Design of three-phase UPS for power system

1 Introduction

In the power system, in order to ensure that important equipment with high power supply reliability requirements can work normally, power plants and substations should be installed with UPS. With the development of power electronics technology, the capacity requirements are getting higher and higher. Large-capacity UPS are all three-phase [1], so the demand for three-phase UPS is gradually increasing.

Power plants and substations all have DC operating power systems. In order to make full use of the energy stored in the batteries in the DC operating system, the UPS used in the power system should be able to output the required AC power under the condition of DC 220V/110V input. This requires the UPS used in the power system to have a wide voltage input range, generally 100V~300V.

In the power system, it is usually required that the AC and DC power supply systems can ensure good isolation, so the UPS system is required to have an output isolation transformer. Because the 220V input voltage of the DC bus cannot get a 220V AC output after passing through the SPWM inverter, an output isolation power frequency transformer is used to step up the voltage before output.

In addition, there are many noise sources that send electromagnetic interference outward from power plants and substations in the power system. Since motors, relays, transmission and distribution lines, etc. may generate electromagnetic interference, the UPS used in the power system is required to have strong anti-interference capabilities.

2 System Structure

Unlike general UPS, the UPS used in power systems no longer contains batteries, and the input voltage is 220V/110V from the DC bus. The system structure is shown in Figure 1. The input is connected to the DC bus and the three-phase four-wire 380V AC power, and the output is a three-phase 380V sinusoidal AC with stable voltage and frequency. The main circuit of the three-phase UPS used in power systems includes: rectifier, starting circuit, three-phase inverter, three-phase isolation transformer, static switch and filter, etc.

The rectifier converts the three-phase AC mains into DC voltage, and the starting circuit limits the current during the DC power-on process to prevent the large current impact during the starting process from damaging the device; the three-phase inverter converts the DC input into a three-phase sinusoidal AC output with a stable voltage and frequency; the output of the three-phase inverter is boosted by a three-phase isolation transformer, and smoothed and filtered by an LC low-pass filter to filter out high-frequency harmonics, and then connected to the output through a static switch. The three-phase isolation transformer has two functions: one is boosting, which converts the phase voltage output by the three-phase inverter from 110V to 220V. The second is isolation, which isolates the inverter from the load, which can buffer the load changes and also eliminate the DC current component on the load; the static switch is used to realize the switching between the mains bypass output and the inverter output; the EMI filter at the input and output ends is used to suppress electromagnetic interference signals and has a bidirectional isolation effect.

2.1 Three-phase SPWM inverter

Figure 1 Main circuit structure diagram

Figure 2 Classic three-phase inverter main circuit

Figure 3 Three-phase inverter adapted to three-phase unbalanced load

Figure 4 Connection of static switch

The three-phase PWM inverter is to convert the DC input into a three-phase sinusoidal AC output. In many practical devices, the classic three-phase inverter structure is usually used, as shown in Figure 2. This is a three-phase bridge inverter composed of three basic bridge arms. Its control method generally adopts a bipolar method. Phases A, B, and C usually share a triangular carrier. The three-phase modulation signals are 120° apart in sequence, and the control rules of the power switches of each phase are the same.

For three-phase output inverters, the balance of the three-phase load is an important issue that must be considered. Currently, large and medium-sized UPSs with good performance are required to have the function of 100% three-phase unbalance [2]. In order to meet the needs of three-phase load imbalance, the three-phase inverter is best to use three single-phase bridges to transform separately, and then combine the outputs with a phase difference of 120°. As shown in Figure 3, each phase has a single-phase bridge inverter (inverters 1 to 3 in Figure 3), and each single-phase bridge inverter requires four switching tubes. Therefore, a total of 12 switching devices are required, and the structure is relatively complex. The benefits are: the three single-phase inverters work independently, and their outputs do not affect each other. In fact, the three single-phase inverters are connected in parallel, but the phases are different. This allows the three-phase load to be 100% unbalanced, and severe three-phase imbalance will not affect the input of any inverter.

The circuit shown in Figure 3 is selected in this design. Each of the three phases has an independent SPWM waveform generating circuit. The three phases share the same clock reference signal AC50Hz from the mains. The sinusoidal wave data stored in the EPROM of each phase waveform generating circuit are 120° out of phase with each other, thereby ensuring that a three-phase synchronous sinusoidal wave control signal is obtained.

