The function of the fuse in the compensation loop in the low-voltage reactive power compensation cabinet is to ensure the safe and reliable operation of the entire loop, so as to achieve the purpose of reactive power compensation. Then the capacitor (and series reactor) is used as the core component of the compensation loop, and the fuse is connected to it. It is necessary to provide reliable protection performance.
Because there is no specific requirement for the selection of the protection circuit protection fuses in the current related standards, the choices of everyone in practical applications are not consistent, and sometimes the differences are even quite different.
In the case where the type of load in the low-voltage distribution system becomes more and more complex, the selection of the compensation circuit fuse cannot be generalized, and the scientific analysis and selection must be made depending on the specific type of low-voltage reactive power compensation.
Below we analyze how to reasonably and correctly select the fuses of the compensation circuit according to the relevant national standards and the types of low-voltage reactive power compensation.
I. Relevant national standards 1. In the low-voltage shunt capacitor standard GB/T12747.1-2004, the relevant requirements and instructions for the maximum current and protection of the relevant capacitors are as follows:
The capacitor unit shall be suitable for continuous operation with a line current rms value of 1.3 times the unit's current at the rated sinusoidal voltage and rated frequency, except during the transition. Taking into account the capacitance deviation, the maximum capacitance up to 1.10CN, so the maximum current up to 1.43IN.
These overcurrent factors are the result of a combination of harmonics, overcurrents, and voltage deviations.
33 Overcurrent The capacitor must not operate at currents exceeding the maximum values ​​specified in Chapter 21.
34 Switches, Protection Devices and Connections Switches, protection devices and connections shall be designed to withstand continuously 1.3 times the current drawn at nominal frequency and sinusoidal voltage equal to the rated voltage. Because the capacitor's capacitance may be 1.10 times the rated value, this current is 1.31.10 times the rated current, which is 1.43IN.
2. In the standard GB7251 for low-voltage capacitors and their complete sets of equipment, the requirements for the selection of capacitor protection fuses are as follows:
5.3.5 b)
The rated operating current of the fuse (square root value) shall be selected from 2 to 3 times the rated current of the single capacitor.
3. In the design specification for parallel capacitor devices GB50227-2008, the requirements for capacitor protection fuses are as follows:
5.4 Fuse 5.4.2 The fuse rated current of the external fuse for single capacitor protection shall be selected according to the capacitor rated current of 1.37~1.50 times.
Second, the standard analysis Through the analysis of the above criteria, we can draw the following conclusions:
1. In the above standards concerning low-voltage capacitors or capacitance compensation devices, the selection requirements for protection fuses are based on the application of simple capacitor compensation, that is, we often say pure capacitance compensation. However, for the application of more and more widely used non-tuned compensation (capacitor series reactor), there is no explanation on how to select a protection fuse.
2. Even for pure capacitance compensation applications, the selection requirements for protection fuses vary from standard to standard. However, from the technical basis and the degree of detail of the standard requirements, GB/T12747.1-2004 has a more reasonable choice of protection fuses (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, not 1.3 times that required in the standard.
3. For non-tunable compensation (capacitor series reactor) application of the fuse selection, there is no standard for reference.
Third, how to correctly choose low-voltage reactive power compensation protection fuse According to the analysis of the relevant national standards, and the two main types of low-voltage reactive power compensation mentioned above, the correct choice of low-voltage reactive power compensation fuses should be from the following two Aspects are considered and selected separately:
1. Pure capacitance compensation application, only low voltage capacitors for compensation components:
Due to the rationality of the national standard GB/T12747.1-2004, the maximum allowable current requirement in the standard can be used for reference, and the rated current of the capacitor of the overcurrent capacity 1.10 can be used to select the protection fuse.
An example of a capacitance compensation loop with 25KVAR/400V:
1) The rated current of the capacitor in the circuit is 36A. If the capacitor only meets the 1.3 times overcurrent capability in the standard, the maximum allowable current of the circuit is 1.101.336=51.5A. The fuse's standard current range includes 50A, 63A, so the correct fuse rating should be 50A.
2) If the capacitor is larger than the overcurrent capacity in the standard, such as 1.5 times, the maximum allowable current of the circuit is 1.101.536=59.4A. The fuse's standard current range includes 50A, 63A, so the correct fuse rated current should be 63A
2. Non-tuning compensation applications. Compensation components include low-voltage capacitors and series reactors:
This kind of application is a reactor with a certain reactance rate connected in series in the traditional capacitor compensation loop. Its advantage is that it can prevent the harmonic amplification of the system and avoid resonance, and at the same time it can absorb some harmonics in the system, thus providing reliable reactive power. At the same time, the compensation has the purpose of improving the power quality, and it has been increasingly applied in industrial and building distribution systems.
When considering fuse protection in this application, the capacitor and the series reactor should be analyzed as a whole, because in addition to the normal fundamental reactive current, the compensation loop should allow it to carry a certain amount of harmonics. Wave current.
More importantly, since the capacitor rated voltage of this application is much larger than the voltage of the distribution system (eg, the rated voltage of the capacitor is 525V, much larger than the 400V system voltage when the 14% reactance rate is applied), the capacitor can actually carry the load in a 400V system. The current (including fundamental and harmonic currents) is greater than its rated current at rated voltage (eg, 525V). Therefore, in this type of reactive power compensation application, the parameters of the series reactor, that is, the maximum overcurrent capability of the reactor, determine the loop overload capability.
Therefore, the choice of fuse should be based on the rated current of the circuit and the maximum over-current capability of the reactor. Take a 25KVAR/400V compensation circuit with a reactance rate of 14% as an example:
The capacitor rated current of the circuit is 36A. If the maximum overcurrent capability of the reactor is 1.8 times the rated current, the maximum allowable current of the circuit is 1.836=64.8A, so the rated rated current of the correct fuse should be 63A.
