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A potential transformer is a device used to transform voltage. The purpose to transform the voltage is mainly for measuring instruments, as the power supply of relays, measure the voltage, power, and energy of the line, or protecting the valuable equipment, motors, and transformers in the line when the line fails.
A potential transformer is a device used to transform voltage. The purpose to transform the voltage is mainly for measuring instruments, as the power supply of relays, measure the voltage, power, and energy of the line, or protecting the valuable equipment, motors, and transformers in the line when the line fails. Therefore, the capacity of the potential transformer is very small, generally, only a few volts, dozens of VA, and the maximum does not exceed one thousand VA. This article mainly introduces its basic structure, working principle, main types, wiring methods, precautions, abnormalities and handling, and ferromagnetic resonance.
The potential transformer model is composed of letters and symbols, and each part has different meanings.
The first letter: J一potential transformer;
The second letter: D一single phase; S一three phase
The third letter: J一oil immersion; Z一pouring;
The fourth letter: number一voltage level (KV).
For example, JDJ-10 means a single-phase oil-immersed potential transformer with a rated voltage of 10KV.
Main Parameters:
● Rated primary voltage: the primary voltage value used as a benchmark for transformer performance.
● Rated secondary voltage: the secondary voltage value used as a benchmark for transformer performance.
● Rated transformation ratio: the ratio of rated primary voltage to rated secondary voltage.
● Accuracy level: the level determined by the transformer system. Its error should be within the specified limit under specified use conditions.
● Rated load: the load value on which the accuracy level of the transformer can be determined.
Figure 1. Potential Transformer Symbol
The basic structure of a potential transformer is very similar to that of a transformer. It also has two windings, one is called the primary winding and the other is called the secondary winding. Both windings are installed or wound on the core. There is insulation between the two windings and between the winding and the iron core so that there is electrical isolation between the two windings and between the winding and the iron core.
When the potential transformer is running, the primary winding is connected in parallel to the line, and the secondary winding is connected in parallel to the instrument or relay. Therefore, when we measure the voltage on the high-voltage line, the primary voltage is very high and the secondary voltage is low, which can ensure the safety of operators and instruments.
Figure 2. Potential Transformer Constructure
The working principle of the potential transformer is the same as that of a transformer, and the basic structure also consists of an iron core and the primary and secondary windings. The capacity is small and relatively constant, and it is close to the no-load state during normal operation.
The impedance of the potential transformer itself is very small. Once the secondary side is short-circuited, the current will increase sharply and burn the coil. And when the primary and secondary sides are damaged, the secondary side will have a high potential to the ground and cause personal and equipment accidents. For this reason, the primary side of the potential transformer is connected with a fuse, and the secondary side is reliably grounded.
The potential transformer for measurement is generally made of a single-phase double-coil structure, and their primary voltage is the voltage to be measured (such as the line voltage of the power system). It can be used in a single-phase, or two units can be connected into a VV shape for three-phase use.
The potential transformer used in the laboratory is often multi-tap on the primary side to meet the needs of measuring different voltages.
The potential transformer for protective grounding has a third coil, which is also called a three-coil potential transformer. The third coil of the three-phase is connected into an open triangle, and the two leading ends of the open triangle are connected with the voltage coil of the ground protection relay.
During normal operation, the three-phase voltage of the power system is symmetrical, and the sum of the three-phase induced electromotive force on the third coil is zero. Once a single phase is grounded, the neutral point will be displaced, and a zero-sequence voltage will appear between the terminals of the open triangle to make the relay operate, thereby protecting the power system.
When a zero-sequence voltage appears in the coil, zero-sequence magnetic flux will appear in the corresponding core. Therefore, this three-phase potential transformer adopts a side yoke core (at 10KV and below) or three single-phase potential transformers. For this kind of transformer, the accuracy of the third coil is not high, but a certain overexcitation characteristic is required. In other words, when the primary voltage increases, the magnetic flux density in the core also increases by a corresponding multiple without damage.
The potential transformer is an indispensable electrical appliance in power transmissions and power supply systems such as power plants and substations. The precision potential transformer is used in the electrical test laboratory to expand the measurement limit and measure voltage, power, and electrical energy.
Why do we need to change the voltage on the line? This is because according to the different conditions of power generation, transmission, and use of electricity, the voltage on the line is different, and the difference is huge. Some are the low voltage of 220V and 380V, and some are the high voltage of tens of thousands of volts or even hundreds of thousands of volts.
