CONTENTS AT A GLANCE Overview
Transformers, General Transformer Characteristics Glossary of Transformer Terms High-Voltage Circuit Breakers Circuit Switchers
Reclosers
Interrupter Switches
Voltage Regulators Power Factor
Primary and Secondary Power Capacitors
Kilovarmeters
Synchronous Condensers Phase Converters
Power Semiconductor Devices
3
Overview
This chapter covers the functions performed by high-voltage power transmission and distribution equipment in the electrical power industry. Basic transformer theory is discussed here to highlight the similarities in construction and operation of all formers, whether they are low-power units or high-voltage transformers used in trans-mission and distribution. The transformer configurations used in single-phase and three-phase applications are discussed. In addition, the more specialized voltage and current transformers for high-voltage instruments as well as autotransformers are explained. The transformer section of this chapter includes a glossary of transformer terms for ready reference.
The circuit breakers discussed in this chapter are rated for more than 600 V. They operate on different principles than the more familiar low-voltage units and have differ-ent distinguishing features. (Residdiffer-ential low-voltage circuit breakers are discussed in Chap. 5.) The functions of high-voltage circuit switchers, reclosers, interrupter switch-es, and voltage regulators in power transmission and distribution systems are explained.
To make the most effective use of AC power, it is important that inductive reactances be minimized to bring the voltage and current waveforms into a closer phase relationship.
The subjects of power factor and power factor correction are discussed, and the role of power capacitors in power factor correction is explained. The use of the kilovarmeter to monitor power factor and capacitive reactance during typical 24-hr periods is discussed.
The way in which rotary or static phase converters convert single-phase to three-phase power is explained. Power semiconductors are playing an increasingly important role in power rectification and control. The semiconductor devices most widely used in the performance of those functions are identified and explained.
Transformers, General
A transformer is a static electrical component with no moving parts that is used for step-ping voltage up or down or isolating one circuit from another. Transformers have the abil-ity to convert low-voltage, high-current AC to high-voltage, low-current AC, or vice versa, with minimal energy losses. Minimizing energy losses is critical in all power generation, transmission, and distribution systems. Transformers work only with AC in accordance with the physical laws of magnetic induction, and they are inherently low-loss compo-nents. The simplest low-voltage transformers can be made by winding separate coils of insulated wire around a ferromagnetic core, typically a stack of steel laminations.
When one coil or winding, called the primaryor input coil,is energized, the core is magnetized so that the resulting magnetic flux induces a voltage in the second wind-ing, called the secondaryor output coil.The change in voltage (voltage ratio) between the primary and secondary coils depends on the number of turns in each winding.
Transformers are widely used in electrical power and lighting circuits as well as many low-voltage electronic products. The large transformers in power generation
sta-tions step up the output voltage of AC generators to higher values for more efficient transmission over transmission lines while also reducing the current values. Somewhat smaller transformers at electrical substations step the transmitted voltage down to the values more useful for regional and local distribution to customers while also stepping up the current.
Some of the smallest commercial transformers are found in the AC-to-DC convert-ers that convert line AC voltages to the low DC voltages required for powering elec-tronic products including cordless telephones, notebook computers, and cellular telephone battery chargers. However, transformers are the largest and heaviest com-ponents in the stand-alone linear power supplies for industrial, military, and commer-cial applications. The 60-Hz transformers built into TV sets and stereo systems are also large and heavy, but the high-frequency switching transformers in desktop and laptop computers are considerably smaller and lighter.
Transformers can also isolate circuits, suppress harmonics, and regulate line volt-age between distribution substations and consumers. Zigzag grounding transformers, for example, derive neutrals for grounding and a fourth wire from a three-phase neu-tral wire. They can be operated at voltages below their nameplate ratings, but they should not be operated at higher voltages unless they have taps intended for that pur-pose. However, when a transformer is operated below its nameplate rated voltage, its kVA output is reduced correspondingly.
Single-phase transformers rated 1 kVA and larger and three-phase transformers rated 15 kVA and larger can be reverse-connected without a loss of kVA rating. This is possible with high-power transformers because their turn ratios are the same as their voltage ratios. The turns ratio compensation on a low-voltage winding of a single-phase transformer rated below 1 kVA rules this out, because the low-voltage winding has a higher voltage than is indicated by the nameplate at no load. Although the trans-former will not be harmed, its output voltage when reverse-connected will be lower than is stated on its nameplate.
