Overhead and underground distribution systems components, part 4. Energy management: Application of energy management: After the power has been generated by steam or hydraulic turbines/generators, steppedup through a stepup transformer, transmitted over towers/insulators/transmission lines, distributed in terminal stations using air or gas insulated systems (bus and breakers), reaching transformer stations where stepdown transformers are used to step down the voltage from transmission to subtransmission/distribution levels, the power passes through medium voltage overhead conductors or underground cables to the distribution transformers that further stepdown the voltage to the utilization level (600v or less). The loads and their characteristics found in industrial plants or commercial institutes or residential subdivisions varies dramatically. The requirements of the different types of loads when it comes to power availability vs quality vary widely. 75% of industrial loads are squirrel cage induction motors, the rest are lighting, heating, computer systems and welding machines (if any). Energy management can be applied to the generation, transmission & distribution side of the electric network (utility side) as well as to the utilization side (user or consumer). The latter may be termed demand side energy management. The main purpose of power system energy management is to achieve the goal of generating, transmitting & distributing electric energy efficiently & reliably. Its main functions are the supervision, the control and management of the electrical network in an integrated manner. It includes SCADA (system supervisory control & data acquisition) & DAC (distribution automation & control). Demand side energy management: The demand side energy management means the use of electric energy efficiently, thus increasing the profitability of the user or power consumer i.e. paying the minimum possible for the electrical power utilization. For industrials, the electric utility bill,usually, include a portion to the maximum demand (an average over a period of lets say 15 minutes) in KW reached for the billing period plus the energy consumption in KWH. To reduce the energy bill any of the following can be done: monitoring the demand so that in batch production line staggering loads to maintain the demand in KW at a minimum possible level can save in the energy bill, if the time of day or time of year is applied, then the process or equipment that utilizes high demands or consumes a lot of energy can be scheduled for the time of lower rates, usually the utility will penalize the user when poor power factor equipment are use, thus improving shunt capacitors close to the loads with low p.f. can reduce the energy bills. Coordination of capacitors design, rating & location can improve p.f. as well as reduce losses in lines and raise power quality in a plant. turnning off the loads (equipment & lights) that are not in use can make a difference in the energy bill. High efficiency motors: High efficiency motors are built with better quality material (steel) to reduce losses & improve
efficiency. They are 3 to 8 % more efficient than the standards one. The lower the motor horsepower the bigger the efficiency gap between the standard & the high efficiency one. Squirrel cage induction motors (SCIM): The power is supplied to the motor through the starter and if speed control is required, then a variable speed drive is connected between the source and the motor terminals. Induction motors have to be modelled for 4 conditions normal operation, starting, during faults and for transients analysis. In induction motors, a.c. is supplied to the stator winding directly and to the rotor winding by induction from the stator. The mmf waves created from the stator and rotor currents are stationary with respect to each other as they are rotating at synchronous speed in the air gap. The mmf waves will have constant amplitudes and their resultant creates the air gap flux density wave. At all speeds (other than the synchronous), the torque produced is steady. The torque angle of an induction motor is equal to 90 degrees plus the rotor power factor. The following equation expresses the relation between the rotor induced emf, induced current and rotor impedance referred to the stator: E2s/I2s = Z2s =r2 + jsX2, where E2s is the slip frequency emf generated in the rotor phase and referred to the stator, I2s is the current in the rotor phase, Z2s is the slip frequency rotor leakage impedance per phase referred to the stator r2 is the referred effective resistance and sX2 is the referred leakage reactance at slip frequency. For the stator: V1 = E1 + I1 (r1 + jX1); where V1 is the stator terminal voltage, E1 is the counter emf generated by the resultant air gap flux, I1 is the stator current, r1 is the effective stator resistance and X1 is the stator leakage reactance. The stator sees a flux and an mmf wave rotating at synchronous speed. The flux wave induces the slip frequency rotor voltage E2s and the stator counter emf E1. The flux wave speed with respect to the rotor is s times its speed with respect to the stator (f2 = sf1). The rotor mmf wave is opposed by the mmf of the load component (I2) of the stator current. (E2s/I2s) = sE1/E2, thus E1/I2 = r2/s + jX2. Now, the equivalent circuit is complete for the steady state analysis. For starting and short circuit contribution of induction motors, the equivalent circuit is reduced to a leakage reactance X' and a resistance r in series. For short circuit motor contribution, the following procedure may be applied: X' = X1 + [( X X2)/(X + X2)]; where X1 is the stator leakage reactance, X2 is the rotor leakage reactance, X is the magnetizing reactance. I1 = E1'/X'; where I1 is the initial rms short circuit current contributed from the motor, E' is the prefault voltage (behind the transient reactance X'). To' = X2 + X /2 f r2; where To' is the open circuit transient time constant, r2 is the rotor resistance. T' = To' [(X')/(X1 + X )] in seconds; where T' is the short circuit transient time constant I1 = (E'/X') et/T amp.
