For the measurement of large currents and high voltages in AC circuits, specially constructed accurate ratio transformers are used in conjunction with low range AC instruments. These specially constructed transformers are known as instrument transformers.
Types of Instrument Transformer
There are two types of instrument transformer. These are:
- Potential Transformers (PTs)
- Current Transformers (CTs)
These instrument transformers are also used in the power system in conjunction with protective relays. For safety purposes, the secondaries of these transformers are grounded.
Working & Construction of Current Transformer
Current transformers are used in AC power circuits to feed the current coils of indicting and metering instruments (ammeters, watt-meters, energy-meters) and protective relays. These transformers make the ordinary low current instruments suitable for the measurement of high current and isolate them from high voltage.
The current transformer basically consists of an iron core on which a primary and one or two secondary windings are wound. The primary winding has one or two turns of thick wire and is connected in series with the load. It carries the actual power system current. Primary current ratings vary from 10 A to 3000 A or more.
The secondary winding has a large number of turns of fine wire. It is connected across current coils of indicting and metering instruments and protective relays. The secondary current ratings are of the order of 5 A, 1 A, and 0.1 A. The latter is used for static relays.
If, for any reason, the instrument connected to the secondary of CT is to be removed, then the secondary of CT must be short-circuited by a fairly thick wire.
The ratio of primary current to the secondary current is known as the transformation ratio of the CT. The transformation ratio of a CT is usually high.
The product of voltage and current on the secondary side when it is supplying its maximum rated value of current is known as the rated burden and is measured in volt-amperes (VA). The volt-ampere rating of CTs is low (5 – 150 VA) as compared to that of power transformers.
Also, current in the secondary of CTs is governed by the current in the primary winding i.e., power circuit current. But in the case of power transformers, it is governed by the load impedance.
A current transformer in combination with a bridge rectifier and a DC milli-ammeter produces a very useful meter known as a clamp-on ammeter. The core of the transformer can be split with the help of a trigger switch. And therefore, the core can be clamped around a live conductor very easily to measure the current.
Thus this arrangement avoids the necessity of breaking the circuit to insert an ammeter to measure the value of current flowing. By changing the shunt resistance of the milli-ammeter circuit ranges from 0 – 5 A to 0 – 600 A.
Example: A 100 : 5 transformer is used in conjunction with a 5-amp ammeter. If the latter reads 3.5 A, find the line current.
Solution: Here, the ratio 100 : 5 stands for the ratio of primary-to-secondary currents, i.e. I1/I2 = 100/5
∴ Primary (or line) current = 3.5 × (100/5) = 70 A
Example: It is desired to measure a line current of the order of 2,000 A to 2,500 A. If a standard 5-amp ammeter is to be used along with a current transformer, what should be the turn ratio of the latter? By what factor should the ammeter reading be multiplied to get the line current in each case?
Solution: I1/I2= 2000/5 = 400 or 2500/5 = 500.
Since I1/I2 = N2/N1 hence N2/N1 = 400 in the first case and 500 in the second case.
It means that N1 : N2 = 1 : 400 or 1 : 500.
Ratio or multiplication factor in the first case is 400 and in the second case 500.
Working & Construction of Potential Transformers
Potential transformers are used in AC power circuits to feed the potential coils of indicting and metering instruments (voltmeters, watt-meters, energy-meters) and protective relays. These transformers make the ordinary low voltage instruments suitable for the measurement of high voltage and isolate them from high voltage.
The PTs are highly accurate ratio step down transformers. Its primary winding has a large number of turns and is always connected across the supply system. Its secondary winding has few numbers of turns and is connected to the potential coil of indicting and metering instruments and protective relays.
In general, they are of the shell-type and do not differ much from the ordinary two-winding transformers, except that their power rating is extremely small.
The primaries of PT are rated from 400 V to several thousand volts and secondaries always for 110 V. Up to voltages of 5,000, potential transformers are usually of the dry type, between 5,000 and 13,800 volts, they may be either dry type or oil-immersed type, although for voltages above 13,800 they are always oil-immersed type.
The ratio of the rated primary voltage to the rated secondary voltage is known as the turn or transformation ratio of PT.
The burden is the total external volt-ampere load on the secondary at the rated secondary voltage.
The rated burden of a PT is the VA burden, which must not exceed if the transformer is to operate with its rated accuracy.
The maximum burden is the greatest VA load at which the PT will operate continuously without overheating its winding beyond the permissible limits.
Let the voltage to be measured of a power system is 11 kV. It is impossible to measure such a high voltage directly by a voltmeter. Therefore, a PT having a secondary to primary turn ratio 1:100 is used in conjunction with a voltmeter, which steps down the voltage from 11 kV to 110 V, as shown in the figure.
For the measurement of power in a high voltage power system, both CT and PT are used. The CT is used to step down the system current, and the PT is used to step down the system voltage up to the required value.
The potential coil (PC) of the wattmeter is connected across the secondary of PT. And the current coil (CC) of wattmeter is connected across the secondary of CT, as shown in the figure.
Capacitor Potential Transformer
The above discussed conventional PTs becomes very expensive to measure voltages exceeding 100 kV due to the insulation requirements. So to measure voltages above 100 kV, a capacitor potential transformer is used.
It is a combination of a capacitor potential divider and a magnetic transformer known as an intermediate transformer of a relatively small ratio.
The stack of high voltage capacitors forms a potential divider. C1 and C2 are capacitors of the two sections, and Z is the burden. The voltage applied to the primary of the intermediate transformer is usually about 10kV.
To get the satisfactory performance of the complete unit, the intermediate transformer must have a very small ratio and phase angle error.
Advantages of Instrument Transformers
The instrument transformers are extensively used for very precise measurements as well as for routine measurements. They are so important for insulating and range extension purposes that it is difficult to imagine the operation of a high voltage system without them. They have many advantages, and these are:
1. When instruments are used in conjunction with instrument transformers, their readings do not depend upon their constants (R, L, C), as is the case with shunts and multipliers. The instrument transformers produce the same instrument reading regardless of the constants or the number of instruments connected in the circuit.
2. We can use moderate size meters for measurements, i.e., 5 A for current and 100 to 120 V for voltage measurements.
3. Instruments and meters can be standardized so that there is saving in overall costs. Replacement of damaged instruments becomes easy.
4. The metering circuit is isolated from the high voltage circuits. Hence the safety is assured for the operators.
5. Power consumption in the metering circuit becomes low.
6. Several instruments can be operated from a single instrument transformer.