Development of single-phase inverter power supply for power system

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Development of single-phase inverter power supply for power system [Copy link]

With the continuous development of the domestic power industry, more and more AC loads in power plants and substations require uninterruptible power supply in the event of a fault, and the requirements for the quality of AC power supply are getting higher and higher. Traditional square wave inverters can no longer meet the application requirements, and UPS is too expensive, so it is difficult to use all of them. Therefore, it is of great practical value to develop a special inverter power supply that meets the requirements of power system use.

The development of this power supply is based on the use of power systems, referring to the technical conditions of the national standard "7260-87 Uninterruptible Power Supply Equipment", and is designed according to UPS requirements in terms of output performance; in terms of system completeness, it is considered to be compatible with the power DC system and meet the needs of central signal monitoring. It adopts new technologies such as IGBT high-frequency inverter, digital frequency division, phase locking, and waveform instantaneous feedback. The test and application results show that it fully meets the design and use requirements.

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The principle block diagram of the inverter power supply is shown in Figure 1. The power supply has two inputs, DC and AC. The DC input is the DC bus of the power system, and the AC input is the industrial frequency AC mains. When the AC mains is in a normal state, it is rectified and filtered by the rectifier to become DC, and then converted by the inverter to a frequency-stabilized and voltage-stabilized AC to supply power to the load. When the mains or the rectifier fails, the DC is directly converted by the inverter to a frequency-stabilized and voltage-stabilized AC to supply power to the load. The input DC power is isolated from the DC power after rectification of the AC mains by a diode. This solution not only stabilizes and purifies the AC mains, but also, when the AC power is cut off, there is no delay in the AC-DC power conversion, thereby greatly improving the power supply quality.

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This power supply adopts the AC-DC-AC static inverter scheme, and its main circuit includes rectifier, DC filter, inverter, AC filter and transformer. Among them, the AC-DC part adopts bridge rectification, and then passes through electrolytic capacitor filtering to obtain stable DC. When the power supply is turned on, the rectifier charges the electrolytic capacitor through the starting resistor, which can reduce the starting current and realize soft starting. After starting, the resistor is short-circuited by the contactor. The DC input is connected before the starting circuit, and soft starting can also be realized. In order to isolate the DC system from other electrical circuits, the AC input of this power supply is isolated by a transformer. The input transformer ratio should be selected to ensure that when the AC mains is undervoltage, the rectifier output DC voltage is still higher than the DC system voltage, so as to ensure that when the AC is normal, the DC system does not discharge to the load.

The DC-AC inverter part adopts a single-phase full-bridge structure and is the core of this power supply. The inverter uses IGBT as the switching element. Taking advantage of the high switching frequency of IGBT, the bipolar sinusoidal pulse width modulation (SPWM) is used to control the inverter, converting the smooth DC into pulse width modulated output AC, and the AC fundamental frequency is the required power output frequency. The pulse width modulated wave output by the inverter is filtered by the output LC filter circuit, and after the transformer is transformed and isolated, the required sinusoidal AC is output.

In order to improve the electromagnetic compatibility performance, anti-interference filters are connected to the input and output ends of the power supply.

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2.3 IGBT drive and protection circuit The drive and protection circuit of the IGBT of this power inverter bridge is made into a circuit board, which together with the inverter bridge constitutes an inverter unit module. The drive circuit is composed of the integrated drive module M57959 of Mitsubishi Corporation of Japan as the core. M57959 is a dedicated drive circuit for IGBT modules, which can drive components of up to 400A/600V. The circuit has fast optical coupling isolation inside, which is suitable for high-frequency switching operation of about 20kHz, and has overcurrent protection function. The drive circuit is powered by +15V/-10V dual power supply to improve anti-interference ability. The front stage of the drive circuit is a PWM signal processing circuit, which transforms the single-channel PWM signal transmitted by the control circuit into two signals with a difference of 180° after being shaped and inverted by the voltage comparator, as the control signal of the upper and lower bridge arm IGBT components. The signal passes through the dead zone circuit, and its rising edge is delayed by 3~4μs to ensure that the upper and lower bridge arms have a dead zone of not less than 3μs before being sent to the drive circuit. This power driver board is equipped with three protections: IGBT overcurrent, power device overheating, and DC bus undervoltage. IGBT overcurrent protection is completed by the internal protection circuit of M57959 by detecting the on-saturation voltage drop of IGBT. The overcurrent protection threshold is adjusted by connecting a voltage regulator in series in the detection circuit. The four protection signals of the four IGBT components of the single-phase bridge circuit are converted into a high-level fault signal through the NAND gate and sent to the fault logic circuit. The overheat protection of the power device is completed by installing a temperature relay on the radiator, giving a disconnect contact when overheating, and the action value of the temperature relay is 75℃. When the voltage of the DC bus undervoltage protection circuit is normal, the voltage regulator tube of the detection circuit is broken down and turned on, so that the optocoupler connected in series with it is turned on, and the secondary side outputs a low level; when the voltage is too low, the voltage regulator tube of the detection circuit is blocked, so that the optocoupler connected in series is turned off, and the secondary side outputs a high level of fault; the protection threshold is determined by the voltage regulator value of the voltage regulator tube. The fault logic circuit on the driver board first latches the overcurrent and undervoltage signals through the D flip-flop, and then integrates the latched signals with the overheating signal through the gate circuit to block the PWM pulse sent to the driver module to complete the protection. At the same time, the integrated fault signal and the overcurrent, undervoltage and overheating signals are sent to the control circuit to complete the power system monitoring.

