[Date:2006-11-13] Source:Power Supply Technology Application Author:Wang Linbing He Xiangning, Zhejiang University [Font:Large Medium Small]
Abstract:The three-phase uninterruptible power supply (UPS) system has made a general comment on the topology of the circuit of each module, the structure of the whole machine circuit as well as the popular control strategies, and it points out that there are problems and new hotspots in the design and application of the UPS and current research. It points out the problems in the design and application of uninterruptible power supply (UPS) and the new hotspots in the current research, and finally predicts the development trend of UPS
Keywords: three-phase uninterruptible power supply; parallel connection of inverters; digital control
O INTRODUCTION
In the future, for quite some time, China's utility grid power supply is insufficient, the voltage fluctuation is large, and the interference is serious, the situation will still exist. And the rapid development of various industries and fields of the quality of power supply put forward more and more high requirements, especially the real-time is very important system, important departments and important power equipment on the quality of power supply requirements and the actual situation of China's power grid contradiction is increasingly acute. Therefore, the uninterruptible power supply (UPS) as a kind of stabilized voltage and frequency purification of the green power supply has become more and more people's focus of attention. In order to continuously improve the performance of the UPS, researchers have done a lot of research on the UPS system, proposed a lot of circuit topology and control strategies.
1 UPS circuit topology
The reliable operation of the UPS can not be separated from the coordination of the work of the modules, the following is a brief analysis of the circuit topology of the main functional modules of the UPS.
1.1 rectifier and power factor correction circuit
The rectifier circuit in the application of the DC power supply device, is the public **** grid and power electronics interface circuit, its performance will affect the operation of the public **** grid and the quality of electricity. High-performance UPS requires a high input power factor and minimizes the harmonic component of the input current. Traditional single-phase UPS mostly use analog methods, three-phase UPS mostly use phase-controlled rectifier circuit and voltage-based single-tube rectifier circuit.
1.1.1 Traditional three-phase phase-controlled rectifier circuits and voltage-type single-tube rectifier circuits
Phase-controlled rectifier circuits use half-controlled power devices as switches, and there are the following problems:
1) The existence of harmonic currents on the grid side will reduce the equipment grid-side power factor and increase the reactive power;
2) Phase-controlled rectifier converter mode, resulting in the converter period in the Grid voltage distortion, not only the performance of their own circuits are affected, but also interference with the grid, the same connection between the network of other equipment to bring about adverse effects;
3) phase-controlled rectifier link is a time-lag link, can not realize the output voltage of the rapid adjustment.
Voltage-type single-tube rectifier circuit is a three-phase uncontrolled rectifier bridge plus Boost circuit, which has the following disadvantages: large current peaks, which not only prevents the improvement of the system power, but also increases the conduction loss and switching loss; in order to maintain the improvement of the grid-side power factor, the Boost circuit must have a certain step-up ratio, which will lead to a three-phase circuit with a too high DC output voltage.
1.1.2 Current-type three-phase bridge rectifier circuit
Current-type three-phase bridge rectifier circuit shown in Figure 1, the advantage is that the feedback control is simple, do not need to join the current feedback in the control circuit, only to adjust the duty cycle of the switching tubes to achieve the input current sinusoidalization; DC side of the voltage is lower. The disadvantage is that the input current sinusoidal degree is not very good, in the input side must be added to the parallel capacitor, to realize the phase shift. This circuit is now beginning to become one of the hot spots of research. This circuit is suitable for high-power rectifier circuit and the power factor requirements are not high.
1.1.3 Voltage-type three-phase bridge rectifier circuit
Voltage-type three-phase bridge rectifier circuit shown in Figure 2, which is characterized by the use of high-frequency PWM rectification technology, the device is in a high-frequency switching state, because the device can be controlled by the opening and closing state, so the rectifier current waveform is controllable. The advantages of this circuit are that the input current in phase with the input voltage can be obtained, that is, the input power factor is 1, and the harmonic content of the input current can be close to zero; the energy can flow in both directions, and the energy flows from the ac side to the dc side during normal times, and the energy flows from the dc side to the ac side when the dc output voltage is higher than a given value, which provides a high conversion efficiency. The disadvantage is that it belongs to Boost type rectifier circuit, and the DC side voltage requirement is high. This circuit is also a hot spot of research in recent years.
1.2 battery pack and charge/discharge circuit
Battery pack is the UPS energy storage unit, utility power when it absorbs energy from the utility and stored in the form of chemical energy, once the utility power is interrupted, it is converted to stored chemical energy into electricity to the inverter power supply, to maintain the continuity of the power supply to the load. In small and medium power UPS systems, the voltage of the battery pack is usually low, therefore, charging and discharging circuits are usually used where the energy can flow in both directions [4]. In high-power systems, in order to improve efficiency and simplify the circuit is usually connected directly to the battery pack in parallel to the DC bus.
