LESSON 7 INSTRUMENT TRANSFORMERS
LEARNING OUTCOME:
▪ Diagram the connection of three single-phase
transformers to form delta-wye transformer bank
▪ Explain the operation of an instrument potential
transformer bank
PERFORMANCE STANDARDS
▪ explain the operation of an instrument potential transformer.
▪ explain the operation of an instrument current transformer,
▪ diagram the connections for a potential transformer and a current transformer in a
single-phase circuit.
▪ state how the following quantities are determined for a single-phase circuit containing
instrument transformers: primary current, primary voltage, primary power, apparent
power, and power factor.
▪ describe the connection of instrument transformers in a three-phase, three-wire
circuit,
▪ describe the connection of instrument transformers to a three-phase, four-wire
system.
Instrument transformers are used to measure and control AC circuits. Direct measurement
of high voltage or heavy currents involves large and expensive instruments, relays, and other
circuit components of many designs. Using instrument transformers, however, makes it possible
to use relatively small and inexpensive instruments and control devices of standardized designs.
Instrument transformers also protect the operator, the measuring devices, and the control
equipment from the dangers of high voltage. The use of instrument transformers results in
increased safety, accuracy, and convenience.
There are two distinct classes of instrument transformers: the instrument potential
transformer and the instrument current transformer. (The word "instrument" is usually omitted for
brevity.)
POTENTIAL TRANSFORMERS
The potential transformer operates on the same principle as a power or distribution
transformer. The main difference is that the capacity of a potential transformer is small compared
to that of power transformers. Potential transformers have ratings from 100 to 500 volt-amperes
(VA). The low-voltage side is usually wound for 115 volts or 120 volts. The load on the low-voltage
side usually consists of the potential coils of various instruments, but may also include the
potential coils of relays and other control equipment. In general, the load is relatively light, and it
is not necessary to have potential transformers with a capacity greater than 100 to 500 volt-
amperes.
The high-voltage primary winding of a potential transformer has the same voltage rating
as the primary circuit. When it is necessary to measure the voltage of a 4,600-V01t, single-phase
line, the primary of the potential transformer would be rated at 4,600 volts and the low-voltage
secondary would be rated at 115 volts. The ratio between the primary and secondary windings is
4,600 _ 40
115 1
A voltmeter connected across the secondary of the potential transformer indicates a value
of 115 volts. To determine the actual voltage on the high-voltage circuit, the instrument reading
of 115 volts must be multiplied by 40 (115 X 40 = 4,600 volts). In most cases, the voltmeter is
calibrated to indicate the actual value of voltage on the primary side. As a result, the operator is
not required to apply the multiplier to the instrument reading, and the possibility of errors is
reduced.
Figure 7-1 illustrates the connections for a potential transformer with a 4,600-volt primary
input and a 115-volt output to the voltmeter. This potential transformer has subtractive polarity.
(All instrument potential transformers now manufactured have subtractive polarity.) One of the
secondary leads of the transformer in Figure 7-1 is grounded to eliminate high-voltage hazards.
Figure 7-1 Connections for a potential transformer.
Potential transformers have highly accurate ratios between the primary and secondary
voltage values; generally, the error is less than 0.5 percent. Power transformers are not designed
for highly accurate voltage transformation.
CURRENT TRANSFORMERS
Current transformers are used so that ammeters and the current coils of other instruments
and relays need not be connected directly to high-current lines. In other words, these instruments
and relays are insulated from high currents. Current transformers also step down the current to a
known ratio. The use of current transformers means that relatively small and accurate
instruments, relays, and control devices of standardized design can be used in circuits.
The current transformer has separate primary and secondary windings. The primary
winding, which may consist of a few turns of heavy wire wound on a laminated iron core, is
connected in series with one of the line wires. The secondary winding consists of a greater
number of turns-of a smaller size of wire. The primary and secondary windings are wound on the
same core.
The current rating of the primary winding of a current transformer is determined by the
maximum value of the load current. The secondary winding is rated at 5 amperes regardless of
the current rating of the primary windings.
For example, assume that the current rating of the primary winding of a current transformer
is 100 amperes. The primary winding has three turns, and the secondary winding has 60 turns.
The secondary Winding has the standard current rating of 5 amperes; therefore, the ratio between
the primary and secondary currents is 100/5 or 20 to l. The primary current is 20 times greater
than the secondary current. Because the secondary winding has 60 turns and the primary winding
has 3 turns, the secondary winding has 20 times as many turns as the primary winding. For a
current transformer, then, the ratio of primary to secondary currents is inversely proportional to
the ratio of primary to secondary turns.
In Figure 7-2, a current transformer is used to step down current in a 4,600-volt, single-
phase circuit. The current transformer is rated at 100 to 5 amperes, and the ratio of current step-
down is 20 to 1. In other words, there are 20 amperes in the primary winding for each ampere in
the secondary winding. If the ammeter at the secondary indicates 4 amperes, the actual current
in the primary is 20 times this value, or 80 amperes.
