Distribution
Transformer
Guide
Distribution Transformer Division Athens, Georgia
Jefferson City, Missouri June, 1979
Revised March, 2002
Foreword
The purpose of this guide is to assemble fundamental information concerning common ratings, connections, and applications of distri-bution transformers. The information presented is a summary of these fundamentals and is intended as a reference for those who deal occasionally with distribution transformer applications. This guide does not purport to cover all aspects of selection and application; if ques-tions arise or further details are required, contact ABB Inc.
2
Index
I. General
PageA. Application ... 4
B. Physical Description ... 4
C. Protection and Accessories ... 11
II. Performance
A. Designation of Winding Voltage Ratings ... 17B. Polarity ... 20
C. Terminal Designations ... 21
D. Short Circuit Ratings ... 22
E. Sound Level Ratings ... 22
F. Tolerance Definitions ... 23
G. Impedance Calculations ... 23
H. Efficiency Calculations ... 24
I. Regulation Calculations ... 24
J. Performance Example ... 26
K. Secondary Fault Current—120/240 Volt Systems ... 27
III. Three-Phase Transformers and Banks
A. Application Considerations ... 34B. Summary of Common Connections ... 41
C. Common Three-Phase Banks Using Single-Phase Transformers ... 48
IV. Loading
A. Paralleling ... 51B. Delta-Delta Bank Loading ... 51
C. Overloading ... 52
D. Single-Phase and Three-Phase Loading of Symmetrical and Unsymmetrical Transformer Banks ... 53
E. Dedicated Motor Loads ... 66
V. Voltage Unbalance
A. Effects of Voltage Unbalance ... 71B. Voltage Unbalance Definitions ... 71
C. Causes of Voltage Unbalance ... 73
D. Voltage Unbalance With Three-Phase Loading ... 73
1. Delta-Delta and Floating Wye-Delta Banks ... 74
2. Open-Delta Banks ... 75
VI. Reference Data
Solid and Concentric Stranded Aluminum and Copper Conductors ... 80Temperature Correction Factors for Resistance of Aluminum Conductors ... 81
Logarithm Tables ... 83
Nominal direct-Current Resistance, Ohms per 1000 Feet, at 20°C and 25°C of Solid and Concentric Stranded Conductors .... 85
Natural Functions of Angles ... 86
Typical Isokeraunic Map ... 87
I. General
PageA. Application ... 4
B. Physical Description ... 4
1. Pole Mounted ... 4
2. Pad Mounted ... 6
C. Protection and Accessories ... 11
1. General ... 11
2. Types of Accessories and Transformer Protection Packages — Pole Mounted ... 11
4
I. General
A. ApplicationABB single-phase and three-phase, oil-filled, pole- and pad-mounted distribution transformers are specifically designed for servicing residential distribution loads; they are also suitable for light commercial loads, and industrial lighting and diversified power applications.
The transformers described herein are designed for the applica-tion condiapplica-tions normally encountered on electric utility power distribution systems. As such they are suitable for use under the “usual service conditions” described in ANSI C57.12.00 General Requirements for Liquid-Immersed Distribution, Power and Regulating Transformers. All other conditions are considered “unusual service” and should be avoided unless specific ABB Division approval is obtained.
B. Physical Description 1. Pole Mounted•
Meets Industry Standard ANSI C57.12.20•
0.5 - 1000kVA•
65° C temperature rise•
Insulation levels:Rated Insulation Basic Impulse
Voltage Ranges Class Level (kV)
480- 600 1.2 30 2160- 2400 5.0 60 4160- 4800 8.7 75 7200-124701 15.0 95 13200-14400 18.0 125 19920-229002 25.0 150 -34400 34.5 200
1Optional 125 kV BIL 12000 volts available 2Optional 125 kV BIL 19920 volts available
Type CSP Type S
kVA High Low
Voltage Voltage
Pole Mounted (Single-Phase)
0.5 2400 through 120/240 1.5 34,400 volts 240/480 3 5 10 15 25 371/2 50 75 100 167 250 333 500 667 750 833 1000
Pole Mounted (Three-Phrase)
15 2400 to 208/120 30 13,800T 240x480T 45 480/277 75 1121/2 150 225 330 Three-Phase 500
JUMBO® Step-Down Transformer. The ABB JUMBO single-phase step-down transformer is especially use-ful during utility system voltage conversions when it is desirable to convert a portion of a substation or a feeder to a higher voltage and still be able to supply the remaining customers at the existing voltage. The
6
2. Pad Mounted
A single-phase, single service, low pro-file distribution padmount transformer available in loop or radial feed. Designed to aesthetically, safely and economically provide underground electrical service to single loads, particularly, rural residences, farms and ranches.
Micro-Pak, 10-50 kVA
A single-phase, multi-service, full-line, low profile padmount transformer designed for loop feed or radial feed on a grounded wye underground dis-tribution system.
The Mini-Pak can be furnished in a complete line of ratings and in a wide range of configurations to fully meet the reliability, safety and operating require-ments of any distribution system.
Mini-Pak, 10-167 kVA
The Maxi-Pak is designed specifically for those customers requiring straight-up feed (Type I) rather than cross feed (Type II). The additional height of the Maxi-Pak allows installation of air load break switching in this low-profile design.
ABB single-phase padmounted Distribution Transformers meet the following Industry Standards:
ANSI C57.12.00 - IEEE Standard General Requirements for Liquid Immersed…Transformers
ANSI C57.12.25 - Pad-Mounted…Single-Phase Distribution Transformers with Separable Insulated High-Voltage Connectors…
ANSI C57.12.28 - Pad-Mounted Equipment - Enclosure Integrity or
ANSI C57.12.29 - …Pad-Mounted Equipment - Enclosure Integrity for Coastal Environments
ANSI C57.12.70 - …Terminal Markings and Connections ANSI C57.12.80 - IEEE Standard Terminology… ANSI C57.12.90 - IEEE Standard Test Code… NEMA Tr-1 - Transformer Standards
IEEE 386 - …Separable Insulated Connectors
ABB recommends the use of ANSI C57.91 - IEEE Guide for Loading…for the establishment of proper distribution transformer loading practices. Ratings @65° Rise
kVA: 10,25,371/2, 50, 75, 100, 167, 2501
HV: 4160GY/2400 through 34500GY/19920V3 BIL: 60, 75, 95, 125, 150 kV
LV: 240/120, 480/240, 277 V, 120/2403, 240/4802 1Maxi only
2Available only on micros with cable lead secondary 3Mini and Maxi only (micros available thru 24940GY/14400)
Standard Features:
1. Equipped with two universal high voltage bushing wells for loop feed. (Only one bushing well is provided for radial feed.)
2. A removable flip-top hood and heavy-duty 3/8'', stainless steel hinge
pins provide safe and durable service.
3. A recessed locking assembly with padlock provisions and a pentahead locking bolt is standard for tamper resistant operation. A hex-head locking bolt is available.
4. All tanks are constructed of heavy gauge steel. Tank seams are welded and each unit is pressure tested and inspected for leaks prior to shipment. In addition, all single phase transformers are sup-plied with:
a. 5/8''-11 stainless steel lifting bosses
b. Oil level/fill plug c. Oil drain plug
d. Self-actuating pressure relief device
e. Two ground bosses, 1/2''-13 NC tapped hole 7/16'' deep.
