Shenzhen City ROHOX Technology CO.,LTD.
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Composition and working principle of solar photovoltaic water pump system
PV system：Composition and working principle of photovoltaic water pump system
System composition and working principle
1.1 structure of photovoltaic water pump system
From Figure 1, the system uses the solar cell array to convert solar energy directly into electrical energy. After DC / DC boost, and with TMPPT function of the inverter output AC voltage to drive the AC asynchronous motor and pump load, to complete the water tower water storage function. Including 4 parts: solar cell array; inverter with TMPPT function; water pump load; water storage device.
1.2 inverter main circuit and hardware structure
The main circuit and hardware control block diagram of the system are shown in Figure 2. Main circuit DC / DC part is the superior performance of the push-pull forward converter circuit voltage; DC / AC part of the three-phase bridge inverter circuit. The main power devices with ASIPM (integrated intelligent power module) PS12036, the core of the control system is composed of 16 bit digital signal controller dsPIC30F2010. The peripheral control circuit includes an array of bus voltage detection and dry level detection circuit. System first through the initial set of work and PI parameters and by MPPT subroutine real-time search out the voltage value as the inner ring of CVT is given. Through the PI regulating working frequency value is obtained and calculated the PWM signal for cycle, to achieve photovoltaic array of the true maximum power tracking (TMPPT), and keep the asynchronous motor V / F ratio is constant. The system will combine MPPT and inverter, fault detection using ASIPM module built-in detection and protection, simple structure, convenient control.
Brief introduction of DC DC / 1.2.1 boost circuit
22.214.171.124 main circuit selection
For medium and small power photovoltaic water pump and PV array voltage mostly (24V, 36V and 48V) low, for the choice of the main boost circuit, people generally choose the push-pull circuit, because of the push-pull circuit transformer primary side voltage is DC input voltage while driving without isolation, and therefore more suitable for the occasions with low input voltage. But bias problem is to restrict a big disadvantage in its application, tube power difference of the parameters and the transformer winding process have may make the work of the push-pull circuit in an unstable state. Based on the consideration of many factors, the system using novel push-pull forward circuit. The circuit topology not only overcome the bias and closed loop control is relatively easy to second order system.
1.2.l.2 push-pull forward circuit simple analysis
Push-pull forward excitation circuit as shown in Figure 2, by the power tube S1 and S2, the capacitors C8 and transformer T and t of the transformer primary winding N1 and N2 with the same number of turns, the end of the same name as shown in Figure 2. When S1 and S2 and shutdown, the C8 on both ends of the capacitor voltage is negative and equal to the array voltage, when S1 is opened, S1, N2 and photovoltaic arrays constitute a loop, N2 is negative, also C8, N1 and S1 constitute a loop, C8 discharge, N1 is negative, this work is equivalent to two forward DC-DC converter parallel. In the same way, when the S2 is turned off by S1, it is also equivalent to the parallel connection of two forward converters. Through theoretical analysis, the push-pull forward excitation circuit is a second-order system, therefore closed-loop control is simple, while the output filter inductance and capacitance is greatly reduced.
Dspic30f2010 1.2.2 brief introduction
In the 16 bit single chip microcomputer skillfully add DSP function. The microchip dsPIC30F digital signal controller (DSC) also has the control function of the microcontroller unit (MCU) and digital signal processor (DSP) computing power and data throughput capacity by microchip company. Because it has the DSP function, also has the size and price of the single chip microcomputer, so the system by the chip as the controller. The chip is mainly used in motor control, such as DC brushless motor, single-phase and three-phase induction motor and switched reluctance motor; it can also be used in.Dspic30f2010 uninterrupted power (UPS), inverter, switch power supply and power factor correction pin signal as shown in Figure 3.
Main structure of 126.96.36.199
12KB program memory;
512 byte SRAM:
1024 byte EEPROM;
16 3 bit timers;
4 input capture channels;
2 output comparison / standard PWM channel;
6 motor control PWM channel;
10 6 bit 500kspsSA / D converter channels.
Main features of 2.2.2 L
A / D sampling speed and multi channel can be sampled at the same time;
6 independent / complementary / center aligned / edge aligned PWM:
2 programmable dead zones;
In noise environment 5V power supply can work properly;
Minimum operating voltage 3V;
D / A sampling and PWM synchronization.