1.3 Inverter Circuit
The inverter is the core of the UPS, which converts the DC power into the AC power required by the user to stabilize the voltage and frequency. The following three-phase inverter as the object to analyze the inverter research hotspots in recent years.
1.3.1 Three-phase half-bridge inverter circuit
In the three-phase inverter circuit to the three-phase half-bridge bridge circuit is the most common, this circuit is characterized by the use of fully-controlled devices composed of inverters, the existence of a high power density, good performance, small and lightweight and other advantages. This circuit facilitates the use of new control strategies to improve the quality of the inverter. However, it is difficult to realize with 100% independent load.
1.3.2 H-bridge inverters
For ultra-large-capacity inverters, due to the significant increase in the power level of the inverter structure puts forward new requirements, the H-bridge arm inverter is one of the options. This inverter output transformer using multi-winding connection, the primary side of the output transformer using three independent windings, the inverter output using three independent H-bridge. This is easy to control, but the cost is higher.
1.3.3 Three-phase four-bridge arm conversion technology
Due to the three-phase circuit, the three-bridge arm inverter itself has inherent defects, people began to seek a new circuit structure, so there is a three-phase four-bridge arm inverter, as shown in Figure 3. This circuit structure output for three-phase four-wire system, three-phase voltage can be controlled independently, the control method is flexible, but the algorithm of this topology is more complex, the PWM vector is rotating in three-dimensional space, and digital control methods must be used to realize the generation of spatial PWM waveforms, and this circuit became one of the hot spots of the research in recent years.
1.4 Three-phase UPS circuit
1.4.1 Traditional three-phase UPS circuit structure
Traditional three-phase UPS structure, the input is rectified by thyristor, the output is used inverter, the battery is directly hooked up to the DC bus, the rectifier is used as a charger at the same time. The output is isolated by a transformer, which can realize the complete isolation of input and output, and ensure that the disturbance of the power grid will not cause interference to the load. When the utility power is cut off, the battery outputs stable AC power through the inverter; when the inverter fails, the output voltage is bypassed to ensure the reliability of power supply. The main disadvantage of this structure is that the volume and weight are relatively large.
1.4.2 High-frequency chain three-phase UPS
In order to reduce the cost, reduce the size and weight of the UPS, the emergence of high-frequency chain three-phase UPS, as shown in Figure 4. This circuit eliminates the bulky industrial frequency transformer, the input is rectified with high frequency, you can get a high input power factor and low input harmonic current. The disadvantages are that there is no transformer isolation of the input and output, and disturbances in the grid may cause disturbances to the output of the UPS; the output three-phase voltage relies on the battery and capacitor midpoints to form the midline, so it is necessary to keep the positive and negative dc voltage amplitudes equal in the control, otherwise there will be a large dc component in the output midline, which is not good for the load and the transformer in the load; the input uses a three-phase four-wire system, and there is current flowing in the midline, which may cause a The potential shift in the center line may cause interference with the load; the input and output are not isolated, and it is more difficult to solve the problem of circulating current when connected in parallel.
1.4.3 New on-line interactive UPS
As the above two UPS are subject to two full power conversion, so the system efficiency is low, from the perspective of improving system efficiency, a series-parallel compensated high-capacity structure has emerged, is a new on-line interactive structure, as shown in Figure 5. This topology input and output also have no transformer isolation, so it will have the disadvantage of high-frequency chain UPS. The output frequency of this UPS must be kept in line with the grid, and the suppression of grid disturbances is not strong, thus the quality of the power supply is poorer than that of the traditional three-phase UPS. It is characterized by the energy from the input to the output is not through the full-power conversion, also consists of two high-frequency converter, but the converter 1 maximum only withstand 20% of the power, in terms of cost, the cost of this structure is lower. In the control method, converter 1 is a voltage compensator to compensate for the distortion of the grid voltage, and converter 2 is a current compensator to compensate for the harmonic currents of the loads and to supply power to the loads as a full-power voltage inverter when the utility power is off.
1.4.4 High-frequency Chain UPS with Input-Output Isolation
Since the input and output of the traditional industrial frequency UPS have isolation transformers with good isolation characteristics for the output and good input characteristics for the high-frequency chain, this high-frequency chain with input-output isolation UPS has appeared as shown in Fig. 6. Due to the disadvantages of high-frequency rectification, an autotransformer must be connected to the input side to step down the voltage, which increases the weight and cost of the whole machine; in addition, due to the use of a high-frequency converter for the input, the efficiency of the whole machine is lower than that of both the high-frequency chain type and the conventional type UPS. However, since the input power factor is 1 and there is no harmonic current, the total power consumed is lower than that of a conventional three-phase UPS.
