AC Systems, Switchgear, and Safety PDF

3rd Class Edition 3 • Part A2 AC Systems, Switchgear, and Safety LEARNING OUTCOME When you complete this chapter, you should be able to: Identify the components of typical AC systems and switchgear, and discuss safety around electrical systems and equipment. LEARNING OBJECTIVES Here /s what you...

3rd Class Edition 3 • Part A2 AC Systems, Switchgear, and Safety LEARNING OUTCOME When you complete this chapter, you should be able to: Identify the components of typical AC systems and switchgear, and discuss safety around electrical systems and equipment. LEARNING OBJECTIVES Here /s what you should be able to do when you complete each objective: 1. Using a one-line electrical drawing, identify the layout of a typical industrial AC power system with multiple generators, and explain the interaction of the major components. 2. Explain the function of the typical gauges, meters, and switches on an AC generator panel. 3. Explain the purpose and function of the circuit protective and switching equipment associated with an AC generator: fuses, safety switches, circuit breakers, circuit protection relays, and automatic bus switchover. 4. Explain the components and operation of a typical uninterruptible power supply (UPS) system. 5. Explain safety procedures and precautions that must be exercised when working around and operating electrical system components. Explain grounding. ^'f^sm. •w. ^-^1 479 AC Systems, Switchgear, and Safety • Chapter 9 ^ INTRODUCTION TO AC SYSTEMS, SWITCHGEAR, AND SAFETY An AC power distribution system has many electrical components that make it functional, economical, and safe to operate. These components consist of items such as the generator, breakers, disconnects, transformers, high-voltage distribution lines and networks, and switchyards and substations. Each of these components has safety devices and operational logic incorporated into its design to ensure safe and reliable operations of the system. Electrical energy is transmitted from the generators to the many points of use. The system must be designed to provide an uninterruptable power supply that meets the needs and requirements of all users, from the largest to the smallest, safely and economically. Primary voltage, which is the voltage that is applied to the primary side of the transformer, can range from 2300 to 39 000 volts. Common primary line voltages are 2300, 4160, 12 470, 13 800, 25 000, and 34 500 volts depending on which distribution voltages a utility is designed to use. High-voltage lines can transmit electricity at 138 kV, 240 kV, or 500 kV. This allows for electricity to travel great distances through the high distribution grid at reduced costs with less line losses. This stepped-up electricity is eventually stepped down as secondary line voltages to various end users. Common secondary line voltages are 120,208,240, 277, and 480 volts. A regulatory body assigns an independent body, the electric system operator, to manage and monitor the power distribution system constantly. This body is responsible for the planning and operations of the interconnected electric system. The independent body ensures the power grid is operated in a safe, reliable, and economical manner. Within the interconnected system, the plant operator controls the operation of an individual generator. However, the electric system operator issues directives to the individual power producers to control parameters such as system loading, MVAR adjustments, and frequency to maintain a balanced system. 3rd Class Edition 3 • Part A2 481 ^ Chapter 9 • AC Systems, Switchgear, and Safety OBJECTIVE 1 Using a one-line electrical drawing, identify the layout of a typical industrial AC power system with multiple generators, and explain the interaction of the major components. INDUSTRIAL AC POWER SYSTEM The main function of a power distribution system is to provide consistent, reliable electrical power to various users at their required voltages in a safe and dependable manner. This is accomplished using a variety of electrical equipment such as generators, transformers, switchgear, breakers, and motor control centres (MCCs). Normal Utility Power Supply Systems To ensure a reliable source of power, most plants use a dual supply system, which uses two independent lines (buses) to supply power to the plant loads. Figure 1 shows a typical industrial system, which is described as follows: • The power is supplied from a power plant with two, separate on-site electrical generators (generators 1 and 2), which produce power at 13.8 kV. There is also an alternate external supply (i.e. backup power source), being supplied at 138 kV, which is transformed down to 13.8 kV. This is known as a station services transformer and is capable of supplying power to the plant when it is not online. • The power is supplied to the users through two individual power lines, or buses. These are identified as "A-bus" and B-bus . • The 15 kV main feeder breakers, identified as 11-01 and 11-02, supply the 13.8 kV/4.16 kV-10 MVA power step-down power transformers, PTR-101 and PTR-102. Here, the voltage is reduced from 13.8 kV to 4.16 kV. • This 4.16 kV power is then fed to the 5 kV incoming switchgear breakers, 11-201 and 11-202. These two breakers, together with the tie breaker, 11-203, comprise the first level of automatic power transfer. • Under normal operation, both incoming breakers, 11-201 and 11-202, are closed and the tie breaker is open. If one of the incoming breakers loses its power supply, that breaker will open and the tie breaker will then close, re-establishing power to the bus through the other breaker. • The power from the 4.16 kV transformer then feeds the 4.16 kV feeder breakers. Power of 4.16 kV is also supplied to JV[CC#1 to provide power for the 4160 V boiler feed and cooling water pumps. • Feeder breakers, 11-211 and 11-212, supply the 3 MVA, 4160/480 V power transformers, PTR-211andPTR-212. • Feeder breakers, 11-221 and 11-222, supply the 2 MVA, 4160/480 V power transformers, PTR-221 and PTR-222, which, in turn, supply substation #1. • Two other feeder breakers, 11-231 and 11-232, also receive power from the main 4.16 kV supply. These two 5 kV switchgear breakers and the tie breaker, 11-233, make up the second level of automatic power transfer. This system supplies power to critical 4.16 kV motor drivers, which receive their power from the 4.16 kV emergency MCC. 482 3rd Class Edition 3 • Part A2 AC Systems, Switchgear, and Safety • Chapter 9 ^ Figure 2 is a continuation of Figure 1: • 4.16 kV power from PTR-211 and PTR-212 supplies feeder breakers, 11-301 and 11-302. • These feeder breakers, together with the tie breaker 11-303, make up the third level of automatic power transfer. This system provides power to the many 480 V users. Figure 1 - Circuit Diagram of High-Voltage Power Supply Alternate external supply Generator breaker Generator breaker Generator Generator 15kV Feeder breakers 11-02 11-01 Power transformers 13.8/4.16 kV T PTR-101^ 5kV Switchgear breakers 11-201 Tie breaker 4.16 kV BUS MCC SWGR T 11-203 Feeder ^ ^ ^ breakers MCC #1 oa: Boiler Cooling | feed water 11-221V 'Vll-211 111-212 4160/480V Transformers 11-232, Uy w pump pump [^^i r 4.16 kV BUS MCCSWGR -^-D->>- 2MVA ^ UUUu 3MVA ^^ PTR-221 PTR-211 -<<HD-> 4.16kV Emergency MCC 2MVA 5kV Motor To substation To Automatic transfer switches nm PTR-212 PTR-222 #1 #1 (900 Kw) 11-222 y^y 13MVA substation Natural gas turbine generator 5 ^ t See Fig 2 for Continuation 3rd Class Edition 3 • Part A2 483 ^ Chapter 9 • AC Systems, Switchgear, and Safety Figure 2 - Medium- and Low-voltage Power Supply ^ ^ Feeder breakers 11-302 11-301 11-303 480 V BUS -^3-^ 480 V BUS M.C.C.L2J 480V Users 11-312 Distribution panel 208V 120V Users Users T—ATS-3 Emergency generator 250kW .Automatic r •JK._^ transfer _ys-4 switches 480V Emergency M.C.C. Emergency Power Supply Systems There are two emergency power supplies within this system. One, shown in Figure 1, consists of a 900 kW natural gas turbine generator, two automatic transfer switches, and one 4.16 kV motor control centre. The other, shown in Figure 2, consists of a 250 kW diesel generator, two automatic transfer switches, and a 480 V motor control centre. The purpose and the method of operation of these systems are described below. 