Requirements and basic design of inverters for off-grid photovoltaic power generation systems

1. Requirements for inverters in off-grid photovoltaic power generation systems

Solar photovoltaic power generation is a new power generation technology that directly converts solar radiation energy into electrical energy. Solar radiation energy is converted into electrical energy through solar cells, and then through energy storage, control and protection, energy conversion and other links, so that it can provide DC or AC power to the load according to people's needs. The power generated by the solar cell array is DC power, but most electrical equipment uses AC power supply, so an inverter is needed in the system to convert DC power into AC power for the load.

In off-grid photovoltaic power generation systems, the efficiency of the inverter will directly affect the efficiency of the entire system. Therefore, the control technology of the inverter of the solar photovoltaic power generation system has important research significance. In the design of the inverter, the analog control method is usually used. However, there are many defects in the analog control system, such as the aging and temperature drift effects of components, sensitivity to electromagnetic interference, and a large number of components. The typical analog PWM inverter control system uses the natural sampling method to compare the sinusoidal modulation wave with the triangular carrier to control the trigger pulse, but the triangular wave generation circuit is easily disturbed by factors such as temperature and device characteristics at high frequencies (20kHz), resulting in DC offset in the output voltage, increased harmonic content, and changes in dead time. The development of high-speed digital signal processors (DSPs) makes digital control of inverters in solar photovoltaic power generation systems possible. Because most of its instructions can be completed within one instruction cycle, more complex and advanced control algorithms can be implemented, further improving the dynamic performance and steady-state performance of the output waveform, and simplifying the design of the entire system to make the system have good consistency.

The inverter is a power electronic circuit that can convert the direct current generated by the solar cell array into alternating current to power the AC load. It is a key component of the entire solar power generation system. The off-grid photovoltaic inverter has two basic functions: on the one hand, it provides power for the DC/AC conversion to the AC load, and on the other hand, it finds the best working point to optimize the efficiency of the solar photovoltaic system. For specific solar radiation, temperature and solar cell type, the solar photovoltaic system has a unique optimal voltage and current, so that the photovoltaic power generation system can generate the maximum power. Therefore, the following basic requirements are put forward for the inverter in the off-grid solar photovoltaic power generation system:

1) The inverter should have a reasonable circuit structure, strict component screening, and various protection functions, such as input DC polarity reverse protection, AC output short circuit protection, overheating, overload protection, etc.

2) It has a wider DC input voltage adaptation range. Since the terminal voltage of the solar cell array varies with the load and sunlight intensity, although the battery has a clamping effect on the voltage of the solar cell, the battery voltage fluctuates with the changes in the remaining capacity and internal resistance of the battery. Especially when the battery is aged, the terminal voltage varies greatly. For example, the terminal voltage of a 12V battery can vary between 10V and 16V. This requires the inverter to ensure normal operation within a wider DC input voltage range and ensure that the AC output voltage is stable within the voltage range required by the load.

3) The inverter minimizes the intermediate links in power conversion to save costs and improve efficiency.

4) The inverter should have a high efficiency. Since the current price of solar cells is relatively high, in order to maximize the use of solar cells and improve system efficiency, the efficiency of the inverter must be improved.

5) The inverter should have high reliability. At present, off-grid solar photovoltaic power generation systems are mainly used in remote areas, and many off-grid solar photovoltaic power generation systems are unattended and unmaintained. This requires the inverter to have high reliability.

6) The output voltage of the inverter has the same frequency and amplitude as the domestic mains voltage, so it is suitable for general electrical loads.

7) In medium and large capacity off-grid solar photovoltaic power generation systems, the output of the inverter should be a sine wave with low distortion. Because in medium and large capacity systems, if square wave power supply is used, the output will contain more harmonic components, and high-order harmonics will generate additional losses. The loads of many off-grid solar photovoltaic power generation systems are communication or instrumentation equipment, which have high requirements for power quality. For the inverter of the off-grid solar photovoltaic power generation system, there are two indicators for high-quality output waveforms: one is high steady-state accuracy, including small THD value, and no static error in phase and amplitude of the fundamental component relative to the reference waveform; the other is good dynamic performance, that is, fast adjustment under external disturbances and small changes in the output waveform.

