Abstract: Parallel operation of power electronic transformers is beneficial to further improving the power supply reliability and capacity of the power system, and has important research value. To achieve stable and reasonable load distribution among parallel-operated power electronic transformers and to ensure good dynamic response characteristics, a control strategy and model for power electronic transformers were established based on the active and reactive power droop characteristic equations. Simulation studies of different typical control operations of two transformers operating in parallel were conducted using a Matlab/Simulink model. The results show that the proposed control strategy can achieve stable and reasonable distribution of active and reactive loads among parallel-operated power electronic transformers while maintaining the rated power supply frequency, and exhibits good dynamic characteristics.
Keywords: power electronic transformer; parallel operation; droop characteristics; Matlab/Simulink
0 Introduction
Power-Electronic Transformer (PET) is a new type of power transformer that has attracted increasing attention from researchers at home and abroad. It is a transformer device that contains a power electronic converter and achieves magnetic coupling through a high-frequency transformer. While performing the transformation, isolation and energy transmission functions of conventional transformers, PET can also act as a power quality controller, making it a multifunctional new type of transformer. Using it in the power distribution system can both reduce voltage and ensure power quality [1] .
Parallel operation of two or more PETs is an important operating mode of transformers and has important research value. However, current research on PETs at home and abroad mainly focuses on their topology and control strategies, while research on their application in power systems and their operating characteristics is relatively weak. Reference [2] uses a master-slave control scheme to solve the problem of parallel current sharing on the AC output side of parallel PETs; Reference [3] conducts simulation research on the dynamic process of step change of parallel PET load and nonlinear load. This paper studies the control strategy of parallel PET load distribution for power distribution and performs dynamic simulation of typical operations [2,3] .
1. Basic structure and control strategy of PET
The basic topology of PET (Polyelectric Transformer) is divided into AC-AC-AC converter and AC-DC-AC-DC-AC dual DC converter. The former has a simple structure but low controllability; the latter has a complex structure, a sophisticated control strategy, and is more practical. A typical AC-DC-AC-DC-AC dual DC topology is shown in Figure 1.
The voltage-type PWM rectifier circuit on the primary side of the PET adopts decoupled voltage and current double closed-loop control. Regardless of whether the load of the transformer is inductive or capacitive, as long as it is within a certain range, the power factor of the power grid can be close to 1. The single-phase inverter circuit on the primary side realizes high-frequency inversion and can be controlled by open-loop control. In order to reduce the size and weight of the transformer, the transformer magnetic material adopts high-permeability magnetic cores such as ferrite. The transformer secondary rectifier circuit is used to realize high-frequency rectification. For the distribution transformer, the bidirectional flow of energy is not considered, so it adopts an uncontrolled rectifier circuit. In order to output constant voltage and constant frequency AC voltage, the PET secondary inverter circuit adopts voltage closed-loop control [4] .
2. PET Parallel Operation Control Principle
Parallel operation of two or more PET inverters is an effective way to improve system reliability and expand capacity, but research on parallel operation of PET inverters is not yet in-depth. The working principle of PET secondary inverters is the same as that of UPS inverters, while there are relatively rich results on the parallel operation of multiple UPS units [5-7] , which can be used as a reference when studying the parallel operation of PET inverters.
The proposed parallel control methods for PETs are mainly: centralized control, master-slave control, distributed logic control and control without interconnection [2] . This paper mainly studies the parallel operation of two PETs without interconnection. Figure 2 is a structural diagram of a parallel system of two PETs, which are originally connected to the same common bus.
To avoid circulating current in the parallel transformers, the frequency, amplitude, and phase of the secondary voltage of each PET unit must be kept consistent. To achieve a stable distribution of active and reactive loads among the parallel transformers, each PET unit should have active droop and reactive droop characteristics. The control structure diagram of a PET secondary inverter with droop characteristics is shown in Figure 3.
The frequency, amplitude, and phase of the PET secondary voltage depend on the sinusoidal modulation signal of the inverter's PWM pulse. The characteristics of the sinusoidal modulation signal are related to the frequency setpoint f <sub>0 </sub>, phase setpoint ρ <sub>0 </sub>, and amplitude setpoint. f <sub>0</sub> = 50Hz is chosen to ensure the rated frequency. ρ <sub>0 </sub> corresponds to the initial phase angle of the voltage when the active load P<sub> 0 </sub> (generally taken as 0, with an active power compensation coefficient K<sub> p </sub> > 0), thus forming the active power droop characteristic.
