Analysis and Mitigation of DC Voltage Imbalance for Medium-Voltage Cascaded Three-Level Neutral-Point-Clamped Converters

Abstract

The cascaded three-level neutral-point-clamped (3L-NPC) converter and the modular multi-level converter (MMC) are attractive solutions for medium-voltage direct-current (MVDC) applications. Due to their low cost compared to MMCs, cascaded 3L-NPC converters have been adopted in ANGLE-DC-a 30 MVA MVDC link demonstration project in North Wales, UK. DC voltage imbalance across submodules (SMs) is a common challenge for both types of MVDC converters. Such imbalance is topology dependent and remains under-researched for cascaded 3L-NPC converters. In this paper, small-signal model-based analysis has been done to reveal that the dc voltage imbalance in cascaded 3L-NPC converters is caused by an unstable system pole. Two voltage balancing methods are presented. The first method is based on PI controllers to precisely regulate SMs' voltages without influencing output power. However, it relies on communication between a central controller and local controllers within SMs. The second method uses inverse-droop based control to take over the dc voltage regulation upon loss of communication. Both balancing methods are experimentally validated using a 30 kVA testbed based on the ANGLE-DC project. It has been demonstrated that the dc voltages of SMs can be effectively balanced with both methods during changes of load conditions and dc bus voltages.

Decentralized Control for Multi-Terminal Cascaded Medium-Voltage Converters Considering Multiple Crossovers

“…Thus, the fault-tolerant operation of system can be achieved. The P −𝑉 𝑑𝑐 droop control can contribute to the voltage balancing across SMs if each SM uses a 𝑃 𝑖 − 𝑉 𝑑𝑐𝑖 droop controller, where 𝑃 𝑖 and 𝑉 𝑑𝑐𝑖 is the power and dc voltage of the i th SM [23] , where 𝑃 0 and 𝑉 𝑑𝑐0 are given references of the converter and 2N is the number of cascaded SMs. Due to the droop characteristic, the dc voltage balancing between SMs can be automatically achieved [23].…”

“…The P −𝑉 𝑑𝑐 droop control can contribute to the voltage balancing across SMs if each SM uses a 𝑃 𝑖 − 𝑉 𝑑𝑐𝑖 droop controller, where 𝑃 𝑖 and 𝑉 𝑑𝑐𝑖 is the power and dc voltage of the i th SM [23] , where 𝑃 0 and 𝑉 𝑑𝑐0 are given references of the converter and 2N is the number of cascaded SMs. Due to the droop characteristic, the dc voltage balancing between SMs can be automatically achieved [23]. For example, if dc voltage of a SM is decreased from 𝑉 𝑑𝑐𝑖0 to 𝑉 𝑑𝑐𝑖1 after suffering from a disturbance, the power is also decreased according to the droop curve.…”

“…When the droop methods are employed to the converters with dc series connection, the smaller the slope is in Fig. 1, the better the dc voltage balancing would be, at the sacrifice of accuracy of power control [19], [23]. Thus, how to find a control solution that balances these three factors is a challenge.…”

Modelling and Control Development of a Cascaded NPC-Based MVDC Converter for Harmonic Analysis Studies in Power Distribution Networks

“…The MVDC systems are appointed as a promising technology for today's distribution networks to accommodate the high penetrations of Distributed Generation (DG) systems with increasing power demand [6][7][8][9][10][11][12][13]. These technologies with a suitable control system can also help reduce power losses, improve power supply, and compensate for load unbalancing.…”

“…Two switchable control schemes were introduced for the normal operating conditions and post-fault restoration, and a smooth transition method between the control modes based on the design of the Phase-Locked Loop (PLL) circuit was proposed. The control performance during fault conditions was discussed in [18], and a control system for a balanced DC voltage operation of a scaled-down MVDC system was recently proposed in [13].…”