Instrument transformers are used with instruments and relays to measure and control ac circuits. Large and expensive devices are required to measure high voltages and currents directly. However, instrument transformers used with small standard devices provide a way of measuring high voltage and current values safely. The use of instrument transformers also means increased safety for the operator, less chance of control equipment damage due to high voltages, more accurate measurements, and greater convenience.
There are two types of instrument transformers: the instrument potential transformer and the instrument current transformer.
The Potential Transformer
The potential transformer is similar to a power transformer or a distribution transformer. However, its capacity is relatively small when compared to that of power transformers. Typical ratings of potential transformers range from 100 to 500 VA. The low-voltage side of the trans- former generally is wound for 120 V. The load on the secondary (low-voltage) side of the potential transformer consists of the potential coils of various instruments. In some cases, the potential coils of relays and other control equipment are also connected to the secondary. In almost all cases, the load is light. Thus, potential transformers require a capacity no greater than 100 to 500 VA.
The primary circuit voltage and the voltage rating of the primary winding of a potential transformer are the same. As an example of the use of a potential transformer, assume that the voltage of a 4800-V, single-phase line must be measured. In this case, the primary of the potential transformer is rated at 4800 V. The low-voltage secondary is rated at 120 V. The ratio between the primary and secondary voltages is 4800/120 40/1.
A voltmeter can be connected across the secondary of the potential transformer to mea- sure the primary voltage, or 120 V. To determine the actual voltage of the high-voltage circuit, the voltmeter reading is multiplied by the transformer ratio. Thus, 120 X 40 4800 V. In some cases, the voltmeter is calibrated to indicate the actual value of the voltage on the primary side. As a result, it is not necessary to apply the multiplier to the instrument read- ing. This means that errors are minimized.
Figure 15–1 shows the connections for a potential transformer with a 4800-V pri- mary input and a 120-V output to a voltmeter. Note that the transformer has subtractive polarity. All instrument potential transformers now manufactured have subtractive polar- ity. One of the secondary leads is grounded to minimize high-voltage hazards.
The ratio between the primary and secondary voltages of a potential transformer is very accurate. Normally, the percentage error is less than 0.5%.
The Current Transformer
The second type of instrument transformer is the current transformer. Current trans- formers are used to avoid the need to connect ammeters, instrument current coils, and relays directly to high-voltage lines. Current transformers step down the current by a known ratio. Thus, small and accurate instruments and control devices can be used because they are insulated from the high-voltage line.
The primary winding of a current transformer is connected in series with one of the line wires. The primary winding consists of a few turns of heavy wire wound on a laminated iron core. The secondary coil has more turns of smaller wire and is wound on the same core as the primary coil. The current rating of the primary winding is the maximum current that the winding will be required to conduct. For higher currents, the line in question may be fitted through a toroidal core with no turns at all. The core than acts as the primary winding and has no connection to the line. The secondary winding is always rated at 5 A.
To illustrate the operation of a current transformer, assume that the current rating of the primary winding is 100 A. The secondary winding has the standard rating of 5 A. The primary winding consists of three turns of wire, and the secondary winding has 60 turns. The ratio between the primary and the secondary currents is 100 A/5 A, or 20:1. In other words, the primary current is 20 times greater than the secondary current. Note that the number of turns and the current in the primary and secondary windings are related by an inverse proportion.
Stepping Down Current. The current transformer shown in Figure 15–2 is used to step down the current in a 4800-V, single-phase circuit. The primary is rated at 100 A. Because the secondary has the standard 5-A rating, the transformer ratio is 20:1. In other words, there is 20 A in the primary winding for each ampere in the secondary winding. If the ammeter indicates 4 A, the actual current in the primary is 20 times this secondary current, or 80 A.
Polarity Markings. The polarity markings of the current transformer are also shown in Figure 15–2. The high-voltage primary terminals are marked H1 and H2. The secondary terminals are marked X1 and X2. Electrons enter H1 and leave X1 at the same instant. Some manufacturers mark only the H1 and X1 leads. The H1 lead connects to the line wire feeding from the source. The H2 lead connects to the line wire feeding to the load. The secondary
leads connect directly to the ammeter. One of the secondary leads is grounded to reduce the high-voltage hazard. A common current transformer with a ratio of 200:5 is shown in Figure 15–3.
Current Transformer Precaution. The secondary circuit of a current transformer must not be opened when there is current in the primary winding. An absence of current in the sec- ondary winding means that there is no secondary magnetomotive force opposing the primary mmf. The primary current becomes an exciting current. The flux of this current is unopposed by a secondary mmf. The primary flux causes a high voltage to be induced in the secondary
winding. This voltage is great enough to be a hazard. If the instrument circuit must be opened when there is current in the primary winding, a short-circuiting switch is installed at the secondary terminals of the current transformer. This switch is closed before the instrument circuit is opened to make repairs or rewire the metering circuit.
Primary-to-Secondary Ratios. Current transformers have very accurate ratios between the primary and secondary currents. The percent error generally is less than 0.5.
Construction. A bar-type construction is often used when the primary winding of a cur- rent transformer has a large rating. The primary winding consists of a straight copper bus bar that passes through the center of a hollow metal core. The secondary winding is wound on the core. All standard current transformers rated at 1000 A or larger have this type of structure. In some cases, current transformers with ratings smaller than 1000 A also have the bar-type structure.