5. The front sill latches with the flip-top hood, is attached on the side of the tank, and is removable.
6. The high voltage universal bushing wells are externally clamped and removable. A parking stand between the bushing wells is pro-vided for attachment of bushing accessories.
7. Externally clamped low voltage epoxy bushings. 8. Tamper-resistant design that exceeds ANSI C57.12.28. 9. NEMA safety labels per NEMA Publication 260-1982.
8
Minimum/Maximum Design Dimensions11111
Micro-Pak Mini-Pak Maxi-Pak
A B C D A B C D A B C D
Min. 24 24 30.25 14 24 32 30.25 14 32 32 30.25 14 Max. 24 24 35.50 16 42 44 46.00 20 42 44 46.00 20 1Actual dimensions will vary according to voltage, loss evaluation,
and accessories.
Optional Accessories 1. Overcurrent Protection
a. An internal primary protective link to remove the transformer from the system in the event of an internal fault.
b. A secondary breaker provides protection against secondary overloads and short circuits. Dimensions are in Inches c. An oil-immersed bayonet-type
fuse link to remove the trans-former from the system in case of an internal fault (fault sensing) or secondary short overload (overload sensing). This fuse is a drawout design and is supplied in series with an isolation link. A drip plate is provided to prevent oil from dripping onto the bush-ing or elbow.
d. A current limiting fuse mounted in a dry well loadbreak canister.2 5.0 5.0 CABLE OPENING “B”+6 “C”+6
• The high interrupting rating of the CL fuse permits its use on systems where the available fault current exceeds the rat-ing of normal expulsion fuses. e. A partial range current limiting fuse mounted under oil within the transformer tank.2
•
An expulsion fuse is supplied in series with the partial range CL fuse.Recommended Pad
•
Available at 95, 125, and 150Dimensions kV BIL.
2. Switching
a. Externally operated tap changer. b. Externally operated dual voltage
switch.2
c. Externally operated loadbreak oil rotary (LBOR) switch.2 d. EFD CL fused air loadbreak
switch available for either radial or loop feed.3
2Not available on Micro 3Maxi only
3. Primary Connection
a. Universal bushing wells (stan-dard) and loadbreak inserts. b. Integral (one piece) loadbreak
bushings.
4. Secondary Connections
a. Copper studs with rotatable spade type bushings.
•
Four-hole, NEMA type, tin-plated copper alloy spade.•
Four-hole, in line, tin-platedcopper alloy spade. b. Cable lead secondary.4 5. Corrosion Resistance
a. ANSI C57.12.29 Full 400 Series Stainless Steel
b. Partial Stainless Steel
•
Mini-Skirt™ and Sill•
Sill Only•
Sill and Hood•
Mini-Skirt™, Sill, and Hood 6. Miscellaneousa. Cleats for anchoring sill to pad. b. Polypad mounting base.4 4Micro only
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The ABB MTR is an oil-filled, three phase, commercial padmounted distribu-tion transformer specifically designed for servicing such underground distribution loads as shopping centers, schools, institutions and industrial plants. It is available both live front and dead front construction, for radial or loop feed applications, or without taps.
Industry Standards
ABB three-phase MTR units meet the following industry standards:
The ABB MTR Padmounted ANSI C57.12.00 - IEEE Standard General
Transformer Three-Phase Requirements for Liquid Immersed…
45-2500 kVA Transformers
ANSI C57.12.22 - Pad-Mounted…
Three-Phase Distribution Transformers with High Voltage Bushings
ANSI C57.12.26 -
Pad-Mounted…Three-Phase Distribution Transformers… With Separable Insulated High-Voltage Connectors
ANSI C57.12.28 - …Pad-Mounted Equipment
- Enclosure Integrity or
ANSI C57.12.29 - …Pad-Mounted Equipment - Enclosure Integrity for Coastal Environments ANSI C57.12.70 - Terminal Markings
and Connections…
ANSI C57.12.80 - IEEE Standard Terminology…
ANSI C57.12.90 - IEEE Standard Test Code…
NEMA Tr-1 - Transformer Standards IEEE 386 - Separable Insulated
Connectors
ABB Recommends the use of ANSI C57.91 - IEEE Guide for Loading…for the estab-lishment of proper distribution transformer loading practices.
Ratings
•
45 through 2500 kVA•
65°C average winding rise over 30°C average ambient.•
Low voltages: 1208Y/120, 216Y/125, 460Y/265, 480Y/277, 480d, 240d and 240d with 120 volt mid-tap in one phase.•
High voltages: 4160 Grd Y/2400 through 34,500 Grd Y/19,920 for Grounded Wye systems; 2400 through 34,500 for Delta systems; various dual high voltages.•
Taps: All voltages are available with or without taps.•
Insulation classes: 35 kV (200 kV BIL) and below.1208Y/120, 216Y/125, 240d not avail-able above 1500kVA.
The distribution transformer functions as an integral part of the distribution system and consideration must be given to proper protection of the transformer from system disturbances. In addition, it is normal practice to apply overcurrent protection on the primary side of the transformer so that a faulted trans-former is isolated from the primary system. Protection from excessive voltage transients and severe overcurrents should be provided. Protection considerations include:
(1) Protective devices must be rated for the conditions anticipated.
(2) When the transformer(s) is provided with overcurrent devices — coordination with system devices should be achieved to allow proper fault isolation.
Caution: Operation of a primary protective device may indi-cate a faulted transformer. Re-energizing should be performed from a remote location unless the cause of device operation is positively identified and corrected. To do otherwise presents a hazard to life and property in the event of violent transformer failure.
2. Types of Accessories and Transformer Protection Packages— Pole Mounted
ABB offers four basic transformer types: S, SP, CP and CSP®.
Together they represent a wide range of protective capabili-ties to meet nearly every application.
•
Conventional “S” TransformersThis type transformer contains no protective equipment. Therefore, lightning, fault and overload protection for these transformers must be provided by the purchaser.
•
Surge-Protecting “SP” TransformersThe “SP” transformers include transformer-mounted lightning arresters and internally-mounted high voltage protective links, but omit the internally-mounted low voltage circuit breaker.
•
Current-Protecting “CP” TransformersThe “CP” transformers are equipped with the internally-mounted low-voltage circuit breaker and high voltage protective links, but omit the lightning arresters.
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Types of Accessories and Transformer Protection Packages— Pole Mounted (Continued)
• Self-Protecting “CSP®” Transformers
In a “CSP” transformer the arrester protects the transformer from over-voltage caused by lightning and/or high voltage switching surges. The protective link operates to remove a defective transformer from service if an internal failure occurs, thereby protecting the system. The breaker provides the trans-former a degree of protection from overloads and short circuits on the secondary side of the transformer. This type trans-former offers the most protection of all protected transtrans-formers except for a “CSP” with a current limiting fuse.
a. CL Fuses
Two basic types of current limiting fuses exist—partial range and general purpose (full range). The partial range fuse requires a protective link applied in series while the general purpose fuse does not. The partial range fuse is available on pole-type transformers (bushing mounted) and padmounted transformers (internally mounted). The general purpose fuse is only available on padmounted transformers.
b. The Distribution Surge Arrester protects the transformer (and other electrical equipment) from dangerous over-voltages, whether caused by lightning surges, switching surges or other transients.
