The relationship between the residual voltage of the arrester and the voltage protection level
May 23, 2022
SPD Residual Voltage and Voltage Protection Level Terminology
Residual voltage Ures is the peak voltage across the arrester or surge protector when the discharge current flows.
The voltage protection level (voltage protection level Up) characterizes the performance parameter of the surge protector to limit the voltage between the terminals, and its value can be selected from a list of priority values. The voltage protection level value should be greater than the maximum value of the limiting voltage.
The figure below is the voltage-current curve of the varistor type lightning arrester (MOV). According to the characteristics of the MOV, different discharge currents correspond to different residual voltages. For the voltage-limiting surge arresters tested for class II, the voltage protection level is the maximum residual voltage value under the nominal discharge current In.
The voltage protection level Up can limit the surge voltage to the allowable range. In order to effectively protect the protected equipment, the inductance voltage drop ΔU of the upper and lower leads of the lightning arrester should be considered, and the effective voltage protection level Up/f should not be greater than The withstand voltage limit Uw of the protected equipment, namely:
Up/f = Up + ΔU ≤ Uw (1)
Up/f —— the effective voltage protection level of the surge protector, kV;
Up - the voltage protection level of the surge protector, kV;
Uw —— equipment withstanding impulse voltage rating, kV.
A case has been discussed in the literature: SPD1 with In 20 kA and Up = 1.8 kV, its effective voltage protection level is: Up/f = Up + ΔU = Up + L × (di/dt) = 1.8 + 0.5 × 20 /8 = 1.8 + 1.25 = 3.05 kV.
When replacing SPD1 with SPD2 with In of 80 kA and Up = 2.5 kV, the following problems arise:
a. Question 1: For SPD2, Up/f = Up + ΔU ≤ Up + L × (di/dt) = 2.5 + 5 = 7.5 kV, which is much larger than the Uw (4 kV) required by the protected equipment, therefore, SPD2 The device will not be protected, and formula (1) is unreasonable.
b. Question 2: SPD2 with In of 80 kA actually uses 4 varistors in parallel. When a surge current of 20 kA occurs, each varistor will shunt about 5 kA, and the residual voltage on SPD2 is higher than that on SPD1. Small, the actual protection effect is also better, which contradicts problem 1.
In fact, the selection of SPD surge protector depends on two important parameters:
a. The withstand voltage limit Uw of the protected equipment at the installation site is used to select the voltage protection level Up of the SPD.
b. The In of the SPD should be selected according to the expected discharge current at the installation site, and the nominal discharge current of the SPD declared by the manufacturer should meet 15 times of 8 / 20 μs positive polarity In .
First of all, SPD1 with In is 20 kA and Up = 1.8 kV is selected for the design. Maintenance and replacement should also follow this specification. SPD2 with In of 80 kA and Up = 2.5 kV cannot be selected. energy coordination.
Secondly, the inductance voltage drop ΔU on SPD2 in question 1 cannot be calculated by taking the nominal discharge current parameter of SPD (In = 80 kA) into the formula L × (di/dt), but the expected discharge current at the installation place (In = 80 kA) 20 kA), so "ΔU = L × (di/dt) = 5 kV" is wrong.
Third, for the voltage-limiting SPD, according to Article 6.4.6 of GB 50057 - 2010, the ΔU at the point where the outdoor line enters the building is calculated as 1 kV/m, and then it can be calculated as ΔU = 0.2UP. Item 8 of Article 5.4.3 of GB 50343 - 2012 requires that the "total length of the leads at both ends of the SPD should not be greater than 0.5 m", that is, the ΔU where the outdoor line enters the building should not be greater than 0.5 kV; Article 4.3.8 of GB 50057 - 2010 4 "The voltage protection level value of the surge protector should be less than or equal to 2.5 kV", and the subsequent SPD's ΔU = 0.2UP≤ 0.5 kV is calculated. The calculation method "ΔU = L × (di/dt) = 1.8 + 0.5 × 20/8 = 1.8 + 1.25" is not intended by the norm.
Fourth, in order to improve the current capacity of the module rail type SPD surge arrester, it is necessary to increase the area and the number of parallel connection of the varistor; in order to improve the service life, the thickness of the varistor will also be increased to cope with different grades and different places. The need for protected equipment. It is not recommended to replace SPD1 with In 20 kA and Up = 1.8 kV with SPD2 with In 80 kA and Up = 2.5 kV.