Low-voltage reactive power compensation circuit protection fuse selection

The function of the fuse of the compensation circuit in the low-voltage reactive power compensation cabinet is to ensure the safe and reliable operation of the entire circuit to achieve the purpose of reactive power compensation. Then the capacitor (and series reactor) is the core component of the compensation circuit, and the fuse is for it. It is very necessary to provide reliable protection performance.

Since there are no specific detailed requirements for the selection of the compensation circuit protection fuse in the current relevant standards, the choices in practical applications are not all consistent, and sometimes the differences are even quite wide.

As the load types in the low-voltage power distribution system become more and more complex, the selection of the fuse of the compensation circuit cannot be generalized. It should be scientifically analyzed and selected depending on the specific type of low-voltage reactive power compensation.

Below we analyze how to choose the fuse of the compensation circuit reasonably and correctly according to the relevant national standards and the types of low-voltage reactive power compensation.

1. Relevant national standards

In the low-voltage shunt capacitor standard GB/T12747.1-2004, the relevant requirements and instructions for the maximum current and protection of the capacitor are as follows:

The capacitor unit shall be suitable for continuous operation under the current generated by the unit under the rated sinusoidal voltage and rated frequency when the root mean square value of the line current is 1.3 times, except for the transition process. Considering the capacitance deviation, the maximum capacitance can reach 1.10CN, so the maximum current can reach 1.43IN. These overcurrent factors are the result of taking into account the combined effects of harmonics, overcurrent and voltage deviation.

2. Standard analysis

Through the research and analysis of the above standards, the following conclusions can be drawn:

1. In the above standards for low-voltage capacitors or capacitance compensation devices, the selection requirements for protective fuses are based on the application of pure capacitor compensation, which is what we often call pure capacitance compensation. For the application of non-tuned compensation (capacitor series reactor), which is more and more widely used at present, it is not explained how to choose a protective fuse.

2. Even for pure capacitance compensation applications, the requirements for the selection of protective fuses are different in each standard. But judging from the technical basis and detailed level required by the standard, GB/T12747.1-2004 requires more reasonable selection of protective fuse (1.101.3=1.43IN), but it is not perfect because of the overcurrent of some capacitor products The capacity has reached 1.5 times, instead of 1.3 times required by the standard.

3. There is no standard for the selection of fuse for non-tuned compensation (capacitor series reactor) applications.

Three, how to correctly choose the protection fuse of low voltage reactive power compensation

According to the analysis of relevant national standards and the two main low-voltage reactive power compensation methods mentioned above, the correct selection of low-voltage reactive power compensation protection fuses should be considered and selected from the following two aspects:

1. Pure capacitance compensation application, the compensation component is only low voltage capacitor:

Due to the rationality of the national standard GB/T12747.1-2004, the maximum allowable current requirement in the standard, 1.10 rated current of the capacitor with overcurrent capability, can be used for reference to select the protection fuse.
Take a 25KVAR/400V capacitor compensation circuit as an example:

1) The rated current of the capacitor of this circuit is 36A. If the capacitor only meets the 1.3 times overcurrent capacity in the standard, the maximum allowable current of this circuit is 1.101.336=51.5A. The standard current range of the fuse includes 50A and 63A, so the rated protection current of the correct fuse should be 50A.

2) If the capacitor is larger than the overcurrent capacity in the standard, suchas 1.5 times, the maximum allowable current of the loop is 1.101.536=59.4A. The standard current range of the fuse includes 50A and 63A, so the rated protection current of the correct fuse should be 63A. 

2. Non-tuned compensation applications, compensation components include low-voltage capacitors and series reactors:

This application is to connect a reactor with a certain reactance rate in series in the traditional capacitor compensation circuit. Its advantage is that it can prevent system harmonic amplification, avoid resonance, and can absorb part of the harmonics in the system, thereby providing reliable reactive power. Compensation has the purpose of improving power quality at the same time, and it has been increasingly used in industrial and building power distribution systems.

When considering fuse protection in this application, the capacitor and series reactor should be analyzed as a whole, because in addition to carrying the normal fundamental reactive current in the compensation circuit, it should also be allowed to carry a certain amount of harmonics. Wave current.

More importantly, because the rated voltage of the capacitor for this application is much greater than the voltage of the distribution system (for example, when the reactance ratio is 14%, the rated voltage of the capacitor is 525V, which is much greater than the 400V system voltage), the capacitor can actually carry in a 400V system The current (including fundamental wave and harmonic current) is greater than its rated current under rated voltage (such as 525V). Therefore, in this kind of reactive power compensation application, it is the parameter of the series reactor that determines the loop overload capacity, that is, the maximum overcurrent capacity of the reactor.

Therefore, the selection of the fuse should be based on the rated current of the loop and the maximum overcurrent capability of the reactor. Take the 25KVAR/400V compensation circuit with a reactance rate of 14% for example analysis:
The capacitor rated current of this circuit is 36A. If the maximum overcurrent capacity of the reactor is 1.8 times the rated current, the maximum allowable current of this circuit is 1.836=64.8A, so the rated protection current of the correct fuse should be 63A.