For starting motor analysis the following procedure may be used: V1 = I1Z1+{(I1)/[(1/Z2')+Ym]} = I1 K; where V1 is the stator terminal voltage for 1 phase (of the 3 balanced phases), I1 is the primary current per phase, Z1 is the primary (stator) impedance = r1+jX1, Z2' is the rotor impedance referred to the stator = r2' + r2' [1s/s] + jx2' and Ym is the main flux admittance = gm jbm (i.e. rm = gm/(gm²+bm²), Xm = bm/(gm²+bm²) & Zm = rm + jXm). The motor parameters: r1, X1, r2',X2', rm and Xm (or gm and bm) are determined from the no load and the locked rotor tests. From the no load tests the following data are obtained: the stator terminal voltage (V1), the no load stator current (Io) and the power consumed (Po), from which the main flux admittancerelated parameters are calculated. From the locked rotor (full voltage) test, the following data are obtained: the stator voltage the stator current (under locked rotor condition) and the power consumed, from which r1, r2' and the saturated reactance are calculated. From the locked rotor test with reduced applied voltage, the following data are obtained: the stator voltage and current, from which the unsaturated impedance and stator plus rotor reactances are calculated. The parameters r2' and X2' (unsaturated) have to include for the skin effect (by multiplying or dividing each value by an appropriate factor). The performance of the induction motors is related to its parameters and is defined by the following factors: the full load torque slip) at the full load torque), the total active power loss in the rotor circuit, the output HP at the full load torque, the slip at pull out (maximum) torque, the rotor current at maximum torque, the starting torque and the starting current. The torque of a polyphase induction motor can be given by: T=(7.04) (Pwatts)/n in LBft=7.04 (Prot)/ns LBft, where Prot is the power transferred by the rotating field, ns is the synchronous speed, Prot = 3Io'² r2'/s in watts. At standstill, the rotor frequency equals the stator frequency, as the motor accelerates, the rotor frequency decreases to a very low value (2 or 3HZ). By use of suitable shapes and arrangements, the rotor barsAfter the power has been generated by steam or hydraulic turbines/generators, steppedup through a step up transformer, transmitted over towers/insulators/transmission lines, distributed in terminal stations using air or gas insulated systems (bus and breakers), reaching transformer stations where stepdown transformers are used to step down the voltage from transmission to subtransmission/distribution levels, the power passes through medium voltage overhead conductors or underground cables to the distribution transformers that further stepdown the voltage to the utilization level (600v or less). The loads and their characteristics found in industrial plants or commercial institutes or residential subdivisions varies dramatically. The requirements of the different types of loads when it comes to power availability vs quality vary widely. 75% of industrial loads are squirrel cage induction motors, the rest are lighting, heating, computer systems and welding machines (if any). (squirrel cage) can have effective resistance at 60HZ which are several times that at 2 or 3 HZ. These rotor slots arrangements are the deep bar rotor and the double cage (Boucherot) rotor. At the higher frequency (i.e. 60HZ), the rotor current flows only in a part of the cross section of the rotor conductor and the rotor resistance
appears high. At the rated load and when the frequency is 2 or 3 HZ, the whole cross section of the rotor conductor is effective and r2 is small. The characteristics of the different 3 phase SCIM are best described by the NEMA designations: class A (normal starting torque, normal starting current and low slip), class B (normal starting torque, low starting, current low slip), class C (high starting torque, low starting current) and class D (high starting torque, high slip). Larger motors that may cause high voltage drops on the system during starting at full voltage (direct across the line) can be started at reduced voltage. In this case, the starting current will be lower though at the expense of the starting torque which will be lower also by a factor equal to the squared voltage applied during starting compared to direct across the line voltage magnitude. The methods used to have a reduced starting voltages are: autotransformers, Y transformation of stator windings and resistance starting. To have a variable speed motor any of the following principles may be used: changing the number of poles in the motor, varying the line frequency supplied to the stator terminals, varying the line voltage magnitude, varying the rotor resistance and the application of the appropriate voltage of the right frequency to the rotor circuit (if wound rotor motors are used). When motors are starting, there will be an imrush current flowing which can be anything from 4 to 10 times the full load current. The calculation procedure for the starting current magnitude was given previously, the duration can be taken as 10 seconds or calculated if the speed (slip) torque characteristics curves of the motor and load are known. The time required to attain a speed
