xac

limitations are met. The voltage-drop calcu- lation should .be based on the starting of the plant’s largest motor with all other required plant loads in operation. Reduced-voltage starting or an auxiliary starting driver may be required when the utility system is not stiff enough to allow full-volta...

limitations are met. The voltage-drop calcu- lation should .be based on the starting of the plant’s largest motor with all other required plant loads in operation. Reduced-voltage starting or an auxiliary starting driver may be required when the utility system is not stiff enough to allow full-voltage starting. 4.3.2.12 Reclosing Procedures Utilities may employ automatic reclosing schemes on overhead lines because the faults which occur on overhead lines are often transient in nature. The delay time and number of automatic reclosures are based on a review of such factors as the voltage level of the feeder and the feeding of the plant from either a radial distribution feeder or a tap on a transmis- sion tie line. The delay time before and between automatic reclosures and the number of reclosures are required for the design of protective relaying and system control schemes. 4.3.2.13 Substation and Metering Requirements Characteristics of a facility substation, should take into consideration: a. Largest single load. b. Total connected load. c. Maximum allowable voltage drop. d. Utility reliability. e. Substation ownership (utility versus user). f. Primary versus secondary metering. g. Spare transformer capacity. h. Future load growth and expansion requirements. i. Grounding. j. Isoceraunic (lightning frequency) level and protection schemes. k. Facility life. 1. Maintainability as it would affect substation design. 4.3.2.14 System Maintainability The electrical distribution and utilization system must be designed so that it can be inspected and maintained on a regular basis to assure reliable operation. Often, the maintenance of electrical facilities for a petroleum facility are maintained at the same maintenance intervals as other process equipment. The connection of unrelated process equipment should be avoided if it cannot be shutdown during the primary facility shutdown. 4.3.2.15 Facility Additions or Modifications Design of electrical systems should take into consider- ation any potential additions or modifications of the facility. This is especially important when selecting equipment fault duty ratings and designing provisions for future expansion or additions. 4.3.3 Parallel Operation with Purchased and Generated Power When a utility supplies a part of the facility power require- ments and operates in parallel with plant generated power, the following must be considered: a. Division and interchange of real and reactive power. b. Protective relaying. c. Service restoration procedures. 4.3.3.1 Division and Interchange of Real and Reactive Power Power interchanges between utility and industrial sys- tems can vary due to excess plant power generation, utility power restrictions, or load adjustments to maintain constant demand on the utility system. Contracts for the purchase of utility power should include the amount of kilowatts and kilovars and the division or interchange of them between the utility and the facility. If the utility line voltage is subject to wide variations, a method for controlling kilovar inter- change as well as regulating voltage should be considered. Possible methods include automatic load-tap changers on transformers, power factor correction capacitors, and power factor control of the generator or synchronous motor excita- tion systems. 4.3.3.2 Protective Relaying Protective relaying must be provided to protect the plant and its generation from faults or power loss in the utility system. The relaying must protect against adverse interac- tions between the systems and, if necessary for system sta- bility, must act to isolate plant generation from the utility system. Impedance, reverse power, directional overcurrent, or underfrequency relays may be used to trip the incoming supply circuit breakers. If plant load exceeds generation, an automatic load-shedding system should be provided r0 relieve generation overloads after isolation and to maintain plant system stability. 4.3.3.3 Service Restoration Procedures Various fault or switching conditions may cause facility and utility generating systems to separate. The usual sequence involves a fault transient disturbance affecting both systems until separation occurs, followed by recovery of the isolated systems. The fault transient may cause a voltage dip which will cause motors to drop off the line; however, if pro- cess conditions allow, important drives. can have their control equipped to permit their restarting automatically when plant voltage recovers. Operation of plant generation after separation may require automatically adjusting the load on plant generation to the load level that it can successfully restart automatically and supply continuously with acceptable voltage and frequency levels. If the plant has been receiving power from the public utility, voltage and frequency will fall unless load-shedding restores the proper balance or the plant turbine-generators can increase output to the proper level. If the plant had been send- ing power to the public utility before isolation, electrical out- put must be reduced. The effect of this reduction on plant steam system conditions must also be determined. Paralleling of the plant generation and the utility system should be possible only at selected circuit breakers that are equipped with synchronizing switches connecting these breakers into the plant synchronizing system. Other circuit breakers where inadvertent paralleling is possible should be equipped with synchronizing-check relays that prevent clos- ing unless voltage and frequency conditions at both terminals of the breakers are within prescribed limits. Relays which protect the system during fault conditions must be applied and set carefully to prevent separation from occurring during synchronizing swings. A power system study is normally required to provide the proper protective relaying and settings as well as the generat- ing system and load shedding system parameters necessary for system stability during the fault and recovery transients. 