The grid-connected inverter is the core component of the grid-connected photovoltaic power generation system. Compared with off-grid photovoltaic inverters, grid-connected inverters not only convert the direct current generated by photovoltaic modules into alternating current, but also control the voltage, current, frequency, phase and synchronization of the alternating current. Technical issues such as electromagnetic interference to the power grid, self-protection, independent operation, islanding effect, and maximum power tracking, so there are higher technical requirements for grid-connected inverters. Figure 3-33 is a schematic diagram of the structure of the grid-connected photovoltaic growatt inverter system.
- Technical requirements for grid-connected inverters
The grid-connected operation of the photovoltaic power generation system puts forward higher technical requirements for the inverter. These requirements are as follows.
(1) The system is required to be able to automatically start and shut down the system according to the sunshine conditions and the specified sunshine intensity, under the condition that the electricity generated by the photovoltaic array can be effectively utilized.
(2) It is required that the inverter must output a sine wave current. The power fed into the public grid by the photovoltaic system must meet the specified indicators of the grid. For example, the output current of the inverter must not contain DC components, and the high-order harmonics must be minimized to avoid harmonic pollution to the grid.
(3) The inverter is required to operate efficiently when the load and sunshine vary widely. The energy of the photovoltaic system comes from solar energy, and the sunlight intensity changes with the climate, so the input DC voltage changes greatly during operation, which requires the inverter to operate efficiently under different sunlight conditions. At the same time, the inverter itself is required to have high inverter efficiency. Generally, the inverter efficiency of medium and small power inverters at full load is required to reach 88% to 93%, and the inverter efficiency requirements of high power inverters at full load are required. reach 95% to 99%.
(4) The inverter is required to make the photovoltaic array always work at the maximum power point state. The output power of the battery module is related to the changes of the sunlight intensity and the ambient temperature, that is, its output characteristics have a nonlinear relationship. This requires the inverter to have the maximum power point tracking control function (MPPT control), that is, no matter how the sunshine, temperature, etc. change, the maximum power output of the battery module square array can be achieved through the automatic adjustment of the inverter, which is to ensure the solar energy. It is an important part of the high-efficiency work of photovoltaic power generation systems.
(5) High reliability is required. Many photovoltaic power generation systems are in remote areas and unattended and maintained states, requiring the inverter to have a reasonable circuit structure and design, with certain anti-interference ability, environmental adaptability, instantaneous overload protection ability and various protection functions. Such as input DC polarity reverse protection, AC output short circuit protection, overheat protection, overload protection, etc.
(6) A wide range of DC voltage input is required. The output voltage of the battery module and the square array will change with the change of sunlight intensity and climatic conditions. For the grid-connected photovoltaic system connected to the battery, although the battery has a certain clamping effect on the output voltage of the battery module, the voltage of the battery itself fluctuates with the change of the remaining power and internal resistance of the battery, especially if the battery is not connected. When the photovoltaic system or the photovoltaic system of the battery is aging, the terminal voltage of the photovoltaic system varies widely. For example, for a photovoltaic system connected to a 12V battery, its terminal voltage will vary from 11 to 17V. This requires that the inverter must be able to work normally within a wide DC voltage input range and ensure the stability of the AC output voltage.
(7) The inverter is required to have the function of grid detection and automatic grid connection. Before the grid-connected inverter is connected to the grid to generate power, it needs to take power from the grid, detect the parameters such as the voltage, frequency, and phase sequence of the grid, and then adjust the parameters of its own power generation to keep it synchronized and consistent with the parameters of the grid, and then enter the grid. Power generation status.
(8) It is required that in the event of a power failure in the power system, the grid-connected photovoltaic system can not only operate independently, but also prevent the islanding effect, and can quickly detect and cut off the power supply to the public power grid to prevent the occurrence of electric shock accidents. After the utility grid restores power supply, the inverter can automatically restore the grid-connected power supply.
(9) Zero (low) voltage ride-through function is required. When an accident or disturbance occurs in the power grid system, causing a voltage sag at the grid-connected point of the photovoltaic power generation system, the inverter must be able to ensure continuous operation without being disconnected from the grid within a certain voltage drop range and time interval.
1) The circuit principle of the three-phase grid-connected inverter
The output voltage of the three-phase grid-connected inverter is generally AC 380V or higher, and the frequency is 50Hz/60Hz, of which 50Hz is the Chinese and European standards, and 60Hz is the American and Japanese standards. Three-phase grid-connected inverters are mostly used in large-capacity photovoltaic power generation systems. The output waveform is a standard sine wave, and the power factor is close to 1.0.
