Data Acquisition

26 Nov 2020 : Hioki Memory HiCorder MR8880 For Troubleshooting RCCB Failure In TL Inverter Solar PV System Due to DC Backflow

Hioki Memory HiCorder MR8880 For Troubleshooting RCCB Failure In TL Inverter Solar PV System Due to DC Backflow

What is RCCB? 

Residual Current Circuit Breaker (RCCB) is a device containing a mechanical switch to disconnect a circuit when it detects a current difference between the live and the neutral wire [1]. Under normal condition, the current in the live wire and the returning current from the neutral wire is the same [2]. The current difference, called Residual Current, is often caused by leakage current (mostly due to old or damaged insulation of equipment) flowing to the ground through a conductive path such as a human body. The Residual Current is used to disconnect the circuit in the RCCB according to their residual current rating design. Figure 1.0 shows the components of a RCCB while Figure 2.0 shows the pole types, residual current trip sensitivity rating, and type of RCCB available in the market.

Figure 1.0 Components of a RCCB
Figure 2.0 RCCB pole type, residual current trip sensitivity rating and type

Leakage current can be dangerous and even fatal to users if it exceeds certain threshold, as can be seen from Figure 3.0 below. Exposure to a leakage current of 30mA exceeding a mere 40miliseconds can kill a person. Therefore, RCCB plays a crucial role in user protection; any failure in the functionality of RCCB will have a devastating effect.

Figure 3.0 Leakage current effect on human

TL Inverter Solar PV System and DC Backflow

Solar Photovoltaic (PV) installation is becoming increasingly popular for domestic usage. Most home Solar PV system comes without a transformer (Transformer-less (TL) Inverter Solar PV system) for cost-effectiveness purpose. In this system, the parasitic capacitance between the PV system and earth forms a loop leading to a voltage which produces a current [3] (Figure 4.0). A parasitic capacitance is a phenomenon of unwanted capacitance when two nearby conductive elements are at or possess different charge levels. Due to the low impedance of this system, the common mode voltage will form a large common mode current (leakage current) on the parasitic capacitance.

Figure 4.0 Leakage current path for TL inverter Solar PV system

However, in a Solar PV system with a transformer, the high parasitic capacitance between the transformer windings loop can suppress the leakage current loop between the solar PV system and earth to a certain extent.

TL Inverter Solar PV System DC Backflow impact on RCCB

In a situation without any leakage current, the magnetic field of the RCCB core between the live wire and the neutral wire are balanced and cancels out. When leakage current occurs (ΙLive > ΙNeutral), the magnetic field changes and generates a current in the relay windings connected to the same iron core to disconnect the circuit if it hits the designated RCCB current rating.

When a certain amount of DC backflow occurs in the AC line supply, the total current flowing in the AC line live wire will be higher in one direction of the AC waveform (Figure 5.0). The higher current in one direction of the AC line causes the RCCB’s iron core magnetic field to magnetize in one direction more than the other [4]. The magnetization results in a higher tripping current required for the relay switch to mechanically disconnect the circuit. In short, the RCCB’s required tripping current is NOT functioning according to its designated rating anymore. This situation is very hazardous, as it means there’s no leakage current tripping protection for the users. Therefore, it is crucial to monitor DC backflow in the AC line to ensure no compromise on the safety of users. 

Figure 5.0 Effect of DC flow on AC magnitude** in live wire

Hioki MR8880-20 for DC backflow measurement  in TL Inverter Solar PV System

As illustrated above, DC backflow at certain level can cause RCCB functionality failure which could be lethal. Therefore, there's a need to monitor and ensure this parameter is at a safe level at all times. Hioki’s Memory Recorder MR8880-20 offer the following features that are useful to check on the DC backflow level.

• Four analog channels to measure up to 3-phase 4-pole RCCB

• Eight logic channels to monitor the trip sequence

• Measure individual phase’s DC current presence by using the four analog channels

The following accessories and attachments paired with MR8880-20 are used for measuring various AC and DC parameters:

1) Display Unit CM7290 with AC/DC Auto-zero Current Sensor CT7731
The AC, AC RMS and DC Current waveforms for RCCB can be measured

2) L9197 Alligator Test Leads with Magnetic Adapter  9804

a) AC and DC Voltage measurement
 The AC, AC RMS and DC voltage waveforms can be measured

b) Power disruption event trigger
The AC, AC RMS and DC voltage waveforms can be measured

3) Logic Probe MR9321-01
The trip time for the three-phase RCCB can be determined and compared to the requirement.

In conclusion, the reliance on RCCB alone for protection from residual current is insufficient for Solar PV System without a transformer. The DC backflow is a common and well-known phenomenon in this type of system. Without proper monitoring to ensure it is below the hazardous level, it can result in injury and worse, the loss of human life.

For more information on Hioki Memory HiCorder MR8880, please click the link here:  Hioki Memory HiCorder MR8880

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References

1. https://www.electgo.com/rccb-2/

2. http://engineering.electrical-equipment.org/electrical-distribution/residual-current-circuit-breaker-rccb.html

3. https://www.inverter.com/leakage-current-control-in-solar-inverter

4. https://www.youtube.com/watch?v=9qX89cPGgp4&feature=youtu.be

**Sine wave sample generated using sine wave generator on https://www.desmos.com/calculator/w9jrdpvsmk