4.4 SYSTEM VOLTAGES 4.4.1 Selection The selection of system voltages in a facility is based pri- marily on economics, with consideration given to the follow- ing factors: a. b. C. d.  e. f.  g: h. 1. Class of service available from the utility. Total connected load. Planning for future growth. Plant standardization of equipment. Density and distribution of the load. Safety. Interconnection to existing systems. Equipment availability. Practical conductor and equipment sizes. 4.4.2 Voltage Levels The voltage levels in a facility can be divided as follows: a. Less than or equal to 600 V (low). b. From 601 V to 69,000 V (medium). c. Greater than 69,000 V (high). The low-voltage level is normally used to supply small motors, lighting, and controls. The medium-voltage level is normally used for larger motors and for distribution of small and medium blocks of power. Voltage levels of 34,500 V to 69,000 V may be used for large blocks of power. The high- voltage level is used for the transmission and distribution of bulk power. 4.5 POWER SYSTEM ARRANGEMENTS Four basic types of power system arrangements are avail- able: the radial, the primary-selective radial, the secondary- selective radial, and the secondary-selective parallel. The selection of a system arrangement is governed by factors such as service continuity, flexibility, regulation, efficiency, operat- ing costs, investment costs, and reliability of the power source. Maintainability of equipment should be carefully considered because it affects all of these factors. Systems that utilize mul- tiple supplies, loops, and ties can be quite complex. The num- ber of relays, switches, and interlocks required by these systems necessitates careful engineering to avoid shutdowns resulting from equipment failures or improper operation. 4.5.1 Simple Radial System The easiest system to understand, operate, and troubleshoot is the simple radial system shown in Figure 10. It is the least expensive system to install and is expandable. The disadvan- tage of the simple radial system is that it provides no alternate source of power. A failure in the primary breaker, cable, switch, or transformer can result in a process shutdown. Plac- ing a single load group or process unit on a radial feeder will reduce the effects of a circuit failure on the overall facility. 4.5.2 Primary-Selective Radial System The primary-selective radial system shown in Figure 11 provides better service continuity and more flexibility than the simple radial system because only half the transformers are on one feeder. Should a feeder fail, the affected loads can be switched to the other feeder. Voltage regulation in a pri- mary-selective radial system is comparable to that of a simple radial system; however, the initial investment in a primary- selective radial system is higher. 4.5.3 Secondary-Selective Radial System The secondary-selective radial system, shown in Figure 12, provides service continuity and voltage regulation. A feeder fault will cause half the load to be dropped, but service can be restored quickly through manual or automatic operation of the secondary tie breakers. Investment costs of this system are relatively high. 4.5.4 Secondary-Selective Parallel System The secondary-selective parallel system, shown in Figure 13, provides uninterrupted service continuity and voltage reg- ulation to all loads. The unit substation tie breaker is normally closed and an intemption of either of the source supplies will not interrupt any of the loads. This configuration is by far the most complex and costly, but may be justified based on con- sequences associated with process disruption. A consider- ation is that disturbances on one bus may affect loads connected to the other bus. Equipment fault ratings must be sized for the total fault duty from all sources. 4.6 POWER SYSTEM STUDIES 4.6.1 General The planning, design and operation of a power system requires continual and comprehensive analyses to evaluate current system performance and to establish the effectiveness of alternative plans for system expansion. 4.6.2 System Studies Studies that will assist in the evaluation of initial and future system performance, reliability, safety and ability to grow with production and/or operating changes are: a. Load flow. b. Cable ampacity. c. Short-circuit. d. Protective Device Coordination. e. Stability. f. Motor starting. g. Insulation Coordination. h. Reliability. i. Grounding. j. Harmonics. The procedures for performing the above system studies are outlined in many publications devoted to the subject. Included among these are the following: a. IEEE Std SO. b. IEEE 141 (Red Book). c. IEEE Std 242 (Buff Book). d. IEEE Std 399 (Brown Book). e. IEEE Std 493 (Gold Book). f. IEEE Std 519. 4.7.1 Fault Considerations When a facility’s electric power system has to be designed and the appropriate equipment needs to be selected, the fol- lowing fault considerations should be considered: a. Possible or likely places where faults may occur. b. Amount of fault current that the system can deliver. c. Possible damage that may result from faults. 