What are the consequences of an open circuit in the current transformer

When CT (abbreviation) is working normally, the secondary load is a relay current coil and other small impedance, which basically runs in a short-circuit state. As a result, the magnetic flux generated by the primary and secondary currents demagnetizes each other, resulting in a low magnetic flux density (below) in the core and a low secondary voltage. 2. When the secondary winding is open and the primary current remains unchanged, in the case of the secondary current is 0, its demagnetizing flux is gone.

At this time, the primary current is all changed to excitation current, which saturates the core (abruptly), and its magnetic flux density is as high as above. 3. After the second situation occurs, let's briefly talk about the consequences: A secondary generates thousands of volts of voltage, which may break down the secondary insulation, which is dangerous to personnel and equipment.

b. The loss of the iron core will increase if it is suddenly saturated, and the iron core will heat up, which is easy to damage the insulation. c. Make the measurement inaccurate, because the change in magnetic flux is too high, which will produce remanence in the core, and the CT ratio difference and angular difference will increase.

The primary current of the current transformer has nothing to do with the current of the secondary load. When the transformer works normally, because the impedance is very small and close to the short-circuit state, most of the magnetization force generated by the primary current is compensated by the secondary current, the total magnetic flux density is not large, and the secondary coil potential is not large. When the current transformer is open, the impedance increases infinitely (q2=), the secondary current is equal to zero, the magnetizing force is equal to zero, and the total magnetizing force is equal to the magnetizing force of the original winding ( It produces a high electric potential in the secondary coil, and its peak value can reach several thousand volts, threatening personal safety, or causing secondary insulation damage to the instrument, protection device and transformer.

On the other hand, the magnetizing force of the original winding increases the magnetic flux density of the iron core excessively, which may cause the core to overheat strongly and be damaged.

It is clearly stipulated in the electrical safety work regulations that the secondary line side of the current transformer must be grounded. The purpose is to prevent the high voltage of the primary side insulation from entering the secondary side, causing harm to people and the secondary side instrument. Therefore, the secondary line side of the current transformer must be grounded.

When the secondary open circuit fault of the current transformer occurs, the correct and safe way to deal with it is to cut off the power on the high-voltage side and deal with it (restore the open circuit point to the normal circuit state), but this will cause an unplanned power outage;

The unconventional way of dealing with it is: because there is a discharge phenomenon at the open circuit point when the current transformer is open, it is easier to find. You can use a wire (the wire diameter should meet the requirements of the rated secondary current of the current transformer, the rated secondary current of the general current transformer is 1 A or 5 A, and the use of 2 square wires is completely OK), first connect one end of the wire with the grounding end of the open circuit coil, and connect the other end of the wire to the farther fixed connection point of the open circuit point with an insulating tool (the insulation of the general screwdriver handle can) to achieve the purpose of shorting the open circuit point, then you can easily deal with the open circuit point and avoid the equipment power failure. This method is not recommended from a safety point of view, but in order to avoid accidents, it can be completed by experienced secondary system staff.

The current transformer will break down the equipment twice and make the equipment have high voltage, endangering personal safety.

When the transformer is open, the transformer becomes a no-load state, and the magnetic flux is much higher than the rated time, in addition to generating a large amount of iron loss to damage the transformer, it also induces a dangerous high voltage in the secondary winding, endangering personal safety.

The secondary winding of the live current transformer is strictly forbidden to operate in open circuit.

Simply put, this is because there are very few turns at a time. The number of turns of the second is a lot relative to one time, when the secondary winding is open, it will produce a high overvoltage, which poses a threat to people and equipment, so the current transformer is strictly forbidden to open the circuit, which is strictly stipulated in Article 221 of the "Electrical Safety Work Regulations".

But now someone has invented it"Current transformer open circuit protector"。The protector is mainly composed of varistors connected to both ends of the secondary winding, when the secondary winding of the current transformer is short-circuited or connected with a load, because the voltage at both ends of the secondary winding is very low, the varistor presents a very high resistance value, and no current flows through the protector, which does not affect the normal operation of the transformer. When the secondary winding is open and the overvoltage is generated, the varistor is in a low resistance state, which is equivalent to a short circuit of the secondary winding, which inhibits the generation of overvoltage and achieves the purpose of protecting equipment and personal safety.

Current transformers.

The hazards of secondary open circuits are:

The high voltage of thousands of volts generated for the second time may break down the insulation of the current transformer, make the entire power distribution equipment shell live, and may also make the maintenance personnel electrocuted, which is life-threatening.

Secondly: The sudden saturation of the core will increase the core loss of the transformer, and the core will heat up.

Damage to the transformer.

