Advanced Control and Protection of Modular Uninterruptible Power Supply Systems

This chapter presents a reduced-order generalized proportional-integral observer (RGPIO)-based resonant super-twisting sliding mode controller (RST-SMC) for the three-phase AC/DC converters. On contrary to utilizing the proportional-integral (PI) controller in regulating the DC-link voltage, which may cause large undershoot/overshoot under the disturbance, the proposed voltage control strategy for the DC-link has high disturbance rejection ability, and the settling time has been greatly reduced. In addition, the proposed RST-SMC in the current control loop not only preserves the merits of the sliding mode controller but also achieves the current tracking without steady-state error in the stationary α − β frame. The effectiveness of the proposed method has been verified by a lab-constructed experimental prototype.

To regulate the DC-link voltage of three-phase AC/DC converters, an enhanced state observer (ESO)-based controller is presented in this chapter. The proposed controller, contrary to the traditional ones, does not require the DC-link current measurement and offers a “plug and play” capability, a rather high disturbance rejection ability and robustness against the DC-link capacitance parameter variation. The design procedure of the suggested controller is discussed, and its effectiveness is verified using experimental results.

In this chapter, an average active power sharing control strategy based on distributed concept for the parallel operation of voltage source inverters (VSIs) is proposed to be applied to a modular uninterruptible power supply (UPS) systems. The presented method is named distributed adaptive virtual impedance control (DAVIC), which is coordinated with a droop control method. Low-bandwidth CAN-based communication is used for the requirement of data sharing of the proposed method in the real modular UPS system. Unlike the conventional virtual impedance control techniques, the virtual impedance of a converter module is adjusted automatically by using global information when DAVIC is applied and further to tune the output impedance of the power modules. The adaptive virtual impedance is calculated by using the difference between the active power of a local module and the average active power of all the modules in a modular UPS. The proposed method DAVIC overcomes the drawback of conventional virtual impedance control, since control since an accurate value of the real output impedances of different converter modules is not required. Simulations using PLECS and experimental results on a real commercial modular UPS are presented developed in order to verify the effectiveness of the proposed control methodology. These results have shown that a superior power sharing performance is obtained when using the proposed method.

This chapter presents a distributed average integral secondary control (DAISC) method for modular uninterruptible power supply (UPS) systems-based microgrids. For each UPS unit, the local primary control level encompasses droop control and virtual impedance loops, which is commonly used in parallel inverter systems. In order to provide a fast voltage recovery performance, along with excellent power sharing capability among the parallel UPS modules, a distributed secondary control method based on CAN communication is proposed. In a sharp contrast to the existing distributed secondary control strategies, in which the output voltage and frequency of the modules are not shared through the CAN bus, in the proposed approach, the inverter modules of the modular UPS share the integral output value of the secondary controller. By using the proposed novel DAISC approach, a better dynamic power sharing performance along with an inherent anti-windup capability of the integral controller is achieved. Simulation results using PLECS and experiments from a modular UPS platform have been developed to verify the feasibility and effectiveness of the proposed distributed secondary control. The results shown that good performance of voltage recovery and power sharing of the proposed control method is obtained.

In this chapter, a regeneration protection solution is proposed to address the DC-link overvoltage issue and the unequal power sharing in the parallel Uninterruptible Power Supply (UPS) systems. First, a DC-link Voltage Protection (DCVP) control strategy is proposed to protect the inverter against the excessive DC-link voltage that may trigger the protection mechanism of the UPS system. In addition, an active power sharing control strategy by regulating the virtual resistance is proposed to solve the circulating current caused by UPS regeneration issue. Finally, the feasibility of the proposed regeneration protection solution is verified by experimental results from the parallel UPS system prototype.

In this chapter, a DC-link voltage protection (DCVP) control method is proposed to address the DC-link overvoltage issue due to power back-feeding in parallel Uninterruptible Power Supply (UPS) system. The proposed control method is able to protect the inverter against the excessive DC-link voltage, which increases the system reliability and robustness. Moreover, a current sharing control strategy is proposed by online regulating the virtual resistance of each UPS module. The proposed current sharing control strategy is able to address the circulating fundamental and harmonic current caused by the line impedance mismatching or power back-feeding issue in the UPS system. In addition, an improved consensus-based distributed controller is proposed to alleviate the overshoot issue during the transient process in voltage amplitude and frequency restoration. Finally, the feasibility of the proposed methods is verified by experimental results from the parallel UPS prototypes.

In this chapter, an overload and short-circuit protection method is proposed for voltage-source inverter-based uninterruptible power supply (UPS) system. In order to achieve high reliability and availability of the UPS, a short-circuit and overload protection scheme is necessary. When overload or short circuit happens, using the proposed control method, the amplitude of the output current can be limited to a constant value, which can be set by the customer to avoid the destruction of the power converter and to obtain a faster recovery performance as well. The detailed principle of the proposed protection method is discussed in this chapter. It mainly contains three parts in the control diagram for current limit, first is the anti-windup in the voltage and current controllers, then the feedforward of the capacitor voltage to the current control loop, the last is the fast reset of the resonant part of the current controller when overcurrent happens. The procedure of developing the control method is also presented in the paper. Experimental results on a commercial UPS system are presented to verify the effectiveness of the control method.

To enhance the robustness and disturbance rejection ability of an on-line uninterruptible power supply (UPS) system, an internal model control (IMC)-based DC-link voltage regulation method is proposed in this chapter. Furthermore, the multi-mode operations of the on-line UPS system are investigated, and their corresponding control strategies are proposed. The proposed control strategies are capable of achieving the seamless transition in traditional normal mode, PV-aided normal mode, enhanced eco-mode, and burn-in test mode. Meanwhile, the uninterruptible load voltage is promised during the mode transition. The small-signal analysis is also conducted to investigate the stability of enhanced eco-mode and burn-in test mode. Finally, extensive experimental results are provided to validate the effectiveness of the proposed methods.

In this chapter, a distributed hierarchical control is proposed for AC microgrid, which could apply to both grid-connected mode and islanded mode as well as mode transitions. The control includes three control levels: (i) the basic droop control is adopted as the primary control; (ii) the secondary control is based on the distributed control with a leader–follower consensus protocol; and (iii) the tertiary level is a mode-supervisory control, which manages the different control targets of four operation modes. Under the proposed control framework, the following targets are achieved: (1) the frequency/voltage recovery and accurate power sharing in islanded mode (IS); (2) flexible power flow regulation between utility grid and microgrid in grid-connected mode (GC); (3) universal control strategy from GC to IS modes without control switching; (4) smooth active synchronization from IS mode to GC mode. In this sense, the proposed method can adapt to all four operation modes of microgrid. Compared with central-standard hierarchical control, the proposed method only requires local neighbor-to-neighbor interaction with a sparse distributed communication network. Thus, the scalability, flexibility, reliability, and robustness are greatly improved in practical application. In addition, stability analysis is added to facilitate the control parameter designs, and substantial simulation cases are provided to validate the control feasibility, link-failure resiliency, and plug-and-play capability.