troubleshooting transformers

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CHAPTER

5

93

Connecting andTroubleshooting

Transformers

Topics to Be Covered

5.1 Checking out a Transformer

5.1.1 Testing for Turns Ratio

5.1.2 Insulation Resistance and Continuity Testing

5.1.3 Changing Transformer Taps

5.1.4 Testing for Transformer Polarity

5.1.5 Always Check the Voltage5.1.6 Checking Transformer Loading

5.2 Specific Hazards Working with Transformers

5.2.1 Energizing an Overhead Transformer

5.2.2 Energizing an Underground Transformer

5.2.3 Protection from Transformer Backfeed

5.2.4 Potential Explosive Flash at Transformer Secondary

5.2.5 Causes of Ferroresonance

5.2.6 Prevention of Ferroresonance

5.3 Single-Phase Transformer Connections

5.3.1 Transformer Protection

5.3.2 Making Neutral Connections and Ground Connections

5.3.3 Parallel or Series Connection of Center-Tapped Secondary Coil

5.3.4 Connecting Two Transformers in Parallel

5.3.5 Connecting a Transformer to a Secondary Network

5.3.6 Troubleshooting a Single-Phase 120/240-Volt Service

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94 Chapter 5

5.4 Three-Phase Transformer Connections

5.4.1 Banking Three Single-Phase Units

5.4.2 Making Wye or Delta Connections?5.4.3 Delta Primary Transformer Connections

5.4.4 Wye Primary Transformer Connections

5.4.5 Delta Secondary Transformer Connections

5.4.6 Open Delta Connections

5.4.7 Wye Secondary Transformer Connections

5.4.8 Three-Phase Secondary Voltages

5.5 Troubleshooting Transformers

5.5.1 Taking Voltage Checks at the Secondary

5.5.2 Troubleshooting a Wye Secondary (120/208, 240/416, or 347/600 Volt)

5.5.3 Troubleshooting a Delta Secondary (600, 480, or 240 Volt)

5.6 Working on a Voltage Conversion

5.6.1 Preparing the Circuit for Voltage Conversion

5.6.2 Working at Transformer Installations

5.6.3 Working at Underground Installations

5.6.4 Working at Primary Step-Down Transformer Installations

5.1 Checking out a Transformer

5.1.1 Testing for Turns Ratio

Check the transformer nameplate for the rated primary voltage and the expected sec-

ondary voltage. A test can confirm the actual turns ratio and will ensure that there

are no shorts between turns in the windings (figure 5.1). Energize the high voltage

H-1

H-2

Apply120 Volts AC

Figure 5–1 Turns Ratio Field Test.

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Connecting and Troubleshooting Transformers 95

coil with a low voltage such as 120 volts. Measure the input voltage and the output

voltage with a voltmeter. The ratio is calculated as:

Input Output = Transformer Ratio

CAUTION! Transformers work both ways! For this test, the low-input voltage

must be connected to the HIGH-voltage winding. If the input voltage was connect-

ed to the low-voltage winding, the transformer would be a step-up transformer, and

a lethal voltage will appear at the high-voltage terminal.

5.1.2 Insulation Resistance and Continuity Testing

Testing a transformer for damaged insulation or coil continuity can be carried out

using a 1000-volt insulation tester (megger). Remove any ground from the X2 ter-

minal for these tests.

Insulation Test

The readings between the high-voltage terminal and the low voltage ( X1, X2, or X3)

terminals should be infinite. Readings between the low voltage terminals and the

transformer tank should be infinite. The insulation test readings with a three-phase

transformer should be as shown in table 5–1.

Continuity Test

Readings between each of the primary terminals and each of the secondary terminals

should be zero as shown in table 5–2.

Table 5–1. Insulation Resistance Tests on Three-Phase Transformer

HV to LV Insulation Resistance LV Insulation Resistance to Tank

Test Connections Readings Test Connections Readings

H1 to X1, X2, or X3 Infinite (∞) X1 to Tank Infinite (∞)



Table 5–2. Continuity Tests on Three-Phase Transformer Windings

Continuity Tests of HV Windings Continuity Tests of LV Windings

Test Connections Readings Test Connections Readings

H1 to H2 Zero X1 to X2 Zero



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96 Chapter 5

5.1.3 Changing Transformer TapsSome distribution transformers have no-load tap changers which can apply addition-

al or fewer turns to the primary winding. Depending on the manufacturer, each tap

will raise or lower the secondary voltage by 4.5 percent or 2.5 percent. The nameplate

shown in figure 5.2 has 2.5 percent taps.

