DC-Link Protection and Control in Modular Uninterruptible Power Supply

Abstract

In this paper, 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 are verified by experimental results from the parallel UPS prototypes. Index Terms-Uninterruptible Power Supply(UPS), DC link voltage protection, current sharing, consensus control, Anti-windup. I. INTRODUCTION RIVEN by the increasing importance of the Uninterruptible Power Supply (UPS) in the industry including enterprise IT, commercial telecom, data center and cloud computing area [1], the global market for the UPS system is projected to soar dramatically in the next few years. Meanwhile, the strong need for the UPS system to provide more reliable, efficient and secure electrical power supply for the modern digital equipment propels the UPS technology advancement by engineers and academic researchers [2]. According to the European Standard EN 62040-3 [3], the UPS systems are divided into on-line, off-line and lineinteractive UPSs. In an on-line UPS system, the load is always powered by the inverter regardless of the grid condition; the only exception is that when the overloading occurs and the bypass switch closes to connect the load with the grid. In offline and line-interactive UPS systems, however, the load power is supplied from the grid or a combination of the inverter and the grid, respectively [4]. The on-line UPS system is the most

A Digital Control System for UPS Systems With Smart-Grid Capability

“…Without the UPS systems, any power outages and fluctuations can result in hardware damage or data loss. The vast growth in data centers and cloud storage systems in the recent years has tremendously increased the demand for the UPS systems [18][19][20][21][22].…”

“…Offline (standby), Line-interactive, and Online [18][19][20][21][22]. In the following subsections, these configurations will be described.…”

Multimode Operation for On-Line Uninterruptible Power Supply System

“…The inner current loop is designed to provide a sufficient damping and protect the inductor's current from an overcurrent. Finally, it should be noted that if the nonlinear load is connected as the sensitive loads, the harmonic compensator should be added in the voltage controller of DC/AC converter to enhance the harmonic rejection ability of the output voltage [23]. Note that the control strategy for the rectifier has been shown in Fig.…”

“…This method, however, requires additional sensors, which may not be desirable from the reliability and cost points of view [19][20][21]. This is particularly true for the UPS systems where the "plug and play" function needs to be realized by expanding more UPS modules according to the client's requirement [22,23]. Therefore, a DC-link voltage control with sensor-less control strategy needs to be investigated.…”

Modelagem de Pequenos Sinais no Espaço de Estados para Estruturas de Controle Hierárquico em Eixos Síncronos Aplicado ao Paralelismo de UPS Trifásicas

“…As fontes ininterruptas de energia (UPS -Uninterruptible Power Supplies) são equipamentos utilizados para proteger cargas críticas, como é o caso de sistemas de telecomunicações, data center e computação em nuvem, contra distúrbios que ocorrem na tensão fornecida pelas concessionárias de energia [1], [2]. Podem ser classificadas como off-line, interativa ou dupla conversão, sendo esta última reconhecida por apresentar o melhor desempenho dentre os tipos citados, alimentando a carga independentemente das condições da rede pública [1]. Cabe salientar que, por apresentarem dois estágios de conversão de energia, estas UPSs geralmente possuem um menor rendimento e um custo mais elevado que as demais [3].…”