With the rapid development of communication, data exchange and new energy industries, DC power distribution systems with voltages higher than 60 V have also developed rapidly. The safety design of these DC power distribution systems and the selection of related products have increasingly attracted the attention of engineering designers. China's standardization organizations and international IEC organizations have also continuously introduced new relevant standards to provide a standard basis for the safe use of DC power distribution systems.
Overcurrent fault protection for DC power distribution systems
The overcurrent fault protection of the DC power distribution system is mainly based on the protection of power distribution equipment such as power distribution lines, battery packs, and rectifier equipment. Commonly used overcurrent protection components are fuses and DC breakers that provide overload and short-circuit fault protection for DC distribution systems.
1 Protect the DC power distribution system with a low voltage fuse
Low-voltage fuses are the most traditional and reliable over-current fault protection components. The fuses have the following advantages for DC power distribution system protection: 1 Fault breaking speed is fast, no arc injection. 2 Small size, no directionality. 3 The fault point is obvious and easy to overhaul. 4 The selectivity between the systems is good.
Since the low-voltage fuse achieves the purpose of fuse protection through the heat capacity flowing through, and the breaking and arc-extinguishing are performed inside the fuse casing, the action curve of the fuse does not change. According to the GB/T 13539 low-voltage fuse standard, In addition to the dedicated fuses protected by photovoltaic panels, fuses for the protection of low-voltage AC distribution systems can be used for DC system protection without any change in protection characteristics.
Figure 3 illustrates the overload and short-circuit protection of the low-voltage fuses in the AC power distribution system and the DC power distribution system. The consistency of the protection characteristics of the low-voltage fuses can be seen. For fuses used directly for photovoltaic panel protection, photovoltaic fuses are required.
Table 3 shows the agreed current range of g-type low-voltage fuses in accordance with GB / T 13539. Since the short-circuit current of the DC power distribution system is usually no more than 25 kA, as long as the general requirements for overload protection according to IEC 60364: IB ≤ IN ≤ IZ (IB is the line operating current, IN is the fuse rated current, IZ is the conductor allowed Current carrying capacity) Select a fuse to achieve a satisfactory protection.
When the fuse is used in a DC power distribution system, it is usually used in combination with a fuse disconnector or an isolating switch fuse set that satisfies the DC closed breaking function.
2 Protect the DC power distribution system with a low voltage circuit breaker
Because the low-voltage circuit breaker has the dual tasks of breaking and protection in the power distribution system, the protection characteristics of the low-voltage circuit breaker and the ability to break the DC current are particularly important in the DC power distribution system.
2. 1 Protection characteristics of low-voltage circuit breakers in DC distribution systems
Due to the difference between the DC current and the AC current, the DC current detection and protection action characteristics are also different from the AC current. Therefore, the circuit breaker used in the DC power distribution system should be a dedicated DC circuit breaker that is calibrated according to the DC protection characteristic curve or an AC circuit breaker that can provide a verified DC correction factor. AC circuit breakers that do not provide a DC factor cannot be used directly for DC circuit protection.
2. 2 Breaking of low-voltage circuit breakers in DC distribution systems
According to the previous analysis, the circuit breaker installed in the DC power distribution system reliably disconnects the short-circuit current while reliably breaking the small current including the critical current. This requires that the DC circuit breaker be provided with a dedicated magnetic system for arc extinguishing, a molded case circuit breaker, etc. can be used as a non-polar circuit breaker that is independent of the component breaking arc and current flow, and for small space circuit breakers with small space, more installations are attached. The magnet has a polarity circuit breaker.
2. 3 DC circuit breaker related standards
Tables 4 and 5 show the current IEC standards for low-voltage DC circuit breakers and the revision of China's standards.
3 Protective coordination of overcurrent protection components of DC power distribution system
As with the protective fit of the overcurrent components of the AC power distribution system, the coordination of the protection components in the DC power distribution system is also of interest to designers and users. Figure 4 shows a schematic diagram of the overcurrent protection coordination commonly used in DC power distribution systems.
3. 1 Selective fit between fuses
The compatibility between the fuses has a good selectivity. As with the selection of the AC power distribution system, the rated current difference between the upper and lower circuit breakers is greater than 1.6 to achieve full selectivity.
3. 2 Selective coordination between circuit breakers
Since the current detection link of the DC circuit breaker is better than the AC circuit breaker by the electronic trip unit, the DC circuit breaker used in the DC power distribution system is mainly a thermal magnetic type B circuit breaker, and the circuit breaker is mainly used. It is difficult to achieve full selectivity with the cooperation of the upper and lower levels.
For the cooperation between the circuit breakers used in the photovoltaic system, since the short-circuit current of the photovoltaic power source is substantially equal to the maximum rated current in the system, the DC circuit breaker for the photovoltaic system can be achieved as long as the difference between the upper and lower stages is twice. Selective.
3. 3 Fuse between the fuse and the circuit breaker
The fuse is used as the upper protection of the circuit breaker, which can achieve good selective protection, as long as the difference between the upper and lower stages is between 2 times, good selectivity can be achieved.
Ground fault and arc fault protection of DC power distribution system
In the case of overcurrent fault protection, the DC power distribution system should also protect against ground faults and arc faults. As the supply voltage of the DC power distribution system continues to increase, the probability of personal electric shock accidents and fire accidents caused by ground faults and arc faults also increases.
1 Ground fault protection of DC system with power supply grounded
For DC power distribution systems with TT and TN grounding with power supply grounding, the use of residual current protection should be the most effective solution. The IEC also noted such a requirement. In November 2015, IEC / TC 23E completed the 23E / 925 / NP "General Rules for Residual Current Protection Devices for DC Systems" project proposal to guide the development of standards for DC residual current protection products. The basic technical requirements are: rated voltage < 400 V; rated current < 125 A; grounding pole in the system; Id n: 85 mA (IEC 60364 - 4 - 41) for personal shock protection; Id n: 300 mA (IEC 60364 - 4 - 42); pole number: 2P, 3P; operating time: general type (momentary), time delay type.
2 Ground fault protection of DC system with unpowered power supply
For DC power distribution systems with unpowered IT grounding at the power supply end, the internationally common solution is to use an insulation monitor (IMD). Currently IEC 61557 - 8:2014 "AC 1 000 V and DC 1 500 V low voltage power distribution. Test, measurement or monitoring equipment for system electrical safety protection test Part 8: Insulation monitoring device for IT systems has the following requirements: only requires alarm, does not emphasize breaking; maximum measuring current ≤ 10 mA (Rj = 0 Ω); maximum measurement Voltage ≤ 120 V and 110% of supply voltage Un; minimum insulation resistance to ground ≥ 18 kΩ (30 Ω / V).
For the insulation alarm resistor, according to different regulations of different usage fields, when there is no special requirement: Rj ≥ 10%, the leakage current at rated voltage is 1 mA.
3 DC system arc fault protection
Arc fault protection is a kind of safety protection concept that has developed rapidly in the past 20 years. It can protect the DC power distribution system by protecting the series fault arc, parallel fault arc and grounding arc in the system.
Figure 5 illustrates the concept of arc protection for DC photovoltaic systems, providing protection against series arcs, parallel arcs, and fault arcs between photovoltaic panels in the system.
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