o = J
(1/
t)(d
o); where
o in
mechanical radians per sec., J is the rotor + load inertia in MKS, t is the differential torque between that produced by the motor and that required to turn the load (available to accelerate the mass). A simplified approximation for starting time is given as follows: t(s) = WK² (rpm1 rpm2) 2 /(60 Tn g); where g = 32.2 ft/sec², Tn is the net average accelerating torque between rpm1 and rpm2, WK² is the total inertia constant in LB ft² and t is the time to accelerate in sec. Before covering briefly, single phase induction motors, lets differentiate between rotating mmf (flux) and alternating flux. The former having a constant magnitude and rotating (travelling) in space, (in the gap) the latter having a variable magnitude and in fixed in space. The alternating mmf can be replaced by two rotating mmfs travelling in opposite directions and each having an amplitude equal to half of that of the alternating mmf. Any 3 phase induction motor can be made to operate as a single phase induction motor by opening one of the 3 stator phases. The two remaining stator phases constitute a single phase winding distributed over 2/3 of the pole pitch. Contrary to the polyphase induction motor, where the rotor emf is induced by only one rotating flux, it is induced in single phase motors by 2 rotating fluxes. The two rotating fluxes have an opposite influence upon the rotor (one to motor it and the other to brake if). The developed torque due to the forward rotating flux = 7.04 (I2f'² r2')/s(ns) LBft., that due to the backward one = 17.04 (I2b'² r2')/ns<(2s). The resultant developed torque = Tf + Tb; where I2f' and I2b' are the currents produced in the rotor by the forward and backward fluxes, respectively. At stand still, s=1 and 2s=1, thus the rotating starting torque =0.
Regardless of the direction of rotation, a driving torque will be present as soon as the rotor starts to rotate. The equivalent circuit of the single phase induction motor can be produced using Kirchoff’s mesh equations of the stator and the 2 rotor circuits (the 2 emfs, currents and frequencies sf1 and (2s)f1 induced in the rotor). To start a single phase motor, either a rotating flux (such that in the polyphase motor) has to be produced or the addition of a commutator with brushes to the rotor is necessary. The first type of starting will achieve a time phase shift between the currents in the main and starting windings through the following designs: split phase motor, resistance start split phase motor, reactor start split phase motor, capacitor start motor, permanent split capacitor motor. The last type of starting will include the following designs: repulsion start induction motor, repulsion induction motor. For very small output and a small starting torque, shaded pole motor design is used. The parameters of the single phase induction motors (r1, X1, rm, Xm, r2'/s, X2', r2'/(2s) can be determined by performing the no load and locked rotor tests on the motors. The no load test is run at rated voltage V1 with the starting circuit open. Io (no load current) and Po (no load power input) are measured. The power input at no load will include the iron losses due to the main flux, friction and windage losses, the rotational iron losses, the copper losses in the stator windings and the copper losses in the rotor windings (can be assumed to be equal to those in the stator). The following parameters are determined from the test: the main flux reactance and resistance, provided that the stator parameters (r1 and X1) and the rotor parameters (r2' and X2') are known. The stator and rotor parameters are calculated from the locked rotor tests. This is run with the starting winding open. The following data are measured: the short circuit (locked rotor) stator voltage, current and power consumed. For the equivalent circuit of a three and single phase induction motors, see below.
