low voltage side because of the distribution system requirements as in 480/277 V and 208/120 V installations. 41

42 Transformer protection Picture 4.18 page 123 Transformers, especially power transformers, are expensive. The transformer must be protected from overcurrent due to faults on its secondary circuit, and from over voltage, which is usually caused by lightning. The protective devices: Lightning protection is provided by lightning arresters. Zinc oxide lightning arresters are the most popular now. The voltage at which the lightning arresters begins conducting, absorbing power, and preventing further voltage rise on the line is set at a voltage below the maximum insulating voltage of the transformer and above the maximum operating voltage of the transformer. Lightning arresters are used on both the primary and secondary side of the power transformers because lightning can strike on either side. Overcurrent protection is provided by circuit breakers and their associated protective relays, and fuses. A fuse opens on overcurrent. Circuit breakers open electrical contacts when they receive a trip signal from one of their associated relays The opening is done by driving the contacts apart with powerful springs. Circuit breakers are used on the secondary side of the transformer with a fuse back up on the primary side. 42

43 Circuit Breakers Circuit breakers for electrical power distribution include both medium (between 600 V and 34.5 kv) and high voltage (above 34.5 kv), high current devices that must automatically disconnect faulted equipment to protect people, prevent damage to upstream equipment, and minimize damage to downstream equipment in two to five cycles. The circuit breaker should not damage itself when it operates. Breakers are classified by: 1) Voltage 2) Continuous current 3) Interrupting capacity (maximum fault current the breaker can interrupt without becoming dangerous themselves), and methods of extinguishing the arc. 43 Example

44 The Arc When current carrying contacts open, the initial electric field between the just parted contacts is very high. The high electric field causes any gas between the contacts to ionize and support current flow through it, or arc. The higher the voltage that the contacts are breaking the more severe the arcing. The arc must be extinguished to interrupt the current. Many methods are used to extinguish an arc. They use one or both of the following two principles: 1. To lengthen the arc until it is long and thin. This causes the arc resistance to rise, thus the arc current to drop, the arc temperature to decrease, and ultimately results in insufficient energy in the arc to keep it ionized. 2. To open the arc in a medium that absorbs energy from the arc causing it to cool and quench. Air, oil, and insulating gas are normally used as the medium. 44

45 Air Circuit Breakers Air circuit breakers use air as the arc interrupting medium. Because air at atmospheric pressure ionizes easily some auxiliary equipment must be used to break the arc except for the very lowest voltage and capacity breakers. Figure5.4 page 162 Convection causes an arc, which is hot, to rise if the contacts are properly oriented. As the rising arc stretches its resistance increases, its current drops, and its increased surface area is exposed to cooler air, causing its temperature to drop until the arc is finally extinguished. The longer an arc can be drawn out the easier it is to extinguish. Arc tips break after the main contacts break Arc horns work on the same principle except convection drives the arc up the spreading horns causing the arc to leave the load current carrying contacts and stretch. Interrupting fins placed in the path of the rising arc will stretch the arc farther, cool it more, and aid in extinguishing the arc. Large low voltage breakers will have interrupting fins. 45

46 Figure 5.5 page 163 Magnetic blowout refers to the use of a transverse magnetic field near the contacts to stretch and drive the arc into the interrupting fins. The magnetic field interacts with the ions of the arc to provide the driving force. The magnetic field can come from a permanent magnet in small breakers, but is provided by a properly positioned coil through which the contact current flows in larger low voltage breakers. Circuit breakers with magnetic blowout and interrupting fins can even be used for lower medium voltages. 46

47 Air Blast Circuit Breakers Figure 5.6 And figure 5.7 Page 164 Cross air blast circuit breakers are special purpose medium voltage circuit breakers used where noise is an important factor. A blast of compressed air (to 800 psi) is blown across the circuit breaker contacts as the contacts open. The blast of high pressure air blows the arc into the interrupting fins, stretches the arc, and cools it. Axial air blast circuit breakers blow high pressure air along the axis of the contacts to stretch and cool the arc. The air is blown from a port next to the stationary contact toward the moving contact. Axial air blast breakers are usually high voltage breakers. They can be built to interrupt currents as high as 63 ka at 800 kv. 47