4.16 kV Emergency MCC If the incoming 4.16 kV power is lost, critical power users are supplied with 4.16 kV emergency power from the 900 kW natural gas turbine generator. Referring to Figure 1, the automatic transfer switch, ATS-1, can be fed with 4.16 kV power from either of the feeder breakers, 11-231 or 11 -232. If both of these feeder breakers suffer a loss of incoming power, the emergency supply system will sense a loss of this voltage and will then start the natural gas turbine generator. The automatic transfer switch, ATS-2, will switch over to this generator in order to supply the emergency MCC. 480 V Emergency MCC If the incoming utility power is lost, critical users are supplied with 480 V emergency power from the 250 kW diesel generator. The operation of the automatic transfer switches, ATS-3 and ATS-4, is the same as the automatic transfer switches described above for the 4.16 kV emergency MCC system. 484 3rd Class Edition 3 • Part A2 AC Systems, Switchgear, and Safety • Chapter 9 OBJECTIVE 2 Explain the function of the typical gauges, meters, and switches on an AC generator panel. AC GENERATOR PANEL A generator control panel is the interface which allows a Power Engineer or operator to control the generator as well as check system diagnostics, overall performance, and current status of the generator. Various control parameters are displayed including such parameters as voltage, current, and frequency. Indicators and Controls The following is a description of the panel shown in Figure 3 which serves a steam turbine coupled to an AC generator. The generator is rated at 250 kW, 600 VAC, and 60 Hz. 1. Kilowatt hour meter: This is a meter that measures and indicates the total, accumulated power delivered by the generator over time. 2. Exciter field voltage: This shows the DC voltage that is being supplied to the generators field windings. 3. Exciter field current: This shows the DC current that is being supplied to the generators field windings. 4. AC kilowatt meter: This shows the AC kilowatts that the generator is producing. 5. Phases A, B, and C: These show the AC current (expressed in amps or amperes) produced by the three-phases of the generator. 6. Voltage adjustment: This adjusts the generator excitation voltage. 7. Frequency meter: This meter shows the AC frequency produced by the generator, in hertz. 8. Power factor meter: This meter is used to check the power factor of the generation system. 9. AC volt meter: This meter indicates the AC voltage being produced by the three-phase generator. 10. KiloVAR meter: This meter indicates the reactive power being generated by the AC generation system. 11. Volt meter selector: This knob is used to verify the generated phase and line voltages. 3rd Class Edition 3 • Part A2 485 ?& Chapter 9 • AC Systems, Switchgear, and Safety Figure 3 - AC Generator Control Parrel 3rd Class Edition 3 • Part A2 AC Systems, Switchgear, and Safety • Chapter 9 fS OBJECTIVE 3 Explain the purpose and function of the circuit protective and switching equipment associated with an AC generator: fuses, safety switches, circuit breakers, circuit protection relays, and automatic bus switchover. CIRCUIT PROTECTIVE AND SWITCHING EQUIPMENT The protection of generators involves the consideration of more possible abnormal operating conditions than the protection of any other system element. In unattended power stations, automatic protection against all harmful abnormal conditions should be provided. Fuses The simplest form of automatic over-current protection is the fuse. It contains a conductive fusible link connected in series with the electric circuit. The fusible link is directly heated by the passage of the overload current. The link is sized so that the heat created by the normal flow of current through it is not sufficient to melt the link. Plug fuses are used on circuits rated 125 volts or less, to ground. The maximum continuous current carrying capacity of plug fuses is 30 A, and the commonly used standard sizes are 10, 15,20, 25, and 30 A. These fuses do not have published interrupting capacities since they are ordinarily used on circuits that have relatively low values of available short circuit current. Cartridge fuses are used on circuits with voltage ratings up to 600 volts; the standard voltage ratings of these fuses are 250 and 600 volts. The non-renewable cartridge fuse is constructed with a zinc or alloy fusible element enclosed in a cylindrical fibre tube. The ends of the fusible element are attached to metallic contact pieces at the ends of the tube, which is filled with an insulating porous powder. On overloads or short circuits, the fusible element is heated to a high temperature, causing it to vaporize. The powder in the fuse cartridge cools and condenses the vapour and quenches the arc, thereby interrupting the flow of current. Figure 4 shows types of plug and cartridge fuses. Figure 4 - Types of Plug and Cartridge Fuses (Kup 1984 and Marcel Derweduwen/Shutterstock) 3rd Class Edition 3 • Part A2 487 r®- Chapter 9 • AC Systems, Switchgear, and Safety Cartridge fuses, both in the 250 and 600 V ratings, are made to fit standardized fuse clip sizes. These sizes are the 30, 60,100,200,400, and 600 A sizes. Each fuse clip size has several continuous ratings of 70, 80, 90, and 100 A. Time delay fuses are made in both the plug and cartridge types. These fuses are constructed so as to have a much greater time lag than ordinary fuses, especially for overload currents. They do operate, however, to clear short circuit currents in about the same time as standard fuses. (B Side Track The line side of a fuse is the side where power is coming from and going into the fuse/ while the load side of a fuse is the side that the power is going to be used. When replacing a cartridge fuse, the new fuse must be the same type as the old one and have the same amperage and voltage ratings. Installation of the wrong fuse can pose serious risk for fire or electrical shock. Time delay fuses have two parts: a thermal cut-out part and a fuse link. The thermal cut-out, with its long-time lag, operates on overload currents up to about 500 percent of normal current. Currents above this value are interrupted by the fuse link. The greatest application of time delay fuses is in motor circuits where it is desirable for the fuse to provide protection for the circuit, and yet, not operate during the starting period of a motor when the current may be only momentarily high. High-voltage fuses are used for the protection of circuits and