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The role of high-voltage current limiting fuses


Source:Yueqing Xujing Fuse Factory

High-voltage fuses are simple protective appliances, which are used to protect electrical equipment from overload and short-circuit currents; According to the installation conditions and uses, choose different types of high-voltage fuses, such as outdoor drop type, indoor type, for some special equipment high-voltage fuses should be selected special series; What we often call fuses is a fuse. It is mainly used for overload and short-circuit protection of electrical equipment such as high-voltage transmission lines, voltage transformers, voltage transformers, etc.

Parameters and functions of high-voltage current limiting fuses

The function of high-voltage fuses is to protect circuits and equipment from short circuits and vehicles. It uses the melting of wires (sheets) with low melting points (also known as fuses) to cut off the circuit. Its structure is simple, low cost, and easy to maintain.

High-voltage fuses can be divided into indoor and outdoor types according to the installation location, and can be divided into current limiting and non-current limiting types according to whether the current is limited. The so-called high-voltage current limiting fuse is the porcelain tube of the fuse is filled with quartz sand, when the short circuit current makes the fuse fuse fuse and produce arc, due to the limitation of the narrow groove between the quartz sand particles, the diameter of the arc column becomes smaller, coupled with the cooling effect of quartz sand, the arc is extinguished before the current reaches the stable value, playing the role of current limiting.

Drop fuse is a non-limiting fuse, there is a gas-producing material in the inner layer of the fuse tube, when the fault current flows through the fuse, the fuse blows to produce an arc, under the action of high temperature of the arc, the high-pressure gas is generated through the nozzle to eject outward, the arc is elongated and cooled, and it is extinguished when the current crosses zero. After the fuse is blown off, the movable joint is released, and the fuse tube falls and forms a clear fracture.

High-voltage current limiting fuses are generally used indoors, but should not be used in the power grid lower than the rated voltage of the fuse, so as to avoid the overvoltage generated by the interception of the fuse fuse exceeding the working phase voltage allowed by the power grid by 2.5 times.

Drop fuses are generally used outdoors because when they extinguish arcs, they eject a large amount of gas and make a loud noise.

What are the main parameters and characteristics of high-voltage current limiting fuses? The main parameters and characteristics of the high-voltage current limiting fuse:

(1) Rated voltage: refers to the voltage that the high-voltage current limiting fuse bears for a long time after it is broken, which is generally equal to or greater than the rated voltage of the equipment or line.

(2) Rated current: The rated current of the fuse and melt refers to the current that passes through for a long time. Generally, it is about 1.5~3 times the rated current of the equipment.

(3) Rated breaking capacity: refers to the ability to reliably break overload or short-circuit current under fault conditions. It is generally expressed in terms of breaking current or breaking capacity.

(4) Insulation level: generally expressed by power frequency withstand voltage and lightning impact withstand voltage.

(5) Ampere-second characteristics: refers to the relationship between the action time of the high-voltage current limiting fuse and the current through the fuse, which is provided by the manufacturer.

This is necessary to ensure the selectivity of the fuse, otherwise the bad consequences of misoperation such as skipping and tripping will occur.

How is the medium voltage fuse selected?

(1) Environmental conditions selected by the high-voltage current limiting fuse.

Environmental conditions refer to ambient temperature, wind speed (there are requirements for fuses used outdoors), pollution conditions (requirements for fuses used outdoors), altitude, seismic intensity, etc., the above requirements can be determined by referring to the requirements of outdoor switchgear.

(2) Technical conditions for the selection of high-voltage current limiting fuses.

The technical conditions include normal working conditions (voltage, current) and protection characteristics (breaking current, small breaking current, fusing characteristics, etc.).

1) Select the rated voltage and current according to the normal working conditions.

Un >= Ue In >= Inr >= IE: Un - manufacturer's guaranteed operating voltage, kV; Ue - the operating voltage of the circuit, kV; In - rated current of the fuse,

a) Inr - rated current of the fused (body), A; Ie - continuous operating current of the grid, A. The selection of Inr includes the following two ways: a) In order to prevent the melt from malfunctioning when passing the transformer excitation inrush current and the short-circuit current outside the protection range and the inrush current of the motor self-starting, the medium voltage fuse protecting the power transformer of 35kV and below, the rated current of the melt can be selected as follows.

Inr = kI1max in k - reliability coefficient, excluding the motor self-starting, take 1.1~1.3, consider the motor self-starting, take 1.5~2.0; I1max - the working current of the power transformer circuit.

b) Used to protect the melt of the power capacitor, when the system voltage rises or waveform distortion causes the loop current to increase or the inrush current is generated during operation, it should not be fused by mistake, and the rated current of the melt can be selected as follows.

Inr = kInc K - reliability coefficient (for current-limiting medium voltage fuses, when a power capacitor, k=1.5~2.0, when a group of power capacitors, k=1.3~1.8; Inc - Rated current of a power capacitor circuit.

2) Select parameters by protection characteristics.

a) The rated breaking current of the high-voltage fuse.

Idn >= I" for current-limiting fuses Idn - rated cut-off flow of the fuse, kA; I" - Sub-transient current of the short-circuit current. Idn > of fuses acting on infinite flow = Idn in Ish formula - rated breaking flow of the fuse, kA; Ish — the rms value of the inrush current of the short-circuit current.

b) The fuse to protect the voltage transformer only needs to be selected according to the rated voltage and rated breaking current, and the rated current does not need to be selected.

c) When the breaking capacity of the medium-voltage fuse cannot meet the requirements of the protected circuit, a current-limiting resistor can be installed in the fuse circuit to limit the short-circuit current, except for capacitors used to protect lightning protection.

d) The rated current of the melt should be selected according to the protective fusing characteristics of the medium voltage fuse, and meet the requirements of reliability, selectivity and sensitivity of protection.

When selecting, the selectivity of action between the front and rear fuses, the high-voltage current limiting fuse and the power supply side and the load side relay protection should be guaranteed. In the event of a short-circuit fault in this section, the fault should be cut off within a short time. When the grid is connected to other grounding protection, the sum of the grounding current and the load current in the loop should not exceed the small fusing current. In addition, the selection of the fuse current when selecting the fuse should meet the following conditions:

1) The fuse fuse current protecting the medium voltage line should be greater than the calculated current of the line (generally 1.1~1.3 times).

2) The fuse fuse current protecting the power transformer is generally 1.5~2.0 times the rated current of the primary side (considering overload, no-load closing current, etc.).

3) The fuse fuse current of the protection voltage transformer is generally 0.5A.