The current transformer in Figure 7-2 has polarity markings in that the two high-voltage
primary leads are marked H1 and H2, and the secondary leads are marked Xl an X2. When H1 is
instantaneously positive, Xl is positive at the same moment, some current transformer
manufacturers mark only the H1 and X1 leads or use polarity marks. Polarity marks can be any
similar marks that are placed on the HI lead and the X 1 lead. Common marks are Xs, dots, or
squares. When connecting current transformers in circuits, the H1
Figure 7-2 A current transformer used with an ammeter.
lead is connected to the line lead feeding from the source, while the H2 lead is connected to the
line lead feeding to the load.
The secondary leads are connected directly to the ammeter. Note that one of the
secondary leads is grounded as a safety precaution to eliminate high-voltage hazards.
Caution: The secondary circuit of a transformer should never be opened when there is
current in the primary winding. If the secondary circuit is opened when there is current in the
primary winding, then the entire primary current is an exciting current that induces a high voltage
in the' secondary winding. This voltage can be high enough to endanger human life.
Individuals working with current transformers must check that the secondary winding circuit
path is closed. At times, it may be necessary to disconnect the secondary instrument circuit when
there is current in the primary winding. For example, the metering circuit may require rewiring or
other repairs may be needed. To protect a worker, a small short-circuiting switch is connected
into the circuit at the secondary terminals of the current transformer. This switch is closed when
the instrument circuit must be disconnected for repairs or rewiring.
Current transformers have very accurate ratios between the primary and secondary current
values: the error of most modern current transformers is less than 0.5 percent.
When the primary winding has a large current rating, it may consist of a straight conductor
passing through the center of a hollow metal core. The secondary winding is wound on the core.
This assembly is called a bar-type current transformer. The name is derived from the construction
of the primary, which actually is a straight copper bus bar. All standard current transformers with
ratings of 1,000 amperes or more are bar-type transformers. Some current transformers with
lower ratings may also be of the bar type. Figure 7-3 shows a bar-type current transformer.
Figure 7-3 Bar type current transformer.
(From Keljik, Electric motors and Motor Controls (Delmar Publishers, 1995))
Figure 7-4 Clamp-on style ammeters/multimeters.
In window-type current transformers, the primary passes through an opening in the
transformer windings. The primary of the transformer is the line conductor, much like the bar forms
the primary in the bar-type current transformer. The shape of the transformer resembles a donut.
Therefore, this type of current transformer (CT) is often referred to as a "donut."
Figure 7-4 shows a clamp-on ammeter that uses the concept of a window-type current
transformer. By opening the clamp and then closing it around the current-carrying conductor, the
current in the conductor is measured on the meter.
INSTRUMENT TRANSFORMERS IN A SINGLE-PHASE CIRCUIT
Figure 7-5 illustrates an instrument load connected through instrument transformers to a
single-phase, high-voltage line. The instruments include a voltmeter, an ammeter, and a
wattmeter. The potential transformer is rated at 4,600 to 115 volts; the current transformer is rated
at 50 to 5 amperes. The potential coils of the voltmeter and the wattmeter are connected in parallel
across the low-voltage output of the potential transformer. Therefore, the voltage across the
potential coils of each of these instruments is the same. The current coils of the ammeter and the
wattmeter are connected in series across the secondary output of the current transformer. As a
result, the current in the current coils of both instruments is the same. Note that the secondary of
each instrument transformer is grounded to provide protection from high-voltage hazards, as
provided in Article 250 of the National Electrical Code.
Figure 7-5 Single-phase metering connections
The voltmeter in Figure 7-5 reads 112.5 volts, the ammeter reads 4 amperes, and the
wattmeter reads 450 watts. To find the primary voltage, primary current, primary power, apparent
power in the primary circuit, and the power factor, the following procedures are used:
Primary Voltage
Voltmeter multiplier = 4,600/115 = 40
Primary volts = 112.5 x 40
= 4,500 volts
Primary Current
Ammeter multiplier = 50/5 = 10
Primary amperes = 4 x 10
= 40 amperes
Primary Power
Wattmeter multiplier = Voltmeter multiplier x ammeter multiplier
Wattmeter multiplier = 40 x 10
= 400
Primary watts = 450 x 400
= 180,000 watts or 180 kilowatts Apparent
Apparent Power
The apparent power of the primary circuit is found by multiplying the primary voltage and
current values.
Apparent power (volt-amperes) = volts x amperes
Volt-amperes = 4,500 x 40
180,000
= 180,000 volt-amps = = 180 KVA
1,000
Power Factor
Power in kilowatts
Power factor = Apparent power in kilovolt-amperes
= 180/180
= 1.00 or 100 percent
In DC instruments needed to monitor large values of DC, another type of system is needed.
Because it is not possible to use standard transformers to change DC levels, instrument shunts
are used to develop the smaller values needed for DC monitoring equipment. In effect, these DC
shunts are high-power, low-resistance resistors that are placed in line with the DC power. They
typically have connection points that are used to connect to meters. The resistance of the shunt
is designed so that when full line current flows through the resistive material, it will develop 50
millivolts across the resistance and therefore the meter movement. (See the DC meter in Figure
7-6.)
