Why PV Output Grounded is Forbidden in Transformerless Inverter?

2KW 3KW Single Phase Grid-tied Photovoltaic PV Solar Inverter 1

PV inverters had been generally classified into isolated inverter and non-isolated inverter. Non-isolated inverter also call transformerless inverter. Because it eliminates the transformer, this type of inverter has much higher efficiency. But on the other hand, the input of the transformerless inverter must not connect to the ground with the same point of AC side.

1.Isolated and Non-isolated

We call it isolated converter which using the transformer to isolate the input side and output side. For PV inverter, the topology is shown below. The inverter has a high frequency modulated DC/DC circuit to convert the DC input to another voltage level of DC. Because of isolated transformer, the PV side can connect to ground or potential earth, this will contribute to the scenario like some system use thin film PV module. Thin film PV module has a disadvantage of polarization by leakage current from positive or negative to PE, so the positive or negative.

Fig.1 High Frequency Transformer Isolated Inverter

Fig.1 High Frequency Transformer Isolated Inverter

For non-isolated inverter which is shown in Fig.2, the high frequency transformer is eliminated and the MPPT tracker will substitute by a boost converter. For anther hand, this kind of inverter use less power switch that will reduce power lose and increase efficiency from 94% to 97%.

Fig.2 Transformerless PV Inverter

Fig.2 Transformerless PV Inverter

2. Effect of PV conductor grounded

Generally, the neutral of the grid is connected to ground (potential earth) and the case of inverter is also connected to the ground for personal safety. If the PV conductor is connected to the ground too, there will occur a circuit loop from the PV side through to grid side directly. This directive current is very easy to trigger over current protection because of the current detective component is in the current loop. But it would be the worst situation that the over current is occurred in another loop that didn’t has a current detective component, and this will make the inverter failed.

Fig.3 Current Loop Between PV and Grid through Neutral of Grid

Fig.3 Current Loop Between PV and Grid through Neutral of Grid

3. Safety consideration of IEETek

When the PV conductor is connected to ground by mistake, it will make the inverter failed or further more to let the grid down. In order to avoid this situation, inverter should detect if the PV conductor is grounded. IEETek use a resistance matrix to detect both the positive and negative side. The detection circuit is shown in Fig.4.

Fig.4 Insulation detected circuit of IEETek’ inverter

Fig.4 Insulation detected circuit of IEETek’ inverter

More over, the detected circuit should be designed in redundant. Inverter of IEETek has two controllers in PV inverter, so we can design two detected circuit to make sure this fault can be detected in case one of them is failed.

We design a high accuracy detection circuit which has the resolution of 10k ohm. This will reduce the false detection of the inverter which can let the PV system to produce more electricity. We have the test result below. When the resistor connect to the PV with the PE is larger than 210kohm.

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