Download (direct link):
1. Describe an autotransformer and compare it to a conventional transformer.
2. Sketch the connections for an autotransformer.
3. What is an induction regulator, and how does it work?
4. Sketch and describe fully a booster transformer and its connections.
In Chapter 14, voltmeters, ammeters, etc., were covered. It is not practical to use coil-wound voltmeters with resistance in series on high-voltage AC circuits, because of the excessive insulation that would be required and the large amount of resistance that it would take.
It is likewise not practical to use shunts and millivoltmeters for measuring currents on high-voltage AC circuits. In both instances, zero-temperature-coefficient resistors could be used, but they would have reactance changes with any change of frequency.
Use of Instrument Transformers
Instrument transformers are the practical answer for measuring current and voltages where high AC voltages and currents are encountered. There are two chief reasons for this:
1. Electricians and station operators are protected from contact with high voltage.
2. The instruments may be constructed with reasonable amounts of insulation and current-carrying capacity.
Instrument transformers are not only used for current and voltage measurements, but also for watt-hour meters, ground relays, overcurrent relays, synchroscopes, ground fault interrupters, and other uses that require their operation by high currents and/or high voltages.
Types of Instrument Transformers
Instrument transformers supply low currents and low voltages for the voltmeters, ammeters, etc. These transformers have very high accuracy of transformation and constants (K) are used as multipliers to convert the readings on the voltmeter or ammeter to the value of the actual voltage or current of the line or equipment being measured. Current transformers are usually designed for 5 amperes output maximum on the secondary, and potential transformers are usually designed for 120 volts output on the secondary.
In order to secure the greatest accuracy, both types of transformers must be made of the highest quality of core steel available, and precision of winding is also necessary.
308 Chapter 29
The potential transformer is an isolation type of transformer, such as was just covered in Chapters 26, 27, and 28. That is, the primary winding is in shunt with the load, and the secondary is in shunt with the instrument(s) used in the measuring. See Figure 29-
1. Potential transformers are often called PTs.
The current transformer is also an isolation-type transformer, but the primary is the line conductor or bus; or it may be more than one conductor, that is, two or more turns of conductors capable of handling the total line current. The primary winding is in series with the load, instead of in shunt as with the potential transformers. The primary current is thus the load current, and the primary emf is the drop of potential across the transformer due to its primary impedance. These transformers are commonly called CTs. Figure 29-1 illustrates a CT as indicated by C.
Figure 29-1 Current and potential transformers in circuit.
The primary emf of a potential transformer is the line voltage of the circuit being measured. In Figure 29-1, the alternator supplies
13,800 volts to a line. Potential transformer P steps the voltage down to 120 volts for measurement. This is a ratio of 115 to one, so K for the potential transformer is K = 115. This indicates that the reading on a 120-volt meter has to be multiplied by 115 to obtain the value of the primary voltage.
This sketch shows 1000 A in the line. Assume this to be the maximum value of the CT. Then 1000/5 = 200, or K = 200 for the
Instrument Transformers 309
current transformer. This indicates that the ammeter reading must be multiplied by 200 to get the line current.
The scales on the meter may be marked to take the K into account in the direct reading on the scale, or the reading may have to be multiplied by K.
Figure 29-2 shows the arrangement of a current transformer. This is the doughnut type, which is used extensively at service entrances for measuring high current capacities.
In Chapter 14 a picture of an Amprobe clamp-on ammeter was shown (Figure 14-9). The difference between the CT in Figure 29-2 and the clamp-on ammeter is that the laminated iron core, C, is hinged so that it may be opened for clamping around a conductor to avoid having to open up a circuit to use the ammeter.
In Figure 29-2, C is a laminated iron core and B is the line conductor or bus and also the primary winding of the CT. S is the secondary winding, made up of a number of turns of small-size
Figure 29-2 Theoretical arrangement of a current transformer.
CTs are usually rated with the voltage and the maximum amperes anticipated on the conductors with which they are to be used. There is also a rating, such as 100 to 5, 200 to 5, 1000 to 5, etc. This means they are rated 100 amperes in the line conductor to 5 amperes on the meter, etc.