Office equipment commonly used and their approximate power consumption: Example for office equipment & their average consumption: desktop PC (130 W), monochrome monitor (30 W), colour monitor (60 W), laptop PC (10 W), dot matrix printer (50 W while printing & 25 while idle), laser printer (300 w while printing & 150 W while idle), inkjet printer (10 W while printing & 3 while idle), photocopier (3001000 W), fascimile machine (100 W while transmitting & 15 W while idle), modem (20 W) and electric typewriter (130 W). Data to compare the total cost of the different lighting options or to calculate the payback period of the different installations: A detailed cost spread sheet can help in comparing the cost of the available new installations or to calculate the pay back period, the main elements of such sheet: total wattage/fixture, burning hours/year, rated life in hours, ratio of burning hours to rated life in hours, number of lamps, operating cost (annual energy cost, annual demand cost, labour cost for relamping) & fixed cost (fixture cost including the lamp plus labour cost for installation). Data required to compare the required wattage for the available alternative bulbs/fixture options: In order to compare the wattage required for different alternative fixtures/bulbs, the following parameters are to be known: wattage/bulb, actual wattage/fixture, mean lumens/fixture, the required lumens for the area where the fixtures/bulbs will be installed, the number of fixtures required & the total wattage. Main tasks of a SCADA system: The main tasks that a SCADA system has to provide are: controlling the plant/system field devices (obviously), alarm handling, limits changing, providing more than 1 operation mode, data archiving provision, events logging and the production of report & trend charts (graphs). Distribution automation functions: Distribution automation and control functions can be classified into: load management, real
time operational management and remote metering. The first function may be subclassified into: discretionary load switching, peak load pricing, load shedding and cold load pickup. The second function is subclassified into: load reconfiguration, voltage regulation, transformer load management, feeder load management, capacitor control, fault indication/location/isolation, system analysis/studies, state/condition monitoring and remote connect/disconnect of services. Fig. 4 shows the underground arrangements and fig. 5 shows the typical overhead system.
Distribution system automation: In any distribution system automatically (remotely) controlled, certain elements have to exist: controlled equipment/devices (like switches, breakers, pad mounted switchgear, tap changers), control station (where the man machine interface equipment are located: work stations, terminals, RTU and software), the controlling equipment like the RTU/sensors interface and the motors/springs mechanisms) and finally the communication medium (power line cariers, packet radio, fiberoptics). The presentation of this subject will be based on a hypothetical system representing the equipment in the transformer/ distribution stations, the components of overhead and the underground distribution systems. A typical transformer station may have the following elements: a high voltage disconnect switch (eg. 230KV, 200A, vertical break), a power transformer/oil immersed/forced cooled (eg. 100/125 MVA 230/27KV) with on load tap changer and gas pressure relays, line ups of switchgear assemblies (eg. SF6 circuit breakers, 27.6KV, 2000 Amps), relay panels (including bus differential protection, transformer differential protection, back up protection, feeder protection or located on the switchgear assembly, transmission line impedance relays, reclosing relays, breaker failure protection schemes, remote tripping for terminal breakers), battery/charger system with protection indicators and metering. The distribution system, overhead and underground may include the following: motor operated sectionalizers, motorized load break switches, reclosures, capacitor switching devices, faulted
circuit indicators (o/h & u/g), padmounted transformers/switchgear, pole mounted transformers, lightning arresters, overhead conductors and underground cables. Data to be collected and equipment to be controlled can be classified broadley into digital (onoff, 1 or 0) and analog (continuous) processes. The number of points to be monitored or controlled and the number of channels required to build a system wide SCADA are to be known in order to achieve an optimum system with provision for future expansion. Other automated tols that are found integrated in a DA system are automated mapping/facility management, primary analysis software, transformer load management. A few of the pitfalls of automating are: hardware interface incompatibility, communication protocols are different, the available polling techniques and the limitations when considering expansion oradditions in hardware ports and communication medium bandwidth. The required indications from the load break switch are whether it is opened or closed (through the use of a dry "C" contact N.O. and N.C. contacts), close or open the switch (through contacts in the closing coil and trip coil of the switch). To monitor the transformer and control the fans, analog and digital inputs and outouts would be required. Examples for such points are: the change in the step of the load tap changer and the indication of such steps, the temperature of the oil and the remote indication of windings thermometers (on the transformer tank), the indication of the fans running (one or two banks), the gas relay contacts (energized or denergized), the oil level indication, forcing the fans to run remotely (or to stop). The main medium voltage circuit breaker should be closed and opened from the conrol station (through contacts in the close & trip coils of the C.B.), the indication of the main contacts position within the breaker i.e. closed or opened, the indication of the C.B. in its cubicle (ie. in the connect, disconnect or test positions). The feeder breakers would have the same states/positions/conditions monitored as the main breakers. The power measurements and protection should also be indicated on the terminals in the control room. Examples for metered analog quantities and digital control are: currents (demand and instantaneous in the the three phases), the real and