Backup protection of surge protectors in low voltage power distribution systems

Lightning disaster is one of the natural disasters that humans cannot avoid at present. Surge protector (SPD) is a device that suppresses the impact voltage generated by lightning, electrical system operation or static electricity. Surge protector is an indispensable device for protecting electronic information technology products. As various electronic information technology products are increasingly infiltrating into various fields of social and family life, the scope of use of surge protector (SPD) is expanding, and the market demand is growing. Surge protector is a device used to protect electrical and electronic equipment from damage by lightning electromagnetic pulses. It is regarded as the "guardian" of many high-tech products. Therefore, the safety and reliability of the product itself are particularly important.

2. Surge protector failure

The core component of the voltage-limiting surge protector (SPD) product is the zinc oxide varistor (MOV) component, and its failure manifestations are short circuit and open circuit. The main cause of the short circuit failure is the thermal breakdown caused by the surge current passing through the MOV or the discharge caused by the surge current on the surface of the MOV, which causes a metallic short circuit between the two poles. The power frequency short circuit current value varies from hundreds to thousands of amperes depending on the system capacity. Another manifestation of the short circuit failure of the SPD product is that because the failed MOV is not completely short-circuited and has a certain impedance, the fault current flowing through the MOV causes it to heat up and burn. During the combustion process, the fault current is between hundreds of milliamperes and several amperes before it is completely open. For low-voltage power distribution systems, the main manifestation of system failures is the overcurrent failure of the phase line to the PE line or N line through the SPD.

3. The role of backup protection components

After the disconnector fails, the surge protector (SPD) may have two types of fault conditions. One is thermal breakdown causing an indirect ground short circuit between the LN/PE line, and its current value can cause the backup overcurrent protection element to operate; the other is that due to the small ground fault current, the overcurrent protection element does not operate, and the element (MOV) catches fire due to heat. Therefore, a backup protection element must be installed at the front end outside the surge protector (SPD). Its function is that when the surge protector (SPD) cannot cut off the power frequency short-circuit current, the overcurrent protection ... For this type of fault, the standard requires that the casing material of the SPD product pass the corresponding abnormal heat resistance and heat resistance tests. If protection must be provided for this type of fault, its backup protection element should be an element with ground fault protection.

Correct and reasonable use of backup protection components is directly related to the safety and reliability of the system and surge protector (SPD).

Figure 1 Surge protection device (SPD) and fuse, circuit breaker protection coordination

4. Characteristics of backup protection components

1. Characteristics of the fuse

The protection characteristic of a fuse is also called the melting characteristic, which is the main characteristic of a fuse. The melting characteristic characterizes the relationship between the current passing through the fuse and the melting time of the fuse. Like the protection characteristic of a thermal relay , it is inverse time.

There is a dividing line between the melting current and the non-melting current in the protection characteristics of the fuse, and the current corresponding to this is the minimum melting current IR. It is a current value. When the current passing through the fuse is equal to it, the fuse should never melt under the rated current, so IR>Ie.

2. Characteristics of circuit breakers

The function of the circuit breaker is to cut off and connect the load circuit , as well as cut off the fault circuit , to prevent the accident from expanding and ensure safe operation.

The main contacts of the circuit breaker are closed manually or electrically. After the main contacts are closed, the free tripping mechanism locks the main contacts in the closed position. The coil of the overcurrent release and the thermal element of the thermal release are connected in series with the main circuit, and the coil of the undervoltage release is connected in parallel with the power supply. When the circuit is short-circuited or severely overloaded, the armature of the overcurrent release is attracted, causing the free tripping mechanism to operate and the main contacts to disconnect the main circuit. When the circuit is overloaded, the thermal element of the thermal release heats up and bends the bimetallic strip, pushing the free tripping mechanism to operate. When the circuit is undervoltage, the armature of the undervoltage release is released. It also activates the free tripping mechanism. The shunt release is used for remote control . In normal operation, its coil is de-energized. When distance control is required, press the start button to energize the coil, and the armature drives the free tripping mechanism to operate, disconnecting the main contacts.

When using a surge protector (SPD) backup protection component in a power system, the first thing to consider is the size of the surge current it carries, and the second is the size of the power supply short-circuit current. Compared with the short-circuit current, the surge current is inevitable and is a forced current.

5. Selection of backup protection components

After the backup protection element cooperates with the SPD, it should be able to protect the rated surge current from acting without the backup protection element, ensuring the normal discharge of the surge current, and at the same time, the residual pressure Ur acting on the branch line is lower than the protection level Up of the electrical equipment, so as to ensure the safety of the system and electrical equipment.

6. Advantages and disadvantages of using fuses and circuit breakers as backup protection

1. The main advantages and characteristics of fuses

① The use of fuses as backup protection components has low residual voltage and good selectivity. As long as the rated current of the fuses of the upper and lower fuses meets the requirement of the overcurrent selectivity ratio of 1.6:1 specified in the national standard and IEC standard, that is, the rated current
of the upper fuse is not less than 1.6 times of the value of the lower fuse, it is considered that the upper and lower fuses can selectively cut off the fault current;
② Good current limiting characteristics and high breaking capacity;
③ Relatively small size;
④ Cheaper price.

2. The main disadvantages and weaknesses of fuses

① The fuse must be replaced after the fault fuse;
② The protection function is single, with only one section of overcurrent inverse time characteristic, which is used for overload, short circuit and ground fault protection;
③ When one phase fuses, the three-phase motor will cause two-phase operation, which can be compensated by a fuse with an alarm signal. One phase fuse can disconnect the three phases;
④ Remote control cannot be achieved, and it is only possible to combine with an electric knife switch or switch .

3. Main advantages and features of circuit breakers
: ① After the fault is disconnected, it can be reset manually without replacing components, unless maintenance is required after cutting off a large short-circuit current;
② It has two protection functions: a long-delay release with inverse time characteristics and an instantaneous current release, which are used for overload and short-circuit protection respectively, and each performs its own function;
③ Remote control can be achieved when the operating mechanism is energized.

4. Main shortcomings and weaknesses

① The use of circuit breakers as backup protection components has high residual voltage. It is difficult to achieve selective disconnection between upper and lower non-selective circuit breakers. When the fault current is large, it is easy to cause both upper and lower circuit breakers to be disconnected instantly;

②The relative price is slightly higher;

③ The breaking capacity of some circuit breakers is small. For example, if a circuit breaker with a small rated current is installed near a large-capacity transformer, the breaking capacity will be insufficient. Now there are products with high breaking capacity that can meet this requirement, but the price is relatively high.

7. Installation of backup protection components

Taking the TN-CS system as an example, the installation method is shown in Figure 2.