Because there is no specific requirement for the selection of the protection circuit protection fuses in the current related standards, the choices of everyone in practical applications are not consistent, and sometimes the differences are even quite different.
In the case where the type of load in the low-voltage distribution system becomes more and more complex, the selection of the compensation circuit fuse cannot be generalized, and the scientific analysis and selection must be made depending on the specific type of low-voltage reactive power compensation.
Below we analyze how to reasonably and correctly select the fuses of the compensation circuit according to the relevant national standards and the types of low-voltage reactive power compensation.
I. Relevant national standards 1. In the low-voltage shunt capacitor standard GB/T12747.1-2004, the relevant requirements and instructions for the maximum current and protection of the relevant capacitors are as follows:
The capacitor unit shall be suitable for continuous operation with a line current rms value of 1.3 times the unit's current at the rated sinusoidal voltage and rated frequency, except during the transition. Taking into account the capacitance deviation, the maximum capacitance up to 1.10CN, so the maximum current up to 1.43IN.
These overcurrent factors are the result of a combination of harmonics, overcurrents, and voltage deviations.
33 Overcurrent The capacitor must not operate at currents exceeding the maximum values ​​specified in Chapter 21.
34 Switches, Protection Devices and Connections Switches, protection devices and connections shall be designed to withstand continuously 1.3 times the current drawn at nominal frequency and sinusoidal voltage equal to the rated voltage. Because the capacitor's capacitance may be 1.10 times the rated value, this current is 1.31.10 times the rated current, which is 1.43IN.
2. In the standard GB7251 for low-voltage capacitors and their complete sets of equipment, the requirements for the selection of capacitor protection fuses are as follows:
5.3.5 b)
The rated operating current of the fuse (square root value) shall be selected from 2 to 3 times the rated current of the single capacitor.
3. In the design specification for parallel capacitor devices GB50227-2008, the requirements for capacitor protection fuses are as follows:
5.4 Fuse 5.4.2 The fuse rated current of the external fuse for single capacitor protection shall be selected according to the capacitor rated current of 1.37~1.50 times.
Second, the standard analysis Through the analysis of the above criteria, we can draw the following conclusions:
1. In the above standards concerning low-voltage capacitors or capacitance compensation devices, the selection requirements for protection fuses are based on the application of simple capacitor compensation, that is, we often say pure capacitance compensation. However, for the application of more and more widely used non-tuned compensation (capacitor series reactor), there is no explanation on how to select a protection fuse.
2. Even for pure capacitance compensation applications, the selection requirements for protection fuses vary from standard to standard. However, from the technical basis and the degree of detail of the standard requirements, GB/T12747.1-2004 has a more reasonable choice of protection fuses (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, not 1.3 times that required in the standard.
3. For non-tunable compensation (capacitor series reactor) application of the fuse selection, there is no standard for reference.
Third, how to correctly choose low-voltage reactive power compensation protection fuse According to the analysis of the relevant national standards, and the two main types of low-voltage reactive power compensation mentioned above, the correct choice of low-voltage reactive power compensation fuses should be from the following two Aspects are considered and selected separately:
1. Pure capacitance compensation application, only low voltage capacitors for compensation components:
Due to the rationality of the national standard GB/T12747.1-2004, the maximum allowable current requirement in the standard can be used for reference, and the rated current of the capacitor of the overcurrent capacity 1.10 can be used to select the protection fuse.
An example of a capacitance compensation loop with 25KVAR/400V:
1) The rated current of the capacitor in the circuit is 36A. If the capacitor only meets the 1.3 times overcurrent capability in the standard, the maximum allowable current of the circuit is 1.101.336=51.5A. The fuse's standard current range includes 50A, 63A, so the correct fuse rating should be 50A.
2) If the capacitor is larger than the overcurrent capacity in the standard, such as 1.5 times, the maximum allowable current of the circuit is 1.101.536=59.4A. The fuse's standard current range includes 50A, 63A, so the correct fuse rated current should be 63A
2. Non-tuning compensation applications. Compensation components include low-voltage capacitors and series reactors:
This kind of application is a reactor with a certain reactance rate connected in series in the traditional capacitor compensation loop. Its advantage is that it can prevent the harmonic amplification of the system and avoid resonance, and at the same time it can absorb some harmonics in the system, thus providing reliable reactive power. At the same time, the compensation has the purpose of improving the power quality, and it has been increasingly applied in industrial and building distribution systems.
When considering fuse protection in this application, the capacitor and the series reactor should be analyzed as a whole, because in addition to the normal fundamental reactive current, the compensation loop should allow it to carry a certain amount of harmonics. Wave current.
More importantly, since the capacitor rated voltage of this application is much larger than the voltage of the distribution system (eg, the rated voltage of the capacitor is 525V, much larger than the 400V system voltage when the 14% reactance rate is applied), the capacitor can actually carry the load in a 400V system. The current (including fundamental and harmonic currents) is greater than its rated current at rated voltage (eg, 525V). Therefore, in this type of reactive power compensation application, the parameters of the series reactor, that is, the maximum overcurrent capability of the reactor, determine the loop overload capability.
Therefore, the choice of fuse should be based on the rated current of the circuit and the maximum over-current capability of the reactor. Take a 25KVAR/400V compensation circuit with a reactance rate of 14% as an example:
The capacitor rated current of the circuit is 36A. If the maximum overcurrent capability of the reactor is 1.8 times the rated current, the maximum allowable current of the circuit is 1.836=64.8A, so the rated rated current of the correct fuse should be 63A.
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