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osses are sometimes referred to as core losses or iron losses. For a given voltage, no-load losses can be considered to be constant. Load losses vary with the flow of load current and include 12R losses, eddy current losses, and stray load losses. The 12R losses are the most significant and are caus...

osses are sometimes referred to as core losses or iron losses. For a given voltage, no-load losses can be considered to be constant. Load losses vary with the flow of load current and include 12R losses, eddy current losses, and stray load losses. The 12R losses are the most significant and are caused by the flow of load current in both primary and seconday windings. Higher efficiency transformers usually have copper windings to min- imize 12R losses. Where forced cooling is specified, addi- tional energy is consumed by fans or oil circulating pumps. While transformers should be sized on the basis of maxi- mum load, they should be designed for maximum efficiency at nolmal operating load, and efficiency should be evaluated accordingly. When specifying transformers, the load at which efficiency will be evaluated should be given. 3.8.2 Motors and Generators Motor loads are the major consumers of energy in process plants. Typically, they can account for 70% of the electrical energy consumption. Motor efficiencies vary from 65% for the smallest HP motors, to 98% for the largest motors. High efficiency designs are available for motors in the standard frame size range. In the United States, the 1992 Energy Pol- icy Act requires energy efficient motors for most motor cate- gories. Motor losses consist of the following: a. Stator 12R loss. b. Rotor 12R loss. c. Core loss (hysteresis and eddy current). d. Friction and windage. e. Stray-load loss. f. Excitation equipment losses (for synchronous machines). Manufacturing design parameters that affect motor losses include: a. Quality and thickness of lamination steel. . b. Size of the air gap. c. Stator and rotor resistances. d. Slot configurations. e. Number of poles (lower design speeds result in lower efficiencies). Much effort is made to optimize these parameters because they also affect motor power factor, inrush current, and start- ing torque. Operating conditions also affect motor efficiency. Typi- cally, motor efficiency falls off rapidly as motor load is decreased below one-half of rated load. Operating a motor at less than rated voltage will cause a decrease in efficiency due to higher stator losses and rotor losses. Operating at overvolt- age decreases efficiency because higher magnetizing current and saturation cause increased stator and core losses. Operat- ing with unbalanced voltages will increase losses (due to neg- ative sequence torque) and result in higher winding temperatures. Motors connected to variable speed drives experience higher losses because of the harmonic content of the supply voltage and the load current. This is due to higher than normal hysteresis and eddy currents induced in the stator and rotor steel by the harmonic currents. 3.8.3 Lighting Equipment Although lighting does not represent a major percentage of the electlical energy consumption of a petroleum facility, it nonetheless provides another area where energy savings can be achieved. The following guidelines can result in an energy efficient lighting system: a. Use the highest efficacy [lumens per watt (lmnV)] lamp that is capable of directing light to the task area involved. b. Select efficient ballasts (e.g., electronic ballasts for fluo- rescent fixtures). c. Maximize use of floodlights to illuminate general process areas. d. Use photocell or time controls to turn off outdoor lighting during daylight hours. e. Use manual controls for tower lighting with controls located at the tower base. f. Monitor lighting levels and reduce them where appropri- ate. For building lighting, this produces additional energy savings because of the reduced load on air conditioning equipment. g. Keep lamps and reflectors clean to obtain maximum light output. 3.8.4 Adjustable Speed Motor Control - Centrifugal pumps, fans, and compressors constitute a large percentage of the motor-driven loads in a petroleum facility. The torque requirements of these centrifugal loads vary as the square of the speed; thus, the brake horsepower required varies as the cube of the speed. Traditionally, centrifugal loads have been designed to oper- ate at constant speed with the process flow being controlled by some type of throttling means (pump control valves, fan dampers, or compressor inlet guide vanes). The energy losses from throttling can be substantial. As an alternative to throttling, the speed of the centrifu- gal load can be controlled to obtain the desired flow rate without producing excessive pressure. Because the flow rate varies directly with speed while the horsepower requirement varies as the cube of the speed, using speed reduction to lower flow rates will result in a significant horsepower reduction. For example, reducing the flow to one-half its initial value by lowering the speed of the load will cause the brake horsepower of the load to be reduced to one-eighth of its initial value. A typical pump head-flow curve is depicted in Figure 3 to further illustrate the attractiveness of using speed adjustment to control flow rate. The darker-shaded area to the lower left of each operating point indicates the power required for that operating point. The lighter-shaded afëa indicates the power savings that result from using speed reduction rather than throttling to control flow rate. In general, a steep system curve, or a steep pump curve, will accentuate the