[James_H._Harlow]_Electric_Power_Transformer_Engin(BookSee.org)

Recommendations

Info

3.5 Transformer Connections Dan D. Perco 3.5.1 Introduction In deciding the transformer connections required in a particular application, there are so many considerations to be taken into account that the final solution must necessarily be a compromise. It is therefore necessary to study in detail the various features of the transformer connections together with the local requirements under which the transformer will be operated. The advantages and disadvantages of each type of connection should be understood and taken into consideration. This section describes the common connections for distribution, power, HVDC (high-voltage dc) converter, rectifier, and phase-shifting transformers. Space does not permit a detailed discussion of other uncommon transformer connections. The information presented in this section is primarily directed to transformer users. Additional information can be obtained from the IEEE transformer standards. In particular, reference is made to IEEE Std. C57.12.70, Terminal Markings and Connections for Distribution and Power Transformers; C57.105, Application of Transformer Connections in Three-Phase Distribution Systems; C57.129, General Requirements and Test Code for Oil-Immersed HVDC Converter Transformers; C57.18.10, Practices and Requirements for Semiconductor Power and Rectifier Transformers; C57.12.20, Overhead-Type Distribution Transformers; and C57.135, IEEE Guide for the Application, Specification, and Testing of Phase-Shifting Transformers. 3.5.2 Polarity of Single-Phase Transformers The term polarity as applied to transformers is used to indicate the phase relationship between the primary and secondary windings of a given transformer or to indicate the instantaneous relative direction of voltage phasors in the windings of different transformers. This facilitates rapid and accurate connections of transformers in service. Transformer manufacturers have agreed to standardize the marking of terminals to indicate their polarity. For a single-phase, two-winding transformer, the high-voltage terminals are labeled H1 and H2, while the low-voltage terminals are labeled X1 and X2. When transformers are to be operated in parallel, like-marked terminals are to be joined together. Transformers can be either subtractive or additive polarity. When like-numbered terminals such as H1 and X1 are joined together, the voltage between the other open terminals will be the difference of the individual impressed winding voltages for a transformer with subtractive polarity. For additivepolarity transformers, the voltage between the open terminals will be the sum of the individual winding voltages. The standards specify subtractive polarity for all transformers except for single-phase transformers 200 kVA and smaller and having high-voltage windings 8660 volts and below. In either case, the polarity of the transformer is identified by the terminal markings as shown in Figure 3.5.1. Subtractive polarity has correspondingly marked terminals for the primary and secondary windings opposite each other. For additive polarity, like-numbered winding terminal markings are diagonally disposed. Transformers with subtractive polarity normally have the primary and secondary windings wound around the core in the same direction. However, the transformer can have subtractive-polarity terminal markings with the primary and secondary coils wound in the opposite directions if the internal winding leads are reversed. 3.5.3 Angular Displacement of Three-Phase Transformers Angular displacement is defined as the phase angle in degrees between the line-to-neutral voltage of the reference-identified high-voltage terminal and the line-to-neutral voltage of the corresponding identified low-voltage terminal. The angle is positive when the low-voltage terminal lags the high-voltage terminal. The convention for the direction of rotation of the voltage phasors is taken as counterclockwise. FIGURE 3.5.1 Single-phase transformer-terminal markings. FIGURE 3.5.2 Standard angular displacement for three-phase transformers. Since the bulk of the electric power generated and transmitted is three-phase, the grouping of transformers for three-phase transformations is of the greatest interest. Connection of three-phase transformers or three single-phase transformers in a three-phase bank can create angular displacement between the primary and secondary terminals. The standard angular displacement for two-winding transformers is shown in Figure 3.5.2. The references for the angular displacement are shown as dashed lines. The angular displacement is the angle between the lines drawn from the neutral to H1 and from the neutral to X1 in a clockwise direction from H1 to X1. The angular displacement between the primary and secondary terminals can be changed from 0 to 330 in 30 steps simply by altering the three-phase connections of the transformer. Therefore, selecting the appropriate three-phase transformer connections will permit connection of systems with different angular displacements. Figure 3.5.2 shows angular displacement for common double-wound three-phase transformers. Multicircuit and autotransformers are similarly connected. © 2004 by CRC Press LLC © 2004 by CRC Press LLC
