Voltage Fast Recovery Control Scheme for Supercapacitor Interface Bidirectional DC-DC Converters

Issuing time:2023-05-19 16:45

As an extension and supplement to traditional power systems with centralized, long-distance transmission, DC Microgrids integrate DC-coupled Distributed Energy Resources (DER) and Energy Storage Systems (Energy Storage Systems). ESS) and modern electronic loads are integrated into a controlled whole [1-3]. Figure 1 shows a typical configuration of a DC microgrid, in which the DC bus serves as an intermediate for energy pooling and power exchange. Since the power intermittently and load disturbance of micro-sources such as photovoltaic and wind power will threaten the stability of bus voltage, bus voltage has become an important indicator for evaluating the power quality of DC microgrids [4-6]. Previous studies have shown that the ESS unit is connected to the DC bus through a bidirectional DC-DC converter to form an energy buffer between the micro source and the load, which can suppress the voltage fluctuation of the bus to a certain extent and improve the reliability of the DC microgrid power supply [7-9]. Generally, for the suppression of power fluctuations of distributed micro sources, the response time scale of ESS is required to be in the order of seconds to minutes, while for the suppression of load disturbance, the response time scale of ESS is required to be increased to the order of milliseconds [10]. When the DC microgrid has a large load disturbance, the ESS unit needs to generate a higher instantaneous power on the busbar side to quickly compensate the instantaneous power difference between the two sides of the source load on the busbar and prevent the bus voltage from producing a higher instantaneous overrush or drop. Compared with batteries, supercapacitors have advantages such as high power density, long cycle life, fast charge and discharge speed, etc., and have inherent advantages in instantaneous power balance control of DC microgrids [11-13]. As an interface converter, the bidirectional DC-DC converter allows the ultracapacitor to operate in a wide voltage range, which improves its energy utilization and service life. However, the dynamic performance of the bidirectional converter affects the response time of the ultracapacitor to the voltage fluctuation of the bus. Therefore, how to improve the dynamic performance of bidirectional DC-DC interface converter has become an important problem worth studying

The nonlinear control strategy breaks through the bandwidth limitation of traditional linear control, and can improve the transient response performance of DC-DC converter to a certain extent [17,18]. At present, most nonlinear controls are based on the boundary control theory, and the choice of switching surface is the core issue of the boundary control theory [19]. As different types of controllers differ greatly in terms of adjustment time and robustness, the selection of switching surfaces is varied, typical ones being sliding mode control and hysteresis control [20]. In literature [21,22], sliding mode control is applied to control bidirectional DC-DC converter, which makes the system stable under large signals and has good robustness to system parameter changes, but there are problems such as output jitter and high switching frequency on the switching surface. Literature [23,24] uses hysteresis control to control bidirectional DC-DC converter, which has the characteristics of simple control and strong robustness, but also has problems such as output jitter and switching frequency jitter. The geometric area method is used to simplify the complex calculation of the switching surface, and then approximate optimal control is obtained, typical of which is time optimal control [20]. Literature [19,20] realizes the approximate optimal transient response of DC-DC converter through time optimal control, but the choice of switching surface is relatively dependent on the converter system parameters and tolerances, thus reducing the robustness of the controller.

Charge Balance Control (CBC) is a simplified time-optimal control based on the principle of capacitive charge balance, which reduces the dependence on system parameters to a certain extent [25]. At present, the control idea of CBC has been applied to control unidirectional DC-DC converter. Literature [26,27] proposes a variable structure capacitor charge balance control strategy. However, when controlling the capacitor charge balance auxiliary circuit of Buck converter according to the control logic, there is a repeated switching behavior, which increases the switching loss of the power tube. In literature [28,29], capacitor charge balance control algorithm is used to control Buck converter, which has excellent dynamic response characteristics when the load is disturbed. However, because the control strategy is based on voltage peak point detection, complex analog detection circuit is needed, and the control algorithm lacks universality. In literature [30,31], the capacitive charge balance control algorithm is extended to Buck-Boost converter and two-tube forward converter respectively under the condition of limiting the control duty cycle. However, the switching loss increases with the increase of the number of switching tube operations during the control process. Literature [32] proposes a digital control algorithm based on capacitive charge balance control applied to Boost converter to achieve the purpose of studying capacitive charge balance by indirectly predicting the law of capacitor change. The control algorithm is simple and less computational, which is suitable for digital control, but the control object is only for single-output filter capacitor voltage. To sum up, the existing CBC control applications are still limited to the control of unidirectional power output, only for the unilateral adjustment of output voltage. For the application of rapid busbar voltage recovery, when the output power control of the supercapacitor is switched to the input power control, the busbar side capacitor as the control object will change from the output filter capacitor to the input filter capacitor, the role of the control object will change, and the original geometric area method and the corresponding control law will no longer be applicable. Therefore, the existing CBC control can not be directly applied to the power control of bidirectional DC-DC interface converter.

In order to realize rapid recovery of bus voltage and solve the problem of bus voltage fluctuation caused by transient power imbalance, this paper intends to improve the transient response of bidirectional DC-DC converter with super capacitor interface. By further expanding and optimizing the idea of indirect prediction of capacitor current change rule for controlling unidirectional current in literature [32], a more general node current substitution method and corresponding control law are obtained, and the purpose of controlling bidirectional current is achieved. Finally, a compound control strategy combining CBC and traditional linear control is obtained, which is suitable for bidirectional DC-DC converter control, namely: The transient process with large disturbance is switched to the corresponding CBC control to realize fast adjustment. In the small disturbance process, incremental PID is used to generate complementary PWM waves, in which linear PID control can maintain good steady-state control precision without output jitter problem, and the complementary drive of the switching tube can realize smooth switching between the two working modes of the bidirectional converter in small transient state. The realization of bidirectional power flow can ensure good dynamic performance of the system.