STEAM POWER by Mike Brown_8

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Simpo PDF Merge and Split Unregistered Version - http://www.simpopdf.com MIL-HDBK-1003/7 standard short circuit ratios of 0.58 at rated kVA and power factor. If a generator has a fast acting voltage regulator and a high ceiling voltage static excitation system, this standard short circuit ratio should be adequate even under severe system disturbance conditions. Higher short circuit ratios are available at extra cost to provide more stability for unduly fluctuating loads which may be anticipated in the system to be served. f) Maximum winding temperature, at rated load for standard generators, is predicated on operation at or below a maximum elevation of 3,300 feet; this may be upgraded for higher altitudes at an additional price. 6.2.2 Features and Accessories. The following features and accessories are available in accordance with NEMA standards SM 12 and SM 13 and will be specified as applicable for each generator: 6.2.2.1 Voltage Variations. Unit will operate with voltage variations of plus or minus 5 percent of rated voltage at rated kVA, power factor and frequency, but not necessarily in accordance with the standards of performance established for operation at rated voltage; i.e., losses and temperature rises may exceed standard values when operation is not at rated voltage. 6.2.2.2 Thermal Variations a) Starting from stabilized temperatures and rated conditions, the armature will be capable of operating, with balanced current, at 130 percent of its rated current for 1 minute not more than twice a year; and the field winding will be capable of operating at 125 percent of rated load field voltage for 1 minute not more than twice a year. b) The generator will be capable of withstanding, without injury, the thermal effects of unbalanced faults at the machine terminals, including the decaying effects of field current and DC component of stator current for times up to 120 seconds, provided the integrated product of generator negative phase sequence current squared and time (I22t) does not exceed 30. Negative phase sequence current is expressed in per unit of rated stator current, and time in seconds. The thermal effect of unbalanced faults at the machine terminals includes the decaying effects of field current where protection is provided by reducing field current (such as with an exciter field breaker or equivalent) and DC component of the stator current. 6.2.2.3 Mechanical Withstand. Generator will be capable of withstanding, without mechanical injury any type of short circuit at its terminals for times not exceeding its short time thermal capabilities at rated kVA and power factor with 5 percent over rated voltage, provided that maximum phase current is limited externally to the maximum current obtained from the three-phase fault. Stator windings must withstand a normal high potential test and show no abnormal deformation or damage to the coils and connections. 88
Simpo PDF Merge and Split Unregistered Version - http://www.simpopdf.com MIL-HDBK-1003/7 6.2.2.4 Excitation Voltage. Excitation system will be wide range stabilized to permit stable operation down to 25 percent of rated excitation voltage on manual control. Excitation ceiling voltage on manual control will not be less than 120 percent of rated exciter voltage when operating with a load resistance equal to the generator field resistance, and excitation system will be capable of supplying this ceiling voltage for not less than 1 minute. These criteria, as set for manual control, will permit operation when on automatic control. Exciter response ratio as defined in ANSI/IEEE 100, Dictionary of Electrical & Electronic Terms, will not be less than 0.50. 6.2.2.5 Wave Shape. Deviation factor of the open circuit terminal voltage wave will not exceed 10 percent. 6.2.2.6 Telephone Influence Factor. The balanced telephone influence factor (TIF) and the residual component TIF will meet the applicable requirements of ANSI C50.13. 