reactive power, the voltage, the relays operation and if available the faulted current level, the reclosing scheme status, whether enabled or disabled, the programming of the digital relays when it comes to setting the pickup levels/curve shape/time delays, resetting the relays, monitoring the battery system, all the alarms (for levels that exceed or decrease below the set limits) eg. overloads, undervoltage etc. For the distribution system, the degree of complexity and the functions of operation/monitoring would be based on the philosophy to be adopted. The general applications will be given hereafter, the strategic switches can be remotely motor operated (thus 2 digital inputs to indicate position and 2 digital outputs to open and close the switch are required), sensors to indicate at the control station the voltage and current values, thus for each location monitored, 2 (per phase or for 3 phases) analog inputs would be required to monitor each phase in sequence and report the levels, if capacitors are present on the distribution system, the level of the reactive power/voltage should be available at the control stations complete with means to operate and indicate the position of the capacitor associated switches, should the system have faulted circuit indicators, remote indication of the status of the F.C.I. is required (through a digital input), for motor operated pad mounted switchgear remote control and indication ofthe different switches (taps) is required through the use of digital
inputs/outputs, loading of transformers on the system can be indicated through the SCADA system if sufficient communication channels and hardware points are available. With underground systems, the most probable communication system will be the Radio Frequency as hard communication wires may not cover the full subdivision. Other options may be (depending on availability) fiberoptics, copper wire drops or powerline carriers. The distribution automation and control functions can be classified into load management functions, real time operational management functions and remote metering functions. The load management functions may include: discretionary load switching, peak load pricing, load shedding and cold load pick up. The second set of functions may have: load reconfiguration, voltage regulation, transformer load management, feeder load management, capacitor control, dispersed storage and generation, fault detection/location/isolation, load studies, condition or state monitoring and remote connect/disconnect of services. •Discretionary load switching or customer load management activity; this activity is appropriate to loads like water heating, air conditioning and thermal storage heating. It involves direct control of such loads (at individual customer sites), from remote control central location. The purpose of such action would be to reduce the load on a particular substation or feeder if it becomes overloaded (and such loads are part of the connected load to such station or feeder). •Load (peak) pricing activity; this activity would allow the implementation of peak load pricing programs through the remote switching of meter registers. •Load shedding activity; it permits the rapid dropping of large blocks of connected load under certain predefined conditions, according to an established priority basis. •Cold load pickup activity; it entails the controlled sequential pick up of the previously dropped loads. •Load reconfiguration activity: it involves the remote control of switches and breakers to permit routine daily, weekly or seasonal reconfiguration of feeders/feeder sections. The purposes of such actions are to assist in performing the routine maintenance, taking advantage of load diversity among feeders, to reduce losses or serve more loads. •Voltage regulation activity: it allows the remote control of selected voltage regulators (if present on the system), together with distributed capacitors to effect coordinated system wide voltage control from a central location. •Transformer load management activity: this function enables the monitoring and continuous reporting of transformer loading data and core temperature, in order to prevent overloads, burnouts or long time abnormal operation. Reconfiguring the network loading can assist in achieving longer life from distribution transformers and can improve outages scheduling (to replace defective components). •Feeder load management activity: it allows the reporting of feeder loading and sections thereof. The purpose of this activity is to equalize load distribution over several feeders thus reducing unnecessary line losses. Fault detection, location and isolation: sensors located throughout the distribution network can be used to detect and report abnormal conditions, locating and isolation of faults can be performed based on this data. Thus proper sectionalization and circuit reconfiguration can be performed efficiently.
•Conditions and state monitoring: this function involves realtime data gathering and status reporting. Examples are: status of load break/disconnect switches and recloseures, MW and MVAR of monitored circuits or loading on distribution stations. •Remote service connect/disconnect services: it permits remote control of the switches to connect or disconnect an individual customer's electric service from a remote site. •Automatic customer meter reading: the four main elements: the transmitter, the receiver, the communicating media and the power supply. The communicating media can be line of sight (RF systems), fibre optic, dial up or dedicated phone lines, power line with associated wave traps. The purposes of such function are more accurate data gathering regarding a service or a customer consumption of electrical power, reduction in the cost of reading/maintaining the meters over the traditional methods. Information obtained from SCADA system: The major information obtained from a SCADA in a power distribution system are: indications (eg. state change like opening or closing of circuit breakers, load break switches, reclosures, disconnects, operation of a relay or fault indicators) of events or alarms, levels (eg. oil level, tap changer position, reading from pressure gauges), pulses (eg. energy meter counters), measurands (eg. current, voltage, power reading, temperature of oil or windings, leakage current). Fig. 6 shows a typica SCADA and fig. 7 shows a single line for an automated system.