4.7.1.1 Location of Faults Although faults can occur anyplace in an electrical system, the probability of occurrence varies at different locations. Switchgear, transformers, and buses have relatively few short circuits; and rotating machines, when maintained and pro- tected against voltage surges, are not prone to failure. Bare overhead distribution systems, however, experience the high- est incidence of faults. 4.7.1.2 Fault-Current Magnitudes Devices used for fault-current interruption must have interrupting and momentary withstand ratings that can ade- quately handle the available fault currents. Inadequate rat- ings can result in failure of the equipment to perform its intended function. Such a failure can destroy the equipment and can result in potential danger to personnel and to other equipment. Interrupting-device ratings should be based on the maxi- mum fault current of the system at the point of application. The magnitude of fault current which the system can deliver depends on the sources of current and the impedance between the sources and the fault. The sources of current include in- plant generators and motors and connections to external power sources such as electric utilities. The fault-current capabilities of simple systems may be hand-calculated; those of more complex systems will require study using computer programs that are readily available. When selecting equip- ment, future expansion of the electrical system must be con- sidered to ensure that ratings are adequate for the future current duty. Driving point voltage (voltage at the time of the fault) must be determined. It is not uncommon to see systems operating at 1 .O5 to l. 10 per unit voltage. lSAvailable from EEE. 4.7.1.3 Damage From Faults Faults which are not promptly isolated from the source of power can be very damaging: electrical and other apparatus can be damaged, fires can be started, and lives can be endan- gered. Lengthy production interruptions are a likely result of this kind of damage. 4.7.2 Fault Clearing Considerations 4.7.2.1 Procedure To properly remove a fault from the electrical system, it must initially be detected by a fault sensing device. The fault sensing device then sends a signal to one or more fault clearing devices which will operate to isolate the faulted segment of the system. This process takes place automati- cally and quickly in order to minimize the damage caused by the fault. The fault clearing devices must have adequate interrupting and momentary withstand ratings as discussed in 4.7.1.2: 4.7.2.2 Dual-Purpose Devices Some electrical devices provide both the sensing and the interrupting functions in the same enclosure without the inter- action of peripheral devices. Examples of these devices are as . . follows: a. Fuses of all voltage ratings. b. Molded-case circuit breakers and insulated-case circuit breakers. These devices have internal sensors (thermal, magnetic, or static) that detect the flow of fault currents and, through the direct molecular or mechanical action of these sensors, oper- ate to clear the fault. 4.7.2.3 Single-Purpose Devices Many electrical fault clearing devices receive an electrical signal to operate from a relay or set of relays. These devices take the relay signal, process that signal, and then operate a set of mechanical contacts to interrupt the flow of electric cur- rent running through them. Examples of these fault clearing- devices are as follows: a. Circuit breakers of all types other than the types discussed in 4.7.2.2. These include sulfur hexafluoride (SFg), oil, air, and vacuum circuit breakers in a variety of configurations. often, these circuit breakers are mounted in a lineup of switchgear, an arrangement discussed in 4.10. b. Circuit switchers which are usually found on high-voltage circuits for transformer and feeder protection. Since relays are a key element in the proper operation of these protective devices, some of the important consider- ations regarding relays-are addressed in 4.7.3. 4.7.2.4 Coordination of Devices Generally, a zone of protection is established around each system element, such as a bus, transmission line, generator, or transformer, and a fault in a zone should be cleared by the fault clearing devices around that particular zone. For the fault clear- ing devices to properly isolate a faulted section of a system, they must be properly coordinated. A coordinated device must protect the equipment in a zone and should be selective with upstream and downstream devices. This means that the protec- tive device closest to a faulted section should open and inter- rupt the flow of fault current before other devices on the electrical system operate. This does not mean that other devices will not see the fault current flowing, but it means that they should not have had time to take any action toward opening to clear the fault. If a malfunction of the devices occurs around a particular zone, fault clearing devices in the next zone upstream should operate to clear a fault. Obviously, the upstream devices should delay long enough to give the primary clearing devices a chance to operate; however, the upstream fault clearing devices should not delay too long or the fault damage could be more extensive. Selectivity between fault clearing devices in series should be maintained. The procedures for determining coordination margins (time interval between device curves) are outlined in many publications devoted to the subject. Included among these are the following: a. WEE Std 141. b. IEEE Std 242. c. IEEE Std 399. d. Applied Protective Re1ying.I Fault clearing device coordination is vital to a facility power system. Particular attention must be paid to obtaining optimum settings and to recalculating and maintaining set- tings as system conditions change. 