Development of General Frequency Converter

1 Introduction

AC motors have the advantages of reliability, simplicity and low cost. However, most of the inverters sold on the market are ordinary general-purpose inverters . If they are directly used in photovoltaic water pump systems, they cannot realize various protection functions well, and they also do not have the maximum power point tracking function of solar cells, resulting in a huge waste of solar cell capacity and internal resources of general-purpose inverters. Based on the full use of the internal resources of general-purpose inverters, this paper improves the ordinary general-purpose inverter and realizes a general-purpose inverter with photovoltaic water pump system control function. Experiments show that the improved general-purpose inverter can realize the general functions of ordinary general-purpose inverters and the control function of photovoltaic water pump systems, while reducing the use cost and maintenance cost of photovoltaic water pumps.

2 Basic System Structure

The hardware circuit of the inverter system is generally divided into two modules: the power main circuit and the control circuit [1]. According to the system requirements, comprehensive consideration of various schemes and combined with the actual situation, this system adopts the basic structure shown in Figure 1.

Figure 1 Basic structure of a general frequency conversion system with photovoltaic water pump control function

As can be seen from Figure 1, the system consists of four parts, namely: photovoltaic array, main circuit module, control circuit module and water pump. Compared with the general inverter structure, the main circuit module structure is different in that there are two types of input power supply - AC power supply and DC power supply output by the photovoltaic array; the control circuit module uses the TMS320LF2406A DSP digital signal processor, which is similar to the general inverter control circuit. In the peripheral control circuit, in addition to the PWM signal drive, voltage, current and other analog signal sampling circuits, DC bus current detection is added. In the photovoltaic water pump variable frequency speed regulation system, in order to prevent the water level from dropping and causing dry friction and damage to the "machine-pump assembly" components of the photovoltaic water pump system, the general frequency conversion system with photovoltaic water pump control function needs a water level dry detection protection function in addition to the general general frequency conversion protection function. Common detection methods include water level sensor recognition and automatic recognition [2]. Based on the output current sampling function of the general frequency converter, this paper adopts automatic recognition of low water level to avoid dry water.

3 Implementation of photovoltaic water pump control function in general inverter

The outstanding features of general-purpose inverters are complete functions and good versatility. In order to facilitate the closed-loop control, modern general-purpose inverters have built-in pi digital regulators. Using the built-in pi digital regulators of general-purpose inverters, the maximum power point tracking of photovoltaic arrays can be easily achieved.

Connect the PV array lead wire to the system DC input terminal, as shown in Figure 1. The system DC bus voltage is the PV array output terminal voltage. For the PV water pump system, since the output power of the water pump motor is proportional to the cube of the speed, the output load matching of the PV array can be achieved directly by changing the speed of the water pump motor, that is, the output power of the solar array can be directly adjusted by PWM control technology, so that the output power of the PV array always tracks the maximum value of the current sunshine and ambient temperature.

3.1 PV array characteristics

In the photovoltaic water pump system, the DC power output by the solar photovoltaic array is used as the system power supply. The solar photovoltaic array power supply is different from the ordinary DC power supply. It has a strong nonlinear characteristic, and the maximum output power is greatly affected by meteorological conditions such as sunshine and ambient temperature. Figure 2 shows the IV characteristic curve and PV characteristic curve of the solar photovoltaic array under different sunshine intensities. In Figure 2, S is the sunshine intensity, and the unit is watt per square meter (w/m2). From the characteristic curve of the photovoltaic array, it can be seen that the photovoltaic array is neither a constant voltage source nor a constant current source, and it cannot provide arbitrarily large power to the load. It is a nonlinear DC power supply. The characteristics of the photovoltaic array determine the special control requirements when applying the photovoltaic array-maximum power point tracking.

Figure 2 IV and PV characteristic curves under different sunshine intensities 3.2 CVT maximum power point tracking

The constant voltage tracking (CVT) method can approximate the maximum power output of the photovoltaic array. Although in actual applications, the temperature and sunshine intensity change greatly, causing the maximum power point voltage of the solar photovoltaic array to shift, resulting in the CVT method not being able to track the maximum power point well, but because it is relatively simple to handle in software, and many general inverters can achieve the CVT function through simple settings, CVT is also a good choice when there is no true maximum power point tracking (TMPT) method. Its control principle is shown in Figure 3.