6.2.3 Excitation Systems. Rotating commutator exciters as a source of DC power for the AC generator field generally have been replaced by silicon diode power rectifier systems of the static or brushless type. a) A typical brushless system includes a rotating permanent magnet pilot exciter with the stator connected through the excitation switchgear to the stationary field of an AC exciter with rotating armature and a rotating silicon diode rectifier assembly, which in turn is connected to the rotating field of the generator. This arrangement eliminates both the commutator and the collector rings. Also, part of the system is a solid state automatic voltage regulator, a means of manual voltage regulation, and necessary control devices for mounting on a remote panel. The exciter rotating parts and the diodes are mounted on the generator shaft; viewing during operation must utilize a strobe light. b) A typical static system includes a three-phase excitation potential transformer, three single-phase current transformers, an excitation cubicle with field breaker and discharge resistor, one automatic and one manual static thyristor type voltage regulators, a full wave static rectifier, necessary devices for mounting on a remote panel, and a collector assembly for connection to the generator field. 6.3 Generator Leads and Switchyard 6.3.1 General. The connection of the generating units to the distribution system can take one of the following patterns: a) With the common bus system, the generators are all connected to the same bus with the distribution feeders. If this bus operates at a voltage of 4.16 kV, this 89
Simpo PDF Merge and Split Unregistered Version - http://www.simpopdf.com MIL-HDBK-1003/7 arrangement is suitable up to approximately 10,000 kVA. If the bus operates at a voltage of 13.8 kV, this arrangement is the best for stations up to about 25,000 or 32,000 kVA. For larger stations, the fault duty on the common bus reaches a level that requires more expensive feeder breakers, and the bus should be split. b) The bus and switchgear will be in the form of a factory fabricated metal clad switchgear as shown in Figure 22. For plants with multiple generators and outgoing circuits, the bus will be split for reliability using a bus tie breaker to permit separation of approximately one-half of the generators and lines on each side of the split. c) A limiting factor of the common type bus system is the interrupting capacity of the switchgear. The switchgear breakers will be capable of interrupting the maximum possible fault current that will flow through them to a fault. In the event that the possible fault current exceeds the interrupting capacity of the available breakers, a synchronizing bus with current limiting reactors will be required. Switching arrangement selected will be adequate to handle the maximum calculated short circuit currents which can be developed under any operating routine that can occur. All possible sources of fault current; i.e., generators, motors, and outside utility sources, will be considered when calculating short circuit currents. In order to clear a fault, all sources will be disconnected. Figure 26 shows, in simplified single line format, a typical synchronizing bus arrangement. The interrupting capacity of the breakers in the switchgear for each set of generators is limited to the contribution to a fault from the generators connected to that bus section plus the contribution from the synchronizing bus and large (load) motors. Since the contribution from generators connected to other bus sections must flow through two reactors in series fault current will be reduced materially. d) If the plant is 20,000 kVA or larger, and the area covered by the distribution system requires distribution feeders in excess of 2 miles, it may be advantageous to connect the generators to a higher voltage bus and feed several distribution substations from that bus with step-down substation transformers at each distribution substation as shown in Figure 24. 90
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Simpo PDF Merge and Split Unregistered Version - http://www.simpopdf.com MIL-HDBK-1003/7 e) The configuration of the high voltage bus will be selected for reliability and economy. Alternative bus arrangements include main and transfer bus, ring bus, and breaker and a half schemes. The main and transfer arrangement, shown in Figure 27, is the lowest cost alternative but is subject to loss of all circuits due to a bus fault. The ring bus arrangement, shown in Figure 28, costs only slightly more than the main and transfer bus arrangement and eliminates the possibility of losing all circuits from a bus fault, since each bus section is included in the protected area of its circuit. Normally it will not be used with more than eight bus sections because of the possibility of simultaneous outages resulting in the bus being split into two parts. The breaker and a half arrangement, shown in Figure 29, is the highest cost alternative and provides the highest reliability without limitation on the number of circuits. 