Newer transformers have an external switch knob to change the taps, while the

cover must be removed to access the taps for changes on older transformers. The

transformer must be isolated before turning the knob.

Caution: The feeder voltage will change during the day and at different seasons.

Therefore, changing the taps at the transformer could produce extreme voltages

when the primary voltage returns to its normal level in off-peak periods.

5.1.4 Testing for Transformer Polarity

When installing a transformer on a secondary network or when banking the trans-

former together with other transformers, ensure that the transformers are connected

at the same polarity. An additive and a subtractive transformer (figure 5.3) can be

banked together by ensuring that the X-1 of one transformer is connected to the X-

1 of the other transformer (similarly with X-2 and X-3), regardless of the position of

the terminals on the transformer tank.

Taps

%105

102.5

100

97.5

95

A

B

C

D

E

Figure 5–2 Tap Settings as Shown on Name Plate.

H-1

X-1 X-2 X-3

H-2 H-1

X-1X-2X-3

H-2

Figure 5–3 Subtractive and Additive Transformers.

Subtractive

Transformer

Additive

Transformer

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Testing a Transfo rmer f or Addi t ive or Subt ract ive Polari ty

Step 1. When facing the transformer (figure 5.4), install a jumper between the high-

voltage neutral (or the righthand-side high-voltage terminal) and the low-

voltage terminal on the righthand side.

Step 2. Apply 120 volts across the primary of the transformer.

Step 3. Measure the voltage between the left-side high-voltage terminal and the left-

side low-voltage terminal.

5.1.5 Always Check the Voltage

After the installation of a transformer, it is considered essential to perform a second-

ary voltage check as a final test to ensure that the customer is supplied with a proper

voltage. The American National Standards Institute (ANSI) voltage standard is to be

between + 6 percent or – 13 percent for 120, 208, 277, 480, or 575-volt services.

5.1.6 Checking Transformer Loading

The actual load on a transformer can be measured as shown in figure 5.5 and calcu-

lated in the field.

Example: The expected current on the secondary of a transformer with a 120/240-

volt loaded to 100 percent can be calculated as follows;

Current @ 100% Load = kVA Transformer 1000

240

Apply120 Volts AC

Short Out H-2and Adjacent

Secondary Terminal

For a 10 to 1Additive Transformer,Voltmeter Will Read

132 Volts

H-1

H-2

X-1X-3

additive

V120 V

+ 12 V

132 V

Figure 5–4 Test for Transformer Polarity.

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98 Chapter 5

5.2 Specific Hazards Working w ith Transformers

5.2.1 Energizing an Overhead Transformer

An overhead transformer with a blown fuse is often checked by energizing it with

another fuse. If the transformer is defective, the fuse can blow backward violently.

Hot particles will blow back at the powerline worker. Precautions include staying out

from under the cutout, using a stick with an attached shield, and using an extra

length of hot stick. A current-limiting fuse in series with the cutout fuse will reduce

the risk of a violent transformer failure in locations where there is a high fault cur-

rent capability.

5.2.2 Energizing an Underground Transformer

There are very few transient faults on an underground transformer. On a dead-front

transformer, the bayonet-style fuse or the current-limiting fuse in a dry-well canister

have been known to fail explosively when using the fusing device to energize a fault-

ed transformer. This risk can be reduced by energizing the transformer with a load-

break elbow or from a remote location.

5.2.3 Protection from Transformer BackfeedA secondary voltage feeding back into an isolated transformer is stepped up to a full

primary voltage at the transformer primary terminal. Sources of secondary backfeed

are not always obvious:

• A portable generator connected into the customer system without opening the

customer main switch.

H-2

V2

I2

H-1

V1

I1

Neutral

Figure 5–5 Calculating the Actual Transformer Load.

