Fault current of battery components in grid-connected photovoltaic power generation system and DC component entering the grid

1. Fault current in solar cell modules
In the grid-connected photovoltaic power generation system, only solar cell modules with reliable insulation between the solar cell and the frame can be used. Solar modules must have double or super strong insulation measures, and fully consider the system voltage resistance of solar modules to ensure that the surface of the modules can be touched even when the photovoltaic system is running without causing danger. At present, all solar cell modules can reach level II protection, and there are no too strict restrictions on selection.

As described above, for a transformerless inverter, the voltage on the solar cell module during operation may be the synchronous voltage value superimposed on the AC grid. When the surface of the component is touched, a fault current to the ground may be generated. If the insulation of the component is good enough, it is generally difficult to generate such current. However, the intensity of the fault current discharge will increase with changes in some conditions, such as the shortening of the solar cell distance (in this case the thickness of the transparent glass or plastic plate decreases), the increase of the contact area, etc. For example, since the liquid used to clean solar cell components contains conductive substances, the conductive area will be enlarged, leading to unexpected fault currents. In this case, although the dangerous current cannot be detected in advance, if an accident occurs, it will cause certain danger. In order to avoid potential safety hazards and dangers, users should follow the following steps when designing photovoltaic grid-connected power generation systems:
1) Connect the frame of the solar cell module and other conductive gas parts to the ground wire.

2) When maintaining the system or cleaning the solar cell components, the inverter must be disconnected from the grid.

In this type of inverter, it is necessary to continuously monitor the DC or AC leakage current that may be generated by the solar module. Once a fault current (greater than 30mA) is generated, the inverter immediately disconnects from the grid. However, the monitoring of fault current in real applications is more complicated than simply monitoring the magnitude of the leakage current. The leakage current changes at any time in the system operation state, and there is no way to know the current value before connecting to the grid. Therefore, each time the inverter is connected to the grid, the insulation resistance of the solar cell module will be tested. Only when the insulation resistance exceeds the required resistance value (greater than 1MΩ), can it be proved that no fault current is injected into the grid, and the grid can be connected at this time. Therefore, identifying the fault current is not only by monitoring the increase of the leakage current, but also by measuring the rate of change of the current. All fault current monitoring devices must have a leakage current detection function (dual), and each monitoring system must be able to independently identify fault currents. In this way, personal safety will be more guaranteed. RCD protection rarely or does not require manual testing after commissioning, but the above protection measures are far more effective than general RCD protection.
2. The DC component entering the AC grid Direct parallel connection with the power grid usually causes direct current to enter the AC power grid directly. This direct current component will affect the normal operation of the equipment on the grid (local grid transformer) and the working characteristics of the RCD. At the same time, it will cause internal friction in the transformer in the electrical appliance connected to the power grid, resulting in magnetic saturation, which is not an electrical appliance. The required use environment. Although this situation does not necessarily damage the equipment, it can trigger the action of the protective equipment that prevents DC components in the grid. Therefore, in theory, grid-connected inverters are equipped with preventive measures to prevent direct current from entering the grid (entering the grid through a 50Hz transformer or capacitor).

It is also very important that the ability of the inverter to send DC power to the grid depends not only on the presence of an isolation transformer, but also in combination with a capacitor, which can only transmit power in an electrically isolated condition. In fact, the concern is the ability of the electrical components in the circuit to input direct current to the grid. For high-frequency transformer-type inverters that are directly connected to the grid, ordinary inverter bridges can deliver DC current to the grid regardless of whether there is a transformer.

For SMA inverters, the capacitor is part of the bridge. The transformer of the transformer-type inverter is set on the grid side of the bridge, so that only AC current can be provided to the grid (such as SunnyBoy5000TLHC and all transformer-type inverters).

Even if the inverter bridge fails, it is impossible to continue to send DC current to the grid. The reason is that the two bipolar relays connected in series in the inverter will cut off the connection to the grid in this case. This solution is applied to all SMA transformerless inverters. Assuming that the relay fails, the short circuit of the bridge will cause overcurrent, and the overload protection (overload switch) in the inverter will still activate and cut off the connection to the grid.

Post time: Jan-05-2021