The current transformer is an indispensable component in the photovoltaic system. It plays an important role in the incoming cabinets, outlet cabinets and measuring cabinets. Today we have come to understand how to prevent current transformer faults and choose according to the system characteristics. Current transformer configuration.

I. Precautions for burnout of current transformers

Preventing and reducing the re-burning of the user's current transformers is necessary both for the user and the power supply company.

1, the current transformer burning causes

(1) The current transformer is opened twice and generates a high voltage, causing the current transformer to burn out.

(2) Insulation ageing of current transformers is too long. When overvoltage is generated, breakdown or discharge may occur locally, causing the current transformer to heat and burn out.

(3) The current transformer is connected to the aluminum row contact surface and oxidized too much. The contact resistance is too large, and the heating causes the current transformer to burn out.

(4) The user is overloaded for a long time, causing the current transformer to burn out.

2. There is a problem with the circuit breaker and wiring of the dedicated transformer user

(1) Generally, 10kV vacuum circuit breakers and SF6 circuit breakers are equipped with overcurrent and overload protection. The circuit breaker can trip correctly when there is a phase short circuit and overload, and the overvoltage is removed by the installed zinc oxide arrester.

(2) When single-phase grounding occurs on the user side, because the circuit breaker does not trip, it is necessary to pull the off-line branch of the contract on a common line one by one in order to find the grounding point, and the public line will be deactivated to increase the power outage range.

(3) The user-side lightning arrester insulation aging can not effectively prevent over-voltage.

3, take measures

(1) Install a watchdog circuit breaker to avoid branch failures affecting the entire feeder power cut, especially to ensure that the branch circuit breaker can be reliably tripped when the power-using side is single-phase grounded.

(2) Strengthen the user's high-voltage metering current transformer and arrester high-voltage insulation test (regulations no more than 4 years), early detection of metering current transformer and arrester insulation aging degree, timely replacement, to avoid the metering current transformer burned out causing power outage.

(3) Regularly clean the user's equipment once to reduce the pollution flash and avoid reducing the insulation.

Second, current transformer selection parameter configuration requirements

1. Selection of a parameter selection principle of current transformer

The rated primary current of the current transformer is determined by the actual load of the circuit under test. The rated primary current of the current transformer should not be less than the primary rated current of the circuit. Under normal circumstances, the coefficient of the load current is multiplied by a factor of 1.2 to 1.25 to determine the value of the transformer. Rated current.

In addition, the rated continuous thermal current, rated short-term thermal current and rated dynamic current of the current transformer should meet the requirements of the maximum load current and short-circuit current of the primary circuit, and the development of the system should be considered.

2. The principle of selection of quadratic parameters of current transformers

The rated secondary load of the transformer is a key parameter that determines the exact level, size, and cost of the transformer. It should be based on the actual situation of the project.

Current transformer rated secondary current generally use 1A, such as conducive to transformer manufacturing or expansion projects, and in some cases to reduce the current transformer secondary open circuit voltage, rated secondary current can also be used 5A.

The rated output value selection should meet the following principles:

(1) The rated output values ​​of the measuring grade, P grade, and PR grade are expressed in volt-amperes.

When the rated secondary current is 1A, the rated output standard value should be 0.5V·A, 1V·A, 1.5V·A, 2.5V·A, 5V·A, 7.5V·A, 10V·A, 15V·A.

When the rated secondary current is 5A, the rated output standard value should be 2.5V·A, 5V·A, 10V·A, 15V·A, 20V·A, 25V·A, 30V·A, 40V·A, 50V·A;

(2) The rated resistive load value of TPX, TPY, and TPZ current transformers is expressed in Ω.

Rated resistance load standard value should use 0.5Ω, 1Ω6, 2Ω, 5Ω, 7.5Ω, 10Ω;

The rated output value of the current transformer should be selected according to the rated current of the transformer and the actual load.

To meet the requirements of transient characteristics, larger rated output values ​​can also be used.

3, choose other current transformer configuration requirements

Current transformer type, number of secondary windings and accuracy level should meet the requirements of relay protection, automatic devices and measuring instruments. Protection current transformer configuration should avoid the main protection of the dead zone.

Transformer secondary winding distribution should be avoided when a set of protection is deactivated, and there is a protection action dead zone in the protected area.

From actual use, one parameter selection and two-parameter selection are the most considered items. Of course, other factors must also be selected according to the actual situation.

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