TRANSFORMER CLASSIFICATION
Transformers are classified in many different ways: dry- or liquid-insulated, single-phase or polysingle-phase, step-up or step-down, and single-winding or multiwinding. In addition, they are classified by application. For example, there are voltage or potential transformers (VTs) and current transformers (CTs) that are used step high voltage and current down to safe levels for the measurement of voltage, current, and power with conventional instruments. The transformers discussed in this chapter are
■ Autotransformers
■ Auto zigzag grounding transformers
■ Buck-boost autotransformers
■ Current transformers
■ Distribution transformers
■ Substation transformers
■ Voltage transformers
TRANSFORMERS, GENERAL 59
In addition to the many different kinds of transformers, there are many different ways to connect them. These include delta-to-delta, delta-to-wye, wye-to-delta, and T.
NEC TRANSFORMER REQUIREMENTS
NEC 2002 Article 450, Transformers and Transformer Vaults (Including Secondary Ties),states the requirements for the installation of transformers. It covers overcurrent protection for transformers rated for more or less than 600 V, autotransformers rated for 600 V or less, grounding autotransformers, secondary tiers, parallel operation, guarding ventilation, and grounding. Specific provisions apply to different types of transformers such as dry-type and liquid-insulated for both indoor and outdoor instal-lation. Transformer vault location, construction, ventilation, and drainage require-ments are also given.
The NEChas defined a secondary tieas a circuit operating at 600 V or less between phases that connect two power sources of power supply points, such as the secondaries of two transformers.
SUBSTATION TRANSFORMERS
Power is transmitted at high voltages because less current is required to transmit a given amount of energy at a higher than a lower voltage. Consequently, electrical ener-gy can be transmitted with less I2Ror line loss when high transmission voltages are used. Transmission voltages as high as 500 kV can only be obtained from step-up transformers at power stations, because AC generators cannot generate voltages at those values.
Step-down transformers in distribution systems reduce the high transmission voltages to the values needed by industrial, commercial, and residential consumers. Large power transformers rated at 35 kV or more can reach efficiencies of 99 percent at full load.
Figure 3-1 shows a primary or secondary open substation transformer that can trans-form transmission voltages to useful levels. Both three-phase and single-phase substa-tion transformers can be constructed in this configurasubsta-tion. They are made from a wide range of steel cores and winding conductors to meet specific substation requirements.
The transformers can be either wound-core or shell-type (these are discussed later).
Their core and coil assemblies are placed in steel tanks that are filled with either elec-trical-grade mineral insulating oil or another suitable dielectric coolant. These trans-formers can be self-cooled or forced-air-cooled. In addition, they can be built specifically for indoor or outdoor installation.
Transformer Characteristics
Figure 3-2 is a diagram of a conventional (core-type) single-phase transformer. It shows two independent coils or windings, the primary and secondary, wound on a closed-ring core made from a stack of laminated sheet steel.
If an AC voltage is applied to the primary winding of the transformer, an electro-magnetic field or flux forms around the core and expands and contracts at the input frequency. This changing flux cuts the wires in the secondary winding and induces a voltage in it. Because the turns of both windings are cut by the same magnetic flux, the voltage induced in each turn of both windings is the same. Thus the voltages across the windings of a transformer are directly proportional to the turns in each winding:
Primary ampere-turns secondary ampere-turns
From this expression, it can be determined that the ratio of the currents in a trans-former is inversely proportional to the ratio of the turns. The voltage that appears
TRANSFORMER CHARACTERISTICS 61
LIQUID-LEVEL GAUGE
FILLING/FILTER PRESS CONNECTION
PRESSURE RELIEF DEVICE
DISTRIBUTION CLASS ARRESTER
TANK LIFTING LUGS
NAMEPLATE
GROUND PADS
DRAIN/FILTERING VALVE BASE
PRESSURE/VACUUM GAUGE
LIQUID
TEMPERATURE GAUGE
DE-ENERGIZED TAP CHANGER
Figure 3-1 Secondary open substation transformer. Courtesy Cooper Power Systems
across the secondary winding depends on the voltage at the primary winding and the ratio of turns in the primary and secondary windings.
Transformers obey the law of conservation of energy,meaning that the product of voltage and current (power) in the primary winding equals the product of voltage and current (power) in the secondary winding, except for losses. For example, if the voltage at the secondary terminals is twice the voltage at the primary terminals, the current at the secondary terminals must be half that at the primary terminals to keep the product of voltage and current constant.
A step-up transformerhas more turns in its secondary winding than in its primary winding, so the voltage across the secondary winding will be higher than the voltage across the primary winding. Similarly, a step-down transformer,as shown in Fig. 3-2, has fewer turns in its secondary winding than in its primary winding, so the secondary voltage will be lower than the primary voltage.