The Type LV Surgemaster™ valve type arrester has one or more arc gap assemblies connected in series with one or more current limiting “valve” blocks. Under high volt-age surge conditions, the resistance in the blocks drops, providing a low-resistance path to ground. Once the surge has passed, however, the block resistance rises again, restricting the flow of current. The gaps will then interrupt this low-magnitude current flow, restoring the arrestor to an insulator.
The LVBB Surgemaster™ valve type arrester is a big block (heavy duty) design which is capable of discharg-ing a 100 KA surge. The big block arrester operates the same way as the LV with additional protection capability. The HMX gapless metal oxide arrester is a heavy duty design utilizing the non-linearity of a metal oxide resistor to provide protection levels equivalent to gapped silicon carbide arresters. The metal oxide distribution arrester offers the benefits of reduced complexity, improved reli-ability and improved performance characteristics. The LV, LVBB Surgemaster™ and HMX distribution arresters are available for either pole, crossarm or trans-former mounting.
c. The Secondary Circuit Breaker provides the transformer with a degree of protection from secondary overloads and short circuits. It is mounted under oil, usually on the core/ coil assembly, connected between the coil’s secondary leads and the secondary bushings. The breaker is cali-brated to trip when its bimetal reaches a predetermined temperature. An additional instantaneous magnetic trip element which responds to high fault currents is available on some breakers.
3. Types of Accessories and Protection—Padmounted For system and transformer protection from surge currents, short circuits and overloads, ABB offers a number of devices including a protective link, distribution surge arrester, second-ary circuit breaker and current limiting fuse.
a. The Protective Fuse Link is an internal, oil-immersed, expulsion type fuse consisting of a fiber tube supporting and surrounding the fuse element usually made of copper and EVERDUR®. The link is sized to operate only in the
event of a winding failure, isolating the transformer from the primary system. Interrupting rating is 3500 amperes at 7.2kV.
Protective Fuse Link
b. The Bayonet-Type Fuse Cartridge contains an oil-im-mersed expulsion type fuse with an interrupting rating of 3800 amperes at 8.3 kV. It is a hookstick-operable, drawout loadbreak design available through 19.9kV1. Two types of fuse links are available—overload-sensing and fault-sensing—and an internal isolation link is sup-plied in series for additional safety. The fault-sensing link is sized to operate only in the event of a transformer fail-ure; the overload-sensing link is sized for additional pro-tection from secondary system faults or prolonged heavy overload conditions.
Standard Ratings:
Voltage Interrupting L.B. Amps
Class Amps (RMS) At .8 PF
8.3 kV 3800 135
15.5 kV 2000 135
23.0 kV 600 45
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c. Current Limiting Fuses are available through 15 kV in either the EFD air loadbreak switch or in a drawout, loadbreak dry fuse well.
Some applications may require parallel current limiting fuses to obtain sufficient full-load or inrush current ratings. A mechanical interlock with a loadbreak oil switch (LBOR) is recommended when using parallel drawout, loadbreak, dry well fuses. Some of the higher kVA designs may require current fuse ratings that are not available—contact Division.
Partial range, internal, block-mounted current limiting fuses, which are applied in series with an internal protec-tive link, are also available through 23 kV.
Loadbreak, Drywell Current Limiting Fuse Canister
d. The EFD is an loadbreak air switch available for radial feed applications. Switching, flexibility and safety are made possible by a compact, “dead front” type construc-tion that enables the switch to be externally-mounted on the tank in the terminal compartment. A sealed, silver sand current limiting fuse is normally provided to the switch’s transformer connecting pole. High voltage cables are con-nected to the switch contacts by means of solderless, clamp-type connectors capable of accepting cable sizes ranging from #6 to #4/0.
EFD Switch Ratings
Continuous current ... 200 A Loadbreak ... 200 A Close-in ... 5,000 A Momentary ... 10,000 A
e. The LBOR (Loadbreak Oil Rotary) switch is gang-operated and available for either radial or loop feed switching. The stacked deck rotary switch has a unique, springloaded cam-operated kicker system which provides quick make and break action to the contacts.
LBOR Ratings: BIL 95 kV 125 kV 150 kV Maximum Voltage (L-L) 15.5 kV 27 kV 38 kV (L-Grd) 8.9 kV 15.5 kV 21.9 kV Continuous and Interrupting Current 300 A 1 200 A 300 A Momentary and
Making Current 12 kA/ 12 kA/ 10 kA/
(RMS Sym./Assym.) 19.2 kA 19.2 kA 16 kA 1200 A 3c rating also available.
LBOR Switch
f. The Tap Changer and Series Multiple Switch. Both are oil-immersed, externally-operated, and are designed for de-energized operation only.
Tap Changer Operating Handle
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II. Performance
PageA. Designation of winding voltage ratings ... 17
B. Polarity ... 20 C. Terminal designations ... 21 1. Pad-Mounted ... 21 2. Pole-Mounted ... 21 (a) 1c pole-mounted ... 21 (b) 3c pole-mounted ... 22
D. Short circuit ratings ... 22
E. Sound level ratings ... 22
F. Tolerance definitions ... 23
G. Impedance calculations ... 23
H. Efficiency calculations ... 24
I. Regulation calculations ... 24
J. Performance example ... 26
II. Performance
A. Designation of Winding Voltage Ratings (from ANSI C57.12.00)1. Single-Phase
Symbol Example Typical Diagram
E 12000
Usage E shall indicate a winding of E volts which is suitable for g
connection on an E volt system.
E/E1Y 2400/4160Y
Usage E/E1Y shall indicate a winding of E volts which is suitable for
g connection on an E volt system or for Y connection on an E1 volt
system.
E1GrdY/E 12 470GrdY/7200
Usage E1GrdY/E shall indicate a winding of E volts with reduced
insulation at the neutral end. The neutral end may be connected directly to the tank for Y or for single-phase operation on an E1 volt system,
provided the neutral end of the winding is effectively grounded.
E/2E 120/240
Usage E/2E shall indicate a winding, the sections of which can be
connected in parallel for operation at E volts, or which can be con-nected in series for operation at 2E volts, or concon-nected in series with a center terminal for three-wire operation at 2E volts between the extreme terminals and E volts between the center terminal and each of the extreme terminals.
2E/E 240/120
Usage 2E/E shall indicate a winding for 2E volts, two-wire full kVA
between extreme terminals, or for 2E/E volts three-wire service with
1/
2 kVA available only, from midpoint to each extreme terminal.
V x V1 240 x 480
Usage V x V1 shall indicate a winding for parallel or series operation
only but not suitable for three-wire service.
Notes:
(1) E = line-to-neutral voltage of a “Y” winding, or line-to-line voltage of a delta winding.
18
2. Three-Phase
Symbol Example Typical Diagram
E 2400
Usage E shall indicate a winding which is permanently g connected
for operation on an E volt system.
E1Y 4160Y
Usage E1Y shall indicate a winding which is permanently Y connected
without a neutral brought out (isolated) for operation on an E1 volt
system.
E1Y/E 4160Y/2400
Usage E1Y/E shall indicate a winding which is permanently Y
con-nected with a fully insulated neutral brought out for operation on an E1 volt system, with E volts available from line to neutral.
E/E1Y 2400/4160Y
Usage E/E1Y shall indicate a winding which may be g connected for
operation on an E volt system, or may be Y connected without a neutral brought out (isolated) for operation on an E1 volt system.