equipment with voltage ratings above 600 V. Two of the commonly used fuses are shown in Figure 5. Figure 5(a) shows an expulsion fuse. It consists of a fusible element mounted in a fuse tube. When the metal fusible link melts, it builds up pressure in the fibre tube. The gas pressure blows the gases out of the open end of the tube. This takes with it the bottom section of the fuse link, and establishes a gap between the two contacts. Figure 5(b) shows a liquid filled fuse in which the arc is quenched by the liquid. The action is similar to that in an oil immersed switch. A spring is normally held in tension by a high resistance tension wire. This wire is paralleled by the fuse wire that carries the current. When high current melts the fuse wire, the tension wire immediately melts and releases the spring, which then contracts and pulls the contacts apart. Figure 5 - High-Voltage Fuses a Upper ,^7 Fuse wire Strain wire contact Upper contact Reduced^ section Flexible Coil spring cable Glass tube Ision fuse '(a') 488 Lower contact Liquid-filled fuse 3rd Class Edition 3 - Part A2 (b)" AC Systems, Switchgear, and Safety • Chapter 9 ^ Safety Switches A switch is a device for isolating parts of an electric circuit or for changing connections in a circuit or system. When a switch is mounted in a metal enclosure and is operable by means of an external handle, it is called a safety switch. The switch itself is not designed for interrupting the flow of short circuit currents. However, switches and fuses are often incorporated into a single device called a fusible safety switch, as shown in Figure 6. Safety switches are made in two, three, four, or five pole assemblies, either fusible or non-fusible. Safety switches are made in single throw and double throw units. Depending upon the use, safety switches have a variety of design features. One type, known as type A, has a quick make, quick break mechanism. A spring-loaded arrangement causes the contacts to open or close with a quick motion, regardless of the speed at which the operating handle is moved. This type of switch also has a door interlock to prevent the opening of the enclosure door when the switch is closed. Enclosed switches, either fusible or non-fusible, are used as disconnecting devices for main services into buildings, for feeder and branch circuit protective and switching devices, and for motor protection and switching. Safety switches are available in two voltage ratings: 240 and 600 volts alternating current and with ratings of 30 to 1200 amperes. Current ratings are the same as for standard fuse clip sizes. Figure 6 - Fusible Safety Switch • < 3rd Class Edition 3 • Part A2 489 r5> Chapter 9 • AC Systems, Switchgear, and Safety Circuit Breakers A circuit breaker is an automatic device that opens under abnormally high current conditions. In three phase systems, circuit breakers may open all three phases when an overload occurs. A circuit breaker is designed to automatically open when the current exceeds the rating of the breaker. Most circuit breakers employ either a thermal or a magnetic tripping element. Circuit breakers may also be activated by remote control relays. Relay systems may cause circuit breakers to open due to changes in frequency, voltage, or current. In most cases, the circuit breakers must be reset manually. The internal construction of a circuit breaker is shown in Figure 7. Figure 7 - Circuit Breaker Terminal Terminal \ Arc Contacts supression (Dmytro Balkhovitin/Shutterstock) Protective Relays There are several relay devices that are intended to protect the generator and system from undesirable conditions or events. The following are typical conditions that are protected by relays. Loss of Excitation (Loss of Field) This device detects the loss of field excitation on the generator. Modern alternators consist of a stator on which the alternating current voltage producing windings are installed. It also contains a rotating armature, or rotor, on which a direct current excitation winding is placed. When a synchronous generator loses excitation, the rotor accelerates and it operates as an induction generator, running at higher than synchronous speed. As a result, the machine draws inductive reactive power from the system instead of supplying it to the system. This is in reverse to the normal operations of the machine and its operating design. Heavy currents can be induced in portions of the rotor and can cause thermal damage to the machine if it continues to operate. Wound rotor generators are not suited to such operation because they do not have windings that can carry the induced rotor currents. 490 3rd Class Edition 3 • Part A2 AC Systems, Switchgear, and Safety • Chapter 9 Over Excitation When the ratio of the voltage to frequency (volts:Hz) exceeds a set value for a given generator, severe overheating could occur due to saturation of the magnetic core of the generator and the subsequent inducement of stray flux in components not designed to carry flux. Such over excitation most often occurs during startup or shutdown while the unit is operating at reduced frequency, or during a complete load rejection, which leaves transmission lines connected to the generating station. Failure in the excitation system can also cause over excitation. A voltage and frequency relay, with an inverse time characteristic that matches the capabilities of the protected equipment and with definite time setpoints, is used to protect the generator from over excitation. In this scenario, the faster and the more unproportionate the ratio of the voltage to frequency occurs above certain set values, the faster the operation of the relay will be and the less time before it reacts. Loss of Synchronism When two areas of power systems, or two interconnecting systems, lose synchronism, there will be large variations in voltages and currents throughout the systems. When the systems are in phase, the voltages will be maximum and the currents minimum. When the systems are 180 degrees, out of phase, the voltages will be minimum and the currents maximum The resulting high peak currents and offfrequency operation may cause winding stresses, pulsating torques, and mechanical resonances that are potentially damaging to the turbine-generator. Therefore, to minimize the possibility of damage, the unit should be tripped without delay. Unbalanced Currents (Negative Phase Sequence) It is desired that generators and the distribution system operate in synchronization. When this occurs within a three single phase system, it is deemed to have a positive sequence configuration. If all the components of the system are in unison and balanced, the generator components will have the same vector rotation in the same direction as the power systems voltage and current components. If the generator phase currents are equal and the vectors are displaced equally by 120°, and are supplying a balanced load, only positive sequence components flow into the power system. A current or voltage unbalance between phases in magnitude or phase angle out of phase results in an unbalancing occurring within the system components. There are numerous conditions that can cause unbalanced currents in a generator. Some of these are: unbalanced system loading, open phases, line to ground faults, air gap between rotor and stator incorrect, and open circuits in one phase external to the generator. When this happens, the system is said to have a negative sequence. Negative sequence components have the same magnitude as positive sequence components but rotate in opposite direction to them in the power system. This reversed rotating stator current induces double frequency currents in rotor components. These high double frequency currents can generate high and possibly dangerous temperatures in a very short period of time, which can damage the insulation of the machine(s), causing them to fail prematurely. The negative phase sequence relay, shown in Figure 8, is provided to protect the unit before the specified limit for the machine is reached. 