1.4.5 Input-output parallel UPS
In this circuit, the input is made up of a number of rectifiers connected in parallel to supply power to the DC bus, and at the same time, the DC bus provides DC voltages to a number of inverters, and the outputs of the inverters are directly connected to the loads. directly to the outputs of multiple inverters to supply power to the loads at the same time. This approach can enhance the capacity of the UPS, increase the reliability of the system, cost reduction, and enhanced maintainability, but the more modules are connected in parallel, the more difficult to solve the problem of equalizing the current between the modules.
2 uninterruptible power supply control technology
With the rapid development of control theory and feature-rich, high-performance various microcontrollers, a variety of discrete control methods. From the number of control feedback loop can be divided into single-loop, double-loop, multi-loop control. Under the conditions of hardware allows as much as possible to improve the number of feedback loops, can improve the control effect. From the control principle includes digital PID control, state feedback control, no differential beat control, repetitive control, sliding mode variable structure control, fuzzy control, neural network control, space vector control and other methods.
Digital PID control control is adaptable and has strong robustness; the algorithm is simple and clear, easy to realize with a microcontroller or DSP. However, there are two limitations: on the one hand, the sampling and quantization error of the system reduces the control accuracy of the algorithm; on the other hand, the sampling and computation delay makes the controlled system a system with pure time lag, resulting in a reduction of the stability domain of the PID controller, which increases the design difficulty.
Predictive control can achieve very small output current distortion and high noise immunity, however, this algorithm requires knowledge of the exact load model and circuit parameters, so robustness is poor, and the delay due to numerical computation is also a problem in practical applications. Hysteresis loop control has fast response speed and high stability, but the switching frequency of the hysteresis loop control is not fixed, which makes the circuit work less reliable and the spectrum of the output voltage deteriorates, which is unfavorable to the system performance.
The basic idea of beat-less control is to derive the PWM pulse width of the next switching cycle according to the state equation of the inverter and the output feedback signal, so that theoretically the output voltage is very close to the reference voltage in both phase and amplitude, and the output voltage error caused by the load change or nonlinear load can be corrected within one switching cycle. However, differential-free control is a control method based on an accurate mathematical model of the object to be controlled, and the robustness is very poor.
Slip-touch control is a type of nonlinear control that is characterized by control discontinuity. This control can be used for both linear and nonlinear systems. This control method is highly robust. The disadvantage is that it is difficult to obtain a satisfactory sliding mold surface.
Repetitive control is a control method based on the principle of internal mold. The purpose of using repetition control in inverters is to eliminate the periodic distortion of the output voltage waveform caused by the rectifier bridge load. The repetition controller can eliminate the steady state error generated by the periodic disturbances, however, the control characteristics of the repetition control delayed by one industrial frequency cycle makes the dynamic characteristics of the UPS inverter using repetition control alone extremely poor.
Fuzzy control belongs to the category of intelligent control. The design of the fuzzy controller does not require an accurate mathematical model of the controlled object, and therefore has a strong robustness and adaptivity. Fuzzy control is similar to the traditional PD control, thus this control has a fast response speed, but its static characteristics are unsatisfactory. Neuronal network control is a type of control that simulates the intelligent activity of the nerve center system of the human brain. With the advantages of nonlinear mapping ability, parallel computing ability and strong robustness, neural networks have been widely used in the field of control, especially in the field of nonlinear systems. Some results have been achieved in the design of neural network structure and learning algorithms. However, due to the limitations of the hardware system, neural network control is currently unable to achieve online control of the inverter output voltage waveform, and most of the applications use offline learning to obtain the optimized control law, and then use the obtained law to achieve online control.
With harmonic injection PWM technology, the DC bus voltage utilization can basically reach loo%. This method is very effective for the voltage open-loop control system, but in the closed-loop control system due to the harmonic injection of the initial phase must be consistent with the fundamental wave, in the voltage instantaneous value control of the initial phase of the voltage fundamental wave can not be accurately located and difficult to apply.
Space vector PWM has the advantages of small current distortion, high DC bus voltage utilization, and easy digital implementation, so it has been used more often in recent years. This control method also requires an accurate model of the circuit.
All of the above control schemes have their advantages, but also have their shortcomings. At the same time using different control methods to form a composite control control scheme has been widely used in practice, and achieved better results.
