How to control the hazards of leakage current - Photovoltaic inverter black technology

The photovoltaic inverter is a very important device in the photovoltaic system. Its main function is to convert the direct current generated by the photovoltaic components into alternating current. In addition, the inverter also undertakes important functions such as detecting the operating status of components, power grids, and cables, communicating with the outside world, and system security management.

In the photovoltaic industry standard NB32004-2013, the inverter has more than 100 strict technical parameters, and each parameter must be qualified to obtain the certificate. The General Administration of Quality Supervision, Inspection and Quarantine of the People's Republic of China also conducts spot checks every year, inspecting 9 items including protective connection, contact current, power frequency withstand voltage of solid insulation, rated input and output, conversion efficiency, harmonics and waveform distortion, power factor, DC component, and over/under voltage protection on the AC output side of photovoltaic grid-connected inverter products. It takes more than two years from development to mass production for a new inverter. In addition to functions such as over- and under-voltage protection, the inverter also has many little-known black technologies, such as leakage current control, thermal design, electromagnetic compatibility, harmonic suppression, efficiency control, etc., which require a lot of manpower and material resources to develop and test.

This article mainly introduces the leakage current control technology of the inverter

1. Why does the photovoltaic system generate leakage current?

The leakage current of the photovoltaic system, also known as the residual current of the array, is essentially a common-mode current. It is caused by the parasitic capacitance between the photovoltaic system and the earth. When a loop is formed between the parasitic capacitance, photovoltaic system and power grid, the common-mode voltage will generate a common-mode current on the parasitic capacitance. When an industrial frequency transformer is installed in the photovoltaic system, the common-mode current generated by the common-mode voltage in the loop can be suppressed to a certain extent because the parasitic capacitance impedance between the transformer windings in the loop is relatively large. However, in a photovoltaic system without a transformer, the loop impedance is relatively small, and the common-mode voltage will form a large common-mode current, i.e., leakage current, on the parasitic capacitance between the photovoltaic system and the earth.

2. Hazards of leakage current

The leakage current in the photovoltaic system, including the DC part and the AC part, will cause grid-connected current distortion, electromagnetic interference and other problems if it is connected to the grid, affecting the operation of equipment in the grid; the leakage current may also cause the inverter casing to be charged, posing a threat to personal safety.

3. Leakage current standards and detection methods

According to Article 7.10.2 of the NB32004-2013 standard, the inverter should provide leakage current detection in any case where the inverter is connected to the AC grid and the AC circuit breaker is closed. The leakage current detection should be able to detect the total (including DC and AC) effective value current and continuous residual current. If the continuous residual current exceeds the following limits, the inverter should be disconnected within 0.3s and send a fault signal:

1) For inverters with rated output less than or equal to 30KVA, 300mA;

2) For inverters with rated output greater than 30KVA, 10mA/KVA.

There are two characteristics of photovoltaic system leakage current. First, the components are complex, including DC and AC parts. Second, the current value is very small, at the milliampere level, which requires extremely high accuracy and requires a dedicated current sensor. The Ministry of Energy's photovoltaic standard stipulates that Type B must be used for the detection of photovoltaic leakage current, that is, a current sensor that can measure both AC and DC leakage current.

The leakage current sensor is installed at the external ground wire output interface of the inverter to detect the current of the inverter output ground wire.

4. Leakage current control technology

At present, leakage current suppression technology has become a hot issue in the research of photovoltaic grid-connected systems. Various universities, research institutions and manufacturers are studying it. The size of the leakage current depends on the parasitic capacitance Cpv between the photovoltaic PV and the earth, and the common-mode voltage change rate. The value of the parasitic capacitance is related to factors such as external environmental conditions and the size and structure of the photovoltaic panel. It is generally around 50~150nF/kW. The common-mode voltage change rate is related to factors such as the inverter topology and modulation algorithm.

For the traditional single/three-phase transformerless photovoltaic grid-connected inverter topology, the two basic conditions for effectively suppressing the common-mode current (leakage current) are: the inductance values ​​of each bridge arm are selected to be consistent; and a non-zero vector is used to synthesize the reference vector so that the common-mode voltage remains constant.

(1) Full H4 bridge topology

In order to solve the problem of leakage current in full H-bridge photovoltaic inverter, bipolar PWM modulation can be used. This modulation eliminates the high-frequency component of the common-mode voltage to the board, so that the common-mode voltage generally has only the low-frequency component of the first harmonic, thereby reducing the impact of leakage current.

(2) H5 topology

This topology only requires one more transistor than the full bridge, which is why it is named H5. During the current freewheeling period, the photovoltaic cell is disconnected from the grid to prevent the voltage between the two poles of the panel and the ground from fluctuating with the switching frequency, thus keeping the common mode voltage almost unchanged.

(3) HERIC topology

The working principle of HERIC AC bypass topology is as follows: In the positive half cycle, switch S5 is always off and S6 is always on, and S1 and S4 are modulated at the switching frequency. When S1 and S4 are on, the sum voltage is Udc and 0 respectively, and the common mode voltage = Udc/2; when S1 and S4 are off, the current flows through the anti-parallel diodes of S6 and S5, and the sum voltage is Udc/2, and the common mode voltage = Udc/2.

(4) H6 topology

The H6 DC bypass topology works as follows: In the positive half cycle, switches S1 and S4 are always turned on, and S5, S6 and S2, S3 are turned on alternately. When S5, S6 are turned on and S2, S3 are turned off, the common-mode voltage is Udc/2; when S2, S3 are turned on and S5, S6 are turned off, there are two current freewheeling paths: (1) S1, S3 anti-parallel diodes, (2) S4, S2 anti-parallel diodes. Diodes D7 and D8 clamp the voltage to Udc/2, and the common-mode voltage is Udc/2. In the negative half cycle, the common-mode voltage is also Udc/2, so the leakage current can be effectively suppressed.

(5) H6.5 topology

The H6.5 topology is improved on the basis of HERIC. Compared with the traditional HERIC, it has one less diode, so the efficiency is relatively higher than HERIC. In reactive exchange, the bus capacitor is not passed through. In the switching state, the power-mode voltage is half the bus voltage, so the power-mode current will be very small; at the same time, the output is three-level, and the volume of the filter core can be further reduced, further improving the efficiency; at the same time, the middle cross tube is a boost chip, which is further optimized in terms of switching losses, so that the efficiency of the whole machine is further improved. On the other hand, now there is a module package, which makes the junction temperature of the chip offset the traditional single tube will be improved, which can significantly improve the reliability of the product.

In addition to the above topologies, the use of multi-level technologies such as 3-level or 5-level can reduce the voltage between the positive and negative poles of the component and the ground, and can also reduce leakage current.

5. Things to note when installing the system

The leakage current is detected through the current of the inverter ground wire. Therefore, during installation, the inverter ground wire must be connected securely and cannot be connected to the inverter neutral wire and the component's safety lightning protection ground wire. Otherwise, it will affect the detection accuracy and cause the inverter to make incorrect judgments.