potential power savings. Also, the lower the static head involved, the greater the power savings will be as a percentage of overall power consumption. The conventional methods for achieving speed adjustment include hydraulic couplings, adjustable sheave belt systems, eddy current clutches, and wound-rotor motors. These devices are relatively inefficient, however, and usually require frequent maintenance. DC motors allow speed adjustment with improved efficiency but are also prone to requiring fre- quent maintenance and are difficult to apply in classified areas. Electronic adjustable-frequency controllers also pro- vide speed adjustment and have been improved over the last decade. They are now the method of choice when adjustable speed drives are needed. The maintenance level for these con- trollers is the lowest of the alternative methods, however, these drives create voltage aild current harmonics which may require remedial modifications to the power system and drive motor. Adjustable-frequency controllers have relatively high efficiencies and can be used with induction motors whch require low maintenance and are readily available for classi- fied areas. (See also Section 2.) The capital cost of adjustable-speed drive equipment is higher than for constant speed equipment, so an economic evaluation as outlined in 3.6 is required to determine if the potential energy savings offsets the increased cost. A major factor in such an evaluation will be the duty cycle of the equipment involved; i.e., the percentage of time that equip- ment will function at operating points requiring less horse- power than the design point. If the equipment is expected to operate at close to its design point for a high percentage of time, then using an adjustable-speed drive system is probably not warranted. It is also important to remember that the appli- cation of adjustable-speed drives requires the consideration of other design factors, such as avoiding the operation of .equip- ment at critical speeds and evaluating the effects of system harmonics that may be generated by adjustable frequency drive equipment. (See also 6.10.4.) 3.8.5 Conductor Sizing Power cables are another some of energy loss in an electri- cal system. The magnitude of the energy loss depends on the resistance of the cable as well as the amount of current expected to flow in the circuit. After power cables have been sized to meet the governing criteria (voltage drop, spare capac- ity, and the requirements of NFPA 70), a check should be made to determine if the anticipated energy loss in the cable would justify purchasing and installing the next larger size cable. 3.9 RELATIONSHIP TO POWER FACTOR The apparent power consumed by an electrical system is expressed in kilovolt-amperes (kVA), and is composed of a kilowatt (kW) component and a kilovolt-ampere reactive (kvar) component. The kW component represents the real work extracted from the power system. The kvar component represents the magnetizing energy necessary for exciting electrical equipment such as motors and transformers, as well as the inductive and capacitive components of other devices on the system. Power factor is the ratio of kW to kVA and provides a measure of the percentage of kVA that is doing useful work. The total current passing through the power system compo- nents (e.g., transformers, cables, transmission lines, switch- gear) produces heating losses proportional to the square of the current (12R). The total current is proportional to the kVA, so by reducing kVA, losses can be reduced. To reduce kVA, it is only practical to cut exciting energy (kvar). In addition to wasting energy through transmission losses, excessive kvar loading uses up transformer, cable, and transmission line capacity, causing the supplying utility to overbuild their sys- tem. To control this, utilities pass on the excess cost through the use of power factor penalty clauses in power contracts. To avoid paying these penalties, power factor must be kept above a fixed value-normally between 0.90 and 0.94. The large number of induction motors typical in a process plant can result in a low overall power factor on the system (0.85 power factor or less). Motors that are lightly loaded accentuate the problem because motor power factor decreases rapidly with decreasing load. The low power fac- tor results in higher-than-necessary currents on the distribu- tion system, resulting in higher losses. Improving the power factor will increase the overall efficiency of the power sys- tem. An improved power factor can also reduce or even eliminate power factor penalty charges if utility contracts contain such provisions. The following actions can increase power factor, and reduce the associated losses: a. Using high power factor rated equipment, such as high power factor lighting ballasts. b. Using synchronous motors which can be operated at unity, or leading (capacitive) power factor. c. Operating high efficiency induction motors at close to design horsepower. Figure &Power Relationship d. Using power factor correction capacitors to supply the reactive requirements of inductive loads. e. Increasing the excitation from in-plant generators. f. Installing a static var compensator. g. Controlling voltage so as to avoid overvoltage conditions. When capacitors are applied to induction motors, the great- est benefit is obtained if the capacitors are installed at the motor terminals, and are switched on and off with the motor. Care must be taken in the application of power factor correc- tion capacitors. Proper attention must be given to the effect that capacitors have on harmonic resonance, thermal overload sizing, circuit breaker switching capability, the lengthening of motor open-circuit time constants, and the possibility of motor self-excitation. A synchronous condenser can also be used to improve power factor. This device is used mainly by utilities, however, and is not practical in most industrial plants. 