To directly measure these low and high voltages, it is necessary to make corresponding low and high voltage voltmeters and other instruments and relays according to the size of the line voltage. This will not only bring great difficulties to instrument production but more importantly, make it impossible and not allowed to make high-voltage instruments directly and measure the voltage directly on high-voltage lines.
1. According to the installation location, it can be divided into indoor and outdoor types.
Voltage transformers of 35kV and below are mostly made into indoor types, and 35kV and above are made into outdoor types.
2. According to the number of phases, it can be divided into single-phase and three-phase types. Voltage transformers of 35kV and above can not be made into three-phase types.
3. According to the number of windings, it can be divided into the dual-winding and three-winding potential transformer. In addition to the primary side and the basic secondary side, the three-winding potential transformer also has a set of auxiliary secondary sides for grounding protection.
4. According to the insulation method, it can be divided into dry type, pouring type, oil-immersed type, and inflatable type.
The dry-type potential transformer has a simple structure, no fire and explosion hazard, but it has low insulation strength, only suitable for indoor devices below 6kV.
The pouring type potential transformer has a compact structure and is easy for maintenance, suitable for a 3kV-35kV indoor type power distribution device.
The oil-immersed potential transformer has good insulation performance and can be used in outdoor power distribution devices above 10kV.
The gas-filled potential transformer is used in SF6 fully enclosed electrical appliances.
5. According to the working principle, it can be divided into the following 3 types.
(1) Electromagnetic Potential Transformer
It is a device that uses the principle of electromagnetic induction to transform voltage or current proportionally.
Figure 3. Electromagnetic Potential Transformer
(2) Capacitive Potential Transformer(CVT)
The voltage is divided by the series capacitors, and then stepped down and isolated by electromagnetic transformers. It is usually used for relay protection. Capacitive potential transformers can also couple the carrier frequency to the transmission line for long-distance communication, remote measurement, selective line high-frequency protection, remote control, teletype typing, etc.
Figure 4. Capacitive Potential Transformer(CVT)
(3) Electronic Potential Transformer
A device composed of one or more voltage sensors or current sensors connected to the transmission system and the secondary converter used to transmit a quantity proportional to the measured quantity to supply measuring instruments and meters and relay protection or control device.
There are several common wiring methods for potential transformers:
1. Single-phase wiring: it can be used to measure the line voltage of the system with a neutral point of 35kV and below that is not directly grounded or the phase-to-ground voltage of a system with a neutral point above 110kV that is directly grounded.
2. V/V connection: it is to connect the high and low voltage windings of two fully insulated single-phase potential transformers between two phases to form an incomplete triangle. This method is often used in high-voltage three-phase systems of 35kV and below where the neutral point is not grounded or grounded through the arc suppression coil, especially in the three-phase system of 10kV.
3. Three single-phase three-winding potential transformers are used to form YN, yn, d0 or YN, y, d0 wiring form, which is widely used in 3-220KV systems. Its secondary winding is used to measure phase-to-phase voltage and phase-to-ground voltage, and the auxiliary secondary winding to be connected into an open triangle for AC grid insulation monitoring instruments and relays.
Figure 5. Single-phase Potential Transformer
A 5 pole 3 phase potential transformer can replace the wiring composed of the above three single-phase three-winding potential transformers. Except for the iron core, the form is basically the same as that shown in Figure 3. It's generally only used for 3-15KV systems.
4. 3 phase 3 winding 5 pole potential transformer. The primary winding and the main secondary winding are connected in a star shape, the neutral point is grounded, and the auxiliary secondary winding is connected in an open triangle.
This kind of potential transformer can measure line voltage and phase-to-ground voltage, and the auxiliary secondary winding can be inserted into the relay and signal indicator for insulation monitoring of the AC grid.
1. The potential transformer shall be tested and inspected in accordance with the items specified in the regulations before being put into operation, like measuring polarities, connecting groups, shaking insulations, checking phase sequence, etc...
2. The wiring of the potential transformer should be correct. The primary winding should be connected in parallel with the circuit under test, and the secondary winding should be connected in parallel with the voltage coil of the connected measuring instrument, relay protection device, or automatic device. At the same time, pay attention to the correctness of the polarity.
3. The capacity of the load connected to the secondary side of the potential transformer should be appropriate, and the load connected to the secondary side of the potential transformer should not exceed its rated capacity. Otherwise, the error of the transformer will increase and it is difficult to achieve the accuracy of the measurement.
