Instrument transformers are primarily used to provide isolation between the main primary circuit and the secondary control and measuring devices. Instrument transformers are divided into two categories: voltage transformers (VT) and current transformers (CT). The voltage transformers and current transformers continuously measure the voltage and current of an electrical system and are responsible to give feedback signals to the relays to enable them to detect abnormal conditions. This transformers are used for measurement, protection and control purposes.
The main tasks of instrument transformers are:
- To transform currents or voltages from usually a high value to a value easy to handle for relays and instruments.
- To insulate the relays, metering and instruments from the primary high-voltage system.
- To provide possibilities of standardizing the relays and instruments, etc. to a few rated currents and voltages.
Basic Theory of Operation
A transformer comprises of two windings viz., primary and secondary coupled through a common magnetic core. When the primary winding is connected to a source and the secondary circuit is left open, the transformer acts as an inductor with minimum current being drawn from the source. At the same time, a voltage will be produced in the secondary open-circuit winding due to the magnetic coupling. When a load is connected across the secondary terminals, the current will start flowing in the secondary, which will be decided by the load impedance and the open-circuit secondary voltage. A proportionate current is drawn in the primary winding depending upon the turn’s ratio between primary and secondary. This principle of transformer operation is used in transfer of voltage and current in a circuit to the required values for the purpose of standardization.
A voltage transformer is an open-circuited transformer whose primary winding is connected across the main electrical system voltage being monitored. A convenient proportionate voltage is generated in the secondary for monitoring. However, the current transformer is having its primary winding directly connected in series with the main circuit carrying the full operating current of the system. An equivalent current is produced in its secondary, which is made to flow through the relay coil to get the equivalent measure of the main system current. The standard currents are invariably 1 A and 5 A universally.
The burden of the instrument transformer is considered to be everything connected externally to its terminals, such as monitoring devices, relays, and pilot wiring.
Instrument transformers are rated by performance in conjunction with a secondary burden. As the burden increases, the accuracy class may, in fact, decrease.
Current Transformers (CT)
In current transformers the primary usually consists of one or two turns whilst the secondary can have several hundred turns. A current transformer is used to transform a primary current quantity in terms of its magnitude and phase to a secondary value such that in normal conditions the secondary value is substantially proportional to the primary value. The primary winding of a current transformer is connected in series with the power circuit.
Current Transformers Construction
A current transformers consists essentially of an iron core with two windings. One winding is connected in the circuit whose current is to be measured and is called the primary and the other winding is connected to burden, and called the secondary. Two of the most basic construction of current transformers are the bar type and wound type:
Bar Type Current Transformers
Bar type current transformers normally have a single concentrically placed primary conductor, sometimes permanently built into the CT and provided with the necessary primary insulation, but very often the bushing of a circuit breaker or power transformer.
At low primary current ratings it may be difficult to obtain sufficient output at the desired accuracy because a large core section is needed to provide enough flux to induce the secondary emf in the small number of turns.
Wound Type Current Transformers
With this device it is possible to change the number of primary turns, thus increasing the CT output voltage with altering the turns ratio.
Voltage Transformers (VT) or Potential Transformers (PT)
The voltage transformer (VT) is connected in parallel with the circuit to be monitored. It operates under the same principles as power transformers, the significant differences being power capability, size, operating flux levels, and compensation. There are basically, three types of voltage transformers used for protection equipment.
Electromagnetic Voltage Transformers (VT), Capacitive Voltage Transformers (CVT) and Optical Voltage Transducer.
Electromagnetic Voltage Transformers (VT)
The electromagnetic type is a step down transformer whose primary (HV) and secondary (LV) windings are connected as below (see Figure 4).
The number of turns in a winding is directly proportional to the open-circuit voltage being measured or produced across it. The above diagram is a single-phase VT. In the three-phase system it is necessary to use three VTs at one per phase and they being connected in star or delta depending on the method of connection of the main power source being monitored. This type of electromagnetic transformers are used in voltage circuits upto 110/132 kV.
For still higher voltages, it is common to adopt the second type namely the capacitor voltage transformer (CVT). Figure 5 below gives the basic connection adopted in this type. Here the primary portion consists of capacitors connected in series to split the primary voltage to convenient values.
Capacitive Voltage Transformers (CVT)
The capacitor VT is more commonly used on extra high-voltage (EHV) networks. The capacitors also allow the injection of a high-frequency signals onto the power line conductors to provide end-to-end communications between substations for distance relays, telemetry/supervisory and voice communications. Hence, in EHV national grid networks of utilities, the CVTs are commonly used for both protection and communication purposes.
The capacitive coupled voltage transformer (CCVT) is primarily a capacitance voltage divider and electromagnetic VT combined. Developed in the early 1920s, it was used to couple telephone carrier current with the high-voltage transmission lines. The next decade brought a capacitive tap on many high-voltage bushings, extending its use for indication and relaying. To provide sufficient energy, the divider output had to be relatively high, typically 11 kV. This necessitated the need for an electromagnetic VT to step the voltage down to 120 V. A tuning reactor was used to increase energy transfer (see Figure 6).
As transmission voltage levels increased, so did the use of CCVTs. It’s traditional low cost versus the conventional VT, and the fact that it was nearly impervious to Ferro resonance due to its low flux density, made it an ideal choice. It proved to be quite stable for protective purposes, but it was not adequate for revenue metering. In fact, the accuracy has been known to drift over time and temperature ranges. This would often warrant the need for routine maintenance. CCVTs are commonly used in 345- to 500-kV systems. Improvements have been made to better stabilize the output, but their popularity has declined.
Another element is the Ferro resonance- suppression circuit, usually on the secondary side of the VT. There are two types, active and passive. Active circuits, which also contain energy-storing components, add to the transient. Passive circuits have little effect on transients. The concern of the transient response is with distance relaying and high-speed line protection. This transient may cause out-of-zone tripping, which is not tolerable.
Optical Voltage Transducer
A new technology, optical voltage transducers, is being used in high-voltage applications. It works on the principle known as the Kerr effect, by which polarized light passes through the electric field produced by the line voltage. This polarized light, measured optically, is converted to an analog electrical signal proportional to the voltage in the primary conductor. This device provides complete isolation, since there is no electrical connection to the primary conductor.
- Protection Application Handbook by ABB
- Electric Power Transformer Engineering by James H. Harlow