3 Uninterruptible power supply design and application of the problems
The U.S. UPS manufacturer APC. company, summed up and summarized the UPS power supply system is currently facing, but also in the future must be resolved in five areas of the problem:
1) the life-cost cycle problem;
2) uninterruptible power supply system adaptability and scalability issues;
3) uninterruptible power supply system, the problem of adaptability, and the expansion;
4) the problem of uninterruptible power supply system, the problem of uninterruptible power supply systems, and the problem of the uninterruptible power supply system. p>
3) the problem of improving the availability of the UPS;
4) the problem of manageability of the UPS to the power supply system;
5) the problem of serviceability.
4 Uninterruptible power supply of the latest development trends
Uninterruptible power supply development trends are the UPS parallel redundancy, redundant parallel technology to improve the capacity and reliability of the UPS; the use of more feature-rich hardware devices to achieve all-digital control, so that a variety of advanced and complex control algorithms can be used to improve the performance of the UPS, that is, the digital and high-frequency development. UPS; UPS further intellectualization and networking, so that the computer network becomes an uninterruptible network.
4.1 UPS parallel technology to achieve redundancy
UPS parallel technology can bring the following benefits:
1) can flexibly expand the capacity of the power supply system;
2) can be formed to improve the reliability of the operation of the parallel redundancy system:
3) a very high degree of system maintenance, when a single When a single power supply fails, it can be easily replaced and repaired by hot-swapping.
The use of parallel technology can form a redundant power supply system with fault tolerance, from the information currently available, there are mainly the following redundancy configuration:
1) centralized parallel control;
2) master-slave parallel control;
3) decentralized parallel control;
4) chain parallel control;
5) Wireless parallel control;
6) wireless parallel control p> 5) Wireless parallel control.
These parallelism, from the point of view of reliability, centralized is the worst, wireless control is the best, and has become a research hotspot in recent years.
4.2 UPS digital, high-frequency
The initial UPS using analog control methods have many limitations. With the continuous improvement of the digital processor computing speed, so that a variety of advanced digital control methods can be realized, so that the design of the UPS has a great deal of flexibility, the design cycle is shortened, the performance is greatly improved. UPS high-frequency, effectively reducing the size and weight of the device, and can eliminate the transformer and the inductor of the audio noise, and at the same time improve the dynamic response of the output voltage. Digital control methods have become a research hotspot in the field of today's AC power supply, an inevitable development trend is a variety of methods of interpenetration, combined with each other to form a composite control scheme. Digital composite control is a development direction of UPS control.
4.3 UPS intelligent, networked
In order to adapt to the development of computer networks, UPS has begun to configure the RS232 interface, RS485 interface, USB interface, SNMP card and MODEM combination, to become part of the computer network, with the following excellent intelligence, network characteristics.
1) Real-time monitoring function it is the UPS analog parameters and indicate the working state of the switching volume of real-time high-speed sampling, to achieve digital monitoring.
2) self-diagnosis, self-protection function UPS will be collected in real time to the analog parameters and operating state data and the system's key hardware equipment data and the normal value of the analysis and comparison, in order to determine whether there is a failure of the UPS hidden danger exists. If there is a fault, according to the corresponding fault information level in the control panel display with a friendly graphical interface, text prompts the way to alarm, or in the field and the control room to indicator lights, alarm alarms, alarms can also be used to automatically dial the phone and other ways to alarm, and make the appropriate protection action.
3) man-machine dialog control mode Large UPS can provide users with a monitor LCD display, graphical and text-based display of workflow and parameter information. Can be provided to allow users to operate the visualization of the menu. And to help and constantly prompt the way to guide the user in accordance with the established way to deal with the fault, effectively prevent misuse.
4) Remote control function in the era of networking, UPS should not only be able to provide protection to the hardware equipment directly powered by it, but also should be the entire network of operating procedures and data and data transmission paths to comprehensively protect the network so that it becomes an uninterrupted network. This means that the UPS should be configured with the appropriate power monitoring software, SNMP (Simple Network Management Protocol) manager, so that it has the ability to manage remotely, the user can perform UPS and network platforms between the remote monitoring and data network communication operations, so that the UPS becomes an important part of the network system. In this way, by the network administrator through the network management software to monitor multiple UPS, and the managed UPS can be in the same LAN can also be in a different LAN, or even through the Internet, incorporated into the network management system to manage the UPS.
Due to the future of the network's extensive and globalization, inevitably bring the complexity of the network, many forms of network systems connected together. As part of the network system, the UPS is required to be able to achieve in a variety of network platforms to monitor and control, and with the Internet, Intranet and e-commerce of the ultra-high-speed development of the user's availability requirements for the network will be increasingly high, so that the UPS from the protection of the network's key equipment to the extension of the protection of the entire network path