4. Short circuit is not allowed on the secondary side of the potential transformer. Since the internal impedance of the potential transformer is very small, if the secondary circuit is short-circuited, a large current will appear, which will damage the secondary equipment and even endanger personal safety.
The potential transformer can be equipped with a fuse on the secondary side to protect itself from being damaged by a short circuit on the secondary side. A fuse should also be installed on the primary side to protect the high-voltage power grid from endangering the safety of the primary system due to high-voltage winding or lead failure.
5. In order to ensure the safety of people when touching measuring instruments and relays, the secondary winding of the potential transformer must be grounded. Because after grounding, when the insulation between the primary and secondary windings is damaged, it can prevent the instrument and relay from appearing high voltage and endangering personal safety.
6. No short circuit is allowed on the secondary side of the potential transformer.
(1) Three-phase voltage indication is unbalanced: one phase voltage is reduced (can be reduced to zero), the other two-phase voltages are normal. The line voltage is abnormal or accompanied by sound or light signals. This may because of the fusing of the high voltage or low voltage fuse of the transformer.
Figure 6. 3 Phase Potential Transformer
(2) Neutral point is not effectively grounded, and three-phase voltage indication is unbalanced: one phase voltage decreases (maybe zero), the other two-phase voltages increase (up to line voltage), or the pointer swings. It may be due to a single-phase ground fault or basic frequency resonance. If the three-phase voltages rise at the same time and exceed the line voltage, it may because of the frequency division or high-frequency resonance.
(3) If the high-voltage fuse is blown repeatedly, the internal insulation may be seriously damaged, such as a short circuit between winding layers or between turns.
(4) If the neutral point is effectively grounded, when the bus is switched on, the phase voltage rises and swings at low frequency, which is generally caused by a series resonance. If there is no operation and the phase voltage suddenly rises or drops abnormally, the internal insulation of the transformer may be damaged, such as the short circuit fault of the insulation brackets, and between the winding layers or the turns;
(5) In the system where the neutral point is effectively grounded, when the potential transformer is put into operation, the voltmeter indication is unstable, which may be caused by poor grounding of the N (X) terminal of the high-voltage winding.
(6) Disconnect the potential transformer circuit.
1. According to the relevant regulations of relay protection and automatic device, exit the relevant protection to prevent malfunction.
2. Check whether the high and low voltage fuses and automatic air switches are normal. If the fuse is blown, you should find out the cause and replace it immediately. When it blows again, you should handle it carefully.
3. Check whether all connectors of the voltage circuit are loose or disconnected and whether the switching circuit has poor contact.
The primary winding of the current transformer is wound with thick wire, usually only has one or a few turns, and is connected in series with the load of the measured current.
Figure 7. Current Transformer Structure
The potential transformer is a step-down transformer, which has a large number of primary winding turns, connected in parallel with the high-voltage power grid. The secondary winding has few turns and is connected to the voltage coil of the voltmeter or power meter.
The working conditions of the two devices are very different during normal operation, which is shown as:
(1) The secondary side of the current transformer can be short-circuited but not open. The secondary side of the potential transformer can be open but not short-circuited.
(2) Compared with the load on the secondary side, the primary internal impedance of the potential transformer is so small that it can be ignored. The potential transformer can be regarded as a voltage source. While the primary internal resistance of the current transformer is large, and it’s often regarded as a current source with infinite internal resistance.
(3) The magnetic flux density of the potential transformer during normal operation is close to the saturation value, and it decreases when the fault occurs. During the normal operation, the magnetic flux density of the current transformer is very low. And since the short-circuit current on the primary side becomes very large during the short circuit, the magnetic flux density increases greatly, sometimes far exceeding the saturation value.
(1) The Function of Current Transformers
In order to ensure the safe and economical operation of the power system, the operation of power equipment must be monitored and measured. However, general measurement and protection devices cannot be directly connected to primary high-voltage equipment. Instead, the large current of the primary system needs to be converted into a small current proportionally for measuring instruments and protection devices.
(2) The Function of Potential Transformers
It's used to transform the high voltage into a standard secondary voltage of 100V or lower proportionally for protection, metering, and instrumentation.
The difference between the two is that one is measuring current and the other is measuring voltage. The current transformer is connected in series in the circuit, the primary winding has fewer turns than the secondary winding, and the secondary cannot be opened.
The potential transformer is connected in parallel in the circuit, the primary winding has more turns than the secondary winding, and the secondary cannot be short-circuited.
Catalogue
FPGA / CPLD
Memory
MOS 
MCU 
DSP
OCEP
Secondary 
Other 