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How to Defeat Control Power Supply Grounding Defects Based on Insulation Reduction in PCB Design

Insulation Description

In the ordinary 380V AC distribution system, control power supply usually derives from DC power system. As a critical standby power supply and control power supply of power plants, the most ordinary and dangerous defect of DC system lies in DC grounding defect. Based on a frequently-occurred insulation defect, this article finds out a series of causes leading to DC control power supply insulation reduction.

Defects Search and Cause Analysis

• Loop introduction

Secondary circuit that will be mentioned in the following part of this article mainly conforms to 380V AC system. In the switch secondary circuit, control power supply derives from AC control power supply through earth leakage protection assistance power terminal in earth leakage protection device and current transformer part. Terminals 5 and 7 that will be mentioned in the following part of this article respectively refer to positive electrode and negative electrode of input terminal in earth leakage DC power supply while terminals 8 and 9 to K and L of current transformer.

• Defect cause search

a. Frequent insulation defects in AC system

Just after the operation of AC-DC low voltage for about one year, DC grounding alarm takes place frequently and insulation monitoring device inspects that AC system at downstream controls corresponding branch circuit of power. It is alarmed that insulation resistance decreases with alarm value 7kΩ and normal 110V DC busbar voltage is respectively +55V and -55V. However, practical DC negative busbar or positive busbar when alarming is almost 0V. Under this condition, if another DC grounding at the other electrode takes place, loop will be caused between DC positive and negative electrodes.

It can be concluded that in AC system, insulation is qualified between major loop and control loop without DC penetration into AC or grounding, so defect issues only occur in DC control part of AC loop. Each part should be inspected in control loop and defect issue lies in earth leakage protection and CT.

b. Insulation reduction inside earth leakage protection

Concerned with these defects, type number of earth leakage protection is *** M40 (110VDC) and that of CT is earth leakage current transformer with the same brand. Through the disassembling of earth leakage protection device, it can be found that this device is composed by three circuit boards one of which is earth leakage protection control board. After measurement from point to point, it can be seen:
1). Insulation value between Terminal 7 and Terminal 9 is about 5kΩ (most is lower than 5 kΩ);
2). Insulation value between Terminal 5 and Terminal 7 is 12.9kΩ;
3). Insulation value between Terminal 5 and Terminal 8 is 18kΩ;
4). Insulation value between Terminal 8 and Terminal 9 is approximately 50kΩ.

Through comparison, without application of loading, insulation value of earth leakage protection between Terminal 7 and Terminal 9 is approximately 150kΩ in truck type switch while with the frequent application of loading, insulation value is reduced to 5kΩ.

c. CT secondary side protection grounding

Because a protecting ground is arranged at CT in the process of earth leakage protection and CT design and assembly, Terminal L of 001TI coil applies earth leakage assembly. This design aims to stop current transformer coil from loop that will lead to first high voltage penetration into secondary loop with components destroyed such as directly connected earth leakage protection device. What's worse, insulation issue between Terminal 7 and Terminal 9 possibly leads to high voltage penetration into DC control loop.

Nonetheless, owing to the grounding point and insulation reduction of PCB earth leakage protection, negative electrode of power is controlled by DC.

• Defect consequence

Usually, this issue takes place to some loads in the same AC system, which means that DC negative busbar is parallel with some resistors of 5 kΩ, finally leading to nearly zero of DC negative busbar and voltage.

In the process of negative busbar grounding, if another busbar grounding comes into being at the other electrode, short circuit will be caused between positive and negative electrodes. Fuse wire or breaker will make loop broken as a result of overloading and fault protection. In addition, DC power will lose electricity, leading to poweroff of all loads downstream and DC power loss of key loads, all of which will endanger smooth implementation of all equipment. Furthermore, multi-point grounding in DC system leads to numerous consequences such as component malfunction, resist-operation and DC power loss.

Processing Scheme and Principle Analysis

• Make CT coil grounding point vacant

Based on CT loop design, there's a grounding point at the secondary side. Theoretically, a high voltage is produced by current transformer secondary side loop, which will destroy other components in the secondary loop. Super high voltage will even destroy components. Grounding here aims to stop high voltage from being produced in order to protect secondary loop.

However, based on analysis mentioned above, when grounding point is cancelled, DC control loop insulation resistance can be ensured not to be reduced so as to eliminate grounding defects in DC system. Therefore, if the grounding point is cancelled, it has to be testified whether secondary side loop voltage value of low-voltage current transformer is within acceptable range. Put it differently, the risk has to be lower than that brought by DC system grounding.

For low-voltage current transformer that is similar to 0.5kV, high pressure may not be definitely produced by secondary side loop. When one side passes through rated current with secondary side loop existing, iron core could be either far from saturate or far from too saturate, core flux and induced electromotive force basically only have fundamental wave and secondary side won't produce high pressure, which fully indicates that current transformer core has a relatively big design margin, that is, a relatively high tetsushige ratio. As a result, downstream load is operated normally with current lower than rated current, which is acceptable to make CT a little vacant.

However, for this type of secondary side loop CT, if a large current occurs to downstream load or short circuit takes place at single phase or between phases, iron core will definitely be saturate when high pressure will be produced at secondary side. Therefore, whether high voltage will be produced by CT at secondary side loop totally depends on saturation degree of iron core. Increasing curves of voltage value depends on saturation curves of CT. Under such condition, a little vacant CT is a little risky. Nevertheless, thanks to protection loop, component destroy risk will be relatively decreased.

So with physical structure of CT fully considered, electricity distribution devices are operated in relatively good environment and first coil features a relatively low possibility power off. Even though downstream current takes place with coil cut and loop protection action features a relatively long delay, secondary high voltage will destroy components, which has extremely low possibility. Therefore, our processing scheme for this defect lies in vacant grounding point.

• Changeover of corresponding earth leakage protection

Although this CT protection grounding point has been eliminated and DC defects eliminated, the fundamental cause for grounding lies in PCB of earth leakage. Under the privilege of none damp or corrosion, insulation value decreases in one to two years' operation.

Based on measurement situation, up to now insulation value is only low between single electrode and ground and low insulation value isn't found between electrodes so short circuits don't occur between electrodes. In the future, this item of data can be recorded in periodical maintenance. If this value tends to decrease or one-time loop takes place to CT at its beginning, changeover should be considered to earth leakage protection.

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