3.10 DEFINITIONS AND CONVERSION FACTORS The following is a list of definitions and conversion factors that are often useful in energy discussions: a. 1 British thermal unit (Btu) equals the heat required to raise the temperature. of 1 pound of water by 1 OF. b. 1 quad (quadrillion Btu) equals 1,015 Btu. c. 1 therm equals 100,000 Btu. d. 1 horsepower (HP) equals 0.746 kW. e. 1 kWh equals 3,413 Btu. Note: Due to thermal losses, approximately 10,000 Btu of raw fuel are consumed to produce 1 kWh of electricity in a conventional util- ity generating station. f. One 42-gallon barrel of fuel oil contains about 6 million Btu. g. One cubic foot of natural gas contains about 1,000 Btu. h. One ton of coal contains about 25 million Btu. 4.1 PURPOSE This section discusses the design considerations that must be evaluated for the development of a reliable and cost effec- tive power system for continuously run petroleum facilities. 4.2 SCOPE All aspects of facility power systems, from the point at which power is introduced into the facility to the points of uti- lization, are covered by this section. Topics include incoming lines for purchased power, in-plant generation, substations, transformers, switchgear, overhead distribution systems, volt- age levels, system arrangements, protective relaying, fault currents, and system stability. 4.3 POWER SOURCES 4.3.1 Generated Power Facility power stations not connected to public utility sys- tems must be designed with redundancy to ensure a self-suf- ficiency for various operating contingencies. These power stations should have provisions for a cold (black) start and, as a minimum, should be designed to supply 100% of plant electrical loads after the loss of any single major component of the power generating system. Other contingencies, such as the capability of motor starting at reduced generation, should be considered. 4.3.1.1 Power Station Auxiliaries Facility power stations that produce process steam and electricity must be provided with highly reliable station auxil- iaries. The auxiliaries should be spared and supplied from a minimum of two independent sources devoted solely to pro- viding auxiliaries. Critical auxiliaries for air, fuel, and water supplies should have steam-driven spares. 4.3.1.2 Power Station Bus Arrangements The size and importance of the power station will deter- mine the type of bus arrangement utilized for the main electri- cal connections. Small stations (less than 10 MW) frequently have only a single main bus as shown in Figure 5. Bus fail- ures are not common, and fair reliability is obtained. It is nec- essary to shut down, however, when performing preventive maintenance to the main bus or when additions are made to the main bus. Circuit breakers must be taken out of service to be worked on, but this problem can be minimized by using drawout-type breakers. These disadvantages can result in dis- ruption to essential station auxiliaries, such as draft fans, pre- heaters, boiler feed pumps, air compressors, and lighting. Unit construction, illustrated in Figure 6, can be used in iso- lated power stations. With this construction, each generator has its own boiler or turbine, main bus, and boiler, or turbine auxil- iary bus. Normally, the tie circuit breaker between the main buses is closed but wili open in the event of a fault on either bus. In effect, the arrangement operates much the same as two independent power stations tied together. The main buses are tied together during normal operation, so each side must be rated for the total fault duty resulting from both generators. A synchronizing bus scheme, shown in simplified form in Figure 7, is often used for a power station bus. This scheme offers a high degree of flexibility to add or remove generators and loads. The reactors serve to limit the amount of fault duty imposed on any one bus and to isolate voltage dips, to a degree, during faults. In this arrangement the loads on each bus are matched to the generating capacity on that bus to min- imize the amount of load transfer through the synchronizing bus under normal operation. The design of the power station bus arrangement should allow for future expansion, such as expanding a single or dual bus arrangement to a synchronizing bus arrangement. The design should also minimize the loss of generating capacity which would occur during a single fault or operating error. 4.3.1.3 Power Station Excitation Systems The reliable generation of reactive power is a vital task per- formed by the generator field. The field is powered from an excitation system controlled by a voltage regulator system that maintains desired bus voltage conditions when operated .in isolation. When operated in parallel with a utility, the volt- age regulator system is biased to maintain a fixed reactive power or power factor. The two types of excitation systems in general use are brushless exciters, which are similar to those used on brushless synchronous motors; and static exciters, which feed power through slip rings to the generator field. With either system, means should be provided to ensure con- tinued- generator fault-current output for faults in close elec- trical proximity to the generator terminals, since these faults will severely depress the generator bus voltage. This will require the use of power current transformers for static excita- tion systems or a constant voltage source for the exciter field on a brushless system. 