4.7.3 Relaying Considerations 4.7.3.1 Relay Dependability A relay detects abnormal system conditions and often ini- tiates breaker operation. Although a breaker may be prop- erly selected and applied, it is useless if it fails to operate at the proper time and a nuisance if it operates when it should not because of improper setting or application of the relay. 4.7.3.2 Relay Selection Electromechanical and static relays must be selected with care because types are available for almost every system requirement. The proper selection and application of relays is important to the electrical system and requires thorough study. Relays associated with facility power systems are used primarily for the protection of feeders, transformers, and rotating machines. 4.7.3.3 Relay Selectivity Many relays have settings which allow their operating characteristics to be changed. Relays can be made, for example, to operate at different times when installed to look at the same set of abnormal system conditions. Used in conjunction with fault clearing devices, they can be used to establish the coordination of the fault clearing devices discussed in 4.7.2.4. Desired protection can only be ensured by choosing relays of the proper types and opera- tion ranges and by determining the correct relay settings. Careful studies must be made of each switching configura- tion for various plant operating load conditions to arrive at the proper settings that will permit maximum load to be placed on line and carried while still protecting for mini- mum fault levels. 4.7.3.4 Relay Testing and Inspection Testing and inspection of protective relays for proper set- tings and operation should be conducted when the relays are placed in service; testing and inspection should then be con- ducted at established, intervals throughout the life of the relays. As much as possible, testing should be done by simu- lating appropriate current and voltages on the primaries or secondaries of the instrument transformers that serve the relays. Many static relays have self-test programs to alarm if the relay is malfunctioning. 4.8 FUSES 4.8.1 Uses Fuses are used on facility power systems to protect equip- ment and cables from overload conditions and to interrupt fault currents when they occur. When applying fuses, single- phasing possibilities should be considered. Since fuses are single-phase devices, only one fuse may blow on a shgle- phase fault, leaving single-phase potential on a polyphase cir- cuit. Also to be considered is that after repeated operation at a current near the fuse’s melting point, the fuse may become damaged and operate quicker than desired. 4.8.2 Availability Fuses may be obtained in every voltage level up to at least 138,000 V. There are varieties which are current limiting so that the energy allowed to flow during a fault condition is lim- ited to a lower level than available on the supply side of the fuse, and there are many fuses designed for special applica- tions such as for use in motor starter circuits. 4.8.3 Coordination Considerations The variety of fuses available allows some flexibility in protective device coordination. Each fuse has a time-current characteristic envelope curve which ,is used to develop cmr- dination plots for the power system. 12t and let-through cur- rents must be considered when coordinating fuses. 4.9 CIRCUIT BREAKERS 4.9.1 Uses Circuit breakers, used on both AC and DC systems, are widely used in facility power systems. Found at almost .every voltage level on the system, circuit breakers protect electrical system components from overloads and faults and isolate parts of the system when these conditions occur. Many circuit breaker applications involve switchgear, and switchgear applications are discussed in 4.10. 4.9.2 Types Circuit breakers used on electrical systems with nominal system voltages less than or equal to 600 V come in a variety of styles. Some, like the molded-case circuit breakers, do not depend on external relays for sensing overloads and faults while others, like the air-break power circuit breakers, have .external current sensors and static relay modules for sensing abnormal conditions. These devices are installed in a variety of equipment, such as panelboards, switchboards, and switchgear. Circuit breakers used on systems with nominal system voltages of 600 V-15,000 V are generally installed in switch- gear lineups. These breakers use relays for sensing fault con- ditions. The interrupting medium is generally air or vacuum in this voltage class. Circuit breakers used on systems with nominal system voltages of 15,000 V-35,000 V may be installed in switch- gear. Most breakers for higher voltage systems are individual free-standing outdoor types, and these higher voltage devices use oil or SF6 as the interrupting medium. Relays are used for sensing fault conditions at all these installations. 4.9.3 Location Circuit breakers are not listed by NRTL for direct use in classified locations. They are, therefore, installed in nonclas- sified locations, either indoors or outdoors, or when installed in a classified location, they must be installed in approved enclosures suitable for the location. 4.9.4 Inspection and Testing Because many types of circuit breakers are available, it is not possible to discuss in this recommended practice inspec- tion and testing procedures for all circuit breakers. A regular preventive maintenance program should be estabfished. The manufacturer’s installation and operating instruction books or a reliable electrical testing firm should be consulted, to estab- lish maintenance and testing requirements and intervals. 4.10 SWITCHGEAR 4.1 0.1 General The term s