The Analysis of Using Surge Test Equipment and Surge Voltage Testers

I. Surge Test Equipment Standard
The national standard for surge voltage tester is GB/T17626.5 (equivalent to the International Standard IEC61000-4-5).
The standard mainly simulates various cases due to indirect lightning strikes, such as:
(1) lightning strikes on external wires, resulting in a large current flowing into the external wires or ground resistance, thus generating interference voltages;
(2) Indirect lightning strikes (such as lightning strikes between cloud layers or within cloud layers) inducing voltages and currents on external wires;
(3) lightning strikes near objects, a strong electric and magnetic field established around, inducing voltages on external wires;
(4) lightning strikes near the ground, introducing interference due to the ground current passing through the public grounding system.
In addition to simulating lightning strikes, the standard also simulates interference introduced by switch operations in cases such as a substation, such as:
(1) interference due to the switching of the main power system (such as the switching of the capacitor bank);
(2) interference due to the jumping of small switches near the equipment;
(3) interference due to the switching of silicon thyristor devices involving resonant circuits;
(4) various systematic failures, such as short circuit and arc faults between the grounding networks or grounding systems of the equipment.

Surge Generator SG61000 5 

The standard describes two different waveform generators: one is the waveform induced by lightning strikes on the power line; the other is the waveform induced on the communication line. Both of these lines are overhead wires, but the impedance of the line is different: the surge waveform induced on the power line is narrower (50uS) and the leading edge is steeper (1.2uS); while the surge waveform induced on the communication line is wider, but the leading edge is slower. In the following, we will mainly analyze the circuit with the waveform induced by lightning strikes on the power line, and also introduce a brief introduction to the lightning protection technology of the communication line.

In the design of common Mode Surge Suppression Circuit to prevent surge, it is assumed that common mode and differential mode are independent of each other. However, these two parts are not really independent, as common mode choking can provide a significant differential mode inductance. This differential mode inductance can be simulated by a separate differential mode inductance. In order to take advantage of the differential mode inductance, in the design process, common mode and differential mode should not be done at the same time, but according to a certain order. Firstly, the common mode noise should be measured and eliminated. By using Differential Mode Rejection Network (DMRN), the differential mode component can be eliminated, so the common mode noise can be directly measured. If the designed common mode filter is to make the differential mode noise not exceed the allowable range at the same time, then the mixed noise of common mode and differential mode should be measured. Since it is known that the common mode component is below the noise tolerance, only the differential mode component exceeds the standard, and the differential mode leakage inductance of the common mode filter can be used to attenuate. For low power supplies, the differential mode inductance of the common mode choke is enough to solve the differential mode radiation problem, because the source impedance of differential mode radiation is small, so only a small amount of inductance is effective. For surge voltages below 4000Vp, generally only LC circuits need to be used for current limiting and smoothing filtering to reduce the pulse signal to 2-3 times the average level of the pulse signal. Because L1 and L2 have 50-week grid current flowing, the inductors are easy to saturate, so L1 and L2 usually use a common mode inductance with very high leakage inductance.

Adding a common mode inductor is to eliminate common mode interference on the parallel line (both two-wire and multi-wire). Due to the imbalance of resistance in the circuit, common mode interference is ultimately reflected in the differential mode. It is difficult to filter using differential mode filtering methods.

Where exactly does the common mode inductance need to be used? Common mode interference is usually electromagnetic radiation or spatial coupling. In this case, no matter if alternating current or direct current, if you have a long line transmission, you need to add the common mode inductance for common mode filtering. For example, many USB cables add a ring magnet. The entrance of the switch power supply, the AC power is transmitted from a long distance, then need to add. Generally, the DC side does not need to be transmitted from a long distance, so it does not need to be added. Without common mode interference, adding it is a waste and it does not provide gain to the circuit.

The design of a power filter can usually be considered from the common mode and differential mode. The most important part of the common mode filter is the common mode choke. Compared with the differential mode chokes, the most notable advantage of the common mode choke is that its inductance value is extremely high, and the volume is small. The important thing to consider when designing the common mode choke is its leakage inductance, that is, the differential mode inductance. Usually, the way to calculate leakage inductance is to assume that it is 1% of the common mode inductance. In fact, the leakage inductance is between 0.5% and 4% of the common mode inductance. In designing the choke with the best performance, the impact of this error may not be ignored.

II. The importance of leakage sensitivity of Surger Voltage Tester
How is leakage sensitivity formed? Tightly wound and winding around the annular coil, even without the core, all its magnetic current concentrates inside the coil “core”. However, if the annular coil is not wound around for a week, or not tightly wound, then the magnetic current will leak from the core. This effect is proportional to the relative distance between wire turns and the magnetic permeability of the spiral tube core. Common mode choke has two windings, which are designed to make the current flowing through the coil core conduct in opposite directions, so that the magnetic field is 0. If for safety reasons, the coil on the core is not wound with two lines, then there is a considerable gap between the two windings, which naturally causes magnetic current “leakage”, that is to say, the magnetic field is not really 0 at the concerned points. The leakage sensitivity of common mode choke is differential mode inductance. In fact, the magnetic flux related to the differential mode must leave the core at some point, that is to say, the magnetic flux forms a closed loop outside the core, not just limited within the annular core.

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