Why is the secondary short circuit not allowed in PT operation?

In the event of a short circuit during PT operation, it is due to the voltage transformer.

The internal impedance is very small, if the secondary circuit.

When there is a short circuit, there will be a large current, which will damage the secondary equipment and even endanger personal safety. Voltage transformers can be equipped with fuses on the secondary side.

to protect itself from damage caused by a short circuit on the secondary side.

Where possible, a fuse should also be installed on the primary side to protect the high-voltage grid from endangering the safety of the primary system due to the faulty high-voltage windings or leads of the transformer.

When the voltage transformer is running, the primary winding N1 is connected in parallel on the ** road, and the secondary winding N2 is connected to the instrument or relay in parallel. Therefore, when measuring the voltage on the high-voltage line, although the primary voltage is high, the secondary voltage is low, which can ensure the safety of the operator and the instrument.

When PT is in normal operation, because the secondary load is the voltage coil impedance of some instruments and relays, which is basically equivalent to the no-load state of the transformer, the current passed by the transformer itself is very small, and its size is determined by the size of the secondary load impedance, because the impedance of PT itself is small, and the capacity is not large, when the transformer is short-circuited twice, the secondary current is very large, and the secondary insurance fuse affects the normal operation of the correct indication and protection of the instrument, when the insurance capacity is not selected properly, and the secondary short circuit insurance can not be fused, then pt It is very easy to burn out.

The reasons are as follows: 1. Most of the magnetic flux generated by the current on the primary side during the normal operation of the current transformer is offset by the reverse magnetic flux generated by the secondary current, that is, the so-called demagnetization, so that the magnetic flux in the core is kept within the normal range of the design.

2. Due to the magnetic saturation of the iron core, the magnetic flux no longer changes with the primary side excitation current according to the sinusoidal law, but a flat-top wave, close to the square wave, that is, the magnetic flux change rate at the edge of the waveform increases, and the induced electromotive force in the secondary winding will increase according to the principle of electromagnetic induction, resulting in high voltage, and the value is different according to the electromagnetic characteristics of the transformer itself, but it is no longer a sine wave. This high voltage can endanger the safety of people and equipment.

The boost due to the turns ratio is secondary because the voltage of the primary winding is extremely low.

Of course, what is said above is that under the premise that there is a normal load current on the primary side, if the current on the primary side is zero or very small, the above phenomenon will not be produced.

The voltage transformer (hereinafter referred to as PT) is quite a no-load transformer in normal operation, this is because the secondary load of PT is mainly the voltage coil of the measuring instrument and relay, and its impedance is generally very large, so that the current passed by the PT secondary is very smallBecause the capacity of PT is usually very small, the wire of the coil is very thin, and the leakage reactance is also very small, once there is a short circuit for the second time, the large short-circuit current is very easy to burn the PT, so in order to ensure the safe movement of PT, short circuit is not allowed. In order to protect it, it is usually necessary to install a fuse.

The impedance of the voltage transformer itself is very small, such as the secondary short circuit, the secondary current increases, so that the secondary fuse is broken, affecting the meter indication and causing the possibility of protection malfunction, so when the secondary circuit of the voltage transformer is working, special attention should be paid to prevent short circuit. The secondary grounding of the voltage transformer belongs to the protective grounding, which is mainly to prevent.

In addition, due to the low insulation level of the secondary circuit, if there is no grounding point, it will also break through, making the insulation damage more serious, so the secondary must be grounded.

Answer: The voltage transformer is in normal operation, the secondary load impedance is very large, the voltage transformer is a constant voltage source, the internal impedance is very small, the capacity is very small, the primary winding wire is very thin, when the transformer is short-circuited twice, the primary current is very large, if the secondary fuse is not selected properly, the fuse can not be blown out, the voltage transformer is very easy to be burned out.

The voltage transformer can be regarded as a transformer, and a short circuit on the secondary side is considered to be a short power supply.

When PT is in normal operation, because the secondary load is the voltage coil impedance of some instruments and relays, which is basically equivalent to the no-load state of the transformer, the current passed by the transformer itself is very small, and its size is determined by the size of the secondary load impedance, because the impedance of PT itself is small, and the capacity is not large, when the transformer is short-circuited twice, the secondary current is very large, and the secondary insurance fuse affects the normal operation of the correct indication and protection of the instrument, when the insurance capacity is not selected properly, and the secondary short circuit insurance can not be fused, then pt It is very easy to burn out.