2 photovoltaic pump maximum power point tracking (MPPT) design
2.1 the characteristics and shortcomings of the conventional constant voltage tracking (CVT) method
CVT method can approximate the maximum power output of solar cells, the software is relatively simple to deal with. But in fact the sunlight intensity and temperature is constantly changing, especially in the western region, the same day in the different time, temperature and sunlight intensity variations are quite large. All of these will cause solar array maximum power point voltage offset, which especially in temperature change is the most influential. In this case, the CVT is not a good way to track the maximum point.
2.2 TMPPT principle and Implementation
TMPPT (TrueMaximum power point tracking) concept is presented in order to overcome the disadvantages of CVT, the meaning is "the true maximum power tracking control, to ensure that the system, regardless of the in which sunshine and temperature conditions, always make solar cells work at the maximum power point. Because the inverter adopts constant f / V control, the pump motor speed is proportional to the input voltage, therefore, regulating the output voltage of the inverter, it is equal to the output power of the load motor. Therefore, this system uses the TMPPT mode to make the solar cell work at the maximum power point as far as possible, providing the maximum energy for the load.
The characteristic curve of the solar cell array (see Figure 4),
In the maximum power point, DP / dv=O. On the left side of the maximum power point, when the DP / dV>O, P showed increasing trend, DP / dV<O when the, P was decreased; but in the right side of the maximum power point, when the DP / dv>O, P was decreased, DP / D v<O, P showed increasing trend. According to the change of P-V, the maximum power point can be determined according to the change of the.
Figure 5 for the TMPPT type maximum power point tracking control block diagram. System input instruction value is 0, the feedback value for DP / DV, it is assumed that the Z3 + 1, is Usp* command voltage increase, adjusted by the CVT link, system output voltage V tracking Usp* increase, sampling the output current I, power operational links and the power differential link and obtain DP / DV values, such as DP / dV>0, (z1) to + 1, Z2 + 1, Z3 is +, Usp* the command voltage continue to increase. Such as dP / dV<O, the Z1 is -l, Z2 is -1, Z3 is -1, Usp* instruction voltage starts to decrease. Stable operation, the system in the vicinity of the maximum power point swing, if the swing amplitude is smaller, the higher the accuracy. In the specific work, in order to prevent the search direction of the false judgment, software set the search limit amplitude, so that the system's reliability is further improved. Because the ASIPM module which is used in the system has the function of current detection, the current detection circuit can be omitted in the hardware design, the cost is saved, and the peripheral circuit is further optimized.
3 system protection function design
1) over current and short circuit protection function, because of the sampling resistance of IGBT bus in the lower arm of ASIPM, the protection function can be achieved by detecting the bus current. When the detected current value exceeds a given value, which had been considered too current or short circuit, this time under the bridge arm IGBT gate circuit is turned off, while the output signal fault, dsPIC detected signal blocks the PWM pulses further protection circuit.
2) under voltage protection function ASIPM detection under the bridge arm control supply voltage, if the supply voltage continuously below a given voltage 1OMs, is under the bridge arm each phase IGBT are is shut off, while the output signal failure, during the fault period, under the bridge arm three-phase IGBT gate very do not accept external signal.
3) overheating protection function ASIPM built-in detection substrate temperature thermistor, the resistance of the thermistor is directly output, dsPIC by detecting the resistance can be completed overheat protection function.
The above protection is the use of the ASIPM itself with the function, without additional circuit, to further simplify the design of the hardware circuit. System in addition to the above protection function, but also has a photovoltaic water pump system unique low frequency, low sunlight, dry (automatic and manual play), and other protective functions. For the pumps, when the speed is lower than the lower limit value, the photovoltaic array provides most of the energy into long-term loss, low-speed operation, can cause fever and affect the service life of the water pump, therefore, the system design of low frequency protection on the pump, when the liquid level below the water inlet of the water pump, the pump is in the idle state, if not take measures to run for a long time will damage the lubrication of bearings, and the system for unattended outdoor work, so in order to increase the reliability of the detection system, automatic and manual dry dry two kinds of identification, the automatic dry system is based on the output power and the operating frequency of discrimination; manual stem it is realized by the water level sensor to identify the current level. Because of low frequency, low sunlight, and so on, the function of the system is done by the software, and the system structure is simple, so the system is not required to increase the hardware circuit.