Figure 7-6 Panel-mounted meters use transformers to monitor
INSTRUMENT TRANSFORMERS ON THREE-PHASE SYSTEMS
Three-Phase, Three-Wire System
On a three-phase, three-wire system, two potential transformers of the same rating and
two current transformers of the same rating are necessary. It is common practice in three-phase
metering to interconnect the secondary circuits. That is, the connections are made so that one
wire or device conducts the combined currents of two transformers in different phases.
The low-voltage instrument connections for a three-phase, three-wire system are shown
in Figure 7-7, Note that the two potential transformers are connected in open delta to the 4,600-
volt, three-phase line. This results in three secondary-voltage values of 115 volts each. The two
current transformers are connected so that the primary of one transformer is in series with line A
and the primary winding of the second transformer is in series with line C.
Note that three ammeters are used in the low-voltage secondary circuit. This wiring system is
satisfactory on a three-phase, three-wire system, and all three ammeters give accurate readings.
Other instruments that can be used in this circuit include a three-phase wattmeter, a three-phase
watt-hour meter, and a three-phase power factor meter. When three-phase instruments are
connected in the secondary circuits, these instruments must be connected correctly so that the
proper phase relationships are maintained. If this precaution is not observed, the instrument
readings will be incorrect, In checking the connections for this three-phase, three-wire metering
system, note that the interconnected potential and current secondaries are both grounded to
provide protection from high- voltage hazards.
Figure 7-7 Metering connections for three-phase, three-wire system.
50- to 5-ampere current transformers are used in the three line conductors. Three ammeters are
used in the interconnected secondary circuit. Both the interconnected potential and the current
secondaries are grounded to protect against possible high-voltage hazards.
Figure 7-8 Metering connections for three-phase, four wire system.
SUMMARY
Instrument transformers are specifically designed to transform voltage and current in very
precise ratios. Potential transformers are used to transform high voltages to usable values of 115
or 120 volts for use by standard instruments. Current transformers are used to transform large
values of AC down to a 5-amp level so that it can be used by standard instruments. DC levels are
typically reduced to a usable level through the use of shunts. The shunt has a primary-load current
rating and the meter is then connected across the shunt. The meter is designed to operate at 50
millivolts.
Answer the following:
1. What are the two types of instrument transformers?
a. __________________________
b. __________________________
2. Why must the secondary circuit of a current transformer be closed when there is current in
the primary circuit?
_______________________________________________________________________
_______________________________________________________________________
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3. A transformer is rated at 4,600/115 volts. A voltmeter connected across the secondary
reads 112 volts. What is the primary voltage?
_______________________________________________________________________
4. A current transformer is rated at 150/5 amperes. An ammeter in the secondary circuit reads
3.5 amperes. What is the primary current?
_______________________________________________________________________
5. A 2,300/115-volt potential transformer and a 100/5-ampere current transformer are
connected on a single-phase line. A voltmeter, an ammeter, and a wattmeter are
connected in the secondaries of the instrument transformers. The voltmeter reads 110
volts, the ammeter reads 4 amperes, and the wattmeter reads 352 watts. Draw the
connections for this circuit. Mark leads H1, X1, and so forth. Show all voltage, current, and
wattage readings.
6. Complete a circuit using instrument transformers to measure voltage and amperage.
Include terminal markings.
FROM SOURCE TO LOAD
7. What is the primary voltage of the single-phase circuit in question 5?
_______________________________________________________________________
8. What is the primary current in amperes of the single-phase circuit in question 5?
_______________________________________________________________________
9. What is the primary power in watts in the single-phase circuit in question 5?
_______________________________________________________________________
10. What is the power factor of the single-phase circuit in question 5?
_______________________________________________________________________
Select the correct answer for each of the following statements and place the corresponding
letter in the space provided.
________ 11. The secondary for a potential transformer is usually wound for
a. 10 volts. c. 230 volts.
b. 115 volts. d, 500 volts.
________ 12. Potential transformer secondaries are grounded to
a. stabilize meter readings.
b. ensure readings with an accuracy of 0.5 percent.
c. complete a system with the primaries.
d. eliminate high-voltage hazards.
________ 13. A transformer used to reduce Current values to a size where small meters can
register them is a(n)
a. autotransformer. c. potential transformer.
b. distribution transformer. d. current transformer.
________ 14. The primary of a large current transformer may consist of
a. many turns of fine wire.
b. a few turns of fine wire.
c. many turns of heavy wire.
d. a straight-through conductor.
________ 15. The standard ampere rating of the secondary of a current transformer is
a. 5 amperes. c. 15 amperes.
b. 50 amperes. d. 150 amperes.
________ 16. The secondary circuit of a current transformer should never be opened when
current is present in the primary because
a. the meter will burn out.
b. the meter will not operate.
c. dangerous high voltage may develop.
d. primary values may be read on the meter.