3rd Class Edition 3 • Part A2 491 ^ Chapter 9 • AC Systems, Switchgear, and Safety Figure 8 - Negative Phase Sequence Relay B Overvoltage Generator overvoltage may occur during a load rejection or excitation control failure. In the case of hydroelectric or gas turbine driven generators, upon load rejection, the generator may speed up and the voltage can reach high levels without necessarily exceeding the generator s V:Hz limit. The voltage regulating equipment often provides this protection. If it does not, it should be provided by an AC overvoltage relay. This relay should have a time delay unit and operate at about 110% of the rated voltage. It should also have an instantaneous unit that operates at about 130% to 150% of the rated voltage. It is not generally required with steam turbine driven generators. Damages can occur when voltage is higher than that for which the components are rated for. The components are designed to operate at a rated voltage that will carry a current just large enough for the component to operate safely and efficiently. The amount of current in a circuit depends on the voltage supplied. When the voltage is exceeded and an increase in current flow occurs, this results in overheating of the components. The components protective insulation breaks down often, which results in shortened life spans or even complete failure of the device in extreme situations. This can be from a sudden spike or occur over a period of time while the component is m operation. 492 3rd Class Edition 3 • Part A2 AC Systems, Switchgear, and Safety • Chapter 9 Undervoltage An undervoltage condition is a decrease in the rms AC voltage to less than 90% at the power frequency for a duration longer than 1 minute. The term brownout is often used to describe sustained periods ofundervoltage initiated by the utility to reduce power demand. Undervoltages result from events which are the reverse of those causing overvoltages. Unstable governor control, a capacitor bank switching off, or overloaded circuits can also cause under voltages. Ideally the automatic voltage regulator (AVR) will correctly respond to correct the undervoltage condition. Operating in undervoltage conditions can drastically reduce the life of the machine and lead to premature failures due to overheating of components. Reversal of Power For generators operating with another generator, it is imperative that the power direction be supervised. If the prime mover fails, the alternator operates as a motor and drives the prime mover. A relay detects the reversal of power direction and switches off the alternator. Power losses and damage to the prime mover are avoided. A reverse power relay can be activated when synchronizing as well, if a generator load is not established immediately after synchronization has occurred. Dead Generator If a dead generator is accidentally energized, while on turning gear, it will start and behave as an induction motor. During the time when the generator is accelerating, very high currents are induced in the rotor and it may be damaged very quickly. Protection is usually provided by three directional inverse time overcurrent relays, one per phase, and connected to operate for reverse power flow into the generator. Another form of protection is through interlocks. The generator breakers are typically interlocked with the generator disconnects so that, when in an offline state, both cannot be closed at the same time and provide a path for power to flow back to the generator. Over Frequency Faults in the system can result in a system breakup into islands, which leaves an imbalance between available generation and the load. This results in an excess of power for the connected loads. Excess power results in over frequency, with a possible overvoltage from reduced load demands. Full or partial load rejection can lead to overspeed of the generator; therefore, over frequency operation. In general, over frequency operation does not pose any serious overheating problem unless the rated power and about 105% voltage are exceeded. Control action can be taken to reduce the generator speed and frequency to normal, without tripping the generator. Generators can operate in two different modes that have a relationship to frequency: isochronous mode or droop mode. In isochronous mode, generators maintain a constant frequency. This mode is typically used when a generator stands alone, or what is commonly referred to as being islanded. In this scenario, the power producer supplies its own power to run the plant and is not interconnected to the power grid. Droop mode allows changes in frequency, and it allows multiple generators to work in tandem by dividing loads in proportion to their power. Therefore, droop mode is the preferred mode of operation for interconnected power producers and distribution systems. In isochronous mode, generators maintain a constant frequency, whereas droop mode allows for changes in frequency in response to changes in load. Operating in droop mode provides more stability of an interconnected power system. 3rd Class Edition 3 • Part A2 493 rS- Chapter 9 • AC Systems, Swi^jhgear, and Safety Under Frequency When insufficient power is being generated for the connected load, under frequency results with a heavy load demand. The drop in voltage causes the voltage regulator to increase the excitation, which results in overheating in both the stator and rotor. At the same time, more power is demanded with the generator less able to supply it at the reduced frequency. Prolonged operation of a generator at reduced frequencies can cause particular problems for gas or steam turbine generators, which are susceptible to damage from operations outside of their normal frequency band. At reduced frequencies, the turbine is more restrictive than the generator because of possible mechanical resonance in many stages of the turbine. If the generator speed is close to the natural frequency of any of the blades, there will be an increase in vibration, which can lead to cracking of the blade structure. While load shedding is the primary protection against generator overloading, under frequency relays should be provided to provide additional protection. ® Side Track Operating tip: On an interconnected generation grid, when the load exceeds the power supply, the frequency in the system will drop. Conversely, when supply exceeds the load, the frequency will increase. Frequency can be controlled by operating the speed governor or by load control. Typically, this is done through an automated control system, but operator intervention and manual control may be required. Frequency response is very important in maintaining a reliable and stable interconnected power supply during disturbances and in islanding situations. Operators should be aware of their