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The circuit is composed of a 16-bit single-chip microcomputer 80C196 and a waveform control module as the core, as shown in Figure 4. The CPU completes analog detection, output voltage effective value control, fault detection and diagnosis, serial communication with the LCD panel, reading setting parameters, exchanging operating parameters and fault information, etc. The waveform control circuit completes frequency control, output voltage waveform control and other functions. The main control link principle is as follows: (1) Standard waveform generation The standard sine wave generation of this power supply adopts the digital addressing table lookup method. The standard sine data is stored in the EPROM. The EPROM is selected according to the output frequency sequence, and then the sine digital quantity output by the EPROM is converted into an analog quantity through the D/A converter. The analog quantity is positive polarity, and after being symmetrically shifted down by the operational amplifier circuit and the capacitor is isolated from the DC, the standard sine wave signal is output. The storage capacity of one cycle of standard sine data is 1k bytes. (2) Voltage control The voltage control adopts a closed-loop regulation method and is completed by the control software of 80C196. The AC output voltage signal sent by the detection circuit is sent to the A/D input port of 80C196 after amplitude adjustment and absolute value conversion, and then converted into a digital quantity through A/D conversion, and then becomes an effective value digital feedback quantity through effective value calculation. The digital given and digital feedback sent by the LCD panel are compared, and the deviation is PI adjusted. The digital signal output by the regulator is converted into an analog quantity through D/A conversion and sent to the standard sine generation circuit as the reference level of the waveform D/A converter, thereby changing the amplitude of the standard sine wave, so that the effective value of the output voltage is maintained constant, and output voltage regulation is achieved. 3) Waveform control Waveform control adopts a dual-loop control scheme with a current inner loop for phase voltage output. In the voltage waveform control system composed of two control loops, the current loop is the inner loop. The controlled object of this loop is the current IC of the filter capacitor. The purpose of control is to enable the current on the filter capacitor to quickly and accurately track the current command. The current loop can transform the controlled object surrounded by it, overcome the influence of system disturbances such as DC voltage fluctuation △Ud and load current IL on the output voltage, thereby improving the rapidity of the control system and improving the quality of the output voltage waveform; the voltage waveform control loop is located outside the current loop. This loop controls the instantaneous value of the output voltage so that the output voltage tracks the input standard sine wave. The waveform control adopts instantaneous value feedback. The output voltage and filter capacitor current are detected and shaped by the detection circuit and directly sent to the waveform loop. Compared with the standard sine wave, PWM control pulses are generated after double loop adjustment. The filter shape control circuit has been made into a thick film module.

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This power keyboard display circuit is composed of a single-chip microcomputer 89C51 with built-in memory and a real-time clock chip DS12887 as the core, which completes the display setting of operating parameters, display query of fault information, serial communication and information exchange with the main control circuit CPU-80C196, power start/stop control, clock management and other functions. The operation display interface uses a 20×2 character LCD screen and 6-bit touch keys, and is equipped with "power", "operation" and "fault" indicator lights. Considering the requirements of power system signal monitoring, the interface circuit displays all operating fault states through light-emitting diodes and converts them into contact signal outputs. The contact signal is normally open and divided into two groups. There are common terminals in the same group, and the groups are electrically independent of each other.

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The detection circuit of this system includes three parts: output voltage detection, output current detection, and filter capacitor current detection. In order to improve the control speed of the waveform ring and ensure the quality of power supply, the detection element related to the waveform ring adopts a magnetic balance HL sensor. All detection signals are electrically isolated from the main control circuit. The control and drive power supply of this system adopts a switching power supply. The switching power supply input is hung on the inverter DC bus, and there are 6 groups of outputs, of which 3 groups of +25V are electrically independent of each other and supply the drive circuit, and the other 3 groups of ±15V, +5V are grounded and supply the control circuit and protection circuit.

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Technical Performance and Application Results The inverter designed according to the above principles has been fully tested by the manufacturer and the power department user, and has been put into operation in the substation. Its main technical performance is as follows: (1) AC input AC220V±15%, 50Hz±5% (2) DC input normal operating voltage range DC187~264V extreme operating voltage range DC170~276V (3) System output rated frequency: 50Hz Rated voltage: 220V Voltage stability accuracy: ±2% Frequency stability accuracy: ±0.1% Frequency change rate: 0.1Hz/s Total harmonic content: <5% (resistive load test) Dynamic voltage: Overshoot <8% (when the load changes from 0 to 100%) Stabilization time <0.1s (4) Overload capacity: 120% for 10 minutes 150% for 10 seconds (5) Efficiency: Total efficiency>80%, (under full load and COS?=0.8) (6) Protection: input over-voltage and under-voltage alarm, output over-current, internal over-temperature (7) Noise ≤60dB The output voltage waveform of the power supply with resistive load and inductive load is shown in Figure 6. The power supply was installed at the substation site. It has been in operation for more than a year and has been running stably and reliably with good performance, fully meeting the design and use requirements.