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• A nearby recreational vehicle (RV) with a generator connected into a home

wiring.

• An extension cord from a neighboring house.

• When working on a transformer that is networked with other transformers to a

common secondary bus, the primary terminal remains alive if there are no net-

work protectors opened after the transformer is disconnected from the primary.

Prot ect ion f rom Transformer Backf eed

• When working on a transformer, remove the secondary leads to ensure that

there is no possibility of backfeed into the transformer secondary terminals.

• Shorting out the secondary with approved grounds can be effective. Placing

the grounds by hand can violate the minimum-approach distance to the pri-

mary bushing.

• Removing meters from all customers fed from the transformer will eliminate

backfeed from customer operations but not from other transformers connect-

ed to the same bus.

• A small portable generator (less than 5 kW) feeding back into a grounded cir-

cuit will continue to run and not short out. The resistance of the circuit

between the generator and the grounds is too high to short out the generator.

When working on a primary circuit, apply protective grounds. It can still be

safe to work on the grounded conductors because the voltage at the grounded

location has been lowered to an acceptable level. However, there will be cur-

rent flowing in the conductor, and the conductor must be jumpered beforecutting or opening the circuit.

5.2.4 Potential Explosive Flash at Transformer Secondary

A step-down transformer can generate a very high current on the secondary side if

the secondary wires are accidently shorted. The magnitude of the fault current avail-

able on a transformer secondary depends mostly on the size of the transformer. The

larger the transformer, the greater the capability to generate a large fault current.

If the secondary wires are shorted, the transformer can briefly carry a load more

than 10 times its rating and supply a high-fault current to the faulted location.

Depending on the impedance of the transformer, a short at the secondary terminals

of a 100 kVA can be as high as 18,000 amperes. The level of fault current at the loca-

tion of the short will depend on the distance from the transformer and the conduc-tor size, as shown in the figure 5.6 example.

5.2.5 Causes of Ferroresonance—Ferroresonance occurs when there is practically

no impedance to current flow. This happens when the circuit has no load on it, and

the capacitive reactance (dependent on the length of the cable) is equal to, and in

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100 Chapter 5

series (dependent on the transformer connections) with an inductive reactance

(dependent on the size of the transformer). The voltage on the circuit can increase

from two to nines times, thereby causing equipment damage.

The most common occurrence of ferroresonance involves a three-phase wye/delta

or delta/delta transformer bank fed with a length of underground cable as illustrated

in figure 5.7.

A certain length of underground cable can provide a critical amount of capacita-tive reactance in the circuit. A transformer bank can provide an inductive reactance

that matches the capacitive reactance of the cable. The inductive load and capacitive

load must be in series with each other (figure 5.8) and this occurs when only one or

two phases are energized.

5.2.6 Prevention of Ferroresonance—Ferroresonance can be avoided by removing

one of the causes of the phenomenon as shown in table 5.3.

Table 5.3. Prevention of Ferroresonance

Causal Factor Action

Operating the source single-pole switches Install a three-phase gang-operated switch.to energize or deenergize an underground

cable feeding a three-phase transformer

puts the transformer coils temporarily in

series during the switching operation.

(continued)

0206

4012

6018

8024

Length of 4/0 Aluminum Service

10030

120 feet26 meters

6

8

10

12

14

16

18

20

2 4 0 - V o l t S h o r t C i r c u i t

i n T h o u s a n d s o f A m p e r e s

167 kVA Transformer

75 kVA Transformer

Figure 5–6 Magnitude of Secondary Fault Current.

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A-PhaseOpen

A-Phase Open

Resonating A-PhaseCircuit Completed through

Cable Capacitance to Ground

Current through TransformerWindings in Series

-

Figure 5–7 Typical Ferroresonance Setup.

-

Inductive Load in Seriesin Delta Primary

-

Inductive Load in SeriesWhen Wye Point Ungrounded

CableCapacitance

on Each Phase

FuseOpen

Source Source Source

FuseOpen

FuseOpen

-

Not Subject to FerroresonanceWhen Wye Point Is Grounded

CableCapacitance

on Each Phase

InductiveLoad

inParallel

Figure 5–8 Sources of Inductive Reactance in Series.