48 Vacuum Circuit Breakers Vacuum circuit break contacts are enclosed in a container with a high vacuum. No significant arcing can occur because there is no air between the contacts to ionize Oil Circuit Breakers Oil circuit breakers use oil as the arc interrupting medium. Oil has a dielectric strength far in excess of air. When contacts open in oil the arc causes the oil to disassociate which absorbs arc energy. Sulphur Hexaflouride Circuit Breakers Sulphur Hexaflouride gas is a popular interrupting medium for high voltage and extremely big voltage (EHV, above 345 kv) applications. Its voltage withstand rating is about three times that of air and it is extremely electronegative. That means its atoms bind for a considerable time to free electrons, thus becoming negative ions. When free electrons are removed from the arc it is difficult to sustain because no free electrons are available to accelerate and ionize atoms by collision. 48

49 RECLOSERS Most faults (80-95%) on distribution and transmission lines are temporary, lasting from a few cycles to a few seconds. They are caused by such things as tree limbs falling or blowing across the lines and are removed when the limb burns off or is blown out of the line. Reclosers allow temporary faults to clear and then restore service quickly, but disconnect a permanent fault. Reclosers are essentially special purpose, light duty circuit breakers. They can interrupt overloads but not severe faults. Reclosers sense an overcurrent, open, then after a preprogrammed time, reclose. They can be programmed to sense an overcurrent, open, and reclose several times (up to five times is typical) and after the preset number of operations remain open. FIGURE 5.17 page 177 Two types of reclosures are currently manufactured. In one type the times are controlled by pistons in hydraulic cylinders, and in the other by electronic circuitry. Electronic reclosers are more flexible, accurate, and easily tested than hydraulic closers, but are also more expensive so electronic controls are used primarily on heavy duty three-phase reclosers. 49

50 SECTIONALIZERS A sectionalizer is a device that is used to automatically isolate faulted line segments from a distribution system. 1. It senses any current above its actuating current followed by a line de-energization by a recloser. 2. It counts the number of overcurrent allowed by line de-energization sequences and after a preset number of times it opens and locks out. The sectionalizer must be manually reset after lock out. If normal line conditions continue for a preset length of time after an overcurrent, de-energization sequence below the preset lock out number, the sectionaiizer will reset itself to zero count. The delay before reset is usually set between 30 and 90 seconds: Two types of sectionalizers are available: 1. Smaller ones are hydraulically operated in a manner similar to reclosers 2. Higher capacity are electronically operated. 50

51 FUSES Fuses are one-time devices that must be replaced each time they open a fault. They use a metallic element that melts when an overload current passes through it. The melted element separates breaking the circuit. 1. Low voltage and current limiting fuses Low voltage fuses use zinc, copper, or silver as the metallic element, while medium and high voltage fuses typically use tin, cadmium, or silver. Current limiting fuses are fuses that limit the peak fault current to less than it would be without a current limiting fuse, and break the circuit in less than one-half cycle. A current limiting fuse cartridge is filled with sand. The sand melts but does not disassociate so it absorbs heat energy cooling the arc, plus the sand filling leaves little air to support an arc. Current limiting is valuable because both the heating and mechanical damage caused by a fault is proportional to the square of the current. 51

52 Lightning Arresters The job of the lightning arresters is to clip the induced voltage transient caused by a lightning strike at a level below the BIL, but above the normal operating voltage, of the protected equipment. The lightning arrester should be an insulator at any, below the protected voltage, and a good conductor at any voltage above to pass the energy of the strike to ground. Shield wires for lightning protection of lines Shield, or static wires, are conductors strung above the load carrying conductors on transmission and distribution towers and poles to protect the load carrying conductors from lightning strikes. The shield wires provide a place for lightning strokes to terminate instead of the power carrying conductors, thereby protecting the power conductors. Almost all lines 34.5 kv and above use shield wires. 52