Also: the transformer is saturated with the core, the measurement is inaccurate, and the CT ratio difference and angle difference are increased.

An open circuit is never allowed on the secondary side of the current transformer (CT).

Because once the circuit is opened, the primary side current i1 becomes magnetizing current, which causes a sudden increase in m and e2, resulting in oversaturation and magnetization of the core, serious heating and even burning the coil; At the same time, the error increases after the magnetic circuit is oversaturated and magnetized.

The magnitude of the secondary current of CT is determined by the primary current, and the magnetic potential generated by the secondary current is the magnetic potential of the primary current. If it is suddenly opened, the excitation electromotive force suddenly changes from a very small value to a very large value, and the magnetic flux in the iron core presents a seriously saturated flat-top wave, so the secondary side winding will induce a very high spire wave when the magnetic pass through zero, and its value can reach thousands or even tens of thousands of volts, endangering the safety of the staff and the insulation performance of the instrument.

1. The secondary side current produces an unbalanced effect, resulting in a significant increase in the magnetic flux of the iron core. The induced electromotive force is proportional to the magnetic flux, resulting in a significant increase in the voltage on the secondary side, which is prone to the risk of electric shock, and even affects the insulation part of the circuit and components on the secondary side. It's broken.

At this time, the iron core will have obvious symptoms, such as discharge sound, electric sparks, noise, etc.

2. After the magnetic flux of the core increases significantly, the temperature of the core will increase significantly, causing the voltage transformer to burn out. If this happens, we will find that the transformer is smoking and has a burning smell.

3. Due to the great increase in magnetic flux, the iron core becomes saturated, so there will be a large amount of remanence, resulting in a significant decrease in the accuracy of the transformer.

4. The current on the secondary side is zero, and the power meter and ammeter are displayed as zero. The aluminum plate of the energy meter does not rotate, does not make noise, and the current relay cannot operate normally, so the protection of the primary circuit cannot be realized. and surveillance.

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When the current transformer is running, the secondary side is not allowed to open the circuit. Here's why:

1. The current transformer generates a magnetic flux 1 in the iron core with the measured current magnetic potential I1N1;

The current magnetic potential of the secondary measuring instrument i2N2 generates a magnetic flux in the iron core 2;Current transformer core combined magnetic flux: =

1+φ2；Because 1 and 2 are in opposite directions, equal in magnitude, cancel each other out, so =

0；2. If the circuit is opened twice, i2=

0, then: 1, the current transformer core magnetic flux is very strong, saturated, the core is hot, burns out the insulation, and produces leakage;

3. If the circuit is opened twice, i2=

0, then: 1, in the secondary coil N2 of the current transformer produces a high induced electric potential E, forming a high voltage at both ends of the secondary coil of the current transformer, endangering the life safety of the operator;

4. One end of the secondary coil of the current transformer is grounded, which is a protective measure to prevent the danger of high voltage;

Therefore, the fuse is not allowed to be connected to the secondary side circuit of the current transformer, and it is not allowed to remove the ammeter, relay and other equipment without bypassing during operation.

In particular, CT with a high number of turns induces a high voltage when the second open circuit is opened. For example, the theoretical open-circuit voltage peak of the 4000 1 TPY core may reach tens of thousands of volts. It may cause secondary breakdown of the connected secondary equipment or the CT itself.

1. Generate a very high voltage, which causes harm to the safety of equipment and operating personnel;

2. The loss of the iron core increases, the heat is serious, and there is the possibility of burning out the insulation;

3. Remanence magnetism is generated in the iron core, which leads to the increase of the error of the transformer and affects the accuracy of measurement;

4. Due to the open circuit of the secondary circuit, the ammeter indication will be abnormal, and the current monitoring function will be lost, and the relay protection device will not work normally, resulting in the failure of the protection will lose the monitoring of the abnormal operation of the main circuit, and if it is not dealt with in time, it may cause serious consequences.

Because during normal operation, the secondary coil of the voltage transformer is equivalent to an open circuit, and the impedance zl is very large, if the secondary circuit is short-circuited, the impedance zl is rapidly reduced to almost zero, and the secondary circuit will produce a large short-circuit current, which will damage the secondary equipment and even endanger personal safety. The voltage transformer can be equipped with a fuse 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 current transformer is in normal operation, the impedance zl is very small, which is equivalent to the operation of the secondary coil in the short-circuit state. The magnetic flux potential generated by the secondary current plays a demagnetizing role in the magnetic potential generated by the primary current, the excitation current is very small, the total magnetic flux in the iron core is very small, and the induced electromotive force of the secondary winding does not exceed tens of volts.