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102 Chapter 5

Table 5.3. Prevention of Ferroresonance (continued)

Causal Factor Action

If there is no load on the transformers Keep a load on the transformer when it is

when they are energized, the low being energized. The increased resistance

resistance in the circuit causes a higher in the circuit will lower the effects of

voltage when resonance occurs. resonance.

A wye primary with a floating Temporarily ground the floating neutral

(ungrounded) neutral is susceptible of the wye primary which will short out

to ferroresonance. a possible series circuit.

5.3 Single-Phase Transformer Connections

5.3.1 Transformer Protection

Always install the specified fuse, current limiting fuse, and surge arrester at a trans-

former. The specified fuse is fast enough to protect the transformer from the heat

generated by a secondary overload or fault and is slow enough to reduce nuisance

outages due to transient faults. The fuse is usually coordinated to trip before the

instantaneous trip on the feeder protection.

5.3.2 Making Neutral Connections and Ground Connections

Neutral connections have one purpose and the ground connections have another. A

neutral is a grounded conductor. Neutral connections are part of the electrical circuit,

and during normal operation, they carry the current back to the source. A ground

wire is a grounding conductor. Ground-wire connections provide a path for current

under abnormal conditions such as during a lightning storm when an arrester or aninsulator sparks over. The ground connections also bond the transformer tank and

related equipment to keep them all at the same potential.

5.3.3 Parallel or Series Connection of Center-Tapped Secondary Coil

A transformer with a center-tapped secondary coil will have a certain voltage induced

across the full length of the coil and half that voltage on each side of any center tap.

For example, on a 120/240-volt transformer, shown in figure 5.9, the secondary

provides 240 volts when the two coils are connected in series. Placing the two sec-

ondary coils in parallel allows the complete coil to be used to provide 120 volts at the

rated kVA capacity of the transformer.

5.3.4 Connecting Two Transformers in Parallel

Two smaller transformers are sometimes paralleled together to give the equivalent

capacity of one large single-phase transformer. In figure 5.10 the two secondary

coils in each transformer are connected parallel. One transformer feeds one leg at

a positive polarity while the other transformer feeds the other leg at a negative

polarity.

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5.3.5 Connecting a Transformer to a Secondary Network

A secondary bus that is fed radially should have bus breaks installed to prevent anyinterconnection with other transformers. A secondary network system is fed from

many transformers connected in parallel to the same secondary bus.

120V

120V

120V240V

Neutral

Series Connection Parallel Connection

Neutral

X-2 X-2X-1 X-1X-3 X-3

— —

— —

—+

+

+ +

+

Figure 5–9 Series and Parallel Secondary Connections.

Secondary Coils are in Series Secondary Coils are in Parallel

NeutralX-2 X-1X-3

—

—

— —

+

+

+ + — —+ +

H-1

One Cutout

H-2

120V120V240V

X-2 X-1X-3

—

—+

+

H-1

H-2

Figure 5–10 Single-Phase Transformers Connected in Parallel.

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104 Chapter 5

• The load is divided among all the transformers connected to the bus.

• Individual customers with peak loads are supplied by the greater availablereserve capacity of the multiple transformers.

• Fuses or network protectors on the secondary bus between transformers iso-

late faulted transformers and interrupt only the customers near the defective

transformer.

5.3.6 Troubleshooting a Single-Phase 120/240-Volt Service

If Then

A customer has intermittent Check for a voltage unbalance in the two 120-volt legs.

power and flickering lights. If one 120-volt leg is two or more volts different from

the other 120-volt leg, then turn on a large 120-voltload. If the voltage increases on one leg and decreases

on the other, then there is a poor neutral connection.

A loose connection on either leg can increase resistance

to current flow and will also result in arcing and

intermittent power.

The lights in part of a A poor or open neutral connection is blocking the

customer’s premises are normal return path from 120-volt appliances. The

very bright and in another current will travel back to the source through the

part, the lights are very other 120-volt leg or through 240-volt equipment.

dim. Although the two voltages will be unbalanced, they

will add up to 240 volts. The 240-volt appliances will

operate normally.

