The following describes the design of a photovoltaic water pump system based on a digital signal controller (DSC) architecture. The system uses the newly released dsPIC30F2010 chip from Mimochip as its core and employs a practical maximum power point tracking (MPPT) control method to achieve true maximum power point tracking (TMPPT) functionality for the solar cells. The main circuit's DC/DC section utilizes a novel push-pull forward converter , while the DC/AC section employs an integrated intelligent power module (ASIPM) with comprehensive protection functions. Practical application has proven that this system is characterized by its small size , light weight, and reliable and stable operation.
Keywords: frequency converter; solar photovoltaic array; push-pull forward converter; constant voltage tracking; maximum power point tracking; photovoltaic water pump
0 Introduction
Millions of farmers and herders in remote areas of western my country still lack access to electricity. Furthermore, the region suffers from arid climates, desertification, and grassland degradation. The rational development of groundwater resources using photovoltaic (PV) water pump systems is of great significance for solving drinking water and agricultural water problems and improving the ecological environment in these areas. The core of PV water pump technology lies in the design of a dedicated frequency converter. This paper focuses on how to design a frequency converter that is compatible with the solar cell array and possesses maximum power point tracking (MPPT) capabilities as well as various protection functions unique to PV water pump systems.
1. System Composition and Working Principle
1.1 Structural diagram of the photovoltaic water pump system
As shown in Figure 1, the system directly converts solar energy into electrical energy using a solar cell array. After DC/DC boost and a frequency converter with TMPPT function, the output three-phase AC voltage drives an AC asynchronous motor and a water pump load to complete the water storage function in the water tower. It mainly consists of four parts: a solar cell array; a frequency converter with TMPPT function; a water pump load; and a water storage device.
1.2 Main Circuit and Hardware Composition of the Frequency Converter
The main circuit and hardware control block diagram of this system are shown in Figure 2. The DC/DC section of the main circuit uses a high-performance push-pull forward converter for voltage boosting; the DC/AC section uses a three-phase bridge inverter circuit. The main power device uses the ASIPM (Integrated Intelligent Power Module) PS12036, and the system control core is composed of a 16-bit digital signal controller dsPIC30F2010. The peripheral control circuits include array bus voltage detection and water level drying detection circuits. The system first operates through the initially set working mode and PI parameters. Then, the voltage value searched in real time by the MPPT subroutine is used as the reference for the inner loop CVT. The operating frequency value is obtained through PI adjustment, and the duty cycle of the PWM signal is calculated to achieve true maximum power point tracking (TMPPT) of the photovoltaic array and maintain the V/f ratio of the asynchronous motor at a constant value. The system combines MPPT and inverter, and uses the fault detection function built into the ASIPM module for detection and protection. The structure is simple and the control is convenient.
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1.2.1.1 Main Circuit Selection
For small to medium power photovoltaic water pumps, the photovoltaic array voltage is mostly low (24V, 36V, 48V). For the selection of the boost main circuit, a push-pull circuit is generally chosen because the primary voltage of the transformer in a push-pull circuit is the DC input voltage, and the drive does not require isolation, making it suitable for applications with low input voltage. However, the magnetization problem is a major disadvantage limiting its application. Differences in power transistor parameters and transformer winding processes can cause the push-pull circuit to operate in an unstable state. Considering these factors, this system adopts a novel push-pull forward converter circuit. This circuit topology not only overcomes the magnetization problem but also facilitates closed-loop control (for a second-order system).
1.2.1.2 Simple Analysis of Push - Pull Forward Converter Circuit
The push-pull forward converter circuit, as shown in Figure 2, consists of power transistors S1 and S2, capacitor C8, and transformer T. The primary windings N1 and N2 of transformer T have the same number of turns, and their terminals are shown in Figure 2. When S1 and S2 are simultaneously turned off, the voltage across capacitor C8 is positive at the bottom and negative at the top, equal to the array voltage. When S1 is turned on, S1, N2, and the photovoltaic array form a loop, with N2 showing positive at the top and negative at the bottom. Simultaneously, C8, N1, and S1 form a loop, causing C8 to discharge, and N1 showing positive at the bottom and negative at the top. This operation is equivalent to two forward converters in parallel. Similarly, when S2 is turned on and S1 is turned off, it is also equivalent to two forward converters in parallel. Theoretical analysis shows that the push-pull forward converter circuit is a second-order system, thus simplifying closed-loop control and significantly reducing the output filter inductance and capacitor.
1.2 . A brief introduction to the 2dsPIC30F2010
Microchip has cleverly incorporated DSP functionality into a 16-bit microcontroller, enabling its dsPIC30F digital signal controller (DSC) to combine the control capabilities of a microcontroller (MCU) with the computational power and data throughput of a digital signal processor (DSP). Because it offers DSP functionality while maintaining the size and price of a microcontroller, this chip is chosen as the controller for this system. This chip is primarily suitable for motor control, such as brushless DC motors, single-phase and three-phase induction motors, and switched reluctance motors; it is also suitable for uninterruptible power supplies (UPS), inverters, switching power supplies, and power factor correction. The pinout of the dsPIC30F2010 is shown in Figure 3.
