Analysis and treatment of heating of current transformer

 

Causes and hazards of overheating of current transformer

 

Human-caused failure.

One is that due to negligence at work or unclear understanding of the operating principle of the current transformer, the secondary circuit is disconnected or the secondary line of the current transformer is not installed, causing the secondary circuit to be in an open circuit state and energized.

 

The second reason is that the secondary terminal of the current transformer is not connected properly or is loose, causing the secondary circuit to be energized.

 

The third is that the secondary circuit of the current transformer is energized because the terminals of the electrical equipment connected to it are not properly connected or are in a loose state (such as electrical instruments, protective relays, electric energy meters, etc.).

 

Fourth, the secondary wire is disconnected, making the secondary of the current transformer charged. The reasons for disconnection are generally bitten by small animals, broken by aging, and pulled apart by mechanical force.

 

Fifth, the secondary miswiring of the current transformer will cause the secondary to be energized.

Sixth, informal wiring is very unsafe and should be avoided.

 

 

 

Causes of insulation damage.

The current transformer has been in use for too long, the insulation has aged, and local breakdown or discharge has occurred, generating overvoltage, causing the current transformer to heat up and burn out;

 

One is that the insulation is damaged due to overload heat, and the primary voltage flows into the secondary, making it energized.

 

Second, due to poor product quality, insulation aging and other reasons, the secondary conductor may also be energized.

 

Third, some current transformers are in humid places all year round, their insulation is damp all year round and degraded and damaged, and the primary voltage penetrates into the secondary, making it energized.

 

 

Causes of excessive oxidation.

The contact surface of the primary aluminum bar of the current transformer is too heavily oxidized, the contact resistance is too large, and the heat causes the current transformer to burn out;

 

 

Reasons for long operation time: The user runs overload for a long time, causing the current transformer to heat up and burn out.

 

 

The insulation of the lightning arrester on the user side is aging and cannot effectively prevent overvoltage.

 

 

Problems with the transformer itself

Overheating may be caused by loose core or parts, improper electric field shielding, secondary open circuit or potential suspension, end screen open circuit and insulation damage.

 

 

 

Quality issues

Voltage transformers generally overheat for the following reasons

 

Poor product quality: If the product’s insulation, core laminations and winding process are not up to standard, the voltage transformer may overheat and the insulation may be kept at high temperature for a long time, which may lead to accelerated aging of the insulation and form a vicious circle.

 

Overload, three-phase imbalance and resonance cause the heating of the internal winding of the PT to increase, especially when the voltage is higher than the rated voltage of the PT, the heating inside the PT is more serious.

 

 

 

Use current transformers correctly to prevent high temperature or burning:

 

The secondary circuit of the current transformer must not be open during operation. If the secondary side is open, the iron loss will be too large, the temperature will be too high and it will burn, or the voltage of the secondary winding will increase and the insulation will be broken, resulting in the risk of high-voltage electric shock. Therefore, during use and installation, it is necessary to pay attention to the tightness and firmness of the terminal head.

 

 

One end of the secondary side of the current transformer and the casing must be reliably grounded.

 

 

If the current transformer makes noise or generates heat during operation, stop operation immediately and check whether the internal core is loose (if so, tighten the core bolts).

 

 

The secondary line of the current transformer in operation must not be disconnected at will or work must not be done between the secondary lines, otherwise it will cause the danger of open circuit.

 

 

The insulation resistance of the secondary coil of the current transformer shall not be less than 10~20 megohms, otherwise it must be dried to restore the insulation. IV. Countermeasures for current transformer damage 1. Wiring principles of current transformer. The wiring of the current transformer should comply with the series principle, that is, the primary winding should be connected in series with the circuit to be measured, and the secondary winding should be connected in series with all instrument loads;

 

Install a “watchdog” circuit breaker. After the circuit breaker is installed, it can prevent the branch fault from affecting the power outage of the entire feeder, especially to ensure that the branch circuit breaker can trip reliably when the single-phase grounding on the power consumption side; connect the metering current transformer to the back of the circuit breaker to ensure that the circuit breaker and lightning arrester operate correctly to cut off the fault when the metering current transformer fails;

 

Check the degree of insulation aging of the current transformer and lightning arrester. Strengthen the high-voltage insulation test of users’ high-voltage metering current transformers and lightning arresters (regulations stipulate that it should not exceed 4 years), detect the degree of insulation aging of metering current transformers and lightning arresters as early as possible, and replace them in time to avoid power outages caused by burning of metering current transformers;

 

Install a current transformer secondary overvoltage protector. Install a current transformer secondary overvoltage protector according to the current being measured, otherwise the error will increase.

At the same time, one end of the secondary side must be grounded to prevent the high voltage on the primary side from entering the low voltage on the secondary side when the insulation is damaged, causing personal and equipment accidents;

 

The secondary side is absolutely not allowed to be open-circuited, because once the circuit is open, all the current on the primary side becomes magnetizing current, causing the core to be over-saturated and magnetized, causing severe heat and even burning the coil; at the same time, after the magnetic circuit is over-saturated and magnetized, the error increases.

 

Install a short-circuit switch. The secondary side of the current transformer is equipped with a short-circuit switch to prevent the primary side from opening.

 

Install current transformers with secondary windings. In order to meet the needs of measuring instruments, relay protection, circuit breaker failure judgment and fault recording devices, current transformers with 2 to 8 secondary windings are installed in the generator, transformer, outgoing line, busbar section circuit breaker, bus tie circuit breaker, bypass circuit breaker and other circuits.

 

Selection of the installation location of the protective current transformer. Generally, it should be installed in a location other than the main protection device. And the user’s primary equipment should be cleaned regularly to reduce flashover and avoid insulation degradation.

 

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