E/E1Y/E 2400/4160Y/2400
Usage E/E1Y/E shall indicate a winding which may be g connected
for operation on an E volt system or may be Y connected with a fully insulated neutral brought out for operation on an E1 volt system with
E volts available from line to neutral.
E1GrdY/E 12470GrdY/7200
Usage E1GrdY/E shall indicate a winding with reduced insulation and
permanently Y connected, with a neutral brought out and effectively grounded for operation on an E1 volt system with E volts available from line to neutral.
Notes:
(1) E = line-to-neutral voltage of a “Y” winding, or line-to-line voltage of a delta winding.
2. Three-Phase (continued)
Symbol Example Typical Diagram
E/E1GrdY/E 7200/12470GrdY/7200
Usage E/E1GrdY/E shall indicate a winding, having reduced
insula-tion, which may be g connected for operation on an E volt system or may be connected Y with a neutral brought out and effectively grounded for operation on an E1 volt system with E volts available
from line to neutral.
V x V1 7200 x 14 400
Usage V x V1 shall indicate a winding, the sections of which may be
connected in parallel to obtain one of the voltage ratings (as defined above) of V, or may be connected in series to obtain one of the volt-age ratings (as defined above) of V1. Windings are permanently g or
Y connected.
Notes:
(1) E = line-to-neutral voltage of a “Y” winding, or line-to-line voltage of a delta winding.
20
B. PolarityThe lead polarity (or polarity) of a transformer is a designation of the relative instantaneous directions of currents in its leads. Primary and secondary leads are said to have the same polarity when at a given instant the current enters the primary lead in question and leaves the secondary lead in question in the same direction as though the two leads formed a continuous circuit. The lead polar-ity of a single-phase transformer may be either additive or subtractive. If one pair of adjacent leads from the two windings in question is connected together and a small voltage is applied to one of the windings, then the connection behaves as an auto trans-former with the secondary voltage adding to or subtracting from the primary voltage. The polarity determination is as follows: a. The lead polarity is additive if the voltage across the other two
leads of the windings in question is greater than that of the higher voltage winding alone.
b. The lead polarity is subtractive if the voltage across the other two leads of the windings in question is less than that of the higher voltage winding alone.
Additive E3 > E1
Subtrative E3 < E1
By industry standards, single-phase distribution transformers 200 kVA and smaller, having high voltage windings rated 8660 volts or less have additive polarity. All other single-phase transformers have subtractive polarity.
The polarity of a three-phase transformer is fixed by the internal connections between phases as well as by the relative locations of leads; it is usually designated by means of a vector diagram showing the angular displacements of windings and a sketch showing the marking of leads. The vectors of the vector diagrams represent induced voltages, and the recognized counterclockwise direction of rotation of the vectors is used. The vector representing the voltage of a given winding is drawn parallel to that represent-ing the correspondrepresent-ing voltage of any other windrepresent-ing havrepresent-ing the same phase.
The terminal designations for pad-mounted distribution trans-formers are clearly marked at the terminals of both the high and low voltage.
2. Pole Mounted
For pole mounted distribution transformers, the terminal designations follows:
(a) Single-Phase Pole Mounted
Connection Additive Polarity Subtractive Polarity
E/2E with three external low-voltage terminals Series or Three-Wire Parallel E/2E with four external low-voltage terminals Series or Three-Wire Parallel E Note:
The H1 terminal for either additive or subtractive polarity is located on the left-hand side when facing the low-voltage terminals.
22
Terminal Designations (Continued) (b) Three-Phase Pole Mounted
All three-phase pole mounted distribution transformers have terminal designations as shown above regardless of internal connection. Neutral terminals (HV and/or LV) will exist as required by the winding connection and will be noted on the transformer nameplate.
D. Short-Circuit Ratings (ANSI C57.12.00)The short-circuit ratings for distribution transformers are set by industry standards. The maximum magnitude required for units with secondary voltages rated less than 600 V is as follows:
1 c kVA 3 c kVA Rating (times normal)
5-25 15-75 40
37.5-100 112.5-300 35
167-500 500 25
750-2500 1/ZT*
Two winding distribution transformers with secondary voltages rated above 600 volts are required to withstand short-circuits limited only by the transformer’s impedance.
The duration of the short-circuit current is determined by 500 kVA and Below 750-2500 kVA
________________ ___________
t = 1250 t = 1.0
l2
where: t = duration (seconds)
I = symmetrical short-circuit current (per unit)
*1/ZT = The short circuit current will be limited by the trans-former impedance only. ZT is transtrans-former per unit impedance.
E. Sound Level Ratings (NEMA TR-1)The sound level ratings for distribution transformers are set by industry standards. The maximum sound level (A weighted response curve) is:
kVA Rating Sound Level (dB)
0-50 48 51-100 51 101-300 55 301-500 56 -750 57 -1000 58 -1500 60 -2000 61 -2500 62
The impedance of a two-winding transformer with impedance voltage larger than 2.5% shall have a tolerance of ± 7.5% of the specified value; and the tolerance for those with impedance voltage 2.5% or less shall have a tolerance of ± 10% of the specified value.
Differences of impedance between two duplicate two-winding transformers when two or more units of a given rating are pro-duced by one manufacturer at the same time shall not exceed 7.5% of the specified value.
Transformers shall be considered suitable for operation in parallel if impedances come within the limitations of the fore-going paragraphs, provided turns ratios and other controlling characteristics are suitable for such operation. (See Paralleling) 2. Losses
The total losses of a transformer are the sum of the excitation losses and the load losses at rated load (with winding tem-perature at 85°C).
Unless otherwise specified, the losses represented by a test of a transformer, or transformers, on a given order, shall not exceed the specified losses by more than the percentages below:
No Load Total
No. of Units Basis of Losses Losses
On One Order Determination (Percent) (Percent)
1 1 unit 10 6
2 or more each unit 10 6
2 or more average of 0 0 all units
G. Impedance CalculationsTransformer impedance is shown on the transformer nameplate (Note: transformer impedance, reactance and resistance are typi-cally given in percent or per unit). If the transformer load losses are known, the impedance may be separated into its reactive and resistive components.
Z - impedance (percent) R - resistance (percent) X - reactance (percent) kVA - transformer kVA rating
Cu - load loss at rated load at 85°C (watts) R = Cu
10 kVA X = CFFFFFFZ2 – R2
24
H. Efficiency CalculationsThe efficiency of a transformer is defined as the ratio of the output power to the input power. It can be calculated at any load and power factor if the transformer losses are known.