The letters identifying the input and output terminals in Fig. 3-2 follow the industry standard marking code. High-voltage winding terminals are marked H1, H2, etc., and low-voltage winding terminals are marked X1, X2, etc. Thus, the high-voltage winding is commonly called the H winding,the low-voltage winding is commonly called the X winding,and the numbers of turns of each winding are designated as Th and Tx.
POWER TRANSFORMER RATING
Power transformer capacity is rated in kilovolt-amperes(kVA). The output rating for a transformer is determined by the maximum current that the transformer can withstand without exceeding its stated temperature limits. Power in an AC circuit depends on the power factor of the load and the current, so if any AC electrical equipment is rated in kilowatts, a power factor must be included to make its power rating meaningful. To avoid this, transformers and most AC machines are rated in kVA, a unit that is inde-pendent of power factor.
In addition to its kVA rating, the nameplates of transformers typically include the manufacturer’s type and serial number, the voltage ratings of both high- and low-voltage windings, the rated frequency, and the impedance drop expressed as a
per-Figure 3-2 Schematic diagram of a basic transformer.
centage of rated voltage. Some nameplates also include an electrical connection diagram.
Power transformers are generally defined as those used to transform higher power levels than distribution transformers (usually over 500 kVA or more than 67 kV). The kVA terminal voltages and currents of power transformers, defined in ANSI C57.12.80, are all based on the rated winding voltages at no-load conditions. However, the actual primary voltage in service must be higher than the rated value by the amount of regulationif the transformer is to deliver the rated voltage to the load on the secondary.
TRANSFORMER CONSTRUCTION
The two most common types of transformer construction are the core or form type and the shell type. The core-type transformershown in Fig. 3-2 has an open rectangular laminated steel core with the primary winding wound on the left “leg” of the core and the secondary winding wound on the right “leg.”
A transformer could be wound as shown in Fig. 3-2, but the separation distance between the primary and secondary windings would mean that much of the primary winding flux would not cut the secondary winding, resulting in transformer loss called leakage flux. To minimize leakage flux in commercial core-type transformers, the windings are divided, with half of each winding being placed on each leg of the core.
The low-voltage winding is wound around the core and the high-voltage winding is wound over the low-voltage winding.
In the alternate shell-type transformershown in Fig. 3-3, both the primary and sec-ondary coils are wound on the central “leg” of the core. In shell-type transformers the magnetic circuit is short and the length of the windings is long. Typically the shell-type transformer has a larger core area and a smaller number of winding turns than a core-type transformer with the same output and performance. Also, the shell core-type typically has a greater ratio by weight of steel to copper. The laminated steel cores are made from stacks of E- and I-shaped stampings assembled around toroidal bobbins.
Stacked power transformers cores are made from a wide range of core steels to minimize core loss. High-voltage primaries are wound with wire, typically either aluminum or insulated copper magnet wire, and secondary windings are wound from sheet metal strips, typically aluminum. Windings are typically insulated
TRANSFORMER CHARACTERISTICS 63
Figure 3-3 Shell-type trans-former core construction.
between layers with adhesive-coated, thermally upgraded paper. The cores are made of single-turn laminations cut and formed so that each lamination completes a magnetic circuit. The laminations are assembled through and around the coil in a staggered joint pattern to keep core loss and exciting current to a minimum.
Most single- and three-phase power transformers are custom-made by transformer manufacturers to meet customer requirements. These can usually be met with either core- or shell-type construction. The purchase decision is usually based on the manu-facturer’s recommendations for the end-use application, cost, and the firm’s manufac-turing facilities. Customers can, however, state preferences based on field experience or other technical considerations. Both core- and shell-type transformers can be made with ratings up to about 300 kVA, but there could be a significant difference in price. They can be dry or oil-immersion units, for which additional insulation and cooling are required.
The wound-core transformeris yet another type of construction. It has a core that is formed by winding a strip of silicon steel into a tight spiral around the insulated wind-ings. However, its maximum kVA ratings are lower than can be obtained from the other two types of construction.
TRANSFORMER LOSSES AND EFFICIENCY
The efficiency of all power transformers is high, but efficiency is highest for large transformers operating at 50 to 100 percent of full load. However, some losses are pre-sent in all transformers. They are classified as copperor I2R lossesand core losses.
Copper losses, also called load losses,are proportional to the load being supplied by the transformer. These losses can be calculated for a given load if the resistances of both windings are known. As in generators and motors, the core loss is due to eddy-current induction lossand hysteresis(molecular friction) loss,caused by the changing polarity of the applied AC. If the cores are laminated from low-loss silicon steel, both eddy-cur-rent and hysteresis losses will be reduced. Nevertheless, well-designed transformers in all frequency and power ranges typically have efficiencies of 90 percent or more.