8) The inverter should have a certain overload capacity, generally 125% to 150%. When overloaded by 150%, it should be able to last for 30 seconds; when overloaded by 125%, it should be able to last for more than 60 seconds. The inverter should ensure the standard rated sinusoidal output under any load conditions (except overload conditions) and transient conditions.

At present, the main problem of inverters is low reliability. The main factors affecting the reliability of inverters are electrolytic capacitors, optocouplers and magnetic materials. Improving the reliability of inverters should start from the design aspect, such as reducing the junction temperature of the device, reducing the electrical stress of the device, reducing the operating current and using high-quality magnetic materials. Measures such as can greatly improve its reliability. If the first generation of magnetic materials, such as TDK's H35 and FDK's H45, are used in the design, due to the low saturation flux density and Curie temperature point of such magnetic materials, it is very easy to fail when working for a long time at a high power. The use of third-generation magnetic materials, such as TDK's H7C4, FDK's H63B and H45C, Siemens' N47 and N67, can not only effectively improve the conversion efficiency, but also greatly improve the reliability of the inverter.

To improve the efficiency of the inverter, its loss must be reduced. The losses in the inverter can usually be divided into two categories: conduction loss and switching loss. The conduction loss is due to the device having a certain on-resistance Rds, so when there is current flowing through it, a certain amount of power consumption will be generated. The conduction loss power Pc is calculated by the following formula:

Pc=I2×Rds

During the process of turning on and off the device, the device not only flows a large current, but also withstands a high voltage, so the device will also produce a large loss, which is called switching loss. Switching loss can be divided into turn-on loss, turn-off loss and capacitor discharge loss.

Turn-on loss:

Pon = (1/2) × Ip × Vp × ts × f

Turn-off loss:

Poff = (1/2) × Ip × Vp × ts × f

Capacitor discharge loss:

Pcd = (1/2) × Cds × Vc2 × f

Total switching losses

Pcf=Ip×Up×ts×f+（1/2）×Cds×Vc2×f

Where: Ip is the maximum current flowing through the device during switching; Vp is the maximum voltage that the device withstands during switching; ts is the turn-on and turn-off time; f is the operating frequency; Cds is the drain-source parasitic capacitance of the power MOSFET.

To reduce the above losses, it is necessary to implement zero voltage or zero current conversion on the power switch tube, that is, to adopt a resonant conversion structure.

2. Basic design of photovoltaic inverter

The basic design criteria for PV inverters include rated voltage, capacity, efficiency, solar cell efficiency, output AC power quality, maximum power point tracking (MPPT) effectiveness, communication characteristics and safety.

1) Rated voltage. The main function of a photovoltaic inverter is to convert the variable DC voltage from the solar cell (sometimes a regulated DC voltage) into an AC voltage to power the AC load. The most commonly used single-phase and three-phase AC voltages are 120V/220V and 208V/380V respectively; and for industrial applications, 480V is also common. For the selected inverter topology, the range of the output AC voltage will determine the DC bus voltage and the rated voltage of each semiconductor switch.

2) Capacity. It is another way of saying the rated power of a PV inverter, which can range from 200W (solar cell integrated module) to hundreds of kilowatts. The larger the capacity, the larger and more expensive the inverter. The cost of a PV inverter is measured in dollars per watt. For a proper design, surges, overloads, and continuous operation modes must be taken into account when determining capacity.

3) Efficiency. Each photovoltaic inverter has high requirements for efficiency (output power/input power). For example, a typical efficiency requirement for a multi-kilowatt inverter is over 95%. Based on the fact that the energy conversion efficiency of solar cell arrays is relatively low (about 15%), high-efficiency inverters are very important in terms of obtaining the maximum output power with the minimum solar cell capacity.

4) Battery. Installing a battery pack on the DC side of the inverter acts as an energy buffer, which can smooth out possible fluctuations in DC voltage and store unused energy of the load.