E - efficiency (percent) L - load (per unit) kVA - transformer kVA rating
Cu - load loss at rated load at 85°C (watts) Fe - no load (excitation) loss (watts)
a - power factor angle
E = L.kVA.cosa.10
5
percent (L.kVA.cosa.103) + Fe + L2.Cu
At rated load and unity power factor
E = kVA.10
5
percent kVA.103 + Fe +.Cu
Regardless of the load power factor angle, it can be shown that the per unit load which results in maximum efficiency is: L (maximum efficiency) =
I. Regulation CalculationsThe voltage regulation of a distribution transformer is the change in output voltage which occurs when the load is reduced from rated value to zero with the primary terminal voltage maintained constant. The regulation can be calculated from the equations below or by the nomograph which follows:
R - resistance(percent) X - reactance (percent) REG- percent voltage regulation
a - power factor angle (positive for inductive load) REG = [R2 + X2 + 200. (X.sina + R.cosa) + 10,000]1/2 – 100
For unity power factor
Regulation Chart
Place straight edge at percent resistance, scale one, and at percent reactance, scale nine. Read the percent regulation at different power factors as given by scales two to eight inclusive
26
J. Performance ExampleExample Transformer Ratings (Typical)
Single-phase kVA 25
High voltage 7200v
Low voltage 120/240v
No load (excitation) loss 104 watts Total loss at rated load 419 watts
Impedance 1.6%
For the above transformer determine the following: (1) Nominal reactance
(2) Minimum impedance
(3) Minimum efficiency at rated load (4) Expected efficiency at 50% load (5) Expected regulation
Assume an inductive power factor (cosa = 0.85) 1. Nominal Reactance R =______Cu = (419–104)_______ = 1.26% 10.kVA 10.25 X = CFFFFFF = CFFFFFFFFFF = 0.99%Z2 – R2 1.62 – 1.262 2. Minimum Impedance Minimum Z = (1 – 0.10).(Nominal Z) = 1.44%
3. Minimum Efficiency at Rated Load
Maximum total loss = 1.06.419 = 444 watts
E = kVA . cosa . 10
5
kVA . cosa . 103 + (Fe + Cu)
= (25) . (.85) . 10
5
= 98.0% (25) . (.85) . 103 + (444)
4. Expected Efficiency at 50% Load
L = 0.5 E = L . kVA . cosa . 10 5 L . kVA . cosa . 103 + Fe + L2Cu = (0.5) .(25).(.85).105 = 98.3% (0.5).(25).(.85).103 + 104 + (0.5)2.(419 – 104) 5. Expected Regulation
REG = [R2 + X2 + 200. (X.sina + R.cosa) + 10,000]1/2– 100
= [1.262 + 0.992 + 200.(0.99.0.53 + 1.26.0.85) + 10,000]1/2–100 = 1.596
Fault Current Equations
Figure K.1 gives, for convenient reference, the equations necessary for calculating the available currents for both 240 volt and 120 volt bolted faults, and defines the terms appearing in the equations. Before explaining the use of the equations, the as-sumptions used in arriving at these are discussed.
28
The impedance of the primary system supplying the distribution transformer is very small in comparison to that of the distribution trans-former and secondary circuit up to the point of fault. The effect of this assumption is to make the calculated values of current for a bolted fault in the secondary system slightly higher than those which result when the effect of primary impedance is included. Increasing the “stiffness” of the primary system, reducing the kVA size of the trans-former, or increasing the secondary circuit length to the fault point reduces the difference between the approximate and more exact cal-culated values of bolted fault current. In contrast, the difference between the approximate and more exact values will be greater for “weak” primary systems, large distribution transformers, and short secondary circuits. For most cases where the calculations are made to determine avail-able fault current at the service entrance for sizing equipment, or to determine maximum currents at which overcurrent protective devices must coordinate, the difference resulting from the assumption is negli-gible. However, for those cases where the calculated current using methods neglecting primary impedance is slightly higher than the interrupting rating of a fuse or breaker in the secondary system, or where the calculated current is slightly above the value at which overcurrent protective device coordination can be achieved, then including the effect of primary system impedance may show that a “problem” does not exist. Calculations including the effects of primary system impedance are not contained in this guide.
Reference to Figure K.1 shows that the expressions for calculating the available current for the 240 volt and 120 volt bolted faults are different. While the 240 volt fault current can be calculated from a knowledge of the “full winding” impedance of the transformer, the calculation of the 120 volt fault current requires a knowledge of the transformer “half winding” impedance. As the relationship between transformer “half winding” and “full winding” impedance is not fixed and can vary from design to design, the most typical relationship for present day designs was used in arriving at the equation for 120 volt fault current. Letting RT+ jXT be the “full winding” impedance in percent on nameplate kVA rating looking into the primary winding, the “half winding” impedance in percent on nameplate kVA can be approximated by 1.5 RT+ j2.0 XT. Also notice from Figure K.1 that the equations do not include the effect of any metering impedances which may be present in the circuit, or any “fault” impedance. Including these impedances will further reduce the calculated values of fault current.
The steps to follow when using the equations in Figure K.1 to calculate the bolted fault currents are as follows:
1. Calculate the transformer resistance in ohms at secondary termi-nals X1-X3 (RTin Figure K.1). This requires that the transformer total losses at full load in watts, and no load losses in watts be known (WTOT and WNL respectively in Figure K.1).
2. Calculate the transformer leakage impedance in ohms at second-ary terminals X1-X3 (ZT in Figure K.1). This requires that the
transformer nameplate impedance in percent (Z%) be known. 3. Calculate the transformer leakage reactance in ohms at secondary
terminals X1-X3 (XTin Figure K.1).
4. Determine the resistance of the secondary circuit in ohms per 1000 feet for a 240 volt fault (RS). Also determine the reactance of the secondary circuit in ohms per 1000 feet for a 240 volt fault (XS).
Typical values for RSand XSin ohms per 1000 feet are given in Tables 1 and 2 for circuits using aluminum phase conductors under the header “240 V FAULTS”. The values in Table 1 are for triplex cable, and those in Table 2 are for rack mounted conductors. From these tables notice that the resistance values are the same, but the reactance values are greater with the rack mounted conductors. This is due to the larger spacing.
5. Calculate the available current for a 240 volt bolted fault (I240) using
the equation in Figure K.1 and the values calculated in steps 1 through 4.
6. Determine the resistance (RS1) and reactance (XS1) of the second-ary circuit in ohms per 1000 feet for a 120 volt fault. typical values for RS1 and XS1 in ohms per 1000 feet are given in Tables 1 and 2 for circuits using aluminum conductors under the header “120 V FAULTS”. In both tables, the values listed are for circuits using a reduced size neutral conductor. If a full size neutral conductor is used, then the impedance values given under the header “240 V FAULTS” should also be used for the calculation of the 120 volt fault currents.
7. Calculate the available current for a 120 volt bolted fault (I120) using
the equation in Figure K.1 and the values calculated in steps 1 through 3 and step 6.
30
Example Calculations
The use of the equations in Figure K.1 is illustrated with the following: A 50 kVA transformer with total losses at full load of 759 watts, and no load losses of 204 watts has an impedance of 1.75 percent. A service entrance circuit which is 80 feet in length using 3/0 aluminum triplex with reduced neutral is connected directly to the transformer terminals. What is the available current for both a 240 and 120 volt bolted fault at the end of the service? From the statement of the problem:
WTOT = 759 watts Z = 1.75 percent
WNL = 204 watts L = 80 feet kVA = 50
The calculations proceed following the steps outlined.