4.3.2 Purchased Power When power is purchased from a utility, the following items, as a minimum, should be considered: a. Source and number of feeders. b. Reliability of utility system. c. Capacity and voltage of circuits. d. Power contracts and demand limitations. e. Parallel operation of multiple incoming lines. f. Automatic transfer scheme. g. Voltage regulation. h. Harmonic distortion, or harmonic current limitations. i. Short-circuit current. j. Coordination with utility relaying. k. Motor starting requirements. 1. Reclosing procedures. m. Substation and metering requirements. n. System maintainability. o. Future potential additions or modifications to the petro- leum facility. 4.3.2.1 Source and Number of Feeders the frequency and nature of service disturbances, the exist- ence and speed of automatic reclosures, and the’ length of outages. Performance records should also be examined to determine if any actions have been taken to prevent recur- rences of previous interruptions. 4.3.2.3 Capacity and Voltage of Circuits Circuits should be sized so that if any one circuit is out of service, the remaining circuits have the capacity to carry the load continuously. Circuit voltage in most cases will depend on utility standards and the amount of purchased power required. When the voltage of incoming feeders is higher than the voltage selected for the facility, transfomers of proper When major portions of the plant load are supplied by pur- chased power, multiple feeders should be provided to increase voltage rating will generally be included in each substation where utility feeders are terminated. service reliability. Circuits should have maximum electrical isolation or redundancy. Where possible, circuits should be separately routed to minimize the possibility of total outage resulting from exposure to fire or to mechanical damage. 4.3.2.2 Reliability of Utility System Utilities should provide information concerning routing, the type of construction, and the extent to which their cir- cuits are protected against outage. Performance records of pertinent utility feeders should be examined to determine 4.3.2.4 Power Contracts and Demand Limitations The exact form of and terms set forth in power contracts will vary with the utility and region of the country. Energy charges include fuel adjustment costs as part of the contract. Demand charges are based both on kilowatts with power fac- tor adjustments and on kilovolt-amperes. Power contracts may have maximum demand limitations or provisions under which demands beyond specified levels are supplied on an interruptible basis only. To determine the most favorable rate, it is essential to know firm demand and energy requirements as well as daily and seasonal load profiles. 4.3.2.5 Parallel Operation of Incoming Lines The preferred operation of incoming utility lines is to par- allel them on the substation bus. Typical arrangements are shown in Figures 8 and 9. Suitable relaying must be provided for proper system protection, and, before the feeders can be operated in parallel, protection must be provided (e.g., through the use of synchronization check relays) to verify that the voltages of the feeders are equal and synchronized. 4.3.2.6 Automatic Transfer Scheme When incoming circuits cannot be paralleled, automatic transfer between the circuits should be considered. Load requirements should be checked carefully, and a regular test- ing method for the automatic transfer scheme should be included. Fast transfer schemes must consider the effects of residual voltage on motors and driven loads. 4.3.2.7 Voltage Regulation Where utility or plant voltage varies (to unacceptable levels), automatic load-tap-changing transfomers or other methods should be considered for maintaining close voltage regulation. 4.3.2.8 Harmonic Distortion, or Harmonic Current Limitations Harmonic distortion, typically generated by nonsinusoidal waveforms originating from SCR rectifiers, adjustable speed drives, and similar electronic voltage and frequency con- trolled devices, can cause serious problems in electrical sys- tems. Problems can include overloading and overheating of phase and neutral power systems; problems associated with high electrical noise on the system; inability of electronic hardware to synchronize; failure of frequency-sensitive cir- cuits such as lighting ballasts; failure of adjustable speed drives and motor windings due to reflected waves; and many other abnormalities. IEEE Std 399 and Std 519 should be reviewed for systems with appreciable nonsinusoidal har- monics. Often, utilities will impose limitations on the maxi- mum amount of harmonic current being generated by the customer into the utility supply. 4.3.2.9 Short-Circuit Current The electrical system must be designed to accommodate the maximum short-circuit current that would result from the combined effect of both utility and in-plant sources. All sys- tem components, such as circuit breakers, transformers, and buses, must have ratings that can adequately withstand and interrupt the effects of the fault currents to which they are exposed. The minimum utility short-circuit level must also be known so that its effect on motor starting and protective relay settings can be determined. 4.3.2.1 O Coordination with Utility Relaying Proper coordination between substation and utility protec- tive relaying is essential to minimize the number and duration of power outages. The proper relaying should be selected by the user in collaboration with the utility company. 4.3.2.1 1 Motor Starting Requirements The voltage drop which occurs on the plant bus during motor starting should be calculated to ensure that plant and utility company