The basic modules of a PLC system are: the processor, the input/output (they can further be classified into digital and analog), process control (proportional/integral/derivative), stepper motor, interface modules (they can be further classified into: local and remote, local and remote transfer, network, network transfer, multimedia network interface, peripheral devices (they include loader/monitor, process control stations, CRT programmers, hand held programmers, tape loader). Fig. 8 shows the front plates of the local and remote interface units, fig. 9 shows a typical block diagram including the racks, interface/ input/ output/ network interface modules.
Geographic information systems: An introduction to GIS: GIS is a computer tool that allows the user to position, analyze and verify objects & events of geographic nature and produce an output in a geographic form (maps & tables). It is a software that links information about where things are with information about what things are like. The components that build the (digital) maps are distributed on different layers. The user enter different geographic features on different layers (this characteristic is similar to CAD software packages). When the user retrieves the map, to be displayed in front of him/her on the screen, he/she decides which layers to be shown based on the information required. Thus certain layers (information) can be suppressed if they are irrelevant to the task at hand. A digital map created by GIS will have points (dots) that represent features on a map like cities, polygons (small areas) that represent features such as lakes and arc (lines) that represent features like roads. The GIS software can be considered a package made up of sub packages that can communicate and understand each other. It can access data directly from other software packages like geographic data & shapes, CAD drawings, databases, images or can import & export data from/to other programs. These integrated characteristics provide the full functionality of a GIS which includes: statistical analysis & research (similar to commonly used database software packages), entering, storing, manipulation & analysis of data in geographic style (functionality for a mapping or geographic information system) and displaying output results or producing of documents for presentation purposes in tables and/or map forms. The output results are laid out in geographic format that provides a much clearer and easy to understand presentation. These features of the GIS distinguishes it from other information technology systems and makes it a tool of great value to public as well as private users when it comes to presenting well explained, informative documents, to providing the required results of an analysis and showing the assumptions upon which the results are based, to forecasting the outcome based on the available factors & variables or in preparing strategic plans. The preparation of maps and reports based on geographic analysis are not new activities, but with the GIS the outcome is produced faster, consistent & accurate. Before, only few persons had the knowledge and were able to access these geographic data in order to make decisions or solve problems. Now, the GISs are taught in secondary schools and post secondary institutes world wide. The industry is in the millions of dollars and employs hundreds of thousands workers. Components of a GIS: There are 5 main components that build a GIS, they are: hardware, software, data, users & methods. How does a GIS work?: The system stores the geographic features (information) among different layers. Such features will be displayed on the screen as geometric shapes, points, lines or areas. For example a layer may have points that represent the cities in a region (or all the wells in an area), another layer of lines may represent all the streets in an area ( or watercourse in a region), a layer of areas can represent construction areas or similar use areas in a region. Each of the geographic features
will have its own set of attributes (characteristics of such feature) that are described by numbers, characters, images and CAD drawings (typically stored in tabular format and linked to the feature by a user defined identifier). For instance a well might include depth and gallons per minute as its descriptive element. Each geographic feature will also have its exact geographic position expressed as coordinates, eg. Cartesian planar (x,y), 3dimension (x,y,z), vector, which is stored with its attributes. Though the system is built on simple idea but it is powerful & versatile. It solves a lot of real life problems from setting the route of a delivery truck, to deciding on the best way for emergency convoys to take, to storing all the important details of a municipality or a city like the location of Police & Fire