A customer is receiving half If the voltage at the meter base is normal, then

power. Some of the lights one of the main fuses at the service entrance is likely

work and some do not. blown. A voltage reading of about 120 volts between

None of the 240-volt the top and the bottom of the fuse indicates that

appliances work. the fuse is blown.

A customer complains A poor connection could cause low voltage at various

about erratic or low voltage. times. Heavy duty equipment used by a neighbor

on the same bus can cause voltage problems for others

on the same bus.

5.4 Three-Phase Transformer Connections

5.4.1 Banking Three Single-Phase Units

There are four specifications to look for when choosing a transformer to interconnect

into a three-phase transformer bank:

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1. The voltage rating of the transformer primary coil must be compatible with

the applicable circuit. The voltage impressed across the primary coil will

depend on whether the coil is connected in a wye (phase-to-neutral) or a delta(phase-to-phase) configuration.

2. The transformer must be able to deliver the needed secondary voltage (see

Table 5.3). The supplied secondary voltage will be dependent on:

• the voltage rating of the secondary coil.

• whether the transformer secondaries are interconnected in a wye or delta

configuration.

• whether the secondary coils inside the transformer are connected together

in series or in parallel.

3. If equipped with tap changers, the transformers must be on the same voltagetap. Dual voltage transformers must be set on the proper voltage.

4. The impedance of the transformers in the bank should be within 0.2 percent

of each other to avoid having the transformer with the lowest impedance tak-

ing a greater share of the load. In other words, if one transformer has an

impedance of 2 percent, then the impedance of the other transformer should

be between 1.8 percent and 2.2 percent.

5.4.2 Making Wye or Delta Connections?

There are two ways to get a voltage across a transformer coil:

1. When each of the three transformers have their coils connected betweenphases A-B, B-C, and C-A (figure 5.11), the transformers are interconnect-

ed in a delta configuration.

+ ++ — — —

Phase A Phase C Phase B

PhaseA

PhaseC

Phase B

+

+

+

—

—

—

Figure 5–11 Delta Configurations.

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106 Chapter 5

2. When each of the three transformers have their coils connected between a

phase and a common neutral (figure 5.12), the transformers are interconnect-

ed in a wye configuration.

5.4.3 Delta Primary Transformer Connections

The three ways to connect a transformer primary into a delta (phase-to-phase)

configuration is shown in figure 5.13. Note how labeling the polarity of the trans-

formers on a drawing will reduce confusion when making connections.

• Each transformer coil in a delta primary or delta secondary is connected phase-

to-phase.

• When two or more transformers are interconnected in a delta configuration, the

coils are connected in series with each other. To connect a coil in series, each

positive terminal of one coil is connected to a negative terminal of another coil.

• To ensure a proper phase rotation, the coils must be connected in a sequence.

+ ++ — — —

Phase B Phase C

Phase B

Phase A

Phase A

Phase C

Neutral

Neutral+

+

+

—

—

—

Figure 5–12 Wye Configurations.

Single

Phase

Open Delta Closed DeltaClosed Delta

++ ++ ++ ++ ++ – – – – – – ++

Figure 5–13 Three Types of Delta Primary Connections.

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• If a transformer primary is to be delta connected on a wye circuit, the voltage

rating of the primary coil must be equal to the phase-to-phase voltage of the

circuit.

5.4.4 Wye Primary Transformer Connections

The three ways to connect a transformer primary into a wye (phase-to-neutral)

configuration is shown in figure 5.14 Note how labeling the polarity of the trans-

formers on a drawing will reduce confusion when making connections.

• Each transformer coil in a wye primary is connected phase-to-neutral. When

two or more transformers are interconnected in a wye configuration, the coils

are connected in parallel with each other. To connect a coil in parallel, each

positive terminal is connected to a phase, and each negative terminal is con-

nected to a common neutral.

5.4.5 Delta Secondary Transformer Connections

A typical delta secondary is shown in figure 5.15. This figure shows a three-phase

240-volt service with an optional 120-volt lighting load supplied from one trans-

former.

• Each positive terminal is connected to a negative terminal.

• On center-tapped secondaries, the ground strap is removed from the X-2 ter-

minal, and the terminal will be alive at 1 / 2 the full coil voltage.