The inverter structures of each phase in Figure 3 are the same, which is a full-bridge inverter circuit. High-frequency SPWM technology is used, and the power switch tube is IGBT, and the switching frequency is 30kHz. By comparing the sine wave control signal with the triangle wave, an SPWM wave with a fundamental wave of 50Hz is obtained, and then the high frequency is filtered out by a filter, and a low-distortion 50Hz sine wave can be output.

2.2 Static switch switching circuit

The function of the static switch is: when the mains power is normal, the mains power is directly sent to the output end to supply power to the load; when the grid voltage is abnormal, the inverter is used to supply power to the load. The static switch is the key to ensuring uninterrupted power supply. It is required to work reliably and not cause a short circuit between the inverter and the mains power during the switching process. At present, AC solid-state relays (SSRs) are commonly used as static switches for switching. It has the advantages of fast switching speed, easy control, and direct interface with the computer. The power system designed in this paper uses a three-phase UPS, and solid-state relays are selected to achieve switching through logic control, which can meet the requirements of fast and reliable switching of UPS.

The mains bypass is connected to the load through a solid-state relay, and the inverter output is connected to the load through the parallel connection of the solid-state relay and the AC contactor. The mains neutral line, the inverter neutral line and the load neutral line are directly connected together.

As shown in Figure 4, the control logic is as follows:

(1) When the AC power is switched to the inverter, the solid-state relay of the AC bypass is first turned off. After the current of the solid-state relay passes through zero and is reliably turned off, the solid-state relay of the inverter branch is turned on, and at the same time, an instruction to close the AC contactor is issued. Because the solid-state relay has a faster action speed than the AC contactor, after the solid-state relay is turned on for a period of time, the AC contactor is closed, short-circuiting the solid-state relay. This can improve the working reliability of the switching device and reduce losses.

(2) The inverter switches to AC power

① If the inverter fails, first shut down the inverter, then turn off the solid-state relay and AC contactor of the inverter branch, and turn on the solid-state relay of the AC bypass. After the fault is eliminated, the inverter will be automatically turned on depending on the specific situation.

Figure 5 AC filter circuit

Figure 6 Inverter control circuit block diagram

② If the inverter is normal, first disconnect the AC contactor of the inverter branch. After the AC contactor is disconnected reliably, turn off the solid-state relay of the inverter branch. After the current of the solid-state relay passes through zero and is reliably shut down, turn on the solid-state relay of the AC bypass.

2.3 AC filter circuit[3][4]

The output of the high-frequency SPWM inverter is a series of rectangular wave pulses of equal amplitude and unequal width, and the pulse width varies according to the sine law. The switching frequency of the switching device is equal to the frequency of the triangular wave carrier. In order to filter out the high-frequency component and obtain a low-distortion sine wave output, the low-pass filter circuit shown in Figure 5 can be used. The main filtering function in the circuit is the inductance L1, L2 connected to the primary side of the transformer and the capacitor C on the secondary side of the transformer. In the commonly used AC filter circuit, there is no resistor R in series with the capacitor C. In this case, the parameters of the inductance and capacitance are determined by the resonant frequency ωn of the filter, ωn=

The transformer ratio n=1/2, usually ωn≤0.1×2π×f1 (f1 is the switching frequency of the inverter), so the LC product can be calculated. The selection of L and C values ​​must not only meet the filtering requirements, but also take into account the frequency response of the entire closed-loop system and the closed-loop zero-pole configuration of the system. It has been observed in the experiment that the filtering effect can be better when the capacitor C is connected in series with a suitable resistor R.

2.4EMI Filter

Interference is a common problem in the design of various power supplies, and UPS is no exception. With the development of power electronics and automation technology, UPS has gradually become a device that integrates power and electronics, strong and weak current systems, so the problem of electromagnetic interference has become more and more complicated. Harmonics in the power grid may interfere with the operation or switching process of the inverter, and the radio frequency interference generated by the high-frequency PWM converter will also interfere with electrical equipment and the power grid.

The main function of EMI filter is to reduce the electromagnetic interference conducted along the power line of AC power grid and suppress the influence of power grid ripple on UPS. Most filters adopt LC filter circuit. In order to make the effect of filtering electromagnetic interference better, two-stage LC filter can be used. In practice, three-phase EMI filter module is often used. At present, EMI filter generally adopts modular structure, which is easy to install and replace, and has become a replaceable standardized electronic component.