frequency control resources and control concepts. Field-Ground Fault Although a single field-ground fault will not affect the operation of a generator or produce any immediate damaging effects, the first ground fault establishes a ground reference, thereby making a second ground fault more likely. This will increase the possibility to ground at other points in the field. A second ground fault will cause extensive damage by: • Shorting out parts of the field winding • Causing high unit vibrations • Causing rotor heating from unbalanced currents • Arc damage at the points of the fault A field ground relay is installed, which must reliably detect the first ground fault. This will allow action to be taken, either through the tripping of the unit or an operator alarm. This is to avoid continued field winding insulation deterioration that would cause a second ground fault and major damage. Stator Ground Fault A generator stator is grounded to redirect power from abnormal equipment conditions away from operators and the system. However, this means a solid path to ground is available if a problem, typically insulation failure, occurs in the stator. To limit ground fault current, impedance is added to the grounding path. A relay is connected to the impedance to measure fault current and shut off the generator as needed. 0 494 On Track Insulation failure is often a result of moisture or dust building up within electrical equipment. This is why it is important to keep electrical components clean and dry. 3rd Class Edition 3 • Part A2 AC Systems, Switchgear, and Safety • Chapter 9 ^ Stator Overheating Stator overheating is caused by overloading or by failure of the cooling system. Overheating because of short-circuited laminations is very localized and it is just a matter of chance whether it can be detected before serious damage is done. The practice is to embed resistance temperature-detector coils (RTDs) or thermocouples in the slots with the stator windings of generators larger than 500 to 1000 kVA. Figure 9 shows the bridge circuits employed with resistance temperature detectors (RTDs). Enough of these detectors are located at different places in the windings so that an indication can be obtained of the temperature conditions throughout the stator. Several of the detectors that give the highest temperature indication are selected for use with a temperature indicator or recorder, which usually have alarm contacts. The detector giving the highest indication may be arranged to operate a temperature relay to sound an alarm. Figure 9 - RTD Bridge Circuits A.C. voltage source -wswems^ Relay polarizing coil Series bridge resistor (ROC) Relay operating coil (FBR). Fixed bridge resistors (3) (RTD) Resistance temperature device Overspeed Overspeed protection is recommended for all prime mover driven generators. The overspeed element should be responsive to machine speed by a mechanical device, or equivalent electrical connection. If it is electrical, the overspeed element should not be adversely affected by generator voltage. The overspeed element may be furnished as part of the prime mover, its speed governor, or of the generator. It should operate the speed governor, or whatever other shutdown means is provided to shut down the prime mover. The overspeed element should also trip the generator circuit breaker. This is to prevent over frequency operation of the generator if it separates itself from the AC system. The overspeed element should be adjusted to operate about 3% to 5% above the full load rejection speed. Side Track Overspeed events can result in dangerous catastrophic failures that are costly in terms of life, lost production, and lost income. The only way to ensure the overspeed protection is functioning properly is through planned testing on a regular periodic basis. Generator startups and shutdowns are prime times to identify any faults in the overspeed (B protection system. 3rd Class Edition 3 • Part A2 495 ^ Chapter 9 • AC Systems, Switchgear, and Safety Phase Fault Protection Phase faults in a generator stator winding can cause thermal damage to insulation, windings, and the core, and mechanical shock to shafts and couplings. Residual flux within the machine can cause fault current to flow for many seconds after the generator is tripped and the field is disconnected. Primary protection for generator phase-phase faults is best provided by a differential relay. Differential relays will detect phase-phase faults, three phase faults, and double phase-to-ground faults. With low-impedance grounding of the generator, some single phase to ground faults can also be detected. Master Trip Relay There are several relays that can go into fault resulting in a generator trip. These relays are connected with a master trip relay (Figure 10). A master trip relay is designed to indicate a fault condition and to trip an associated main breaker when it receives a signal that one of the relays in its circuit has tripped. Master trip relays (also known as 86 relays or lock-out relays) can be configured to operate with one individual protection relay or a number of protective relays arranged in zones; refer to Figure 11. For example, all relays associated with tripping the generator breakers are linked to an individual master trip relay. Likewise, the transformer or distribution systems protective relays may all be linked to a separate master trip relay. If any of the protection relays activate in a protection zone, the master trip relay's output contacts will be operated. This results in the master trip relay being tripped, thus resulting in the tripping of its associated breaker. In short, when a protection relay is activated in a protection zone, it trips the master trip relay, which trips the breaker. The operator will then have to identify the fault back to the relay that initiated it, correct the fault, and reset the individual relay at its panel. Then, and only then, can the operator reset the master trip relay be manually turning the pistol grip to the reset position. If all faults are cleared, the master trip relay will latch into the reset position. Until this is done, the master trip relays associated breaker cannot be operated. This setup allows for all the protective relays in a zone to be located in one panel, with the need for only one output to be utilized and activated to send a trip signal to the master trip relay, and in turn to the breaker trip coils. (See Figures 10 and 11.) ® Side Track The American National Standards Institute (ANSI) assigns numbers to be used as the standard means of identifying, among other things/ protective devices. These numbers are used to identify the functions of devices shown on schematic drawings. The ANSI number for a master trip relay (lock-out relay) is 86. Often the master trip relay is known only as an 86. 496 3rd Class Edition 3 - Part A2 AC Systems, Switchgear, and Safety • Chapter 9 ^ Figure 10 - Master Lock-Out Relay (86) Figure 11 - Master Lock-Out Relay (86) Diagram Generator protective relays RL1 RL2 RL3 RL4 / / / Master lookout relay 86 86 To trip coil of generator breaker Automatic Bus Switchover The purpose of automatic bus switchover is to provide power to the users on a bus if the generator supply to that bus is lost (e.g. the generator trips off line). The switch automatically closes, allowing power to be fed onto the "dead" bus from an alternate source, including the startup of an emergency, standby generator. One type of automatic bus switchover unit, shown in Figure 12, operates in the following manner. 