53 Shield wires provide a 30 zone of protection on either side of a vertical line drawn from the ground to the wire. Figure 5.35 page 198 Towers in which the power carrying conductors do not fit within the zone of protection of a single shield wire use two. Equipment in station yards can be protected by placing it within the 30 protection zone of a tall mast with a conductor running from the tip to ground. Shield wires must be grounded to provide a path for the lightning current. 53

54 PROTECTIVE RELAVS A relay is an electromechanical- or microprocessor-controlled electronic system. That senses an abnormal or fault condition, such as an overcurrent, under or over voltage, or low frequency, and sends a trip signal to a circuit breaker. They are used to protect generators, transformers, motors, and lines. Monitoring relays verify conditions in the power system or power system protection system and send an alarm when the conditions are abnormal. Monitoring relays often are used in conjunction with protective relays. Programming relays sequence events or detect sequences of events. They are used to control and monitor synchronization and reclosing sequences. Regulatory relays are used to determine if a parameter, such as line voltage, is between programmed limits and send a control signal to force the parameter to return to within the limits Auxiliary relays provide miscellaneous functions within other relaying systems. Timers are an example of an auxiliary relay function. Relays must operate reliably, and quickly, be economical, and selective, operatmg only on the desired input 54 Example

55 Microcomputer Controlled Relays The use of microprocessors in microcomputer relay systems has allowed relay systems to perform several relaying functions with a single central relaying package in a very economical manner. The multifunction capability of microprocessor controlled relay systems has resulted in a drop in the cost per function of such relays when compared to electromechanical relays. The current and potential transformers provide current and voltage information to the relay from which the relay microcomputer calculates any additional parameters needed, such as impedance, VAR and power quantity and flow direction, trends over a fixed time, and running averages of quantities. Figure 5.36 page 199 The relay can also make use of other parameters, such as temperature and vibration sensor outputs, to monitor more conditions than electromechanical relays are able to monitor. A single microcomputer-controlled relay can monitor and respond to abnormal conditions while gathering data for use in control and trend analysis. The relay will react to out of limit parameters by sending a trip signal to a circuit breaker and an alarm signal to a central monitoring point via a telecommunication system. 55

56 DISCONNECT SWITCHES Disconnect switches are designed to open and close a circuit at high voltages. The switches must have a large gap when open. An air gap of about 11 feet is required at 230 kv. Disconnect switches cannot open a fault. Non-load Break Disconnect Switch: High and medium voltage disconnect switches are designed to isolate a section of a circuit after the protective device has de-energized the circuit. Disconnect switches can be operated by motors, as most high voltage switches are, by an insulated lever connected to a actuating arm that moves the switch blade. Load Break Switches: Load break disconnect switches can interrupt normal load currents, but not large fault currents. The wall switch is the most common load break switch. Most load break switches use motors to open and close the switch blades but the interrupters are actuated by strong spring pressure. 56

57 Substations generally have: 1. Switching equipment 2. Protection equipment 3. Control equipment 4. One or more transformers In a large substation: Circuit breakers are used to interrupt any short-circuits or overload currents that may occur on the network. In smaller distribution stations: Recloser circuit breakers or fuses may be used for protection of distribution circuits. Other devices such as capacitors and voltage regulators may also be located at a substation. Substations may be on the surface in fenced enclosures, underground, or located in special-purpose buildings.

58 Distribution substation A distribution substation transfers power from the transmission system to the distribution system of an area. The input for a distribution substation is typically at least two transmission or subtransmission lines. Distribution voltages are typically medium voltage, between 2.4 and 33 kv depending on the size of the area served and the practices of the local utility. Besides changing the voltage, the job of the distribution substation is to isolate faults in either the transmission or distribution systems. Distribution substations may also be the points of voltage regulation, although on long distribution circuits (several km/miles), voltage regulation equipment may also be installed along the line. Complicated distribution substations can be found in the downtown areas of large cities, with high-voltage switching, and switching and backup systems on the low-voltage side.