3.5.4 Three-Phase Transformer Connections Three-phase connections can be made either by using three single-phase transformers or by using a three-phase transformer. Advantages of the three-phase transformer is that it costs less, the weight is less, it requires less floor space, and has lower losses than three single-phase transformers. Circuit computations involving three-phase transformer banks under balanced conditions can be made by dealing with only one of the transformers or phases. It is more convenient to use line-to-neutral values, since transformer impedances can then be added directly to transmission-line impedances. All impedances must be converted to the same side of the transformer bank by multiplying them by the square of the voltage ratio. There are two basic types of three-phase transformers, core type and shell type. The magnetic circuit of the shell type is very similar to three single-phase transformers. This type of transformer has a return circuit for each phase of the magnetic flux. Consequently, the zero-sequence impedance is equal to the positive-sequence impedance. The conditions with respect to magnetizing currents and zero-sequence impedances are essentially the same as for single-phase transformers connected in the same way. The center-phase coil is usually wound in a direction opposite to that of the two outer phases in order to decrease the core yoke required between phases. This reversal of polarity is corrected when the leads are terminated. The magnetic circuit of each phase of a three-limb core-type transformer is mutually connected in that the flux of one phase must return through the other two phases. In this type of transformer, the total instantaneous magnetic flux in each core section due to the fundamental excitation current is zero. However, in the wye–wye-connected transformer, there are third-harmonic voltages in the phases caused by the third-harmonic current. These voltages and the resulting magnetic flux are all induced in the same direction. Since there is no return path for this flux in the core, the flux must return through the relatively low-reluctance path outside the core. The core-type transformer is occasionally manufactured with a five-limb core. In this case, the magnetic circuit and performance characteristics are similar to that of shell-type or single-phase transformers. Unbalanced system faults and loads can cause significant zero-sequence magnetic flux to occur for some three-phase connections. Unless a magnetic return path for this flux is provided, the flux returns outside the core and can cause eddy-current heating in other transformer conductive components, such as the tank. The existence of zero-sequence flux either within or outside the core depends on both core configuration and winding connections. Three-phase transformer-core assemblies do not usually provide full-capacity return legs for zero-sequence flux. Thus, if sufficient zero-sequence flux occurs, it will be forced to return outside the core. Three-phase transformer connections can be compared with each other with respect to: • Ratio of kVA output to the kVA internal rating of the bank • Degree of voltage symmetry with unbalanced phase loads • Voltage and current harmonics • Transformer ground availability • System fault-current level • Switching and system fault and transient voltages In some cases, there may also be other operating characteristics to determine the most suitable connection for each application. 3.5.4.1 Double-Wound Transformers The majority of three-phase transformer connections are made by connecting the individual phases either between the power-system lines, thus forming a delta connection, or by connecting one end of each phase together and the other ends to the lines, thus forming a wye (also referred to as star) connection. For these connections, the total rating of the internal windings is equal to the through-load rating. This accounts for the popularity of these connections. For all other double-wound transformer connections, the ratio is less than unity. For example, in the interconnected star or zigzag connection, the transformer is capable of delivering a load equal to only 86.6% of the internal winding rating. Since the cost of a transformer varies approximately with the three-quarter power of the internal kVA, the cost of an interconnected star or zigzag transformer is approximately 5% higher than for a similar double-wound transformer. All of these types of three-phase connections are shown in Figure 3.5.2. 