1. Normal Utility Power Mode Under normal circumstances, when utility power is available, the utility power runs through the transfer switch control contactors and the power is connected to the distribution panel and then to the electrical loads. A battery charger, installed in the transfer switch control, is powered by the utility to keep the starting battery in the generator set charged. 3rd Class Edition 3 • Part A2 497 ?& Chapter 9 • AC Systems, Switchgear, and Safety 2. Power Outage Occurs When the utility power voltage falls to less than 85% of its normal value, or it fails entirely, the standby power system will automatically go through a start sequence. The transfer switch control circuitry constantly monitors the power quality from both the utility source and the generator set. When the transfer switch control circuitry senses the unacceptable utility power, the control waits for three seconds and then sends a signal to start the generator set engine. If the utility power returns before the three seconds has passed, the generator set will not be signaled to start. When the start signal is received and, providing the manual/auto switch is set to auto, the engine starts, reaches operating speed, and AC power is now available at the generator set. The transfer switch control circuitry senses this, waits for the three seconds and then transfers the generator power to the transfer switch contactors. The sequence of operation usually occurs in less than 10 seconds from the time the power outage occurs to the time when emergency generator power is connected. The transfer switch includes a manually operated handle. If the transfer circuitry does not cause the automatic transfer to generated power, the manual/auto switch can be moved to the manual position and the handle then used to transfer from utility power to emergency power, or visa-versa. 3. Utility Power Returns When the normal utility power source is restored, the transfer switch control circuitry senses this and will watch for acceptable voltage levels, for a period of five minutes. After this five minute period and if the voltage levels have been stable, the control will signal the transfer switch contactors to re-transfer the load back to the utility power source and then disconnect the emergency generator source. At this point, the emergency generator set is off-line, but will continue to run for another five minutes, allowing it to cool properly. After the cool down cycle, the generator will automatically shut down and reset to standby mode. Figure 12 - Automatic Transfer Switch Emergency breaker Emergency. power 0 0- Normal breaker •J_ -0 Generator A Transfer circuitry Engine start logic Man/auto switch 498 Normal 480V M.C.C. 3rd Class Edition 3 • Part A2 0-^- power AC Systems, Switchgear, and Safety • Chapter 9 ^ OBJECTIVE 4 Explain the components and operation of a typical un'mterruptible power supply (UPS) system. UNINTERRUPTIBLE POWER SUPPLY (UPS) An uninterruptible power supply is required for plant systems that cannot tolerate a momentary loss of voltage or frequency. The purpose of this type of system, shown in Figure 13, is to provide a bumpless supply of electrical energy to critical plant control and shutdown circuitry. The term bumpless means that when the normal supply of utility power is interrupted, power is maintained to essential users by backup sources, such as batteries. These would include plant computers, control systems, emergency lighting, and communication systems. UPS SYSTEM COMPONENTS The four main components of an UPS system are a rectifier, battery bank, inverter, and static switch. All components must function well together to provide a bumpless supply of electrical energy to critical plant control and shutdown circuitry when an unexpected event like a power blackout, undervoltage, overvoltage, or voltage surge situation occurs. Rectifier A rectifier converts 600 V three phase AC power to 120 V DC, which provides power to charge the batteries and also supply the inverter. Battery Bank Battery banks are used to store electrical energy and supply the inverter in the event of a loss of AC power. In order to achieve the desired 120 DC volts, and depending on the application, the battery banks consist of several batteries connected in series or in a series-parallel manner. Figure 13 shows a small UPS battery bank. Figure 13 - UPS Battery Bank (Vittee/Shutterstock) 3rd Class Edition 3 • Part A2 499 ?& Chapter 9 • AC Systems, Switchgear, and Safety Inverter An inverter converts the 120 V DC power from the rectifier to 120 V single phase AC for critical plant AC power users to maintain a supply of stable voltage and frequency output power. Static Switch The 120 V single-phase AC power from the inverter is fed through the static switch to the 120 V critical users. If the inverter fails, for whatever reason, the control system will automatically open the switch located upstream of the rectifier and close the switch on the alternate 600 V feed. This 600 V power will then be reduced to 120 V single-phase AC, through the use of a step-down transformer. The static switch itself will also switch over to this alternate power source. This all occurs within 1/240 of a second (1/4 cycle). UPS SYSTEM OPERATION Utility power, at 600 V AC, is supplied to the rectifier/charger section from the MCC (motor control centre). The rectifier/charger converts the 600 V AC to 120 V DC. This 120 V DC is used to supply power to critical control circuitry and also to maintain the battery banks in a fully charged condition. The 120 V DC power output from the rectifier/charger also goes to the inverter where it is converted back to alternating current. The 120 V AC single-phase power passes through the bypass switch to supply the various AC power users. If there is a loss of utility power, the DC and AC users will then be supplied with power from the battery banks. This will occur until the automatic transfer switch starts the emergency generator, which will then supply the power to the UPS system. If the inverter fails, the bypass power will automatically be selected to supply the AC users. The bypass switch is a make-before-break type of switch to provide a bumpless transfer of power. An alarm will annunciate to alert the operator of a "UPS trouble". If there is a loss of utility power while the inverter is on bypass, circuitry in the automatic transfer switch will not allow the inverter to transfer back to normal or utility power when it becomes available. This interlock ensures a bumpless transfer of power to the AC UPS users by employing a time delayed restoration of normal power 60 s after the inverter is placed back in service. Whenever the inverter is on bypass and there is a temporary loss of normal or utility power to the UPS system from the MCC, then the 120 V AC circuits will be lost until power has been restored to the MCC, either from the utility or the generator. 