6.3.2 Generator Leads 6.3.2.1 Cable a) Connections between the generator and switchgear bus where distribution is at generator voltage, and between generator and step up transformer where distribution is at 34.5 kV and higher, will be by means of cable or bus duct. In most instances more than one cable per phase will be necessary to handle the current up to a practical maximum of four conductors per phase. Generally, cable installations will be provided for generator capacities up to 25 MVA. For larger units, bus ducts will be evaluated as an alternative. b) The power cables will be run in a cable tray, separate from the control cable tray, in steel conduit suspended from ceiling or on wall hangers, or in ducts, depending on the installation requirements. c) Cable terminations will be made by means of potheads where lead covered cable is applied, or by compression lugs where neoprene or similarly jacketed cables are used. Stress cones will be used at 4.16 kV and above. d) For most applications utilizing conduit, cross-linked polyethylene with approved type filler or ethylene-propylene cables will be used. For applications where cables will be suspended from hangers or placed in tray, armored cable will be used to provide physical protection. If the cable current rating does not exceed 400 amperes, the three phases will be triplexed; i.e., all run in one steel armored enclosure. In the event that single-phase cables are required, the armor will be nonmagnetic. 92
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Simpo PDF Merge and Split Unregistered Version - http://www.simpopdf.com MIL-HDBK-1003/7 e) In no event should the current carrying capacity of the power cables emanating from the generator be a limiting factor on turbine generator output. As a rule of thumb, the cable current carrying capacity will be at least 1.25 times the current associated with kVA capacity of the generator (not the kW rating of the turbine). 6.3.2.2 Segregated Phase Bus a) For gas turbine generator installations the connections from the generator to the side wall or roof of the gas turbine generator enclosure will have been made by the manufacturer in segregated phase bus configuration. The three-phase conductors will be flat copper bus, either in single or multiple conductor per phase pattern. External connection to switchgear or transformer will be by means of segregated phase bus or cable. In the segregated phase bus, the three bare bus-phases will be physically separated by nonmagnetic barriers with a single enclosure around the three buses. b) For applications involving an outdoor gas turbine generator for which a relatively small lineup of outdoor metal clad switchgear is required to handle the distribution system, segregated phase bus will be used. For multiple gas turbine generator installations, the switchgear will be of indoor construction and installed in a control/switchgear building. For these installations, the several generators will be connected to the switchgear via cables. c) Segregated bus current ratings may follow the rule of thumb set forth above for generator cables, but final selection will be based on expected field conditions. 6.3.2.3 Isolated Phase Bus a) For steam turbine generator ratings of 25 MVA and above, the use of isolated phase bus for connection from generator to step up transformer will be used. At such generator ratings, distribution seldom is made at generator voltage. An isolated phase bus system, utilizing individual phase copper or aluminum, hollow square or round bus on insulators in individual nonmagnetic bus enclosures, provides maximum reliability by minimizing the possibility of phase-to-ground or phase-to-phase faults. b) Isolated phase bus current ratings should follow the rule of thumb set forth above for generator cables. 96
Simpo PDF Merge and Split Unregistered Version - http://www.simpopdf.com MIL-HDBK-1003/7 Section 7. STEAM CONDENSERS 7.1 Condenser Types 7.1.1 Spray Type. Spray condensers utilize mixing or direct contact of cooling water and steam. Cooling water is distributed inside the condenser in the form of a fine spray that contacts and condenses the steam. This type has application where dry cooling towers are used. Part of the condensate from the condenser is circulated through dry cooling towers and returned to and sprayed into the condenser. The balance of the condensate, which is equal to the steam condensed, is pumped separately and returned to the feedwater cycle. 