The output filter attenuates the electromagnetic interference generated inside the UPS to prevent possible damage to other devices at the output end. Three-phase AC EMI filters are usually used as filters at the output end. In addition, in order to reduce the electromagnetic interference between the DC power supply and the inverter, a DC EMI filter is generally connected to the DC power supply input end.

3. Control circuit

The core of the three-phase UPS used in the power system is the three-phase inverter. Its control circuit mainly controls the working state of the three-phase inverter, and mainly includes two parts: analog control circuit and digital monitoring circuit. The function of the analog control circuit is to generate SPWM waves, introduce feedback regulation to the voltage and current to ensure the output voltage is stable and the waveform distortion is small, reduce the DC component in the output voltage, and achieve phase-locked synchronization. The digital monitoring circuit detects the state of the entire system, collects data, handles various faults, and controls the switching of static switches.

3.1 Generation of sine wave control signal and triangle wave carrier

The function of the SPWM inverter is to convert direct current into a rectangular wave of equal amplitude and unequal width, and its fundamental wave is a 50Hz sine wave. The PWM control signal is usually generated by comparing the reference sine wave with the triangle wave. In this design, in order to generate a sine wave control signal with stable frequency, low distortion and adjustable amplitude, the sine wave data is stored in the EPROM, and the sine wave control signal can be obtained through D/A conversion. The triangle wave generator uses a single-chip function generator integrated chip ICL8038, which can generate a triangle wave with stable frequency and low output waveform distortion as long as a small number of external components are connected.

3.2 Control of three-phase SPWM inverter

As shown in Figure 6, the output voltage and current are sampled through the voltage and current Hall elements to form a feedback signal. The feedback voltage is subtracted from the given voltage to form a deviation voltage, which is sent to the VREF terminal of the D/A converter 0832 connected to the output end of the sine wave generator. This is equivalent to multiplying the deviation voltage with the sine wave control signal. The multiplication result is subtracted from the instantaneous voltage feedback and the anti-bias magnetic feedback signal and then passed through the PI regulator and compared with the triangle wave to obtain a PWM wave. This PWM wave is then passed through the dead zone forming circuit to generate two complementary PWM control signals with a certain dead zone.

Figure 7 Block diagram of digital monitoring circuit

In order to prevent circulating current or load power failure during the switching process, synchronous phase-locking technology is used to achieve the same frequency and phase of the UPS and grid voltages. In this design, a monolithic integrated digital phase-locked loop chip CD4046 is used to achieve synchronous phase-locking.

3.3 Digital monitoring circuit [4]

The monitoring circuit is a digital system with 89C52 single-chip microcomputer as the core. It is mainly composed of single-chip microcomputer, keyboard, input and output interface, watchdog circuit, communication interface, etc. The circuit block diagram is shown in Figure 7.

Three buttons are set: start/reset button (RESET), stop button (STOP) and mode button (MOD). The status of each button is detected and sent to the microcontroller, and the control of the inverter and system is determined according to actual requirements. The input interface circuit also monitors the working status and faults of the entire system and inputs them into the microcontroller, which outputs the corresponding control and alarm signals.

The watchdog circuit uses MAX813 to reset the microcontroller on power-on and reset the timer to prevent the software from running out of control. Use an I/O port (P1.0) of the microcontroller to clear the timer. When the program jumps or enters an infinite loop during execution, the program cannot clear it before the timer overflows, which will cause the MAX813 timer to overflow and generate a reset signal to restart the microcontroller.

The communication interface uses the MAX1483 chip to convert the serial port signal from TTL level to RS485 level, forming a half-duplex RS485 communication interface.

During the switching process, the Hall element in the main circuit detects the zero-crossing point of the output current in real time, and after sending a shutdown signal to one group of solid-state relays, the other group of solid-state relays can be sent an opening signal only when the output current crosses zero. This can reduce the switching time and ensure reliable switching.

4 Conclusion

This paper analyzes the requirements of the power system for three-phase UPS and proposes a design scheme for three-phase UPS for power system. The system adopts high-frequency SPWM technology, IGBT as the switching device, and power frequency transformer isolation in the main circuit. The three-phase inverter is composed of three independent single-phase inverters, and the voltage instantaneous value and voltage effective value are double closed-loop controlled. Hall elements are used to detect the voltage and current of the main circuit. The single-chip microcomputer monitors the working status and faults of the system, and the solid-state relay realizes the switching. The system has good output waveform quality, can adapt to 100% unbalanced load, has high working reliability, and strong anti-interference ability, which can fully meet the needs of power plants, substations and other power departments.