Class B and Class C protection for power supply systems

The lightning protection of power supply system is usually divided into Class B protection and Class C protection. The main function of Class B protection is to deal with the strong electromagnetic pulse interference of lightning and resist the lightning of direct lightning. The main function of Class C protection is to complete the fine protection work after Class B rough protection, and limit the transient overvoltage of the power supply system to the range allowed by the electrical insulation. Usually, the coordination of these two levels can greatly improve the lightning protection level of the power supply system, making the equipment work safer and more reliable in the thunderstorm season.

Class 1C protection

According to the requirements of relevant departments of our country, as shown in Figure 1, Class C lightning arresters have been installed on communication power supply equipment in a 3+1 manner (three lightning arresters are connected to the phase line and the neutral line, and one lightning arrester is connected to the neutral line and the ground line), which can effectively protect against general induction lightning. This type of lightning arrester is mainly a varistor lightning arrester, with a current capacity of generally 15kA~20kA (8/20μs), and a residual voltage of ≤1.5kV at rated current capacity (the insulation strength of the communication power supply is 1.5kV). The maximum continuous working voltage of the lightning arrester (IEC standard) is divided into AC275V and AC385V. When selecting, it should be noted that it is greater than the maximum fluctuation voltage of the power grid. At the same time, it should also be noted that the increase in the maximum continuous working voltage will increase the residual voltage of the lightning arrester. In urban stations with good power supply conditions, products with a maximum continuous working voltage of AC275V can be selected. In rural stations with poor power supply conditions, it is best to use products with a maximum continuous working voltage of AC320V, such as VAL?MS320ST.

The main disadvantages of varistor arresters are:

Figure 13+1 protection circuit

(1) The residual voltage will increase with the increase of the current flowing through it. When the system is disturbed by a large lightning pulse, the current flowing through the arrester is often greater than the rated current carrying capacity of the arrester. As the residual voltage increases (exceeding 1.5kV), it will endanger the safety of the equipment and cannot provide reliable protection for the equipment.

(2) Aging occurs during use. The service life of a varistor arrester is usually to withstand about 20 shocks of the rated current capacity, and an avalanche phenomenon will occur when it fails.

In order to ensure that this type of arrester is always in a reliable working state and ensure the safety of the power line, the arrester is usually equipped with failure detection, disconnection and remote signaling functions.

Figure 2 Spark gap working principle

①The starting voltage ignites the arc ②The spark connects the two electrodes

③The spark spreads to the outside ④The spark reaches the impact plate

⑤ Generate sparks ⑥ Sparks are interrupted and extinguished

Class 2B lightning arrester

The main function of Class B protection has been described above. There are two common direct lightning strikes:

(1) Lightning directly strikes the power line and is conducted along the power line into the equipment;

(2) After lightning strikes an outdoor lightning rod, it is transmitted into the ground, causing the ground potential to rise instantly (to more than tens of thousands of volts). Since the power neutral line and the equipment casing are both connected to this ground network, the instantaneous rise in ground potential will cause the insulation of the equipment to break down.

According to the relevant IEC standards, the current carrying capacity of this type of arrester should be greater than or equal to 25kA (10/350μs). Technically, this type of arrester is divided into air gap type and varistor type.

Air gap type lightning arrester, also known as lightning current type lightning arrester, is the preferred product of Class B lightning arrester. This type of lightning arrester is based on the ancient gap discharge principle, and adds the technology of cutting off the arc (see Figure 2), so that after the lightning arrester has processed the lightning current, it can cut off the subsequent current of the network, avoid the short circuit of the power grid to the ground, and restore the lightning arrester to its initial working state. The characteristics of this type of lightning arrester are that the ability to pass lightning current is very strong and can withstand the lightning of direct lightning. Since the electrode is a special alloy resistant to arc corrosion, it can withstand tens of thousands of impacts of the rated current capacity. The main shortcomings of this type of lightning arrester are:

(1) The action delay is 4 times that of the varistor (100ns);

(2) The ignition voltage is between 3kV and 4kV. (To be precise, the high ignition voltage is not a disadvantage. Since the rate of change of lightning current is very large, it can reach 2.5kA/μs. When the lightning current flows through the 10m long power line of Class C protection, the voltage drop generated by the inductance and resistance is much greater than 4kV, which meets the ignition conditions of the gap type arrester.)

Varistor type arresters [current capacity above 60kA (8/20μs)] are substitute products for Class B protection. This type of arrester expands the current capacity by connecting multiple varistors in parallel (capacity increase: 1+1<1.7, the more parallel connections, the lower the efficiency), and also requires special technology to solve the current balance distribution, which makes this type of arrester large in size and expensive. This type of arrester has low characteristic energy and cannot meet the requirements for handling lightning energy, and often explodes and catches fire during use.

Table 1 is a performance comparison of different types of arresters under the same test waveform (10/350μs) and the same current capacity:

Table 1 Performance comparison of different types of lightning arresters*

Item * Note 1 If the varistor type lightning arrester does not have a 10/350μs current carrying capacity

When using indicators, you can convert them as follows:

Current capacity: I(8/20μs)÷5=I(10/350μs)

Note 2: The comparison in this table is based on the same lightning current carrying capacity.

3. Coordination of Class B protection and Class C protection

Both Class B and Class C protection use varistor protection solutions. The advantages and disadvantages they have are also the advantages and disadvantages of varistor arresters. The main disadvantages are that it is difficult to achieve a large current capacity; the energy characteristics are small; the service life is short (the rated current capacity is about 20 times); when the lightning current exceeds the rated current capacity, the residual pressure will increase and the equipment cannot be effectively protected.

The two-stage protection scheme using gap and varistor respectively has the characteristics of complementary advantages, which can give full play to the advantages of both arresters and is an ideal scheme. The gap type arrester has a strong ability to withstand large currents and is durable (>10,000 times). It undertakes the main protection tasks in the scheme coordination. The problems of action delay and high starting voltage are solved by the varistor arrester with Class C protection. During the action delay of the gap type arrester, since the lightning pulse current will not exceed the rated current carrying capacity of the Class C protection, the Class C protection can effectively handle and ensure that the residual voltage value is less than 1.5kV, which effectively protects the equipment. In this scheme coordination, since the Class C protection only undertakes a small number of protection tasks, it can also greatly improve the service life of the varistor.

In order to better coordinate the Class B protection and Class C protection, attention should be paid to the decoupling between the two levels during installation. When the site permits, the straight-line distance of the line should be ≥10m, otherwise a decoupling inductor should be connected in series in the line.

As mentioned before, the communication power supply equipment has been installed with Class C protection in 3+1 mode. When doing Class B protection, it is best to use the 3+1 installation mode. This installation mode has wide adaptability and is suitable for TT power grids, TN power grids and other power grids, and can also ensure personal safety.