7.1.2 Surface Type. Surface condensers are basically a shell and tube heat exchanger consisting of water boxes for directing the flow of cooling water to and from horizontal tubes. The tubes are sealed into fixed tube sheets at each end and are supported at intermediate points along the length of the tubes by tube support plates. Numerous tubes present a relatively large heat transfer and condensing surface to the steam. During operation at a very high vacuum, only a few pounds of steam are contained in the steam space and in contact with the large and relatively cold condensing surface at any one instant. As a result, the steam condenses in a fraction of a second and reduces in volume ratio of about 30,000:1. 7.1.2.1 Pass Configuration. Condensers may have up to four passes; one and two pass condensers are the most common. In a single pass condenser, the cooling water makes one passage from end to end, through the tubes. Single pass condensers have an inlet water box on one end and an outlet water box on the other end. Two pass condensers have the cooling water inlet and outlet on the same water box at one end of the condenser, with a return water box at the other end. 7.1.2.2 Divided Water Box. Water boxes may be divided by a vertical partition and provided with two separate water box doors or covers. This arrangement requires two separate cooling water inlets or outlets or both to permit opening the water boxes on one side of the condenser for tube cleaning while the other side of the condenser remains in operation. Operation of the turbine with only half the condenser in service is limited to 50 percent to 65 percent load depending on quantity of cooling water flowing through the operating side of the condenser. 7.1.2.3 Reheating Hotwell. The hotwell of a condenser is that portion of the condenser bottom or appendage that receives and contains a certain amount of condensate resulting from steam condensation. Unless the condenser is provided with a reheating hotwell (also commonly called a deaerating hotwell), the condensate, while falling down through the tube bundle, will be subcooled to a temperature lower than the saturation pressure corresponding to the condenser steam side vacuum. For power generation, condenser subcooling is undesirable since it results in an increase in turbine heat rate that represents a loss of cycle efficiency. Condenser subcooling is also undesirable because the condensate may contain noncondensible gases that could result in corrosion of piping and equipment in the feedwater system. Use of a deaerating hotwell provides for reheating the condensate within the condenser to saturation temperature that effectively deaerates the condensate and eliminates subcooling. Condensers should be specified to provide condensate effluent at saturation temperature corresponding to condenser vacuum and with an oxygen content not to exceed 0.005cc per liter of water (equivalent to 7 parts per billion as specified in the Heat Exchange Institute (HEI), Standards for Steam Surface Condenser, 1970. 7.1.2.4 Air Cooler Section. The condenser tubes and baffles are arranged in such a way as to cause the steam to flow from the condenser steam inlet toward the air cooler section. The steam carries with it the noncondensible gases such as air, carbon dioxide, and ammonia that leave the air cooler section through the air outlets and flow to air removal equipment. Any residual steam is condensed in the air cooler section. 7.2 Condenser Sizes. The proper size of condenser is dependent on the following factors: 97
Simpo PDF Merge and Split Unregistered Version - http://www.simpopdf.com MIL-HDBK-1003/7 a) Steam flow to condenser. b) Condenser absolute pressure. c) Cooling water inlet temperature. d) Cooling water velocity through tubes. e) Tube size (O.D. and gauge). f) Tube material. g) Effective tube length (active length between tube sheets). h) Number of water passes. i) Tube cleanliness factor. 7.2.1 Condenser Heat Load. For approximation, use turbine exhaust steam flow in pound per hour times 950 Btu per pound for non-reheat turbines or 980 Btu per pound for reheat turbines. 7.2.2 Condenser Vacuum. Condenser vacuum is closely related to the temperature of cooling water to be used in the condenser. For ocean, lake, or river water, the maximum expected temperature is used for design purposes. For cooling towers, design is usually based on water temperature from the tower and an ambient wet bulb that is exceeded not more than 5 percent of the time. The condenser performance is then calculated to 98
