With the development of power electronics technology, many occasions require high-power and high-current DC power supplies. The DC fast-regulating power supply used in EAST's magnetically constrained nuclear fusion device is a high-power DC power supply. Its technical requirements are: voltage response time 1ms peak voltage 50V; maximum current 20kA, can achieve 4 quadrant operation. In response to this requirement, power parallel technology is inevitably required, that is, parallel connection of power pipes or power supply devices. For the 20kA DC power supply, if the power tube IGBTs are connected in parallel, each bridge arm needs at least 15 power tubes in parallel, which not only brings great difficulty to the drive, but also, in general, the power tube with larger current capacity. The voltage capacity is also large, and in the case where the actual voltage is only 50V, this is a great waste for the voltage capacity of the power tube. Therefore, a parallel technical idea of ​​a DC/DC power supply device using a domino structure is proposed.
The basic requirements for a power parallel system are:
1) Keep the output voltage stable under grid disturbance or load disturbance;
2) The modulation frequency of each module is the same. If they are inconsistent, a low frequency ripple signal is generated to increase the ripple component of the output current and voltage;
3) Control the current of each module to share the load current.
1 Topology of high-power DC power supply
DC/DC power supply has two topologies in parallel. One is to use the input DC bus structure. The system block diagram is shown in Figure 1a. It mainly includes rectifier transformer and uncontrollable diode rectifier circuit, N DC/DC converter, pump. The voltage boosting circuit is the same; the other is to use a separate AC-DC/DC power supply in parallel, and the system structure is shown in Figure 1b.
Using the topology shown in Figure 1a, the system requires a large-capacity non-adjustable DC power supply, which can be generally stepped down by a rectifier transformer, diode rectified and obtained by capacitive filtering. Although this structure can ensure that each branch in parallel has a common DC voltage input, avoiding the imbalance caused by the parallel input voltage of the parallel branch, but the DC power supply has a large capacity, the current reaches 20KA, and the DC bus The load on the load is too heavy, and the pre-stage AC-DC equipment is required to be high and difficult to implement. In addition, sharing the busbar at the input end is not conducive to achieving parallel connection of independent power modules in a complete sense. Therefore, a topology in which AC-DC/DC DC power supplies as shown in Fig. 1b are connected in parallel is employed.
The topology shown in Figure 1b guarantees the independence of each AC-DC/DC power supply module, enabling parallel connection of DC power supply units, which can freely increase or decrease the number of modules according to actual voltage, current and power requirements. ! There is a lot of space in practical applications and it has certain research value. But this topology also has its downside! That is, if the output voltage of the transformer is slightly different, the output voltage of each rectifier module will be different, resulting in a serious imbalance in the output current of each rectifier module.
However, this imbalance can be suppressed by the following measures: First, when using independent AC-DC/DC power supplies in parallel, the AC-DC/DC output DC voltage of each module should be as close as possible; secondly, For the imbalance of the output current of each rectifier module caused by the different output voltage of the transformer, the current sharing measure can be set in the DC/DC link. The DC/DC module uses a limited unipolar pulse width modulation (PDW) method, and the load of each loop is balanced by adjusting the duty ratio of each DC/DC module. When the power module is given positive and negative switching, it can realize different quadrant operation and meet the requirements of the system 4 quadrant operation.
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