500 3rd Class Edition 3 • Part A2 AC Systems, Switchgear, and Safety • Chapter 9 f£ Figure 14 - UPS System Automatic transfer switch DC Static Rectifier/ bypass switch charger DC k Battery bank AC / Inverter \ DC \ / / AC 125VDC users UPS System Battery Design The duration of time that batteries can perform their function will depend on their ampere hour rating and the current drawn by the various DC circuits and the inverter. If the power draw is normal, and providing that the batteries are in good condition and fully charged, they should provide adequate power for approximately four hours. The types of batteries used depend on the application and cost. Typical batteries used are: • Lead acid batteries - Sealed lead acid (SLA) or valve regulated lead acid (VRLA) -Vented lead acid (VLA) • Nickel cadmium (NiCad) • Lithium ion (Li-ion) SLA or VRLA batteries are the most common type used in UPS systems. These batteries have a lifespan of up to ten years and are best suited in a dry climate-controlled environment within the temperature range of 20°C-25°C. SLA or VRLA batteries do not require any direct maintenance, such as topping up with pure water as they are sealed. These batteries come in two types of electrolyte composition: absorbed glass mat (AGM), where the electrolyte is held in a porous microfibre glass separator, and gel, where the electrolyte is a gel with a mixture of silica and sulfuric acid. AGM batteries have a lower cost and higher charge and discharge rates. The operating temperature ofAGM batteries can range from -40°C to 55°C. VRLA batteries are sealed inside a case that has a relief vent valve to release gases, which opens at 41.3 kPa and resets between 15 to 20 kPa. Under normal operation conditions, the batteries should not leak or vent hydrogen or acid mist during recharging. 3rd Class Edition 3 - Part A2 501 ?& Chapter 9 • AC Systems, Switchgear, and Safety VLA batteries have plates that are flooded with electrolyte acid. They have a lifespan of up to twenty years. These batteries are used in applications that require a high ampere hour rating. Due to being open, VLA batteries can vent hydrogen gas or vapours directly into the environment which they are stored in. This requires a storage space with high exhaust ventilation, and one with wash down facilities in case of leaks or spills. VLA batteries are top vented and must be kept upright. The electrolyte levels require monitoring and topping up with pure water on a scheduled basis. VLA batteries are more expensive than VRLA batteries. Nickel cadmium batteries are common in communication systems UPS systems. These batteries are more expensive than lead acid batteries and have higher disposal costs since nickel and cadmium are toxic materials. The electrodes consist of a positive nickel hydroxide plate(s) and a negative cadmium hydroxide plate(s). These batteries have a high cycle life and are tolerant to deep discharges. Nickel cadmium batteries have a lifespan of up to twenty years. Lithium ion batteries have a higher capital cost than the aforementioned alternatives. This can potentially balance out due to their longer service life, and reduced storage room cooling requirements. These batteries generate less heat and can function at higher temperatures. Lithium ion batteries are smaller, lighter, can potentially have double the life span of a lead acid battery, have faster charge times, require less storage space, and little maintenance. Lithium ion batteries have built in battery monitoring and management systems which translates into greater reliability of the system. UPS SYSTEM MAINTENANCE Maintenance of an uninterruptible power supply (UPS) system is vital to ensure the reliability of the system when it is needed. Preventive maintenance consists of scheduled activities like testing, inspection, and cleaning to sustain the systems dependability. Testing Regardless of the types of batteries, the batteries should be tested up to four times each year. The voltages should be recorded. A load test should be done but can damage a battery if it is performed too often. Load testing is usually done every six months to one year. Load testing involves removing the AC supply to the rectifier/charger and observing how quickly the battery output power deteriorates when the inverter is loaded. Recovery rates and times should be monitored as well; these will vary depending on the type of battery. Visual Inspection A visual inspection should be completed during each scheduled maintenance. This visual inspection should include any sign of cracks, leaks, swelling, or corrosion of the battery cases. It is also important to make sure there is sufficient space between each battery. This space will allow heat to escape. Torquing the Connections All connections must be torqued each year. It is important to follow the manufacturer s specified torque values. If the connection is too loose, it could overheat during a discharge and cause problems. If it is too tight, the post or terminal could be distorted. Cleaning The exterior of the batteries should be cleaned with a damp cloth. A damp cloth with a mixture of a neutralizer will remove any acid film. Corrosion should be brushed away with a stainless steel or brass brush. Record Keeping The installation date of each battery should be recorded. A clear and accurate log of the findings from each maintenance period should be maintained. 502 3rd Class Edition 3 • Part A2 AC Systems, Switchgear, and Safety • Chapter 9 ^ OBJECTIVE 5 Explain safety procedures and precautions that must be exercised when working around and operating electrical system components. Explain grounding. SAFETY PROCEDURES Safe working habits are largely a matter of common sense. Power plant operators should be aware of the possible danger to themselves and others when operating electrical equipment. An operator must report any electrical malfunction and take equipment out of service for maintenance by qualified personnel. When a power plant engineer is involved in plant design, safety of equipment and personnel is of vital importance. Attention should be placed on the following. Electrical Installations • All motors, generators, and equipment should be installed in such a way that no live parts are exposed. • Sufficient space must be allowed around equipment for inspection and repair to be carried out safely. • Guards must adequately cover all rotating parts. • Identify all feeders and circuits as to their purpose so that correct circuit breakers and switchgear can be observed easily at a breaker panel. Personal Clothing and Habits • Comfortable but close-fitting clothing should be worn. Many plants ban hoodies in the workplace. • Wearing insulated safety shoes is recommended when working on electrical equipment. • Avoid wearing any loose articles such as rings and chains that may come in contact with live equipment. Side Track Arc flash is a phenomenon that is produced from an electrical arc. Commonly caused by the switching of electrical circuits, it can produce enough energy in the form of heat and blast pressure to cause substantial damage to equipment and loss of life. Temperatures produced by arc flash can exceed 19 000°C. Arc flash is extremely dangerous and specialized training is required if work is required where the potential for arc flash exists. (B Arc Flash Protection (CSA Z462 Workplace Electrical Safety) Arc flash is a type of electrical explosion that occurs when electrical energy passes from a highvoltage source, through the air, to a low-voltage conductor. Arc flash can release an extreme amount of heat that produces a hazardous pressure wave. Safety concerns include burns, visual impairment, deafness, and injuries from flying debris. Arc flash is rare, but Power Engineers may encounter it when operating electrical circuit breakers or switchgear. The risk of arc flash can be significantly reduced through proper installation of electrical equipment, preventive maintenance, and reliability programs. Breakers that are not cycled or cleaned of loose dust and debris are at a higher risk of arc flash. The best protection is to limit the interaction that operators have with electrical equipment through controls and procedures. 