1.2.2.1 Main Structure
12KB program memory;
512 bytes of SRAM:
1024 bytes of EEPROM;
3 16-bit timers;
4 input capture channels;
2 output compare/standard PWM channels;
Six motor control PWM channels;
Six 10-bit 500ksps SA/D converter channels.
l2.2.2 Main Features
A/D sampling speed is fast and multiple channels can be sampled simultaneously;
6 independent/complementary/center-aligned/edge-aligned PWMs:
Two programmable dead zones;
The 5V power supply can operate normally in noisy environments.
Minimum operating voltage: 3V;
A/D sampling and PWM synchronization.
2. Photovoltaic water pump maximum power point tracking (MPPT) design
2.1 Characteristics and shortcomings of conventional constant voltage tracking (CVT) method
The CVT method can approximate the maximum power output of a solar cell, and its software processing is relatively simple. However, in reality, solar radiation intensity and temperature are constantly changing, especially in western regions where temperature and solar radiation intensity vary considerably at different times of the day. These variations cause deviations in the maximum power point voltage of the solar cell array, with temperature changes having the greatest impact. In such cases, the CVT method cannot accurately track the maximum point.
2.2 Principles and Implementation of TMPPT
To overcome the drawbacks of CVT (Continuous Voltage Transmission) systems, the concept of TMPPT (True Maximum Power Point Tracking) was proposed. This means "True Maximum Power Point Tracking" control, ensuring that the solar cells always operate at their maximum power point regardless of solar radiation or temperature conditions. Since the inverter uses constant V/f control, the speed of the water pump motor is directly proportional to its input voltage. Therefore, adjusting the inverter's output voltage is equivalent to adjusting the output power of the load motor. Thus, this system employs TMPPT to ensure the solar cells operate at their maximum power point as much as possible, providing the maximum energy to the load.
As can be seen from the characteristic curves of the solar cell array (see Figure 4),
At the point of maximum power, dP/dv = 0. To the left of the point of maximum power, when dP/dV > 0, P tends to increase.
Figure 5 shows the block diagram of a TMPPT (Maximum Power Point Tracking) control system. The system input command value is 0, and the feedback value is dP/dV. Assuming Z3 is in state +1, the Usp* command voltage increases. After adjustment by the CVT (Continuous Voltage Transmission) stage, the system output voltage V tracks the increase of Usp*. The output current I is sampled, and after power calculation and power differentiation, the dP/dV value is obtained. If dP/dV > 0, then Z1 is +1, Z2 is +1, Z3 is +1, and the Usp* command voltage continues to increase. If dP/dV...
3. System protection function design
1) Overcurrent and Short Circuit Protection: Since a sampling resistor is connected in series on the lower arm IGBT bus of the ASIPM, protection can be achieved by detecting the bus current. When the detected current value exceeds the given value, it is considered an overcurrent or short circuit. At this time, the lower arm IGBT gate circuit is turned off, and a fault signal is output. When the dsPIC detects this signal, it blocks the PWM pulse to further protect the downstream circuit.
2) Undervoltage protection function: ASIPM detects the control power supply voltage of the lower bridge arm. If the power supply voltage is continuously lower than the given voltage by 10ms, all IGBTs of the lower bridge arm will be turned off, and a fault signal will be output at the same time. During the fault period, the gates of the three phase IGBTs of the lower bridge arm will not accept external signals.
3) Overheat protection function: The ASIPM has a built-in thermistor that detects the temperature of the substrate. The resistance value of the thermistor is directly output, and the dsPIC can complete the overheat protection function by detecting its resistance value.
The above protections utilize the built-in functions of the ASIPM, eliminating the need for external circuitry and further simplifying hardware design. In addition to the aforementioned protections, the system also features protections unique to photovoltaic water pump systems, including low-frequency, low-sunlight, and automatic/manual drying protection. For pump loads, when the speed falls below the lower limit, most of the energy provided by the photovoltaic array is converted into losses. Prolonged low-speed operation will cause overheating and affect the pump's lifespan. Therefore, this system incorporates low-frequency protection. For the water pump, when the liquid level is below the pump inlet, the pump is in an unloaded state. Without intervention, prolonged operation will damage the lubricated bearings. Since this system operates outdoors unattended, it employs both automatic and manual drying identification methods to increase detection reliability. Automatic drying is determined based on the system's output power and motor operating frequency; manual drying is achieved by using a water level sensor to identify the current water level. Because the low-frequency, low-sunlight, and drying functions are all implemented in software without requiring additional hardware circuitry, the system structure is simple.
4 Conclusion
The push-pull forward converter circuit used in the DC/DC stage of this system outperforms traditional topologies in terms of performance and economy, making it highly suitable for photovoltaic water pump systems. The DC/AC stage utilizes the latest ASIPM module, significantly simplifying the circuitry and improving system reliability. The TMPPT (Maximum Power Point Tracking) control strategy enhances system efficiency and simplifies the system structure. Furthermore, the system employs a digital signal controller (DSC) dsPIC30F2010, which plays a crucial role in improving system operating speed and overall performance. In conclusion, the photovoltaic water pump controller based on the above structure exhibits significant advantages and market competitiveness in terms of structure, function, cost, and reliability.