Simpo PDF Merge and Split Unregistered Version - http://www.simpopdf.com MIL-HDBK-1003/7 determine the condenser pressure with an ambient wet bulb temperature, that is exceeded not more than 1 percent of the time. Under the latter condition and maximum turbine load, the condenser pressure should not exceed 4 inches Hg Abs. Using the peak ambient wet bulb of record and maximum turbine load, the calculated condenser pressure should not exceed 4-1/2 inches Hg Abs. The turbine exhaust pressure monitor is usually set to alarm at 5 inches Hg Abs, which is near the upper limit of exhaust pressure used as a basis for condensing turbine design. 7.2.3 Cooling Water Temperatures 7.2.3.1 Inlet Temperature. Economical design of condensers usually results in a temperature difference between steam saturation temperature (ts) corresponding to condenser pressure and inlet cooling water temperature (t1) in the range of 20 degrees F (11 degrees C) to 30 degrees F (17 degrees C). Table 13 shows typical design conditions: Table 13 Typical Design Conditions for Steam Condensers +)))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))), * * Cooling Water Temp. Condenser Pressure ts - t1 * F(t1) In. Hg Abs Degrees, F * * * * 50 1.0 29.0 * * 55 1.0 - 1.25 24.0 - 30.9 * * 60 1.0 - 1.5 19.0 - 31.7 * * 65 1.5 - 1.75 26.7 - 31.7 * * 70 1.5 - 2.0 21.7 - 31.1 * * 75 2.0 - 2.25 26.1 - 30.1 * * 80 2.0 - 2.5 21.1 - 28.7 * * 85 2.5 - 3.0 23.7 - 31.1 * * 90 3.0 - 3.5 25.1 - 30.6 .))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))- 7.2.3.2 Terminal Difference. The condenser terminal difference is the difference in temperature between the steam saturation temperature (ts) corresponding to condenser pressure and the outlet cooling water temperature (t2). Economical design of condenser will result with t2 in the range of 5 degrees F (2.8 degrees C) to 10 degrees F (5.6 degrees C) lower than ts. The HEI Conditions limits the minimum terminal temperature difference that can be used for condenser design to 5 degrees F (2.8 degrees C). 7.2.3.3 Temperature Rise. The difference between inlet and outlet cooling water temperatures is called the temperature rise that will be typically between 10 degrees F (5.6 degrees C) and 25 degrees F (13.9 degrees C). 99
Simpo PDF Merge and Split Unregistered Version - http://www.simpopdf.com MIL-HDBK-1003/7 7.2.4 Tube Water Velocity. The maximum cooling water velocity through the tubes is limited by erosion of the inlet ends of the tubes and by the water side pressure drop (friction loss). Velocities in excess of 8 feet per second are seldom used. The normal tube water velocity ranges from 6 to 8 feet per second. Higher velocities provide higher heat transfer but will cause increased friction loss. Where conditions require the use of stainless steel tubes, the tube water velocity should be at least 7 feet per second to ensure that the tubes are continually scrubbed with oxygen for passivation of the stainless steel and maximum protection against corrosion. As a general rule, 7.5 feet per second water velocity is used with stainless steel tubes. When using admiralty tubes, water velocities should be limited to about 7 feet per second to prevent excessive erosion. Previous studies indicate that varying cooling water tube velocities from 6.8 to 7.6 feet per second has very little effect on the economics or performance of the entire cooling water system. 7.2.5 Tube Outside Diameter and Gauge. Condenser tubes are available in the following six outside diameters: 5/8-inch, 3/4-inch, 7/8-inch, 1-inch, 1-1/8 inch, and 1-1/4 inch. For power plants, 3/4-inch, 7/8-inch, and 1-inch OD tubes are the most prevalent sizes. As a general rule, 3/4-inch tubes are used in small condensers up to 15,000 square feet, 7/8-inch tubes are used in condensers between 15,000 and 50,000 square feet. Condensers larger than 50,000 square feet normally use at least 1-inch tubes. Condenser tubes are readily available in 14, 16, 18, 20, 22, and 24 gauge. For inhibited Admiralty or arsenical copper, 18 BWG tubes are normally used. For stainless steel tubes, 22 BWG tubes are normally used. 7.2.6 Tube Length. The length of tubes is important because of its direct relation to friction loss and steam distribution over the tube bundle. The selection of tube length depends on condenser surface required, space available for the installation, and cooling water pump power required. Normally, economical tube length for single pass condensers will fall in the ranges as shown in Table 14. Two pass condensers will normally have shorter tube lengths. Table 14 Typical Condenser Tube Length vs. Surface +)))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))), * * Tube Condenser * * Length Surface * * Feet Sq.Ft. * * * 16 - 24 Less than 20,000 22 - 30 20,000 to 50,000 30 - 36 50,000 to 100,000 32 - 44 100,000 to 500,000 .))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))- 100