5.4.6 Open Delta Connections

A three-phase delta service can be supplied with two single-phase transformers. This

hookup is sometimes used as an economical way to feed a small three-phase delta

service. This type of service is called an open delta because the delta configuration is

missing one side. Figure 5.16 shows the open delta loop with the availability of three

phases.

SinglePhase

Open Wye Wye

Neutral

++ ++ ++ ++++++ — — — — — —

Figure 5–14 Three Types of Wye Primary Connections.

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108 Chapter 5

Three primary wires are needed to feed an open-delta, three-phase, secondary

service. A wye primary would need two phases and a neutral, and a delta primary

would need three phases. A three-phase wye secondary service cannot be fed from

two normal single-phase transformers.

When one transformer of a normal three-phase delta-delta or wye-delta trans-

former bank is found defective, the connections can be changed to configurations

shown in figure 5.16 and service restored as an open delta. The customer should be

told to reduce demand on service until the transformer is replaced because the two

good transformers will now have the capacity to supply only 57.7 percent of the

capacity of three transformers.

5.4.7 Wye Secondary Transformer Connections

A typical wye secondary is shown in figure 5.17. The figure shows a three-phase

120/208-volt service with an optional 120/240-volt lighting load supplied from one

transformer.

• Each positive terminal becomes a separate phase and all negative terminals are

connected together.

240-Volt Delta with a 120/240-Volt Lighting Service

1240V

240V

240V

240V

2 4 0 V

2 4 0 V

120V

120V

2 1 0 V

120V1 3

2

120V

240V2

3120/240-V

Single-PhaseService

240-VDelta Service

Vector Drawingof Secondary

X-1

X-2

X-3

— +

+ +

— +

X-1

X-2

X-3

—

—

X-1

X-2

X-3

— +

+

— +

— —

+ — +

Figure 5–15 Typical Delta Secondary Connections.

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• The phase-to-phase voltage is3— or 1.73 times the phase-to-neutral voltage. The

voltage across each transformer coil is equivalent to the phase-to-neutral voltage

• The primary neutral must be connected to the secondary neutral in a wye-wye

transformer bank. This neutral connection provides a path for any fault current

or current from an unbalanced load to get back to the source. There is a poten-

tially lethal voltage between the primary and secondary neutrals if they are not

connected together.

• A standard single-phase 120/240-volt lighting service can be provided from a

three-phase 120/208-volt service. In figure 5.17, the internal secondary coils in

the center transformer are left in series to provide a 120/240-volt supply. The

kVA rating of the center transformer is usually increased to provide the capac-

ity for the extra load.

5.4.8 Three-Phase Secondary Voltages

The secondary voltage from a three-phase transformer bank depends on more than

just the transformer ratio. Three transformers with a given ratio can be interconnect-

ed to provide up to four different types of services (table 5.4).

+ ++ +

+ ++ +

— — — —

— — — —

A

A

A A

NB

B

H1 H1H2 H2

X2 X2X3 X3X1 X1

H1 H1H2 H2

X2 X2X3 X3X1 X1

B BC C

CC

Wye / Open Delta Delta / Open Delta

-

-

+

+ 240V

2 4 0 V

1 3

2

Vector Drawingof Secondary

Figure 5–16 Open Delta Transformer Connections.

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110 Chapter 5

208V

X-3X-2

X-1

— — +

+

+

X-3X-2

X-1

— — ++

+

120V 120V 120V

X-3X-2

X-1

— —

— — —

+

+

+

Neutral

120V 120V

240V

208V208V

120/208-Volt Wye with 120/240 Lighting Service

Figure 5–17 Typical Wye Secondary Connections.

1. The secondary voltage is based on whether the transformer secondary is inter-

connected as wye or delta. The secondary phase-to-phase voltage is:

• equal to the actual voltage across the transformer coil with a delta connection.

• equal to 1.73 times the voltage across the transformer coil with a wye con-

nection.

2. The secondary voltage is also based on whether a transformer with a center-

tapped secondary coil has the two parts of the coil inside the tank arranged in

parallel or in series. The output voltage of series-connected coils is double the

output of two parallel connected coils.