stations, hospitals, parks, overhead & underground distribution systems, electrical service entrance points, meters locations, water pipes routes, sewage & storm sewage paths & locations, telephone lines, electrical power stations location & details, telephone end offices (local central offices), toll offices (tandem offices) & intermediate switching offices, environmental data and weather patterns. Tasks of a GIS? The general objective of a GIS is to essentially fulfill the following 5 tasks: data entering & saving, data manipulation, management of system, research & analysis on the data entered & acquired and displaying & viewing the sought information. Numerical examples: For an overhead conductor with size = 556.5 MCM, aluminum core with 19 strands and operating voltage of 25 KV, calculate the reactance at 60 c/s and 50HZ per 1000 ft, the resistance (AL1350) per 1000 ft, the capacitance, charging current/1000ft and surge impedance. Given: Conductor details: 556.5 MCM/19 strands/Al 1359. Nominal voltage: 25 KV, frequency: 60 & 50 c/s, length 1000 ft, conductor diameter: .855" (from tables), area: .437 inch2 (from tables or 7.854(1000)(556.5)/(10+7) = .437 in.2) L = (2)/(10+7)[ln(d/r')] H/m = (2/10+7)ln 12(2)/.855(.7788) (at 1 ft spacing)
L= 7.17(1000)/3.28(10+6) = 2185/10000000 H/1000ft. XL = 2 pi 60 L = .082 OHM @ 60 c/s and XL = .068 OHM @ 50 c/s , where pi=3.141592654 R = .0927 microOHM ft (1000)/.437/(12)(12) = .0305 ignoring skin effect, taking into account approximate skin effect then R = .0305(1.15) = .035 OHM C = 1/ (10+9)(18)(ln d/r)F/m = 1/ (10+9)(18)(ln 12/.855/2) = .0166/10+9 C = .0166(1000)/3.28(10+9) F/1000ft = 5.08(109) farad/1000ft XC = 1/2 pi f C = 5.22 (10+5) OHM,
charging current = 25000/1.732(5.2)(10+5) = .028 amp/1000ft
Z = surge impedance under these conditions = (L/C).5)= 207 OHM For an underground XLPE cable, size = 250 MCM, aluminum 1350 core with 37 strands and operating on 25 KV system, calculate the reactance at 60 & 50 HZ per 1000 ft, resistance per 1000 ft, capacitance, surge impedance, charging current/1000 ft, speed of propagation of the wave and
the insulation resistance. Given: cable data: 250 MCM, XLPE, AL1350, 37 strands, 25 KV, 1000 ft length, conductor dia.: .575 in, area = .196 in2), dia. over insulation: 1.16 ", e = 3.5, where e is the dielectric constant L = (2/10+7)[ln 12(2)/(.575)(.7788) = 2427/(10+7) H/1000 ft at 1 ft spacing XL = .092 OHM @ 60 c/s and .076 OHM @ 50 c/s R/1000 ft = .0927(1000)(144)/.196 = .068 OHM (1.15) = .078 OHM
C = 3.5/)10+9)(18)(ln 1.16/.575) = .277/(10 )F/m(1000)/3.28 = 84/(10+9) F/1000ft L at insulation neutral or sheath = 2 ln 1.16/.575(.7788)/(10+7) = 2 (107) H/m
Z = surge impedance = (L/C).5) = 27 OHM, XC = 1/2 pi C 60 = 31578 OHM/1000ft Charging current for 1000 ft cable length =(25000/1.732)(31578) = .46 amp.
v = speed of propagation = (1)/(LC).5) = 1.34(10+8) m/sec. Volumetric insulation resistance = (ra) (ln 1.16/.575)/(2pl)(12)(2.54), where ra is the resistivity or specific resistance of the dielectric assume ra = 6(10+14), Rvolumetric = 6(10+14)(.7)/191511 = 2193 MegaOHM/1000 ft.
If the overhead conductor and the underground cable of problems 24 & 25 are connected in series and a voltage wave of 25 KV is travelling through the overhead portion, calculate the reflected and refracted powers at the junction point. Zline = 207, Zcable = 27, KV = 25 reflacted voltage = (27207)25/1.732(207+27) = 11.1 KV refracted voltage = (2)(27)(25)/1.732(207+27) = 3.33 KV refracted power = 3 (Vph)(Vph)/Zcable = 3(3.33)(3.33)/27 = 1232 KW reflected power = 3(11.1)(11.1)/207 = 1785 KW Provide the differential and gas accumulation/sudden release protection to a 100 MVA power transformer, 220/25KV with +/16% tap changer. Assume that the available relays have a pickup setting between 2050% of relay rating with an adjustable slope of 2050% and another with fixed slope and restrained pickup between 20 and 50% and unrestrained pickup of 8, 13, 20 x relay nominal current. The pressure gas relays have two settings, for the trip 5.2 17.2 KPa and for the alarm 200400 CC. The tap changer gas pressure trip can be set between 35390 KPa. I excitation = 5% of full load primary, C.T. error = 2.5%, relay rating = 5A, C.T. primary current = 1.5 x full load current of transformer. I primary = 100 (1000)/(220)(1.732) = 262 A I secondary = 262 x 220/25 = 2300 A Using a 1200/800/200:5A C.T. on the primary side (the C.Ts are delta connected), 3500 : 5A C.T. on secondary side (the C.Ts are wye connected) of the power transformer. Turns ratio of C.T. on primary winding = 800/5 = 160, on secondary = 3500/5 = 700. Relay current due to primary C.T. at f.l. = (262/160) 1.732 = 2.8 amp, relay current due to secondary C.T. at f.l. = 2300/700 = 3.28 amp, relay current ratio = 3.28/2.8 = 1.17. Mismatch at midpoint changer and full load = 17%.