1. RT= 0.0576 759 – 204 = 0.012787 ohms 502
2. ZT= 0.576 1.75 = 0.02016 ohms 50
3. XT= CFFFFFFFFFFFFFFFFF = 0.015586 ohms.020162 – .0127872
4. From Table 1, the resistive and reactive components of the imped-ance for a 240 volt fault with 3/0 aluminum triplex cable are: RS= 0.211 ohms per 1000 feet
XS= 0.0589 ohms per 1000 feet
5. Placing these values of RT, XT, RS, XS, and L into the equation for I240 in Figure K.1 gives:
I240 = 6676.6 amperes rms symmetrical
Note that the large number of significant digits included in these calculations is not to suggest that they are accurate to the last digit, but to aid those who want to check their own calculations. 6. From Table 1, the resistive and reactive components of the
imped-ance for a 120 volt fault with 3/0 aluminum triplex cable (reduced neutral) are:
RS1= 0.273 ohms per 1000 feet XS1= 0.0604 ohms per 1000 feet
7. Placing the above values into the equation for I120 in Figure K.1
gives:
I120 = 4071.1 amperes rms symmetrical
For this example notice that at a distance of 80 feet from the trans-former, the available current for the 120 volt bolted fault is considerably less than that for a 240 volt fault. However, from the equation for I240
and I120 in Figure K.1, notice that for a fault at the transformer
second-ary terminals (L = 0.0 feet), the available current for a bolted 120 volt fault is greater than that for a 240 volt fault. Thus at some distance L from the transformer, I240 and I120 would be equal, and at distances
Figure K.2 is a plot of the available current for both the 120 and 240 volt bolted faults vs. the distance from the transformer terminals to the fault point in feet. The curves are for transformer sizes of 50, 75, and 100 kVA supplying a secondary circuit made with 3/0 aluminum triplex with reduced neutral. From these curves notice that:
Figure K.2
(a) The available current for both the 120 and 240 volt faults is rapidly reduced as the fault is moved away from the transformer, even for the rather large 3/0 aluminum service conductor. (b) With the 3/0 aluminum service conductor, the available current
for a 120 volt fault is less than that of a 240 volt fault at distances greater than about 10 feet from the 50, 75, or 100 kVA trans-former. For most all single-phase services rated 200 amperes or less, the available current at the service entrance for the 120 volt fault is less than that of the 240 volt fault.
(c) As the distance from the transformer to the fault location be-comes large, the available current for both the 120 and 240 volt faults becomes independent of the transformer size, especially for the 120 volt fault.
32 Notes: (1) Resistance values based on a conductor temperature of 25 ° C. (2)
Reactance based on following: (a)
600 volt insulation with all 3 insulated conductors in contact. (b) For 120 volt (Phase-to-Neutral Fault), all current returns in the neutral conductor w ith no current
returning in the earth.
(3) Insulation thickness is 0 .062 inch for #4 to #2, 0.078
inch for #1 to 4/0, and .094 inch for 250 to 500 MCM.
(4) For secondary circuits with full size neutral, use
resis-tance and reacresis-tance values given for 240 volt fault for both 120 and 240 volt faults. Notes:
(1) Resistance values based on a conductor temperature of 25 ° C. (2 ) Reactance values based on secondary rack with 1 2 inch spacing between conductors with neutral in top posi-tion and phase conductors in the two lower positions. Resistance and reactance values given for 120 volt fault assume fault is to phase conductor in m iddle position in rack. (3) For secondary circuits with full size neutral, use
resis-tance and reacresis-tance values given for 240 volt fault for both 120 and 240 volt faults.
T
able 1.
T
y
pical Impedances for 120/240 V
olt Circuits W
ith T
riplex Cable
A
luminum Phase Cond.
A
luminum Neutral Cond.
120 V o lt Faults 240 V o lt Faults ___________________ ____________________ ____________ ____________ Size No. of Size No. of RS1 XS1 RS XS (A WG or MCM) Strands (A WG or MCM) Strands (j /1000 Ft.) (j /1000 Ft.) (j /1000 Ft.) (j /1000 Ft.) ___________ ______ ___________ ______ ________ ________ ________ ________ 2 7 4 7 .691 .0652 .534 .0633 1 1 9 3 7 .547 .0659 .424 .0659 1/0 1 9 2 7 .435 .0628 .335 .0616 2/0 1 9 1 19 .345 .0629 .266 .0596 3/0 1 9 1 /0 19 .273 .0604 .21 1 .0589 4/0 1 9 2 /0 19 .217 .0588 .167 .0576 250 37 3/0 1 9 .177 .0583 .142 .0574 350 37 4/0 1 9 .134 .0570 .102 .0558 500 37 300 37 .095 .0547 .072 .0530 T able 2. T y
pical Impedances for 120/240 V
olt Circuits W
ith Rack Mounted Conductors
A
luminum Phase Cond.
A
luminum Neutral Cond.
120 V o lt Faults 240 V o lt Faults ___________________ ____________________ ____________ ____________ Size No. of Size No. of RS1 XS1 RS XS (A WG or MCM) Strands (A WG or MCM) Strands (j /1000 Ft.) (j /1000 Ft.) (j /1000 Ft.) (j /1000 Ft.) ___________ ______ ___________ ______ ________ ________ ________ ________ 2 7 4 7 .691 .223 .534 .217 1 1 9 3 7 .547 .217 .424 .212 1/0 1 9 2 7 .435 .21 1 .335 .204 2/0 1 9 1 19 .345 .205 .266 .199 3/0 1 9 1 /0 19 .273 .199 .21 1 .193 4/0 1 9 2 /0 19 .217 .193 .167 .188 250 37 3/0 1 9 .177 .189 .142 .184 350 37 4/0 1 9 .134 .182 .102 .176 500 37 300 37 .095 .174 .072 .168
III. Three-Phase Transformers
and Banks
PageA. Application Considerations ... 34 1. Types of distribution systems ... 34 a. Primary (source) systems ... 34 b. Secondary (service) systems ... 34 2. Angular displacement (phase shift) ... 35 3. Neutral grounding ... 35 a. Primary neutral grounding ... 35 b. Secondary neutral grounding ... 35 4. Ferroresonance ... 36
a. Primary winding connections which can result
in Ferroresonance ... 37 b. Primary winding connections which can prevent or
minimize the possibility of Ferroresonance ... 38 B. Summary of Common Connections ... 41 1. Delta-delta ... 41 2. Delta-wye ... 42 3. Wye-delta ... 43 4. Wye-wye ... 44 5. Grounded wye-wye ... 45 6. T-T (O degree angular displacement) ... 45 7. T-T (30 degree angular displacement) ... 46 8. Open Wye-Open Delta ... 46 9. Open Delta-Open Delta ... 47 C. Common Three-Phase Banks Using
34
III. Three-Phase Transformers and Banks
This section presents many important factors to be considered when selecting the connections used for both three phase transformers and phase banks of the single-phase transformers applied in three-phase distribution systems. A summary of commonly encountered connections is provided. In addition, connection diagrams using single-phase transformers for three-single-phase banks are shown.
A. Application Considerations 1. Types of distribution systemsA three-phase distribution transformer, or a bank of single-phase distribution transformers should be thought of as a system component which connects the primary to the second-ary system. Since it is a system component, proper application and determination of permissible connections requires an understanding of the characteristics of both the primary sys-tem which will supply the transformer, and the secondary system which will be supplied by the transformer.
a. Primary (Source) Systems
Distribution systems are either effectively grounded, impedance grounded, or ungrounded. Most electric utility distribution systems in this country are three-phase 4-wire multi-grounded neutral systems which are effectively grounded. (An effectively grounded system is one where at any point in the system the ratio of zero-sequence reactance to positive-sequence reactance is less than three, and the ration of zero-sequence resistance to positive-sequence reactance is less than one.) With a 4-wire effectively grounded neutral system, the primary windings of the distribution transformers can be connected from either phase to phase or phase to neutral. This permits usage of the following connections: Delta, open delta, grounded wye, open wye, floating wye, and T. Whether the neutral point of wye connected primary windings should or should not be connected to the system neutral depends upon the con-nections used for the secondary windings.