Generally speaking: the design of the voltage transformer is the output of small current and low voltage on the secondary circuit, in other words, the output of small power, the load that can be driven is very small, as long as the equipment and instruments that can drive the voltage test can be, but the primary circuit is connected to the high-power power supply end, and it can withstand a small power that is slightly larger than the secondary circuit, so once the short-circuit current of the secondary circuit is too large, the coil will heat up violently, when the power consumption (temperature) exceeds the rated power consumption (temperature) of the voltage transformer, The primary or secondary circuit coil of the voltage transformer will be burned.

1. Short circuit is absolutely not allowed on the secondary side of the voltage transformer.

This is because, because the internal impedance of the voltage transformer is very small, if the secondary circuit is short-circuited, a large current will occurIt will damage the secondary equipment and even endanger personal safety.

2. The secondary side of the current transformer is absolutely not allowed to open the circuit.

Once the secondary side of the current transformer is opened, the current on the primary side will become magnetizing current, which will cause the core to be oversaturated and magnetized, and the heating will be serious and even the coil will be burned. Moreover, when the current transformer is working normally, the secondary side is approximately short circuit.

If it is suddenly opened, the excitation electromotive force will suddenly change from a very small value to a large value, and the magnetic flux in the core will show a severely saturated flat-top wave, so the secondary side winding will induce a very high voltage, the value of which can reach thousands of volts, which will endanger the safety of the staff and the insulation performance of the instrument.

Cause of short circuit. 1. The component is damaged.

Short circuits are often caused by damage to insulation or careless wiring. For example, equipment defects caused by aging equipment insulation materials and poor design, manufacturing, installation, and maintenance develop into short circuits [2].

2. Influence of meteorological conditions.

For example, scintillation discharge caused by lightning strikes, overhead line disconnection due to wind damage, and power pole collapse caused by wire icing.

3. Man-made destruction.

For example, the staff pulled the switch with load, did not rule out the grounding wire closing and power supply when overhauling the line or equipment, the misoperation of the operation personnel, and the stealing of the wire and the carbon fiber bullet used in the Kosovo war and the Iraq war in the United States.

4. Other reasons.

Trenching damages cables, birds and beasts kite jumps on current-carrying bare conductors, etc.

To borrow a sentence from Zhao Benshan, "short circuits are not allowed" in the case of mistakes.

In the operation of the current transformer, a short circuit is allowed on the secondary side, but an open circuit is not allowed, otherwise high voltage will be generated, and the iron core will also enter magnetic saturation.

The current transformer is not allowed to open the circuit twice.

Voltage transformers are not allowed to short circuit.

In the normal operation of the voltage transformer, the secondary load impedance is very large, the voltage transformer is a constant voltage source, the internal impedance is very small, the capacity is very small, the primary winding wire is very thin, when the transformer is short-circuited twice, the primary current is very large, if the secondary fuse is not selected properly, the fuse can not be blown out, the voltage transformer is very easy to be burned out. Therefore, the secondary side of the voltage transformer is not allowed to be short-circuited.

After the secondary side of the current transformer is opened, the current on the primary side remains unchanged, and the current on the secondary side is equal to zero, so the demagnetizing flux generated by the secondary current also disappears. At this time, all the primary currents become excitation currents, which saturates the transformer core and the magnetic flux is also very high, which will have the following consequences:

1) Due to the saturation of magnetic flux, thousands of volts of high voltage will be generated on the secondary side, and the waveform will change, causing harm to people and equipment.

2) Due to the saturation of the magnetic flux of the core, the loss of the core increases, resulting in high heat, which will damage the insulation.

3) Remanence will be generated in the core, so that the ratio difference and angle difference of the transformer will increase, and the accuracy will be lost.

Therefore, the secondary side of the current transformer is not allowed to open the circuit.

Related to the transformer ratio, both voltage transformer and current transformer are equivalent to a transformer.

In short:

Voltage transformer: the number of turns of the primary side winding is more than that of the secondary side, if the secondary side is short-circuited, a larger magnetic flux will be induced, and the primary side is also similar to the short-circuit state, which is the same as the short circuit of the ordinary transformer, which will cause excessive current;

Current transformer: the number of turns on the secondary side is much greater than that of the primary side, if the secondary side is open and the secondary side voltage is much greater than the primary side, the dielectric strength of the cable on the secondary side is not able to withstand such a high voltage, so the secondary side can not be opened.