At 220 + 16% = 255KV tap, maintaining secondary voltage at 25KV, primary full load current = 100 (1000)/255 (1.732) = 226 amp, relay current = 226/160(1.732) = 2.45amp.,voltage of 25 KV on the secondary while primary = 220 16 % = 185 KV primary full load current = 100(1000)/185(1.732) = 312 amp., relay current = 312/160 (1.732) = 3.4 A Mismatch for + 16% = 3.28/2.45, mismatch for 16% = 3.28/3.4 which are 34 % and 4 %, respectively. The maximum mismatch = 34 %, add 6 % as safety margin. Thus the slope adjustment = 40 % (range is 20 to 50 %). The pickup level under full load current = inaccuracies + esciting current + allowance for the limited restraint at emergency load through currents = 2.5(5/100) + (1.732)(5(262)/160)(100) + (3.282.45) = .125 + .14 + .83 = 1.1 amp, the pickup setting = 40%(5) = 2 A (range 20 to 50 %). The unrestrained instantaneous triping current = 13 x 5 = 65 amp. secondary relay current. VERIFICATIONS: •
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Assuming a 100 MVAbase and 220/25 KVbase, Ibase (@ primary side) = 100/1.732)(220) = 262 amp., Ibase (@ secondary side) = 2300 amp. Assuming an infinite source, 11% impedance transformer, a 3phase fault on the secondary of the transformer beyond the differential protection zone will produce 9 p.u. fault current (1/.11), Iprimary = 2358 amp., Isec. = 20700 amp. The current from the primary side into the relay = (1.732)2358/160 25.5 amp., from the secondary side = 20700//700 = 29.6 amp. A mismatch of 29.6/25.5 = 16 %. Assuming the tap changer to be at 220KV + 16% = 255 KV and the impedance = 14%, the short circuit fault current of a 3phase fault = 1/.14 = 7.16 p.u., full load primary current = 232 amp., SCC on the primary side = 1661 A, SCC on the secondary side = 16468 A, relay current from primary side = 1661(1.732)/160 = 18 A, relay current from the secondary side = 16468/700 = 23.5, the mismatch = 30%. Assuming the tap changer at 220 KV 16% = 185 KV and the impedance = 8%, a 3phase fault current = 1/.08 = 12.5 p.u., primary current = 312 A, SCC on primary side = 3900 A, SCC on secondary side = 28750 A, relay current from prim. side = 42.2 A, relay current from sec. side = 41.1 A, mismatch = 3%
GAS RELAYS: Main tank alarm = 200 cc, main tank trip = 17 KPa above static head at relay level, tap changer trip = 100 KPa. SUMMARY: In this chapter, the defining parameters, classifications, tests and typical configurations of distribution systems components were presented. The general properties of medium voltage EPR & XLPE cables were given plus factory & site tests. For low voltage secondary cables, the defining parameters were listed. For transformers, a broad classification was given. The defining parameters and the CSA standards that govern the ratings, design, manufacturing and testing of distribution transformers were presented. Other components found in overhead and underground distribution systems were covered
from their types and defining parameters point of view. These components are: lightning arresters, conductors, terminations, splices, connectors, elbows and insulators. Distribution systems automation and SCADA were presented by covering the functions and data collected from such systems. Typical systems were given to clarify this topic. The major modules found in a typical PLC (programmable logic controllers) installation were presented. The numerical examples at the end of this chapter showed how the line and cable constants (inductance, capacitance, resistance, inductive/capacitive reactances, surge impedance, charging current and propagation speed) and the effect of the surge impedance on travelling waves are calculated. They also demonstrated a method to select/adjust/verify the settings of relays (differential & gas) used in the protection of power transformers. REFERENCES: 1. 2. 3. 4. 5. 6. 7.
Wildi, T "Electrotechnique", Les Presses de l'Université Laval. CSA, "Canadian Electricity Code", Part 1, std C22.1. CSA, "Single phase & three phase distribution transformers", std C2. Gonen "Electric power distribution systems engineering", McGraw Hill. Kurtz, "The lineman's & cableman's Handbook", McGraw Hill. Perry, "Chemical Engineers handbook", McGraw Hill. ICEA S66524, "Cross linked thermosetting polyethylene insulated wire & cable for the transmission & distribution of electrical energy". 8. ASTM 2.03, "Nonferrous metal products electrical conductors. 9. Brady, "Materials handbook", Mcgraw Hill. 10.Kheir, "Computer Programming for Power System Analysts", Kheir.