Research "Exchange of haberdashery" technology of parallel connection of high-power DC and DC power supplies of IGBT devices.
With development of power electronics technology, many occasions require DC power supplies with large power and large current. The fast control DC power supply used in EAST magnetic holding nuclear fusion device is a powerful DC power supply. Its technical requirements are: voltage response time 1ms, peak voltage 50V, maximum current 20kA, which can realize 4 work quadrant. In response to this requirement, it is inevitable to use power supply parallel connection technology, that is, parallel connection of power tubes or parallel connection of power supply devices. For 20kA DC power supply, if IGBT power tubes are used in parallel, at least 15 power tubes should be connected in parallel for each leg of bridge. The load capacity is also relatively large. When actual voltage is only 50V, it is a big waste of capacitance power tube voltage. Therefore, technical idea of parallel connection of DC/DC power devices according to domino scheme is proposed.
Basic requirements for a parallel power system:
1) Maintain a stable output voltage when mains or load is disturbed;
2) The modulation frequency of each module is same. If this is not matched, a low frequency ripple signal will be generated to increase ripple component of output current and voltage;
3) Control current of each module to evenly distribute load current.
1 High Power DC Power Supply Topology
There are two topologies for paralleling DC/DC power supplies. One is to use DC input bus structure. The block diagram of system is shown in Figure 1a. Device, pump voltage suppression circuit, etc.; other is to use independent AC-DC/DC power supplies in parallel, and system structure diagram is shown in Figure 1b.
Using topology shown in Figure 1a, system requires a high capacity, unregulated DC power supply, which can usually be obtained by stepping down a rectifier transformer, rectifying with a diode, and filtering with a capacitor. Although this structure can ensure that each parallel branch has a common DC voltage input and avoid imbalance caused by different input voltages on DC side, DC power supply has large capacity and 20kA current, load is too heavy. , and front stage AC-DC equipment requirements are relatively high, which is not easy to implement. In addition, a common input bus does not contribute to implementation of parallel connection of independent power modules in full sense. Therefore, adopt AC-DC/DC power supply parallel connection topology structure as shown in Fig. 1b.
The topology shown in Figure 1b can ensure independence of each AC/DC/DC power module, which can realize parallel connection of DC power devices, and can freely increase or decrease modules according to actual voltage, current. and power requirements Number! There is a lot of space in practical application, and it has a certain research value. But this topology also has disadvantages: if output voltage of transformer is slightly different, then output voltage of each rectifier module will be different, which will lead to a serious imbalance in output current of each rectifier module.
However, this imbalance can be corrected by taking following appropriate measures: First, when using independent AC/DC/DC power supplies in parallel, try to make AC/DC/DC output DC voltage of each module nearly equal; Secondly, due to unbalanced output current of each rectifier module, caused by different output voltage of transformers, current sharing measures can be set in DC/DC link. The DC/DC module uses limited unipolar pulse width modulation (PDW), and load of each circuit strives to be balanced by adjusting duty cycle of each DC/DC module. When a given power module current is switched between positive and negative, it can realize multi-quadrant operation and meet requirement of 4-quadrant system operation.
2 High power DC power supply control circuit
When using a tokamak fast control power supply, output current of power supply is required to follow set current curve in real time. Therefore, power supply system is a current follower system, and speed of system will be more important to showproductivity teller. The choice of control method will affect static and dynamic performance of entire system.
To improve accuracy of stationary and dynamic performance of system and achieve current control, two-stage current control is used (Fig. 2), that is, mutual interaction of total current. loop and current loop of module can not only improve performance performance, but also realize current balance of each module.
The main function of outer loop is to monitor current in real time and apply a combined feedback and feed-forward control method. Proactive control in composite control does not affect stability of source system. But it can greatly improve steady-state accuracy and dynamic performance of system without increasing open-loop gain. To achieve effect of control. Without complicating structure of direct communication channel. Feedforward control takes first derivative of input signal and adds it to signal input. The main functions of current loop of inner loop module are as follows.
1) Transform transfer function of control object.
2) Limit maximum output current and at same time perform current sharing of each power module.
3 Data transfer topology
EAST vertically biased plasma, fast power management, current sharing is an important issue when devices are connected in parallel. The CPU data transmission between monitoring computer and power module is master-slave mode (Fig. 3), that is, CPU load of each power module implements its own current control and sends power module status information to monitoring computer The function of monitoring computer is to realize a unified control of each power module, including sending start and stop commands to each power module. Send current set signal, collect total DC output current, total voltage, AC input voltage, AC voltage and current of each power module, DC output current, temperature, blown fuse, access control and other physical quantities. At same time, it communicates with upper-level EAST main control computer and each power module of system to complete automatic reporting and download various data. Tasks such as automatic removal and introduction of modules. The monitoring computer transmits same set current to each power module! When adjusting and controlling current loop of power module through single-chip microcomputer software programming, output same load current! Better current separation effect is obtained.
For a high power power system such as a tokamak fast control power supply that has high requirements for quadrant operation and current tracking, several independent low to medium power power modules can be connected in parallel to provide total power of power supply. demand. A key problem facing multiple power supplies in parallel is distribution of current between component modules. Using power module intelligence and automatic control system theory, each component power supply module becomes a closed system with current tracking capability! The current distribution between modules is realized by control law, not by hardware. . A system formed in this way will also be able to meet fast current tracking requirements for fast controlled power supplies. So this design can be implemented. The main thing is that following conditions are met:
1) The embedded application of a single-chip microcomputer in power module and parallel system realizes intelligence of device and greatly improves modulation frequency consistency of module. It is advantageous to reduce output voltage and low-frequency current ripple, and overcome disadvantage that it is difficult to match modulation frequency of each module by conventional method.
2) DC/DC line with PWM technology has fast response capability;
3) Current tracking technology based on control theory, can achieve current sharing between modules with different hardware current sharing ideas, and transmit same current reference to each module CPU by controlling monitoring computer. Implement static current sharing of power module.
In cases where high output power is required. This system has good prospects for application. This is in line with development trend of digital power system management.
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