ρ=ρ 0 -K p P （1）U0 corresponds to the voltage amplitude when the reactive load Q=0. By introducing a reactive compensation coefficient KQ ＞0, a reactive droop characteristic can be formed.
U=U 0 -K Q Q （2）For each PET operating in parallel, the values of ρ0 and U0 should be the same. Due to the introduction of active and reactive power compensation, when the load changes, each PET operating in parallel will automatically adjust the phase angle and amplitude of its output voltage, and automatically realize the stable power distribution among the transformers. In order to reasonably distribute the load according to the transformer capacity, the per-unit values of Kp and KQ of each PET based on its own capacity should be equal, generally taken as 0.01 to 0.05.
References [2,8] propose using frequency droop characteristics for active power distribution between parallel PETs and inverters. Obviously, under this control method, the power supply frequency cannot be maintained at 50Hz under different loads; and to ensure frequency quality, the frequency droop coefficient must be very small, which is not conducive to the stable distribution of active load among parallel PETs. In contrast, the initial phase angle droop characteristic used in this paper can maintain constant frequency power supply and allows for the selection of a reasonable droop coefficient as needed, achieving a stable and reasonable distribution of active load. The output voltage frequency of each PET participating in parallel must be equal to 50Hz to ensure normal operation. In Figure 3, this can be achieved by using closed-loop PI control for the frequency.
The parameters of parallel-running PETs may not be completely consistent. The most common difference is that the parameters of the current-limiting reactor or the inductance of the connecting line are different. The voltage measurement point in Figure 3 is deliberately set on the common bus, so that even if the PET parameters are inconsistent, the power distribution between parallel PETs can be guaranteed to be stable and reasonable. If the voltage measurement point is located at the output end of each PET, this cannot be guaranteed [3] .
3. Simulation Analysis
This paper uses Matlab 6.5/Simulink to build a simulation model and simulates the parallel operation of two PETs with the same parameters. The main system parameters are: PET2 rated capacity 10kVA, rated voltage 240/110V; PET2 rated capacity 10kVA, rated voltage 240/110V; system frequency 50Hz; high-frequency transformer frequency 1000Hz; IGBT switching frequency 9000Hz; KP and KQ are both taken as per unit value 0.01; frequency setpoint f0 is taken as 50Hz; phase setpoint ρ0 is taken as 0; amplitude setpoint U0 is taken as per unit value 1.0.
3.1 Two PET units are put into parallel operation simultaneously (Case 1)
At 1.0s, the two PET units were switched on from no-load operation to parallel operation on the low-voltage side, undertaking a comprehensive load with a power factor of 0.8. The relevant variable waveforms are shown in Figures 4-6. As can be seen from the figures, the waveforms of the corresponding variables for the two PET units are consistent. After parallel operation, the load current they bear is equal, achieving current sharing control and stable distribution of active and reactive loads, while maintaining a constant frequency.
3.2 PET2 is added to parallel operation (Case 2)
PET1 operates under load, and PET2 is switched on from no-load status at 1.0s, with the two PETs operating in parallel. The relevant waveforms are shown in Figures 7 and 8. As can be seen from the figures, after PET1 switches from single-unit operation to parallel operation, the load current, active and reactive loads it bears all decrease, with the decreased portion being borne by PET2. Ultimately, current sharing control and stable distribution of active and reactive loads are achieved between the two parallel PETs, exhibiting good dynamic response performance.
4. Conclusion
This paper establishes a PET control strategy and model based on the active and reactive power droop characteristic equations, and conducts a simulation study on the dynamic process of PET parallel operation based on this model. Simulation results show that the proposed control strategy can achieve stable distribution of active and reactive power loads while maintaining the rated power supply frequency, and exhibits good dynamic characteristics.
References:
[1] Wang Dan, Mao Chengxiong, Lu Jiming, et al. Power quality regulation method based on electronic power transformer [J]. High Voltage Engineering, 2005, 31(8):63-65.
[2] Wang Dan, Mao Chengxiong, Lu Jiming. Parallel operation of power electronic transformers [J]. Electric Power Automation, 2005, 29(6): 66-71.
[3] Liu Haibo, Mao Chengxiong, et al. Parallel technology of electronic power transformers based on interconnection-free control [J]. Automation of Electric Power Systems. 2007, 31(15): 55-60.
[4] Dong Dezhi, Xie Dawei, Hong Naigang. Power electronic transformer based on dual PWM conversion [J]. Journal of Anhui University of Technology, 2006, 23(2): 170-173.
For details, please click: Dynamic Simulation of Parallel Operation of Power Electronic Transformers