5) Output power quality. Due to the inherent switching mode characteristics of the inverter, its AC output waveform is not an ideal sine wave, and usually contains a wide range of high-frequency harmonics introduced by pulse width modulation (PWM). For many electronic loads, these harmonics are harmful rather than beneficial.

6) MPPT efficiency. The output of a solar cell will follow a series of characteristic curves under different lighting conditions in its current-voltage curve. Therefore, in order to obtain maximum power output, the voltage needs to be dynamically adjusted.

7) Communication characteristics. For a multi-kilowatt photovoltaic inverter, it is necessary to build a communication connection for monitoring and data storage. As a general-purpose controller, a microprocessor (MCU) is very suitable for this function.

8) Safety. There are two meanings: one is to ensure the safe and stable operation of the photovoltaic power generation system, and the other is to ensure that there are no safety risks to the workers during operation, maintenance and repair.

3. Key elements of photovoltaic inverter design

Two key factors to consider when designing a photovoltaic inverter are efficiency and harmonic distortion. Efficiency can be divided into two parts: the efficiency of the solar cell and the efficiency of the inverter. The efficiency of the inverter depends largely on the external components used in the design, not the controller; while the efficiency of the solar cell is related to how the controller controls the solar cell array. The maximum operating power of each solar cell array depends largely on the temperature and light of the solar cell array. The MCU must control the output load of the solar cell array to maximize the operating power of the solar cell array. Since this is not a mathematically intensive algorithm, a low-cost MCU can be used to complete the task.

Currently, most photovoltaic inverters can only optimize the overall efficiency of the solar array from a certain optimal position of the solar array. This optimization method seriously restricts the efficiency of the solar power generation system. If the photovoltaic system operates at non-optimal voltage and current levels, the efficiency of the system is very low, and the opportunity to collect solar energy is wasted. In the photovoltaic power generation system, solar cells are formed by multiple series groups connected in parallel. Just like holiday lights, if a solar cell in the series fails, it will cause the entire solar cell group to fail. In addition, this situation also occurs when there is a partial shadow or other factors that block the solar cell.

To solve the above problems, solar cells are currently integrated with bypass diodes so that the current can bypass the shaded and failed solar cell. After the diode is activated, it can reroute the current, that is, redirect it to bypass the failed solar cell. In this way, not only the power supply potential of the shaded solar cell is wasted, but also the total voltage of the entire solar cell group is reduced. Based on the principle of selecting the best working point for solar cells, the inverter must decide whether to optimize the voltage of the affected solar cell string or the energy generated by other unaffected solar cell groups. In most cases, the inverter will choose to optimize the unaffected solar cell group and reduce the energy generated by the affected solar cell group accordingly, or even completely shut down the affected solar cell group. The result is that as long as there is 10% shade in the solar photovoltaic power generation system, the solar photovoltaic power generation will drop by half. The main reason for this phenomenon is that the current solar photovoltaic power generation system cannot match the extremely sensitive solar cells. Therefore, more intelligent technologies and products are needed to develop solar energy.

The bypass diode at the front end of the photovoltaic inverter is not strictly speaking part of the inverter, but as a part of the solar power generation equipment, it is also crucial to the operation of the inverter and even the reliability of the entire system. Microsemi has launched two new products for this application: LX2400 and SFDS1045. LX2400 incorporates the latest heat dissipation packaging technology (Cool RUNTM process), does not require a heat sink, and the temperature rise is less than 10°C when passing a current of 10A. The reliability design with the goal of 30 years of stable operation guarantees a leakage current of less than 100μA, a steady-state current capability of 20A and a bidirectional lightning resistance function. Its biggest feature is the lowest temperature rise in the industry. SFDS1045 is a new generation of Schottky diodes and the thinnest bypass diode in the industry to date. It is only 0.74mm thick and is placed under the glass package, which is particularly suitable for direct application in solar cell arrays. In addition, its unique flexible copper pins have satellite application-level reliability.