3.5.4.1.1 Wye–Wye Connections of Transformers Joining together the terminals of similar windings with the same polarity derives the neutral of the wye connection. This neutral point is available and can be brought out for any desired purpose, such as grounding or zero-sequence current measurements and protection. For high-voltage transmission systems, the use of the wye-connected transformer is more economical because the voltage across the phase of each winding is a factor of 1.73 less than the voltage between the lines. If the neutral point is grounded, it is not necessary to insulate it for the line voltage. If the neutral is not grounded, the fault current during a system line to ground fault is insignificant because of the absence of a zero-sequence current path. If the neutral is grounded in the wye–wye transformer and the transformer is made with a three-limb core, the zero-sequence impedance is still high. As a result, the fault currents during a system line-to-ground fault are relatively low. For wye–wye transformers made of three single-phase units or with a shell-type or five-limb core-type, the zerosequence impedance is approximately equal to the positive-sequence impedance. The fault current during a system ground fault for this case is usually the limiting factor in the design of the transformer. In all types of wye-wye transformer connections, only the transformer positive-sequence impedance limits the fault current during a system three-phase system fault. With the wye connection, the voltages are symmetrical as far as the lines are concerned, but they introduce third-harmonic (or multiples of the third harmonic) voltage and current dissymmetry between lines and neutral. The third-harmonic voltage is a zero-sequence phenomenon and thereby is exhibited in the same direction on all phases. If the transformer and generator neutrals are grounded, thirdharmonic currents will flow that can create interference in telephone circuits. If the transformer neutral is not grounded, the third-harmonic voltage at the neutral point will be additive for all three phases, and the neutral voltage will oscillate around the zero point at three times the system frequency. Thirdharmonic voltages are also created on the lines, which can subject the power system to dangerous overvoltages due to resonance with the line capacitance. This is particularly true for shell-type threephase transformers, five-limb core-type transformers, and three-phase banks of single-phase transformers. For any three-phase connection of three-limb core-form transformers, the impedance to thirdharmonic flux is relatively high on account of the magnetic coupling between the three phases, resulting in a more stabilized neutral voltage. A delta tertiary winding can be added on wye–wye transformers to provide a path for third-harmonic and zero-sequence currents and to stabilize the neutral voltage. The tertiary in this application will be required to carry all of the zero-sequence fault current during a system line-to-ground fault. The most common way to supply unbalanced loads is to use a four-wire wye-connected circuit. However, the primary windings of the transformer bank cannot be wye-connected unless the primary neutral is joined to the neutral of the generator. In this case, a third-harmonic voltage exists from each secondary line-to-neutral voltage because the generator supplies a sinusoidal excitation current. The third-harmonic currents created by the third-harmonic voltages can be a source of telephone interference. If the primary neutral is not connected to the generator, single-phase or unbalanced three-phase loads in the secondary cannot be supplied, since the primary current has to flow through the high impedance of the other primary windings. 3.5.4.1.2 Delta–Delta Connection The delta–delta connection has an economic advantage over the wye–wye connection for low-voltage, high-current requirements because the winding current is reduced by a factor of 1.73 to 58% of that in the wye–wye connection. © 2004 by CRC Press LLC © 2004 by CRC Press LLC
Page 1 and 2:
ELECTRIC POWER TRANSFORMER ENGINEER
Page 3 and 4:
I wish to recognize the interest of
Page 5 and 6:
Contents 1 Theory and Principles Ch
Page 7 and 8:
1.3 Equivalent Circuit of an Iron-C
Page 9 and 10:
designer starts to make a design fo
Page 11 and 12:
• Transient voltages generated du
Page 13 and 14:
the transformer’s bushings and, m
Page 15 and 16:
% regulation = [(V NL - V FL )/V FL
Page 17 and 18:
In core-form transformers, the wind
Page 19 and 20:
FIGURE 2.1.14 Layer windings (singl
Page 21 and 22:
the transformer design, which may b
Page 23 and 24:
consumer’s service circuit is a d
Page 25 and 26:
FIGURE 2.2.3 Single-phase transform
Page 27 and 28:
is installed so that the tank never
Page 29 and 30:
above which persons might be burned
Page 31 and 32:
FIGURE 2.2.16 Two-bushing subway. (
Page 33 and 34:
carbon steel or the very expensive
Page 35 and 36:
FIGURE 2.2.31 Radial-style dead fro
Page 37 and 38:
FIGURE 2.2.36 Complete transformer
Page 39 and 40:
voltage in each winding simultaneou
Page 41 and 42:
mounted transformers can have arres
Page 43 and 44:
V S + V S I V L Z=R+jX a) V L V S V
Page 45 and 46:
Z = jX (2.3.19) 0.7 Then the curren
Page 47 and 48:
X1 L* X1 S =X1 L X2 L* X3 L* a) S L
Page 49 and 50:
150 V(%) 100 50 0 -50 40 80 T(s) 12
Page 51 and 52:
2.3.8.1.2 Series Connection • The
Page 53 and 54:
A six-pulse single-way transformer
Page 55 and 56:
The degree of coupling also influen
Page 57 and 58:
where I h = current for the hth har
Page 59 and 60:
In the case of liquid-filled transf
Page 61 and 62:
FIGURE 2.4.21 A 36-pulse system mad
Page 63 and 64:
Vacuum-pressure encaps ulated — T
Page 65 and 66:
FIGURE 2.6.1 Typical wiring and sin
Page 67 and 68:
a) H1 b) Ip X2 FLUX H2 LEAKAGE FLUX
Page 69 and 70:
and for CTs is TCF = RCF - (PA/2600
Page 71 and 72:
RCF x Bx Bt RCF t - RCF 0 co
Page 73 and 74:
2000 1000 5% Vex BANDWITH FROM NOMI
Page 75 and 76:
This may be more useful when operat
Page 77 and 78:
also some uncertainty in repeatabil
Page 79 and 80:
FIGURE 2.6.25 Standard two-element
Page 81 and 82:
2.7 Step-Voltage Regulators Craig A
Page 83 and 84:
Source Bypass Switch Source Bypass
Page 85 and 86:
2.7.4 Theory A step-voltage regulat
Page 87 and 88:
FIGURE 2.7.22 Equalizer winding inc
Page 89 and 90:
TABLE 2.7.4 Increase in Ampere Load
Page 91 and 92:
References IEEE, Standard Requireme
Page 93 and 94:
300% FIGURE 2.8.4 Schematic of a co
Page 95 and 96:
TABLE 2.8.1 Typical Sizing Workshee
Page 97 and 98:
Input with Interruption Ferro Outpu
Page 99 and 100:
FIGURE 2.8.20A Performance of a rid
Page 101 and 102: • Input frequency or frequency ra
Page 103 and 104: References 1. Sola/Hevi-Duty Corp.,
Page 105 and 106: FIGURE 2.9.5 Typical phase-reactor
Page 107 and 108: FIGURE 2.9.10 Two generator arrange
Page 109 and 110: • Overloading of lines • Increa
Page 111 and 112: FIGURE 2.9.23 34-kV, 25-MVAR (per p
Page 113 and 114: V 90 ( X X ) s T (2.9.10a) where,
Page 115 and 116: Therefore, Line reactive losses = (
Page 117 and 118: ate of rise of transient-recovery v
Page 119 and 120: the LLCR is provided in IEEE Std. C
Page 121 and 122: aluminum shielding, i.e., an oil-im
Page 123 and 124: Leo J. Savio ADAPT Corporation Ted
Page 125 and 126: 3.1.2.2.2 Heat Dissipation A second
Page 127 and 128: FIGURE 3.2.1 Solid-type bushing. ma
Page 129 and 130: 1.6 cm. This means that most of the
Page 131 and 132: where HS = steady-state bushing ho
Page 133 and 134: the conductivity of the water-solub
Page 135 and 136: of 178 ohm-m, and a test duration o
Page 137 and 138: 2. Easley, J.K. and Stockum, F.R.,
Page 139 and 140: FIGURE 3.3.5 Reactance-type LTC, in
Page 141 and 142: FIGURE 3.3.10 LTC with delta connec
Page 143 and 144: 3.3.5 Selection of Load Tap Changer
Page 145 and 146: FIGURE 3.3.19 Effect of the leakage
Page 147 and 148: References Goosen, P.V. , Transform
Page 149 and 150: If i 1 is the full-load current rat
Page 151: TABLE 3.4.1 Loading Recommendations
Page 155 and 156: FIGURE 3.5.4 Interconnected star-gr
Page 157 and 158: 3.6.1.1 Standards ANSI standards fo
Page 159 and 160: ANSI standards. LTC control setting
Page 161 and 162: FIGURE 3.6.7 Discharging the capaci
Page 163 and 164: FIGURE 3.6.10 Standard switching-im
Page 165 and 166: voltage of the excited winding, rea
Page 167 and 168: FIGURE 3.6.18 Current, voltage, and
Page 169 and 170: TABLE 3.6.3 Transformer Short-Circu
Page 171 and 172: FIGURE 3.7.3 Control for voltage re
Page 173 and 174: FIGURE 3.7.6A Feeder with power-fac
Page 175 and 176: 3. Circulating current (current bal
Page 177 and 178: FIGURE 3.7.10 Control block diagram
Page 179 and 180: Primary Current (Amps) 60 40 20 0 -
Page 181 and 182: current transformer having two prim
Page 183 and 184: 87R1 87BL1 (A) Independent Harmonic
Page 185 and 186: Winding 1 Secondary Currents Data A
Page 187 and 188: Y Y 20 18 3 rd Harmonic CTR1=40 CTR
Page 189 and 190: 230 kV 3 180 MVA 138 kV 5 CTR1 = 2
Page 191 and 192: 29. Concordia, C. and Rothe, F.S.,
Page 193 and 194: FIGURE 3.9.1 Measurement of pressur
Page 195 and 196: H 2m 1. Vertical Forced Air 2. Prin
Page 197 and 198: Gade, S., Sound Intensity Instrumen
Page 199 and 200: nected. This steady-state voltage d
Page 201 and 202: where [A] = state matrix [B] = inpu
Page 203 and 204:
where C = capacitance between the t
Page 205 and 206:
L ijo = N i N j ijo (3.10.31) 1.4
Page 207 and 208:
% VOLTAGE % VOLTAGE 100 BIL 90 50 3
Page 209 and 210:
29. Degeneff, R.C., Reducing Storag
Page 211 and 212:
All connections should be cleaned a
Page 213 and 214:
6. Costs to set up and use a mobile
Page 215 and 216:
Records of relay operation must be
Page 217 and 218:
• Has heating occurred as the res
Page 219 and 220:
a reference value) of the currents
Page 221 and 222:
The next data-processing step is to
Page 223 and 224:
temperature. As the transformer coo
Page 225 and 226:
3.13.3.2 Instrument Transformers Th
Page 227 and 228:
FIGURE 3.13.5 Analysis of bushing s
Page 229 and 230:
temperature. Under abnormal conditi
Page 231 and 232:
3.14.1.2 Accredited Standards Commi
Page 233 and 234:
FIGURE 3.14.4 IEC technical-committ
Page 235 and 236:
TABLE 3.14.2 Relevant Documents for
Page 237 and 238:
Page 239 and 240:
Page 241:
TABLE 3.14.13 Relevant Documents fo
show all

[James_H._Harlow]_Electric_Power_Transformer_Engin(BookSee.org)

Create successful ePaper yourself

Delete template?

Save as template?