The circuit principle of the three-phase grid-connected inverter is shown in Figure 3-34, which is divided into two parts: the main circuit and the microprocessor circuit. Among them, the main circuit mainly completes the conversion and inversion process of DC-DC-AC. The microprocessor circuit mainly completes the control process of the system grid connection. The purpose of grid-connected control of the system is to maintain the AC voltage value, waveform, phase, etc. output by the inverter within the specified range. Therefore, the microprocessor control circuit must complete the real-time detection of the grid voltage phase, the current phase feedback control, and the photovoltaic method. array maximum power tracking and real-time sine wave pulse width modulation signal generation. Its specific working process is as follows: the voltage and phase of the utility grid are sent to the A/D converter of the microprocessor through the Hall voltage sensor, and the microprocessor compares the phase of the feedback current with the voltage phase of the utility grid, and the error signal passes through the A/D converter. The PID operator is adjusted and sent to the pulse width modulator (PWM), which completes the power feedback process with a power factor of 1. Another major job done by the microprocessor is to achieve the maximum power output of the photovoltaic array. The output voltage and current of the photovoltaic square array are detected and multiplied by the voltage and current sensors respectively to obtain the output power of the square array, and then the PWM output duty cycle is adjusted. The adjustment of this duty cycle is essentially to adjust the size of the feedback voltage, so as to achieve maximum power optimization. When the amplitude of U changes, the phase angle between the feedback current and the grid voltageφ There will also be some changes. Since the feedback control of the current phase has been realized, the decoupling control of the phase and the amplitude is naturally realized, which makes the processing process of the microprocessor easier.
(2) Circuit principle of single-phase grid-connected inverter
The output voltage of the single-phase grid-connected inverter is AC 220V or 110V, the frequency is 50Hz, and the waveform is sine wave, which is mostly used in small household systems. The circuit principle of single-phase grid-connected inverter is shown in Figure 3-35. Its inverter and control process are basically similar to the three-phase grid-connected inverter.
(3) Detection of independent operation of grid-connected inverter and prevention of islanding effect
In the process of solar photovoltaic grid-connected power generation, due to the grid-connected operation of the photovoltaic power generation system and the power system, when the power system fails due to an abnormality for some reason, if the photovoltaic power generation system cannot stop working or is disconnected from the power system, the It will continue to supply power to the power transmission line, and this operating state is vividly called the “islanding effect”. Especially when the generated power of the photovoltaic power generation system is balanced with the power consumption of the load, even if the power system is powered off, the parameters such as the voltage and frequency at the output end of the photovoltaic power generation system will not change rapidly, so that the photovoltaic power generation system cannot correctly judge whether the power system is Failure or interruption of power supply can easily lead to the phenomenon of “islanding effect”.
The “island effect” can have serious consequences. When the power system fails or the power supply is interrupted, the photovoltaic power generation system will continue to supply power to the power grid, which will threaten the safety of the power supply line repair and maintenance operators and equipment, resulting in electric shock accidents. It not only hinders the maintenance of the power failure and the restoration of the power grid as soon as possible, but also may cause damage to the power distribution system and some load equipment. Therefore, in order to ensure the safety of maintenance operators and the timely recovery of power supply, when the power system is powered off, the photovoltaic power generation system must be stopped or automatically separated from the power system (at this time, the photovoltaic power generation system is automatically switched to an independent power supply system, and will continue to operation to supply some emergency loads and necessary loads). The more photovoltaic grid-connected power generation systems are connected to the power system, the higher the probability of “islanding effect” occurs, so there must be corresponding countermeasures to solve the “islanding effect”.
In the inverter circuit, the function of detecting the independent operation state of the photovoltaic system is called independent operation detection. The function of detecting the stand-alone operation state and making the photovoltaic power generation system stop or automatically separate from the power system is called stand-alone stop or “islanding” prevention. The independent operation detection function is divided into passive detection and active detection.
① Passive detection method. When the power grid fails and the power is cut off, the output voltage, output frequency, voltage phase and harmonics of the inverter will change. The passive detection method monitors the changes of the voltage, frequency, phase and harmonics of the grid system in real time to detect the The power failure of the grid power system causes the voltage fluctuation, phase jump, frequency change, harmonic change and other parameter changes when the inverter transitions to independent operation, and the independent operation state is detected.
Passive detection methods include voltage phase jump detection method, frequency change rate detection method, voltage harmonic detection method, output power change rate detection method, etc. Among them, the voltage phase jump detection method is more commonly used.