Table 5.4 Standard North American Three-Phase Voltages

Transformer Type of Three- Coil Arrangements External

Secondary Phase Service Inside Tank Configuration

120/240 120/208 parallel Wye

240/416 series Wye

120 parallel Delta

240 series Delta

240/480 240/416 parallel Wye

277/480 series Wye

240 parallel Delta480 series Delta

277 277/480 NA Wye

347 347/600 NA Wye

600 600 NA Delta

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5.5 Troubleshooting Transformers

5.5.1 Taking Voltage Checks at the SecondaryTroubleshooting transformers involves checking for problems on secondary services.

Many no power or partial power calls are due to internal customer problems. A utili-

ty responsibility normally ends at the service entrance. Opening the customer’s main

switch and taking a voltage reading will determine whether the utility is the cause of

the problem.

5.5.2 Troubleshooting a Wye Secondary (120/208, 240/416, or 347/600 Volt)

If Then

A customer has an Open the customer’s switch and check the voltage.

abnormal voltage. If the voltage on a phase-to-neutral reads zero, thencheck for a defective transformer or an open phase on

the primary feeder. If the voltages on all three phases

are balanced, close the customer switch and check the

voltage. An unbalanced voltage indicates the customer’s

load is unbalanced.

Three-phase motors The usual cause of trouble on one phase is an

are overheating and the unbalanced load. This type of service has single-phase

thermal overload loads. Unbalance beyond 3 percent can cause the

protection trips out low-voltage leg to draw more current and create

the motor. equipment heating. A faulty winding in a three-phase

motor can cause high current on a phase.

The load is balanced at Open the customer’s main switch and check that the

the service entrance but utility supply voltage is correct. Close the customer’s

there is still a problem. circuit breakers, one circuit at a time, and take voltage

and current readings. A suspected circuit will have an

unbalanced voltage or abnormal current readings for

the equipment being fed.

All three-phase equipment One phase is probably out. A phaseout can be due

and some single-phase to a feeder problem, a transformer problem, or a blown

equipment will not fuse at the service entrance.

operate.

The single-phase A poor or broken neutral connection will cause

equipment has problems with single-phase loads. A customer-owned

intermittent power. secondary (dry) transformer which steps down the

The three-phase load higher 600 or 416-volt supply to 120/208 can be the

is operating normally cause of problems.

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112 Chapter 5

5.5.3 Troubleshooting a Delta Secondary (600, 480 or 240 Volt)

If Then

The customer has One or more phases feeding the customer is out of

abnormal voltage. service. Check the transformers and the primary feeder.

With the customer’s

switch open, the The customer’s switch must be open for this voltage

voltage on the supply check because the back feed in a delta service can show

side shows one or a voltage at the service entrance.

more of the phase-to-

phase voltages at

zero volts.

At the main panel, the When one phase of a primary feeder loses power, thephase-to-phase voltage voltage at a wye-delta transformer bank will have

readings are lower than a reduced voltage on two of the phase-to-phase

normal and one phase- readings and zero volts on the third phase-to-phase

to-phase voltage reading reading.

is zero volts. For example,

on a 240-volt service, the On a wye-delta transformer bank, the primary neutral

readings are 210 volts, is not connected to the system neutral or the secondary

210 volts, and 0 volts. neutral, but it is left ungrounded or floating. If the

transformer neutral was grounded and one primary

phase is opened, the transformer bank would become

an open-delta bank and continue to supply three-phase

power at a 57 percent reduced capacity exposing thebank to a burnout because of an overload.

At the main panel all A ground fault exists on a phase. When one phase

three phase-to-phase of a delta circuit is faulted to ground, it does not

voltage readings are blow a fuse because there is no path for a return current

normal but one phase- to the source. Both the phase and the ground are alive

to-ground is reading and, therefore, a faulted phase-to-ground voltage

very low or zero voltage. reading will show a very low or zero voltage difference.

Both the faulted phase and earth are alive in relation to

the other two phases.

A customer with a delta service often has a ground

fault indicating a light connected between each phase

and ground. The light will go out when there is a

ground fault and the ground becomes alive.