Although they are not commonly used by electric utilities for distribution, impedance grounded or ungrounded systems are frequently found in industrial plants. These systems provide no path to carry neutral load current. Thus, distribution transformers applied must be connected phase to phase using either delta, open delta, floating wye, or T connected windings.
b. Secondary (Service) Systems
Secondary systems supplied from distribution transformers and operating at 600 volts or less usually are either 3-wire ungrounded, or 4-wire grounded. To supply a 3-wire ungrounded (delta) system, the transformer secondary winding may be connected in delta, open delta, floating wye, or T.
Loads which require both single-phase 3-wire 120/240 volt service and three-phase 240 volt service can be supplied by a 4-wire service consisting of transformers with secondary windings connected delta or open delta
with a center tap ground on one leg of the delta. In the 4-wire grounded (wye) system, the transformer second-ary windings must have a neutral point which can be grounded. The 4-wire grounded secondary service can be supplied by either the wye connection or the T con-nection with the neutral point grounded.
2. Angular Displacement (Phase Shift)
For standard three-phase connections the phase-to-neutral voltage on the primary side either leads that on the secondary side by 30° or is in phase with the phase-to-neutral voltage on the secondary side. The delta delta and wye wye connections produce no phase shift. The delta wye and wye delta connec-tions produce the 30° phase shift. The T-T transformer can be designed to exhibit either a 30° or a 0° phase shift.
When paralleling three-phase transformers or banks, the phase shift of each must be the same. In addition, the 30° phase shift has an effect on the coordination of overcurrent protective devices located on the primary and secondary sides of the transformer. For unsymmetrical faults the line currents do not transform in proportion to the voltage ratings. Of particular importance is a line-to-line fault on the transformer secondary. For the connec-tions which have a 30° phase shift, this fault produces a fault current in one primary phase which is 1.15 times the secondary fault current on a per unit basis. This additional 15% must be considered to achieve selective coordination.
3. NeutraI Grounding
Some transformer connections or winding connections (wye or T) have a neutral point on either the primary windings, secondary windings, or both, which can be grounded. That is, the neutral point of the primary windings can be connected to the multi-grounded neutral conductor of the primary system, or the neutral point of the secondary windings can be grounded to establish a 4-wire grounded wye system. Whether the neutral point of windings should or should not be grounded depends on factors discussed below.
a. Primary Neutral Grounding
For the primary neutral point to be grounded, the primary source must be a 4-wire multi-grounded neutral system. In addition, it is generally undesirable that a distribution bank act as a ground source for the primary system. To prevent creation of a grounding bank, a primary wye should only be grounded if the secondary is also con-nected in wye and a T primary should never be grounded. Note however that the open wye connection must be grounded at the neutral point to function properly. b. Secondary Neutral Grounding
To supply phase to neutral connected load on the sec-ondary, a low impedance ground source must be established. This can be achieved by grounding the neutral of a secondary wye connection provided that the primary is connected either delta or wye grounded supplied by a 4-wire multi-grounded neutral (effectively grounded) source. The neutral of a secondary T connection may also be grounded. In addition, a delta or open delta winding
36
4. Ferroresonance
Ferroresonance is a non-linear resonance which can occur during open conductor (single-phase) conditions in the distri-bution system. When ferroresonance occurs, it is characterized by high overvoltages whose waveform contains appreciable harmonics. The transformers involved in the ferroresonant circuit may emit unusual noises which frequently are described as rattling, rumbling, or whining sounds. These are considerably different than those which emanate from the transformer when energized at rated frequency and voltage. Overvoltages of five times normal and higher have been measured during ferroresonant oscillations in test circuits. Some causes of open conductor conditions which may result in ferroresonance are: (1) the operation of single-pole overcurrent protective devices such as fuses or single-pole reclosers, (2) normal switching operations with single-pole devices such as distribution cut-outs to energize or de-energize a transformer, and (3) failure to connect jumpers.
Whether ferroresonance will occur during open conductor con-ditions depends to a great extent upon the connections used for the primary windings in a distribution transformer bank or in a three-phase distribution transformer. Under normal conditions where all three primary phases to the transformer bank are energized through a continuous path from the source, ferro-resonance will not occur for any of the connections used for the primary windings. But when an open conductor condition occurs, the non-linear inductance of a transformer or transformer bank, with certain connections, can be placed in series with system capacitance. If the capacitance lies within a specified range, ferroresonance may result. However, with other transformer connections, ferroresonance will not occur during open conduc-tor conditions because the non-linear inductances cannot be inserted in series with system capacitances.
Figure 4.1: Cable-fed transformer with single-pole switching devices located at the junction between the overhead and underground circuits.
Figure 4.1 illustrates a frequently encountered system condi-tion which produces ferroresonance. An unloaded three-phase pad mounted transformer with delta connected primary wind-ings is supplied from an open-wire line through a cable circuit. At the riser or transition pole, the cable circuit is connected to the open wire line using distribution cutouts. Notice that during the switching operation (open conductor condition) where only
the switch in phase A is closed as illustrated, the non-linear inductances of the transformer windings between phases A and B, and phases A and C, are placed in series with the cable capacitance on the open phases. This makes a series L-C circuit where the L is non-linear, and if the parameters are in the proper range, ferroresonance will occur.
a. Primary Winding Connections Which Can Result in Ferroresonance
Theoretically, ferroresonance can occur during open conductor conditions in either one or two phases if the primary windings of the distribution transformers are connected in delta, open delta, floating wye, or tee. Whether it does or does not occur with these “ungrounded” connections for the primary windings depends upon the amount of capacitance between the open conductor and transformer, the transformer internal capacitances, the transformer size, the system voltage, and the amount of load connected to the secondary terminals of the transformer, or the amount of load on the primary circuit between the open conduc-tor and transformer. Studies have shown that ferroresonance is more likely to occur with cable circuits (due to higher capaci-tance) than open-wire lines, with small transformers, at higher primary voltage levels (more likely at 35 kV than 4 kV voltage level), and with unloaded transformers.
Industry experience has shown that in overhead distribution systems operating at 15 kV and below, overvoltages and ferroresonance usually do not occur during open conductor con-ditions, even when the ungrounded primary winding connections are used for transformers. Ferroresonance became an impor-tant concern in the utility industry with the advent of underground distribution and the use of 25 and 35 kV class voltages. In higher voltage (25 kV and 35 kV) overhead systems, over-voltages and ferroresonance have occurred when single-pole switching is performed at the terminals of small transformer banks with their primary windings connected in floating wye or delta. This is due to the internal capacitances of the transformers. Figure 4.2 summarizes in a qualitative fashion the probability of ferroresonance occurring in 15, 25, and 35 kV class overhead systems when the switching is performed at the terminals of small banks made from single-phase units.