If the secondary side is open, the secondary current is equal to zero, and the demagnetization effect disappears, but the 1 of the primary coil remains unchanged, and its primary current completely becomes the excitation current, causing the magnetic flux in the iron core to increase sharply, and the iron core is in a highly saturated state.

In addition, the number of turns of the secondary winding is very large, and a very high voltage (even up to thousands of volts) will be generated at both ends of the secondary winding, which may not only damage the insulation of the secondary winding, but also seriously endanger personal safety. Therefore, an open circuit on the secondary side of the current transformer is absolutely not allowed.

Because the secondary side i of the same current transformer is fixed, when the secondary side is open by u i, the voltage u on the secondary side will also be infinite, so when the current transformer is open, it is easy to produce high-voltage electric shock accidents.

The current transformer has all the current flow of the line, the number of turns of the secondary side winding is relatively large, and it is connected in series in the measuring instrument and the protection circuit, when the current transformer is working, its secondary side loop is always closed, so the impedance of the series coil of the measuring instrument and the protection loop is very small, and the working state of the current transformer is close to a short circuit.

The current transformer converts the large current on the primary side into a small current on the secondary side for measurement, and the secondary side cannot be opened.

The scope of use of current transformer is very large, and the help to users is great, but it is necessary to pay attention to its use to avoid unnecessary trouble. So why can't the current transformer open the circuit, and what to pay attention to when using the current transformer, do you understand? Now let's take a look.

1. Why can't the current transformer be opened?

As long as there is an open circuit in the current transformer, it will cause the resistance of the secondary coil to be greatly reduced, and the short-circuit current generated will become very large, resulting in the heating of the secondary coil, which will eventually lead to its burning. Therefore, it is necessary to connect a fuse in the current transformer to prevent an open circuit.

2. What should be paid attention to when using current transformers?

1. There are many models of current sensing base spikes, and special attention should be paid to the power of the connected equipment, if the deviation is large, unnecessary trouble may occur. When the current sensor is in operation, it is necessary to make the actual current reach between 30% and 120% of the rated rate, so as not to increase its error.

2. When the current transformer is connected to the circuit, it is necessary to pay attention to whether its polarity is correct. In the current transformer, the two ends of the primary winding will be marked L1 and L2, while the two ends of the secondary winding will be marked K1 and K2, and the current will be from L1 to L2 in the primary winding, and from K1 to K2 in the secondary winding.

3. In the current transformer, one end of the secondary winding must be connected to the ground, so as to prevent the equipment from being damaged when its insulation is broken down, and even cause personal physical and mental health. In addition, when the secondary side is opened, high voltage is released, which causes a dangerous retarder and prevents the circuit from being opened.

As for why the current transformer can't be opened, and what to pay attention to when using the current transformer, I'll introduce it here, do you understand it? The use of current transformers should be special, otherwise there will be trouble, which will lead to trouble.

Summary. Hello dear, this is not possible. Because when the current passes through the primary side of the current transformer, a higher voltage will be induced on the secondary side, and the voltage acts on the load and the internal resistance of the transformer respectively when there is a load on the secondary side (that is, when the circuit is closed); If the secondary circuit is open, a higher no-load voltage will be induced on the secondary side, which may break down the transformer insulation.

Therefore, the two splice terminals of the secondary circuit of the current transformer are provided with short circuit plates, which can be short-circuited here when the secondary load of the transformer needs to be overhauled.

Hello dear, this is not possible. Because when the current passes through the primary side of the current transformer, a higher voltage will be induced on the secondary side, and the voltage acts on the load and the internal resistance of the transformer respectively when there is a load on the secondary side (that is, when the circuit is closed); If the secondary circuit is open, a higher no-load voltage will be induced on the secondary side, which may break down the transformer insulation. Therefore, the two splice terminals of the secondary circuit of the current transformer are provided with short circuit plates, which can be short-circuited here when the secondary load of the transformer needs to be overhauled.

After removing the meter, the tablet is not pressed tightly, resulting in a virtual connection and open circuit. Later, I found that the problem was dealt with. Can this transformer still install an electric meter?

Dear, you can be safe.

This is a multi-winding transformer. There is a current display in the background of the computer, and it is normal. I'm afraid that the meter won't go out of the way.

Dear, no, this can be installed next.

Will this winding still work?

Dear, you can use it.

It's been a while. How can you be sure that it can still be used.

It's been a while, haven't you?

The road was open for a while. 3 hours. Later, I didn't know that a current transformer and a secondary voltage protector were broken.

Relay protection. and computer backend. All have current display normal.

Dear, then you can install it first, and then find a way to solve it, this kind of thing has to be experimented, and there is no way to judge 100%.