Measuring Current, Voltage, and Power
A potential transformer and a current transformer are shown in Figure 15–4. These transformers are being used with standard instrument movements to measure the voltage,
current, and power for a 4600-V single-phase circuit. The potential transformer is rated at 4800/120 V, and the current transformer is rated at 50/5 A.
The voltmeter and the potential coil of the wattmeter are connected in parallel across the low-voltage output of the potential transformer. The voltage across the potential coils of both instruments is the same.
The ammeter and the current coil of the wattmeter are connected in series across the secondary output of the current transformer. This means that the same current appears in the current coils of both instruments.
When lead H1 of the potential transformer and lead H1 of the current transformer are instantaneously positive, the X1 leads of both transformer secondaries are also instan- taneously positive. The current and voltage terminals of the wattmeter (marked ±) have the same instantaneous polarity. Thus, the torque on the wattmeter movement causes the pointer to move upscale. The secondary side of each instrument transformer is grounded to minimize high-voltage hazards.
Calculating the Primary Voltage and Current. In Figure 15–4, the voltmeter reading is 112.5 V, the ammeter reading is 4 A, and the wattmeter reading is 450 W. The primary voltage can be found as follows:
Calculating Power and Power Factor. The primary power must be determined using a power multiplier. Recall that power is the product of the voltage. the current, and the power factor. For this example, the power multiplier is the product of the current multiplier and the voltage multiplier:
Three-phase circuits are used for most high-voltage and current transmission and distribution lines. This means that the three-phase, three-wire system requires two potential transformers having the same rating, and two current transformers having the same rating.
The connections shown in Figure 15–5 are for a three-phase, three-wire system using instrument transformers and measuring devices. The two potential transformers are connected in open delta to the 4800-V, three-phase line. There are three secondary voltages of 120 V each. Two current transformers are used. The primary winding of one current transformer is connected in series with line A. The primary winding of the other current transformer is connected in series with line C.
It can be seen by checking the respective primary and secondary circuit paths for the ammeters that each ammeter is connected correctly. Other instruments that may be added to the circuit include a three-phase wattmeter, a three-phase watt-hour meter, or a three-phase power factor meter. The proper phase relationships must be maintained when three-phase instruments are connected into a secondary circuit. If the instruments are not connected properly, their readings will be incorrect. For the three-phase, three-wire metering system
of Figure 15–5, both the interconnected secondary potential circuit and the interconnected secondary current circuit are grounded. Grounding serves as protection from high-voltage hazards.
Testing Metering Connections
When a test is to be made on industrial equipment such as a three-phase motor, temporary metering connections are commonly made using portable instruments and instrument transformers. Figure 15–6 shows the connections required to obtain measurements of the cur- rents, the voltage, and the power in watts for a 20-hp, 480-V, three-phase motor. Two potential transformers, rated at 480/120 V, are connected in open delta. Two current transformers, each rated at 50/5 A, are connected in line wires A and C.
The circuit in Figure 15–6 shows one method of measuring the three-phase power using the two-wattmeter method. The current coil of wattmeter 2 is in the secondary circuit of the current transformer and is in series with line wire A. The potential coil of wattmeter 2 is connected across the secondary of the potential transformer whose primary winding is
connected to line wire A and B. The current coil of wattmeter 1 in the secondary circuit of the current transformer is in series with line wire C. The potential coil of wattmeter 1 is connected across the secondary of the potential transformer whose primary winding is connected to line wires B and C. The polarity terminals of both wattmeters are marked and are connected into the circuit correctly.
To check the connections, note that when primary line wires A and C are instantaneously positive, line wire B is instantaneously negative. The H1 terminals on the current transformers are positive. The X1 terminals on both current transformers are positive at the same instant. This means that the marked polarity current terminal on each wattmeter is instantaneously positive. Leads A and C from the low-voltage side of the potential trans- formers feed to the marked polarity potential terminals of the two wattmeters. These leads are also positive at this same instant. Therefore, both wattmeters indicate upscale if the power factor of the load is greater than 0.50 lag.
PROBLEM 1
Statement of the Problem
Figure 15–6 shows that the three secondary voltages are 120 V each. Both ammeters indicate 2.7 A. Wattmeter 1 reads 317 W, and wattmeter 2 indicates 121 W. The motor is delivering its rated capacity of 20 hp. Determine
1. the input, in watts.
2. the power factor.
3. the motor efficiency.
Solution
1. The total true power input is determined as follows:
The ratio of the two power values may be called the power factor ratio for the two- wattmeter method given in Unit 10. This ratio can be determined using the two- wattmeter readings or the primary power values:
If the ratio of 0.38 is projected on the power factor ratio curve, it will be 0.78. This value agrees with the calculated power factor of 0.78 lag.
3. When the motor is delivering its rated horsepower output, its efficiency is
Secondary Metering Connections
Figure 15–7 shows the secondary metering connections for a 2400/4160-V, three- phase, four-wire system. The three potential transformers are connected in wye. The three-phase output of these transformers consists of three secondary voltages of 120 V to neutral. A 50-to-5-A current transformer is connected in series with each of the three line wires.
When wattmeters, watt-hour meters, and other three-phase instruments are used in a circuit, their readings will be correct only if they are connected with the proper phase relationship. Three single-phase wattmeters are shown in the circuit in Figure 15–7. These instruments make it easier to check the instantaneous directions of current and voltage. In an actual installation, a three-element indicating wattmeter and possibly a three-element watt-hour meter would be used. However, the connections are the same. Note that both the low-voltage potential network and the secondary current network are grounded to minimize high-voltage hazards.
Labels: Alternating current fundamentals