Figure 3 Schematic diagram of CVT tracking control

Among them, usp* is the command voltage for the solar cell array; the feedback voltage usp of the control system is the DC bus voltage of the general inverter, which is also the terminal voltage of the photovoltaic array. When the built-in pi regulator controller is enabled, the system constitutes a negative feedback control of the DC bus voltage, that is, the output voltage usp of the photovoltaic array. When usp>usp*, the error signal increases the input voltage parameter v of the v/f function generator after passing through the pi regulator. After the v/f function generator is operated, the other input frequency parameter f of the svpwm converter increases. Through pulse width regulation, the system output voltage and frequency increase, the speed of the water pump motor increases, and the output power of the water pump increases, so that the output power of the photovoltaic array matched with the water pump increases, and the output current increases. It is known from the UI characteristic curve of the photovoltaic array that the output voltage usp of the photovoltaic array decreases until usp=usp; when usp

3.3 tmppt maximum power point tracking

The maximum power point tracking of the tmppt method can achieve true maximum power point tracking. When the voltage corresponding to the maximum power point changes greatly, the system can ensure the maximum daily water pumping without any adjustment. The most common method of maximum power point tracking of the tmppt method is to implement it through differential negative feedback of power to voltage. Although the principle of this method is simple, it is relatively complicated to implement because of the use of division in data processing. Therefore, according to the tmppt tracking principle [4][5], the power change characteristics on both sides of the maximum power point are analyzed. This paper adopts a relatively simple method to judge by the change in power change, that is, the change in dp. For a general inverter with photovoltaic water pump control function, its tmppt control principle is shown in Figure 4.

Figure 4 Principle of tmppt tracking control implemented by dp method

It can be seen from the PV characteristic curve of the photovoltaic array in Figure 2 that when the command voltage usp* is on the left side of the maximum power point, increasing the command voltage usp* will increase the power, and decreasing the command voltage usp* will reduce the power; when the command voltage usp* is on the right side of the maximum power point, increasing the command voltage usp* will reduce the power, and decreasing the command voltage usp* will increase the power; therefore, firstly, by giving the command voltage usp* a disturbance amount, the current position of usp* is determined according to the dp sign, that is, whether it is on the left or right side of the maximum power point, and then the value of the command voltage usp* is adjusted according to the direction of power increase. Figure 4 is a control principle diagram of dp change search for the maximum power point. It can be seen from the PV characteristic curve in Figure 2 that the output power of the photovoltaic array is 0 at the maximum open circuit voltage. In order to prevent current shock during CVT adjustment, the search starts from the right side of the maximum power point, and z3 is initialized to -1. As can be seen from Figure 4, the system tracking control process is: the system starts to search from the right side of the maximum power point, and the power increases from 0, then dp>0, and the value of z1 is +1. The value of z2 depends on the product of z1z3. Since the initial value of z3 is -1, z2 is -1. After comparison, z3 is -1, and the command voltage continues to decrease. Because usp* is on the right side of the maximum power point, usp* decreases, the power increases, dp>0, and the command voltage usp* is continuously adjusted according to the above rules; when usp* reaches the left side of the maximum power point, the power decreases, dp<0, z1 takes -1, at this time the value of z2 is +1, the value of z3 is +1, the command voltage usp* begins to increase, and the system searches back for the maximum power point. Finally, the system runs near the maximum power point. The same as dp/dv, when tracking the dp value, the hysteresis comparison method is also used.

4 Experimental Results

Figure 5 PV array output voltage and current waveforms during cvt start-up operation

Figure 6 PV array output voltage and current waveforms during tmppt start-up operation

According to the existing inverter resource platform, a prototype was designed according to the above design ideas. Experiments have proved that the system can operate safely and stably, and all functions can be realized, achieving the design purpose. Figure 5 is the output voltage and current waveform of the photovoltaic array during the start-up operation of the CVT tracking control. The experimental conditions are that the sunshine intensity is 672W/m2, the open-circuit voltage of the solar array is UOC=368V, and the command voltage is USP*=276V. Figure 6 is the output voltage and current waveform of the photovoltaic array during the start-up operation of the TMPPT tracking control. The experimental conditions are that the sunshine intensity is 690W/m2, the open-circuit voltage of the solar array is UOC=363V, and the search start voltage is UDC=350V.