The probability of ferroresonance and the associated overvolt-ages is very high if the circuit between the location of the open conductor and the transformer is made from shielded cable and operates at voltage levels in either the 15, 25, or 35 kV class. This is because the capacitance per unit length of a cable circuit is in the range of 50 times that of open wire lines. A system illustrating this situation is shown in Figure 4.1. Because of the high probability of ferroresonance in underground systems using conventional single-pole switching devices, many system operators will not use the ungrounded primary winding connec-tions in cable-fed transformers.
If, however the transformer primary windings are ungrounded, as with the delta, open delta, wye, and tee connections, and the circuit between the transformer and possible location of an open conductor (single-phase) condition is made from cable, the possibility of ferroresonance can be minimized with the follow-ing measures.
38
(1) Application of only three-pole gang operated switches and fault interrupters. This minimizes the possibility of having single-phase conditions.
(2) Location of the single-pole switches and overcurrent pro-tective devices only at the transformer terminals. (3) Connection of resistive load to the secondary terminals
of the transformer during remote single-pole switching. Although these measures can be very effective, many opera-tors of underground systems consider them unacceptable for either economical, operational, or technical reasons. Instead, they prefer to use transformer connections which have either a zero or very low probability of ferroresonance during open conductor conditions at a location remote from the transformer.
Figure 4.2: Probability of ferroresonance in overhead systems when switching is performed at the terminals of small transformer banks made from single-phase units. b. Primary Winding Connections Which Prevent Or
Minimize Possibility of Ferroresonance
When the primary windings of single-phase distribution trans-formers used in a bank are connected in open wye or grounded wye, or if a three-phase unit with the grounded wye primary windings employs triplex construction, ferroresonance will not occur during most open conductor conditions in the primary system. This is true for both overhead and underground systems operating up through 35 kV. But if either a floating wye or delta connected shunt capacitor bank is installed on the primary line between the transformer bank and location of the open conductor, ferroresonance may occur. However, the use of these connections for capacitor banks is very uncom-mon in distribution systems operating in the 15 kV class and above. If there is a very long length of open wire line between
the location of the open conductor and transformer bank with grounded wye or open wye primary windings, and no other load is connected to the line beyond the open point, ferro-resonance can occur because of the phase-to-phase capacitance of the open wire line. The probability of such con-ditions existing, even in 25 and 35 kV rural distribution systems, is very remote. Thus, for practical purposes, ferroresonance will not occur when the grounded wye or open wye connec-tions are used for the primary windings with single-phase units, or a three-phase unit with triplex construction.
The probability of ferroresonance is zero when the switching is performed at the terminals of transformer banks in over-head systems with the grounded wye or open wye connected primaries at all voltages as illustrated in Figure 4.2.
When the grounded grounded wye or grounded wye-floating wye connections are used in a transformer constructed on a four- or five-legged core, overvoltages and ferroresonance may occur during open conductor conditions at a remote point when cable circuits are involved. Test data shows that crest voltages as high as 2.35 per unit are possible, but usually they are considerably less than this. In contrast, overvoltages of 5 per unit and higher are possible when the transformer has the ungrounded primary winding connections. Furthermore, the length of primary cable circuit which can be used with trans-formers with four- or five-legged core and grounded-wye primary is in the range of 50 times that possible when the ungrounded primary connections are used when the voltage on the open phase is limited to 1.25 per unit.
Although the use of triplex construction essentially eliminates the possibility of ferroresonance in cable-fed three-phase trans-formers with the grounded wye primary, such construction generally makes the transformer larger, heavier, and more costly than conventional four- or five-legged core units. Most system operators, based on the good experience and perfor-mance they have had with the grounded wye primaries on four-and five-legged cores, have not been able to justify the added cost for triplex construction.
If it is necessary to further minimize the possibility of ferro-resonance when the grounded wye primary is used on a four-or five-legged cfour-ore, the measures listed below can be employed:
(1) Application of only three-pole gang operated switches and fault interrupters. This minimizes the possibility of having single phase conditions.
(2) Location of single-pole switches and overcurrent protec-tive devices only at the transformer terminals.
(3) Connection of resistive load to the secondary terminals of the transformer during remote single-pole switching. The preceding discussion of ferroresonance is both very brief and very qualitative in content. As it may be necessary to quan-tify certain aspects of ferroresonance, such as determining the maximum length of cable circuit which can be used between a switch and transformer if voltage is to be limited to a specified value, the reader is referred to the many references which exist on the subject. A few are listed below.
40
References
1. Schmid, R. L. “An Analysis and Results of Ferroresonance”. Trans-mission and Distribution, pp. 114-117, Oct. 1969.
2. Kratz, E. F., Manning, L. W., and M. Maxwell. “Ferroresonance in Series Capacitor-Distribution Transformer Applications.” AIEE Transaction (Power Apparatus & Systems), vol. 78, pp. 438-449, August 1959.
3. Young, F. S., Schmid, R. L., and P. I. Fergestad. “A Laboratory Investigation of Ferroresonance in Cable Connected Transform-ers,” IEEE Transactions on Power Apparatus and Systems, vol. PAS-87, pp.1240-1249, May 1968.
4. Crann, L. B., and R. B. Flickinger, “Overvoltages on 14.4/24.9 kV Rural Distribution Systems.” AIEE Transactions (Power Apparatus and Systems), vol. 73, pp. 1208-1212, Oct. 1954.
5. Smith, D. R., Swanson, S. R., and J. D. Borst. “Overvoltages With Remotely-Switched Cable-Fed Grounded Wye-Wye Transformers.” IEEE Transactions on Power Apparatus and Systems, vol. PAS-94, pp. 1843-1853, Sept./Oct. 1975.
Phasor Diagram:
Angular Displacement (Degrees): 0
Source: Suitable for both ungrounded and effectively grounded sources.
Service: Suitable for 3-wire service or for 4-wire service with a mid-tap ground.
Notes:
1. With one unit out of service, a bank of single-phase units can be reconnected as an open delta, open delta bank. With one of three identical units out of service, the rating of the bank when supplying only three-phase load is about 57.7 percent of the bank rating when all three units are in service.
2. Caution: Each unit in a bank of single-phase units must be con-nected for the same voltage ratio, otherwise high circulating currents can occur. Prior to completing a closed delta second-ary connection, the voltage between the two transformers closing the delta should be checked to verify the voltage ratios and connections.
3. Impedance mismatch among units of a single-phase bank will require derating of the bank.
4. Single-phase units having a secondary breaker should not be used for a bank providing 4-wire (mid-tap ground) delta service. 5. Frequently installed with mid-tap ground on one leg when sup-plying combination three-phase and single-phase load where the three-phase load is much larger than single-phase load. 6. Single-phase transformers with primary windings rated E volts
42
DELTA-WYE Connection Phasor
Diagram:
Angular Displacement (Degrees): 30
Source: Suitable for both ungrounded and effectively grounded sources.
Service: Suitable for 3-wire service or for 4-wire grounded service with a XO grounded.
Notes:
1. With XO grounded, the bank acts as a ground source for the secondary system.
2. Fundamental and harmonic frequency zero-sequence currents in the secondary lines supplied by the transformer do not flow in the primary lines. Instead these zero-sequence currents circu-late in the closed delta primary windings.
3. When supplied from effectively grounded primary system, ground relay for primary system does not see load unbalances and ground faults in the secondary system.
4. Single-phase transformers with primary windings rated E volts usually are used for this bank.