In detail:

The secondary side of the current transformer is not allowed to operate in an open circuit

The impedance of the instrument coil connected to the sub-coil of the current transformer is very small, which is equivalent to operating in the short-circuit state of the sub-coil, and the voltage on the terminal of the sub-coil of the transformer is only a few volts, so the magnetic flux in the core is very small. Although the magnetokinetic potential of the primary coil can reach hundreds of amperes or thousands of ampere turns or more, most of it is offset by the demagnetized magnetic kinetic potential established by the short-circuited secondary coil, and only a small part is left as the excitation magnetokinetic potential of the core to establish the magnetic flux in the core. If the secondary coil is disconnected during operation, and the secondary side current is equal to zero, then the magnetokinetic potential that plays the role of demagnetization disappears, and the magnetokinetic potential of the primary side remains unchanged, and the measured current on the primary side all becomes the excitation current, which will make the magnetic flux in the iron core sharp, and the iron core will be seriously heated to burn out the coil insulation, or make the high-voltage side short circuit to the ground.

In addition, the open circuit of the auxiliary coil will induce a high voltage, which is dangerous for the instrument and the operator, so the secondary side of the current transformer is not allowed to be disconnected.

Short circuits are not allowed in the secondary side of the voltage transformer

If the secondary coil of the voltage transformer is short-circuited, the impedance of the secondary coil will be greatly reduced, and a large short-circuit current will occur, so that the secondary coil will be burned out due to serious heating. Generally, a fuse should be used on the secondary side of the voltage transformer to avoid the fault spread caused by the abnormal transformer.

Because the current transformer is equivalent to a step-up transformer, if the secondary side is open, you want to go down even if it is a voltage of 5V, after 100-500 times of amplification, people encounter...

This is the same as the principle that the current source is not allowed to open the circuit and the voltage source is not allowed to be short-circuited.

The secondary open circuit of the current transformer will induce high voltage, causing the secondary instrument to burn out. The voltage transformer is a voltage source, and a short circuit will burn the secondary coil. The second test is multi-coil

The secondary side of the current transformer must be grounded, and the secondary circuit must not be opened! It turned out to be this reason.

A short circuit on the secondary side of the voltage transformer will produce a large current and burn the voltage transformer. The current transformer will not generate voltage when there is voltage and no current on the primary side, and the secondary side will not produce voltage when there is a certain amount of stable current on the primary side, and the opening of the secondary side will produce a certain value of voltage, but it will not be very high. However, the primary main circuit is generally connected with the switch, when the switch turns on or off the main circuit current, the main circuit current will have a sudden change process, according to V=L*Di DT (L is the inductance of the transformer), at this time, if the secondary side is open, it will instantly excite a very high voltage, which is prone to danger.

Therefore, the current transformer is not allowed to open the circuit.

Although the voltage on the secondary side of the voltage transformer has dropped to a lower voltage, it is still a dangerous voltage. And the rated capacity of the voltage transformer is very small, and the wire cross-section of the coil is very small, so it does not have overload capacity. Once a short circuit occurs on the secondary side, a large short-circuit fault current will be generated, and the voltage transformer can be burned out in a very short time.

Therefore, the voltage transformer cannot be short-circuited during operation. The current transformer in operation, the load connected to the secondary toilet is the instrument or relay current coil, etc., the impedance is very small, and the basic operation is in the short-circuit state. When the secondary coil is opened, the current on the primary side remains constant, and the demagnetized flux generated by the current on the secondary side disappears.

Due to the severe saturation of the iron core, the following consequences will occur: 1) Due to the saturation of magnetic flux, the secondary side of the current transformer will generate high voltage of several thousand volts, which poses a threat to the secondary insulation and is dangerous to equipment and operating personnel. 2) Due to the sudden saturation of the core, the loss of the core increases, and the insulation may burn out due to serious heating.

The secondary side of the voltage transformer has a voltage of about 100V, which should be connected to the circuit that can withstand the voltage of 100V, and the current through it is determined by the impedance of the secondary circuit. The impedance of the voltage transformer itself is very small, if the secondary short circuit occurs, the secondary current increases, causing the secondary fuse to blow, affecting the meter indication and causing protection malfunction. If the fuse capacity is not selected properly, it is very easy to damage the voltage transformer.

Because, when the secondary short circuit, the current of the primary and secondary circuits will be very large, so that the transformer will burn out.

Because the number of turns of the coil on the secondary side of the voltage transformer is less than that of the coil on the primary side, but the wire diameter is larger, according to the principle of the transformer, once the secondary side is short-circuited, it is bound to cause a large short-circuit current on the secondary side, which will cause the transformer to burn out. Therefore, a fuse must be installed on the secondary side of the voltage transformer to prevent it from short circuiting. On the contrary, the current transformer is strictly forbidden to open the circuit on the secondary side, because once the open circuit will induce high voltage on the secondary side, resulting in unsafety.