The detection principle of the voltage phase jump detection method is shown in Figure 3-36. The detection process is as follows: Periodically detect the AC voltage cycle of the inverter. If the cycle deviation exceeds a certain set value, it can be judged as independent operation. state; at this time, the inverter will stop running or run off the grid. Usually, the inverter connected to the power system operates under the condition that the power factor is 1 (that is, the voltage of the power system is in phase with the output current of the inverter), the inverter does not supply reactive power to the load, and the power system Supply reactive power. However, when operating alone, the power system cannot supply reactive power, and the inverter has to supply reactive power to the load, resulting in a sudden change in the voltage phase. The detection circuit detects the change of the voltage phase, and it can be determined that the photovoltaic power generation system is in a separate operation state.
The disadvantage of the passive detection method is that it is difficult to detect the occurrence of the “islanding effect” when the output power of the inverter is exactly balanced with the local load power. Therefore, the passive detection method has limitations and a large non-detection area.
② Active detection method. The active detection method is that the output terminal of the inverter actively sends a disturbance signal such as voltage, frequency or output power to the system, and observes whether the power grid is affected, and detects whether it is in a separate operation state according to the parameter changes. When the power grid is working normally, the power grid has a balancing effect, and these disturbance signals cannot be detected. When the power grid fails, the disturbance signal output by the inverter will be detected.
Active detection methods include frequency offset method, active power variation method, reactive power variation method, load variation method, etc. The most commonly used is the frequency offset method.
According to GB/T19939-2005 “Technical Requirements for Grid-connected Photovoltaic System”, the photovoltaic power generation system should run synchronously with the power grid when it is connected to the grid, the rated frequency of the grid is 50Hz, and the allowable deviation of the frequency after the photovoltaic power generation system is connected to the grid is ±0.5 Hz, when it exceeds the frequency range, it must act within 0.2s to disconnect the photovoltaic power generation system from the grid.
The working principle of frequency offset mode active detection is shown in Figure 3-37. This mode is to make the output AC frequency of the photovoltaic power generation system change within the allowable range according to the load condition in independent operation, and to change the frequency according to whether the system follows the change. Determine whether the photovoltaic power generation system is in a separate operation state. For example, if the output frequency of the inverter is fluctuated by ±0.1Hz relative to the system frequency, this frequency fluctuation will be absorbed by the system when it is connected to the grid, so the frequency of the system will not change. When the system is in the state of independent operation, the fluctuation of this frequency will cause the change of the system frequency, which can be judged as independent operation according to the detected frequency. Generally, when the frequency fluctuation lasts for more than 0.2s, the inverter will stop running or disconnect from the power grid.
The active detection method has high precision and small non-detection area, but the control is complicated, and the quality of the output power of the inverter is reduced. At present, a more advanced detection method is a combined detection method that combines a passive detection method with an active detection method.
(4) Switch structure type of grid-connected inverter
Generally speaking, the cost of grid-connected inverter accounts for 10% to 15% of the total cost of the entire photovoltaic power generation system, while the cost of grid-connected inverter mainly depends on its internal switch structure type and power electronic components. The grid-connected inverter generally has the following three switch structure types.
① Inverter with power frequency transformer. This type of switch is usually composed of a single-phase inverter bridge composed of power transistors (such as MOSFETs) and a rear power frequency transformer. The power frequency transformer can not only easily match the grid voltage, but also play the role of DC-AC. isolation. The inverter using the power frequency transformer technology works stably and reliably, and has good economy in the low power range. The disadvantage of this structure is that it is bulky and bulky, and the inverter efficiency is relatively low.
② Inverter with high frequency transformer. The use of high frequency electronic switching circuits can significantly reduce the size and weight of the inverter. This type of switch structure consists of a DC converter that boosts the DC voltage to more than 300 volts and a bridge inverter circuit composed of IGBTs. The high-frequency transformer is much smaller in volume and weight than the power-frequency transformer. For example, the power-frequency transformer of a 2.5kW inverter weighs about 20kg, while the high-frequency inverter of the same power inverter is only about 0.5kg. The circuit of this type of structure has high working efficiency, but the disadvantage is that the cost of high-frequency switching circuits and components is also high, and even depends on imports. However, the overall cost disadvantage is not obvious, especially for high-power applications, which have relatively good economics.
③ Transformerless inverter. This switching structure has the relatively highest conversion efficiency because the loss caused by the transformer link is reduced, but the cost of anti-interference and safety measures will increase.