3rd Class Edition 3 • Part A2 503 ^ Chapter 9 • AC Systems, Switchgear, and Safety The severity of the arc flash potential or energy is measured in energy released per area at a particular distance from the arc flash. The units of this flash potential or energy are Joules per centimeter squared (J/cm2) or calories per centimeter squared (cal/cm2). This value is used to select the proper personal protective equipment (PPE), which must meet or exceed the thermal energy that could be released during an incident. Arc flash clothing is rated by an arc thermal performance value (ATPV) or energy ofbreak-open threshold (EBT). Both ratings are based on the flash potential or energy measurement, as determined through an arc flash hazard analysis. CSA Z462 Workplace Electrical Safety-2018 Table 3 provides information on the selection of arc-rated clothing and other PPE when the incident energy analysis method is used. Essentially, the table specifies electrical safety requirements in the workplace and includes precautions regarding arc flash safety. Flash suits, flash hoods, face shields, and gloves are examples of arc-rated PPE. The arc-rated clothing must meet or exceed the estimated incident energy potential. Caution Be sure to follow site specific policies, procedures, and training requirements. Consult local jurisdictional regulatory requirements regarding protection against arc flash incidents. Figure 15 shows a worker using proper arc flash PPE to operate an electrical circuit breaker. Figure 15 - Operating an Electrical Circuit Breaker with Proper Arc Flash PPE (Mohd Nasri Bin M.ohd Zain/Shutterstock) 504 3rd Class Edition 3 • Part A2 AC Systems, Switchgear, and Safety • Chapter 9 Working on Live Equipment • Consider all circuits to be alive unless certain that they are dead and cannot, by some human error, be made live. • When opening an electrical circuit, place tags that show equipment is out of service for maintenance. The tag should bear the name of the person who installed it and should only be removed by this same person if available. A lock-out device and lock-out lock should be applied. • An operator must isolate all equipment, such as pumps, before maintenance is started. All switches must be locked open at the source of the power at the breaker. Test the equipment after isolation by attempting to start it at the start/stop station. The circuit may be open but charged capacitors can injure a person. • Always open switches completely before removing fuses. If it is necessary to change a fuse in a live circuit, use approved fuse pullers that can withstand the line voltage. When removing fuses of live circuits, break contact with the line side first. Make contact with the line side first when inserting a new fuse. • Switches should be opened in a firm, positive manner to prevent arcing. • All portable electrical tools should be properly grounded. Fire Hazards • Due to the conductive nature of water, it should never be used on an electrically generated fire. Use CO^ (or dry chemical if C02 is not available) extinguishers. • If electrical equipment is not on fire, but is in the vicinity of a fire, and water is the appropriate fighting medium for the general area, isolate the electrical equipment before using water. Grounding System • A ground is a reference point of zero voltage potential, which is usually an actual connection to the earth. Controlled grounding is very important in that an open ground condition could present severe safety problems to anyone operating the power generation equipment. • A grounding system ensures that any person who touches any of the metal parts will not receive a high-voltage electrical shock. The conductor used for grounding is either a bare wire or a green insulated wire. • Ground straps are used as safety devices to direct energy to ground in the event a circuit becomes unintentionally charged. Lightning Arrestors • Power lines and associated equipment could become inoperable when struck by lightning. Lightning arrestors are used to ground excessively high voltages that are caused by lightning strikes. They are designed to operate rapidly and repeatedly, if necessary. • Lightning arrestors are connected to transformers or to the inside of switchgear. 3rd Class Edition 3 • Part A2 505 r2> Chapter 9 • AC Systems, Switchgear, and Safety SUMMARY OF AC SYSTEMS, SWITCHGEAR^ AND SAFETY Power systems are designed to provide safe reliable operation. Each component in the system has its own safety devices, such as protective relays, fuses, protective grounding systems, and breakers. These safety devices are engineered to work in unison with the other components to provide the end user with an uninterruptable reliable source of energy that meets their load requirements under various operating conditions. Power systems require a coordinated control strategy to ensure voltages and frequency remain constant within their operating parameters, whether interconnected to the power grid or islanded. This chapter provided the learner an introductory understanding of the concepts, components, and design features required to produce a reliable power supply, and the safety devices and systems used to accomplish this. 506 3rd Class Edition 3 - Part A2 AC Systems, Switchgear, and Safety • Chapter 9 ^ CHAPTER QUESTIONS Objective 1 1. Using a simple, single line diagram, sketch a typical industrial AC high-voltage power supply that includes at least two main generators, an emergency generator, high-voltage buses, and the necessary breakers and transformers. Objective 2 2. On an AC generator panel, which component will not normally be on there? a) Synch-scope b) Gas turbine gas pressure gauge c) Prime mover RPM gauge d) Voltage gauge Objective 3 3. Explain the purpose of the following protective relays for an AC generator. a) Over excitation b) Under frequency c) Overspeed d) Loss of synchronism e) Reversal of power 4. Using a simple sketch, explain the purpose and operation of an automatic transfer switch. 5. A/an relay is designed to indicate a fault condition and to trip an associated main breaker when it receives a signal that one of the relays in its circuit has tripped. a) Undervoltage b) Overspeed c) Master trip d) Overvoltage 6. Generator protective relays automatically reset once the fault has cleared. a) True b) False 3rd Class Edition 3 - Part A2 507 Chapter 9 • AC Systems, Switchgear, and Safety Objective 4 7. Using a simple sketch, explain the purpose of a UPS system. 8. Explain the maintenance required on UPS batteries. 9. Which battery type has self-monitoring capabilities: a) Sealed lead acid (SLA) or valve regulated lead acid (VRLA) b) Vented lead acid (VLA) c) Lithium ion (Li-ion) d) Nickel cadmium (NiCad) 10. Battery banks are electrically connected in_ in order to reach the desired voltages required. a) Series b) Parallel c) Closed terminals d) Open terminals Objective 5 11. Explain the procedures and precautions required when working on electrical systems. 12. Th