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5.6 Working on a Voltage Conversion

5.6.1 Preparing the Circuit for Voltage Conversion

Work That Can Be Done on t he Circuit before

the Day of t he Voltage Conversion

1. If going to a higher voltage, change all insulators on the circuit where they have

not already been done through pre-building projects.

2. Change any switchgear that can handle the duties for both voltages.

3. If going to a higher voltage, change any surge arresters that are on the circuit.

The utility will need to decide if the risk of having the higher voltage arresters

on the circuit for a period of time is acceptable.

4. Primary underground cable that cannot accommodate the new voltage will

need to be replaced or planned to be fed at the original voltage using a primary

step-down transformer.

Work t o Be Done on Voltage Conversion Day

1. If going to a higher voltage, existing voltage regulators and capacitors will need

to be removed or replaced.

5.6.2 Working at Transformer Installations

Work That Can Be Done at Transfo rmer Installat ionsbefore the Day of the Volt age Conversion

1. Install dual voltage transformers. There may not be any dual voltage trans-formers for 277/480 or 347/600 three-phase banks.

2. Install surge arresters for the new voltage.

3. Install cutouts and fuses for the new voltage.

4. There may be a need to install current-limiting fuses for a higher voltage dis-

tribution system.

5. Preparations need to be made if existing wye/delta transformer banks need

to be changed to wye/wye during conversion day to accommodate single

primary-bushing transformers. The customer’s service entrance will need to be

upgraded to accept a neutral.


1. Isolate all transformers from the circuit, and energize the circuit at the new

voltage.

2. Change transformers that are not dual voltage.

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114 Chapter 5

3. Change the tap setting on the dual voltage transformers to the new voltage.

4. If the circuit has been energized at the new voltage, energize the transformerand take voltage and phase rotation checks before the customer applies load

(remove meter or open customer breaker).

5. Or, if it is preferred to energize the new higher-voltage surge arresters from a

remote location, leave the circuit out under the “clearance” and attach the riser

and arrester to the circuit. Leave the customers disconnected and perform volt-

age checks at each transformer after the circuit is energized.

6. Or, if the transformer will be fed from a primary step-down transformer, ensure

that there is proper voltage and phase rotation before connecting customers.

7. Having the transformers on a check-off sheet will reduce the risk of forgetting

to change the tap on a transformer and leaving a customer with a high-

damaging voltage.

5.6.3 Working at Underground Installations

Work That Can Be Done at Underground Installationsbefore the Day of t he Voltage Conversion

1. If using existing cable at the new voltage, cable size may not match the bush-

ing inserts of new switching cabinets or transformers. Non-standard equip-

ment may be needed.

2. If a new, higher-voltage cable is installed before conversion day, larger elbows

(separable connectors) may not match existing equipment. Non-standard

equipment may be needed.

3. Install dual-voltage transformers or plan a change during the outage.

4. There may be a need to install current-limiting fuses for a higher-voltage dis-

tribution system.

5. Replace switchgear and fusing to accommodate the new voltage.

6. Change surge arresters at riser poles and any elbow arresters installed at dual-

voltage transformers.

Work t o Be Done on Voltage Conversion Day,

1. Change the tap setting on the dual-voltage transformers to the new voltage.

2. If the circuit has been energized at the new voltage, energize the transformer

and take voltage and phase rotation checks before the customer applies load.

3. If the transformer is to be fed from a primary step-down transformer, ensure

that there is proper voltage and phase rotation before connecting customers.

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5.6.4 Working at Primary Step-Down Transformer Installations

Work That Can Be Done At Underground Installationsbefore the Day of the Volt age Conversion

During a voltage conversion project, primary step-down transformers are often

installed to delay the conversion of some line sections, especially underground. Do a

turns ratio test on the primary step-down transformer because setting the external

switch or tap does not always make the proper internal connection.


Reduce the risk of causing damage to a customer’s appliance by isolating all the trans-

formers on the load side of the primary step-down transformer and doing a voltage

test and phase-rotation tests on an individual basis as the transformers are energized.

Make a clear separation at tie points between the two primary voltage levels.

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116 Chapter 5

Additional Local Safety Regulations

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troubleshooting transformers

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