TRANSFORMER COOLING
When a large transformer is operating under load, heat is generated in both the wind-ings and the core, due to copper and core losses. The methods used for cooling trans-formers depend on their size, rating, application, and location. Power transmission and distribution transformers can be cooled by forced air, circulating water, electrical-grade mineral oil, or other suitable dielectric coolants.
The cooling liquid provides additional insulation between the windings, and it con-ducts heat from the windings to the conductive sidewalls and surface of the tank, where it can be dissipated in the surrounding air. The liquid circulates through the tank by natural convection because of temperature differences in the liquid. Some liquid-cooled transformer tanks are made with cooling fins so that the coolant has contact with a larger radiating surface. Where conditions require it, the oil can be circulated by pumps or the oil cooling can be supplemented by water cooling, forced air, or both.
Air-cooled dry transformers are specified where oil-cooled transformers located indoors in industrial facilities would present a fire hazard. The windings are wound so that the natural circulation of air will dissipate the heat from the windings and core.
These transformers can be enclosed in perforated metal cases to allow maximum cir-culation of the air through the windings. Where conditions require it, natural trans-former cooling can be supplemented by forced-air circulation.
AUTOTRANSFORMERS
An autotransformeris a special transformer consisting of a single continuous winding that is used for both input and output voltages. Because the primary and secondary windings are the same, they are connected electrically as well as magnetically.
Autotransformers have features that can make them superior to two-winding trans-formers for many applications because of their lower cost, greater efficiency, smaller size and weight, and better regulation.
Figure 3-4ais a schematic for a step-down autotransformer. The entire winding acforms the primary winding, and section abforms the secondary winding. It can be seen that section abis common to both the primary and secondary windings. As in the standard two-winding transformer, the ratio of voltage transformation is equal to the ratio of primary to secondary turns, if the losses are neglected. The fol-lowing relationships apply:
The autotransformer winding shown in Fig. 3-4bhas a total of 230 turns with sections aband bc,which have 160 and 70 turns, respectively. If a voltage of 460 V is applied
Ix Ih Th
Tx Eh
Ex
TRANSFORMER CHARACTERISTICS 65
Figure 3-4 Autotransformers: (a) schematic of a step-down autotransformer;
(b) wiring diagram of an autotransformer supplying a load.
to the winding ac,the voltage across each turn will be 2.0 V, and the voltage from ato bwill then be 160 2.0 or 320 V.
If a noninductive load of 50 is connected to winding ab,a current Ix of 320/50, or 6.4 A, flows, and the power output of the transformer is 320 6.4, or 2048 W.
Neglecting the transformer losses, the power input must be 2048 W and the primary current must be 2048/460, or 4.5 A.
Thus the section of the winding that is common to both the primary and secondary circuits carries only the difference between the primary and secondary currents. For this reason, an autotransformer uses less copper wire in its windings and is more effi-cient than a comparably rated conventional two-winding transformer.
If a multivoltage supply is required for an application, an autotransformer with mul-tiple taps can provide the required output voltages. The connections from the various taps are brought out to terminals or a switching matrix so that any of a number of volt-ages can be selected.
Autotransformers are most effective for voltage transformations near unity. For example, an autotransformer can boosta distribution voltage by a small increment to compensate for line drop. Autotransformers can also start AC motors because they permit a reduced voltage to be applied to the motor during the start-up period.
Autotransformers are not recommended for supplying a low voltage from a high-voltage supply, because if the winding common to both primary and secondary should open accidentally, the full primary voltage will appear across the secondary terminals. This could damage connected equipment or circuitry and pose a shock hazard for personnel.
TAP CHANGING
High-voltage windings of substation and distribution transformers are equipped with tap-changing devices to compensate for drops in line voltage and to make small adjustments in the transformer ratio. The winding is tapped in several places, and con-nections are made from these places to a tap-changing switch or a terminal block inside the transformer tank. By operating the switch or changing the connections to the terminal block, changes can be made in the number of active turns in the winding.
Taps are typically either two 2.5 percent above and below or four 2.5 percent below rated voltage. These changes are usually made with the transformer offline, but some transformers have external tap-changer switches that can be operated outside the tank for safe operation under load.
SINGLE-PHASE DISTRIBUTION TRANSFORMERS
Single-phase distribution transformers are usually made with the secondary or low-volt-age windings in two sections, as shown in Fig. 3-5. The two sections can be connected in series as shown in Fig. 3-5ato supply a three-wire, 120/240-V load. This connection is widely used by power companies to provide both 120 and 240 V to residential and commercial customers. Alternatively, the two sections can be connected in parallel as shown in Fig. 3-5bto supply a two-wire 120-V load. Power companies use this