59 Collector substation In distributed generation projects such as a wind farm, a collector substation may be required, which is similar to a distribution substation although power flows in the opposite direction, from many wind turbines up into the transmission grid. For economy of construction the collector system operates around 35 kv, and the collector substation steps up voltage to a transmission voltage for the grid. The collector substation can also provide power factor correction if it is needed, metering and control of the wind farm. Collector substations also exist where multiple thermal or hydroelectric power plants of comparable output power are in proximity.

60 Switching substation A switching substation is a substation which does not contain transformers and operates only at a single voltage level. Switching substations are sometimes used as collector and distribution stations. Sometimes they are used for switching the current to back-up lines or for paralellizing circuits in case of failure.

61 Design The main considerations taking into account during the design process are: 1. Reliability 2. Cost (sufficient reliability without excessive cost) 3. Expansion of the station, if required. Selection of the location of a substation must consider many factors: 1. Sufficient land area 2. Necessary clearances for electrical safety 3. Access to maintain large apparatus such as transformers. 4. The site must have room for expansion due to load growth or planned transmission additions. 5. Environmental effects( drainage, noise and road traffic effects. 6. Grounding must be taking into account to protect passers-by during a shortcircuit in the transmission system 7. The substation site must be reasonably central to the distribution area to be served.

62 Distribution Substation Protection Needs Above a minimum protection needed to avoid injury to people and damage to equipment, the level of protection of a substation is determined by how critical the loss of power is to the load. The loss of electrical power to a hospital is very serious while the loss of power to a residence is inconvenient. In the event of a fault the hospital electricity must be restored in the shortest amount of time possible while the residence can be without electricity several hours without serious consequences. Equipping a substation with automatic switching to restore power when it is lost and to assure the least possible damage and repair time after a fault is expensive. A small substation at the end of a radial subtransmission line that might be used to serve a small group of residences. It consists of two dead end poles to terminate the lines, two manual non-load break switches, and primary fusing.

63 DISTRIBUTION SUBSTATION CONSTRUCTION METHODS Four basic methods exist for substation construction: 1. Wood 2. Steel lattice 3. Steel low profile 4. Unit. Wood pole substations are inexpensive, and can easily use wire bus structures. Wood is suitable only for relatively small, simple substations because of the difficulty of building complex bus and switch gear support structures from wood. Lattice steel provides structures of low weight and high strength. Complex, lattice steel is reasonably economical and is the preferred material for substation construction whenever possible. Solid steel low profile substations are superior to lattice or wood constructed substations. However, low profile construction is more expensive than either wood or lattice steel, and requires more land because multilevel bus structures cannot be used. The unit substation is a relatively recent development. A unit substation is factory built and tested, then shipped in modules that are bolted together at the site. Unit substations usually contain high and low voltage disconnect switches, one or two three-phase transformers, low voltage breakers, high voltage fusing, bus work, and relays.

64 DISTRIBUTION SUBSTATION LAYOUT 1. Single source, single feeder substation The one-line diagram of a single-source, single-feeder substation with the minimum equipment used. A bypass switch is provided so service can continue during circuit breaker maintenance. The probability of a fault during circuit breaker maintenance is small, but still there as a result the transformer is protected by a primary fuse to back up the breaker, and provide some protection for internal transformer faults. The minimum relaying is overcurrent on the secondary side of the transformer. The switches can be manually or motor operated.

65 2. Single bus substation This is the one line of a single bus substation fed by a single radial subtransaission line. Each feeder must has its own overcurrent protection. The primary switch must be able to break the transformer excitation current. The transformer may have differential relaying that trips all of the feeder breakers in the event of a fault. Each distribution voltage the substation supplies must have its own bus. The possibility of a subtransmission circuit fault is much higher than a transformer fault. Two sources allow service to be restored more quickly upon a subtransmission circuit fault.

66 2. Single bus substation This is the one line of a double throw switch on the primary side which allows the transfer to be made quickly from one subtransmission circuit to another. The switch is interlocked with the transformer breaker so it cannot be opened under load. The switch can be replaced by two manual high voltage breakers that can break the load, and expected fault current. The transformer secondary breaker makes possible very effective differential bus protection to detect faults internal to the bus. The bus relays then trip all of the circuit breakers connected to the bus upon a bus fault.

