High-efficiency micro solar inverter test methods

The rapid economic development has led to the rapid depletion of natural resources, which has triggered an increasingly strong demand for renewable energy. Therefore, pollution-free and inexhaustible energy such as solar energy, wind energy, hydropower and geothermal energy will receive more and more attention. In Europe and the United States, renewable energy, led by solar energy, has been widely used. Unlike the situation in China (where the government and large companies come forward to build solar power stations), European and American countries have vigorously developed household solar power supply systems and established small power generation facilities on the roofs of buildings. The maximum output power of a single solar panel they use is generally around 100 to 200 watts. By connecting an inverter, the DC power generated by the solar panel can be converted into AC power and connected to the grid and transmitted to the city power network. This power can be used for your own family, or the excess power can be sold to the power company.

Micro inverters have the advantages of small size, flexible installation on roofs or walls, high conversion efficiency and relatively low price, making them very suitable for home use. Many domestic companies have begun the research and development and production of micro inverters, and have formed a large export scale.

The output of solar panels is different from the output of general DC power supply equipment. Its output IV characteristic curve is closely related to environmental factors such as light and temperature. The voltage and current values ​​of the working point on the curve change with the load. In order to maximize the output power of solar panels, inverters often also need to have peak power tracking function to ensure that the working point is always near the maximum power point on the IV curve. The key to designing, developing and certifying inverters is to test and verify the input and output characteristics of the inverter under different environmental conditions (i.e., on different IV curves).

The main contents of the test include: developing and verifying the performance of the inverter peak power tracking circuit (MPPT) algorithm;

Measure and verify inverter efficiency;

Verify the stability of the grid-level output produced by the inverter under extremely high and low input voltage conditions;

Performance certification test: confirm the output performance under different environmental conditions;

Performance accelerated life test: use only a few weeks to estimate the results after several years of operation;

Certification testing to relevant standards.

To achieve these test purposes, a predictable and repeatable solar illumination condition must be created and its ambient temperature must be controlled to obtain a fixed IV output curve. Natural illumination and other environmental factors are difficult to control, so it is not feasible to test the performance of the inverter directly using solar panels.

In order to accurately simulate the output of solar panels under specific environmental conditions (especially for small power inverters, the simulation accuracy requirements are often higher), many manufacturers have launched dedicated solar array simulators to simulate the output characteristics of solar panels under various environments and accurately reproduce the IV output characteristic curve under different environmental conditions. The data of the IV curve mostly comes from the actual measurement results of the user on the output of the solar panel. In order to simplify the operation, the current internationally common curve setting method is: through the four characteristic values ​​on the IV curve, namely Voc (open circuit voltage value), Isc (short circuit current value), Vmp (maximum power point voltage value), Imp (maximum power point current value) to fit the complete IV curve. The formula used is as follows:

Under the guidance of the above formula, an IV curve can be accurately established inside the solar array simulator to simulate the output of the solar panel under certain environmental conditions, as shown in Figure 1. No matter how the load changes, the operating point of the solar array simulator will always be above this curve.

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Although a standard IV curve can be fitted using four key points. However, under certain test conditions, we will find that the IV curve output by the solar panel is not an ideal monotonic curve. Due to the presence of obstructions on the surface of the panel or the damage of some battery cells, the output characteristic curve will be distorted and multiple ridges will appear on the curve. To simulate this situation, the list method is needed to discretize the IV curve into several groups of voltage-current points and manually input them into the array simulator. In order to obtain better simulation accuracy, the more points describing the curve, the better. Usually, hundreds or even thousands of groups of voltage-current points are required to obtain a more ideal simulation effect.

In the actual working environment, the output IV curve of the solar panel will continue to change due to the constant changes in light illumination and incident angle, as well as the influence of cloud cover. In order to test the working effect of the solar inverter under dynamic conditions, it is necessary to save multiple IV curves in advance and simulate the dynamic light changes by continuously switching these curves. In order to achieve good simulation results, the solar array simulator needs to have sufficient storage space to store hundreds of IV curves and be able to switch between curves in a timely and fast manner to simulate a continuously changing working environment. In addition, by adding different voltage or current biases to the existing curves, the purpose of dynamically changing the IV curve can also be achieved.

If the goal is to verify the performance of the peak power tracking circuit and develop a solar inverter that can always operate at the maximum power point of the IV curve under different environmental conditions, the peak power tracking range and tracking frequency must be considered in the design and development of the circuit. The peak power tracking range is a section near the maximum peak power point of the IV curve, which is also the working range of the inverter peak power tracking circuit algorithm. The tracking frequency is the swing rate of the curve within the working range, as shown in Figure 2. To ensure that the inverter can always find the maximum peak power point when the module IV curve changes, it must have a wide enough tracking range and a high enough tracking frequency. To verify the effectiveness of the design, it is necessary to accurately reproduce the IV curve of the solar panel to verify whether the inverter can stably operate near the peak power point under different curves.

In addition to being able to obtain as much power as possible from the solar panel, a high-efficiency inverter can also convert as much input DC power into AC power as possible. Although adding a fixed DC voltage to the inverter input to study its efficiency can provide some meaningful information, it does not allow designers to fully understand the maximum peak power tracking (MPPT) circuit and DC-AC inverter. Testing the inverter efficiency using a solar array simulator will be more accurate and reliable than testing it using a normal DC power supply.

The E4360A modular solar simulation simulator launched by Agilent is a very efficient micro solar inverter test tool. It provides three IV curve generation methods, which can flexibly generate various IV curves required, and can store up to 512 curves, as well as dynamically simulate the impact of environmental changes on the output of solar panels, so as to determine the performance of the inverter. Users can use E4360A to perform accelerated life tests: To speed up life tests, the speed of environmental conditions changes must be greatly accelerated, while increasing module output, so that the results after several years of work can be obtained in just a few weeks. With E4360A, users can achieve the purpose of accelerated testing by generating IV curves that simulate changes in environmental conditions and repeatedly simulating the impact of changes in temperature and solar radiation on the output of solar panels during the day in a very short period of time.