67 3. Two Transformer Distribution Substations More critical loads implement a two transformer distribution substation allowing to significantly decrease the out of service time. Normally the transformers are rated at 75% capacity when self cooled and equipped with automatic air cooling that is used when one transformer must handle the entire substation capacity. The tie switch between the two transformer connections to the bus which is in open state when both transformers are in use to prevent the transformer secondaries from operating in parallel. Momentary parallel operation during switching is often permissible but must be avoided for the extended operation time due to the high secondary currents. The primary side switching is arranged so that either or both transformers can be fed by either subtransmission line.

68 4. Automatic Switching (Throw-over) Two source, radial arrangement Service outage time can be reduced considerably by using circuit breakers to automatically, or on command from a central control station, disconnect the faulted source or bus and connect the substation so that power can reach all of the feeders. Figure shows a substation connected for automatic switching, also called throw-over or roll-over. Operation: Assume sources 1 and 2 are connected as radial lines. 1. Source 1 is lost, breaker 1 will open under relay control disconnecting source 1 2. Breaker 3 closes connecting transformer 1 to source 2, and vice versa. 3. If transformer 1 fails, breakers 1, 3, and 4 would open to disconnect it. The low voltage bus tie breaker 6 closes to connect all of the feeders to transformer 2 The low voltage tie breaker is interlocked with transformer secondary breakers 4 and 5 to prevent parallel transformer operation.

69 Fault Classifications : TRANSMISSION LINE FAULT CALCULATIONS A system fault is defined as any abnormal condition. 1. Line to ground. Line to ground faults are caused by a line touching the ground. Wind, ice loading, tree falling on a line can cause a line to ground fault. This category accounts for about 70% of all line short circuit faults. 69

70 Fault Classifications : 2. Line to line. These faults are caused by high winds blowing one line into another, or by a line breaking and falling on a line below it. These account for about 15% of line faults. 70

71 Fault Classifications : 3. Double line to ground. This category is caused by the same things that cause single line to ground faults, except two lines are involved instead of one. These account for about 10% of line faults. 71

72 Fault Classifications : 4. Three-phase faults. If a line condition occurs in which all three phases are shorted together, an equipment failure, or all three lines falling to the ground, it is called a three-phase fault. Accounts for only about 5% of all line faults. 72

73 The point at which a conductor touches ground or another conductor during a fault is usually accompanied by an arc. The arc is resistive, but arc resistance varies widely. The usual utility practice is to consider the fault resistance zero to calculate the maximum fault current that can occur at a point of interest on a line. The fault current that flows depends on the source, line, and fault impedances: 73

74 Bus Protection Differential protection is effective for bus faults because the current leaving the bus on feeders and the current entering the bus from sources should be zero at any instant. Additionally, differential protection can distinguish between internal bus faults and external feeder faults. A feeder fault can result in the CTs on the feeder saturating, and the dc offset of a fault worsens the situation. Thus special care must be taken in bus differential relaying to prevent external faults from causing a trip on the circuit breakers supplying the bus. Three major systems are used:. 1. Linear coupler (LC) system, which works by eliminating the iron core of the CTs. 2. Multi-restraint, variable percentage relays (CA-16). 3. High impedance voltage operated differential relays (KAB).

75 SUBSTATION GROUNDING Substation grounding is done safety, and to provide a stable reference voltage for protection systems. The grounding system of a substation consists of a ground mat made of large size bare conductors, connected in a grid pattern, and buried beneath the substation. The perimeter of the grid is connected to metal rods driven about 30 feet into the ground. The grid wires are about 20 feet apart but the spacing varies with the conductivity of the soil. Highly conductive soil can use larger grid wire spacing. All substation structures are to be constructed within the perimeter of the grid. The fence around a substation has two buried ground wires connected to the fence every few feet. One runs about 3 feet outside the fence, and one inside the fence. Both wires are connected to grounding rods every 50 feet.