Oliver Cwikowski

Impact of DC Breaker Systems on Multiterminal VSC-HVDC Stability

The use of VSC-HVDC grids for offshore wind farm integration will require the use of dc breaker systems and presently they require dc reactors to limit the rate of rise of fault current. The introduction of large dc reactors throughout a VSC-HVDC system can have a significant impact on its stable operation and will require additional control. This paper analyzes this problem and proposes a PSS-like control (DCPSS) to aid dc grid stability and cope with this effect. A generalized analytical model for studies on dc voltage control is presented. Key stability and transient performance issues caused by the use of the dc reactors in a multiterminal system are investigated by analyzing poles, zeros, and frequency responses of open-loop and closed-loop models. Design and location identification methods for the DCPSS are provided. An excellent damping enhancement is achieved by this controller. The analytical studies and time-domain simulations in this paper are performed based on two VSC-HVDC models.

Analysis and simulation of the proactive hybrid circuit breaker

High Voltage Direct Current (HVDC) short circuit protection is a fundamental requirement for any HVDC transmission system. Presently, all point-to-point links are protected using circuit breakers on the AC side of the converters. In order to enable HVDC grids, a more advanced protection system must be developed. HVDC circuit breakers are one solution for the protection of future HVDC grids. Several designs have been proposed for DC circuit breakers but few are suitable for Voltage Source Converter (VSC) applications. To date, only a few industrial prototypes have been developed, which are seen to be suitable for the VSC HVDC applications. This paper presents analysis and simulations on one of these prototypes, the Proactive Hybrid Circuit Breaker (PHCB). Equations are derived from a state-space analysis of the circuit breaker. A model of the circuit breaker is suitably parameterized for a +/- 300 kV VSC system in PSCAD. Fault simulations are then performed and compared to the equations developed in a state space analysis. Discussion is then given to the design and testing of the Load Commutation Switch (LCS).

A review of technologies for MVDC circuit breakers

Medium voltage direct current (MVDC) distribution networks have been considered for various applications, such as offshore wind farm collector systems, all-electric naval vessels, and aircraft. MVDC circuit breakers are a critical technology to directly manage faults in multi-terminal DC (MTDC) networks. However, DC current breaking is much more challenging than in AC systems because there is no natural zero-crossing of the current waveform to aid fault isolation. This paper reviews existing MVDC circuit breaker technologies and also discusses their advantages and disadvantages. This paper also introduces new topologies that can be applied in MVDC applications. The operation of several hybrid DC circuit breaker topologies with aided commutation is included. The paper illustrates that a hybrid DC circuit breaker with aided commutation can clear a fault within 2-5 msecs with low losses, this shows great potential for future MVDC applications. The implications for the practical design of commercial MVDC circuit breakers are also discussed.

The impact of traveling waves on HVDC protection

The development of High Voltage Direct Current (HVDC) protection technology is a necessary step in the development of high power Voltage Source Converter (VSC) transmission grids. Presently, only a few industry prototypes have been developed for VSC HVDC grid applications[1-3]. However, before any piece of equipment can be installed, it must be subject to testing to prove it is capable of working. These tests are based upon operational experience and/or theoretical analysis of the system it is to be placed in. To date, no VSC HVDC circuit breaker is in commercial operation. Developing knowledge around the testing of the circuit breaker is an important step on the road to HVDC grids. This paper discusses the impact of traveling wave phenomena on the testing of HVDC circuit breakers for VSC applications, derives theoretical calculations to describe the phenomena and compares this to PSCAD simulations of a VSC under DC side pole-to-pole faults.

Integrated HVDC Circuit Breakers With Current Flow Control Capability

Two key problems in meshed high-voltage direct current (HVDC) transmission grids are managing line power flows and protection against dc faults. Current flow controllers (CFCs) will be required to balance cable currents in meshed dc grids, in order to prevent individual line power capacity limits restricting overall power flow in the grid. Direct current circuit breakers (DCCBs) will be also required to protect HVDC grids from dc faults. This paper demonstrates that the current flow controller functionality can be added into a hybrid CBs design. This paper proposes integrating an interline CFC into the load commutation switch (LCS) of a hybrid DCCB. The integrated design LCS/CFC is analyzed and a state-space model is derived. The control of the CFC is designed and the performance of the LCS/CFC during normal operation is verified by means of MATLAB Simulink and PSCAD simulations. A comparison of the integrated LCS/CFC and the separate design is given. The case studies show a reduction in total power losses, and improved protection operation times can be achieved.

Modular Multilevel Converter DC Fault Protection

High-voltage direct current (HVDC) grids will require the development of dc protections that provide fast fault isolation and minimize the disturbance caused to the existing ac power networks. This paper investigates how the dc fault recovery performance of a half-bridge modular multilevel converter (HB-MMC) is impacted by different dc protection design choices. An HB-MMC point-to-point HVDC system that is protected with dc circuit breakers (CBs) is simulated on a real-time digital simulator using detailed switch models of the converters and switch gear. A dc CB controller has been developed and implemented in a software-in-the-loop fashion, and has been made available free for download. A novel blocking scheme for the HB-MMC is proposed, which limits the prospective dc-side fault current, benefiting dc switch gear. A comparison of circulating current controllers shows that the standard d —q controller is likely to be unsuitable for fault studies. Finally, benchmarking shows that a 48% reduction in power-flow recovery time and a 90% reduction in the energy dissipated in the circuit breaker can be achieved, along with other benefits, depending on the protection design.

Operating DC Circuit Breakers With MMC

High voltage direct current (HVDC) grids may be protected from dc faults through the application of HVDC circuit breakers. Recent advances in dc circuit breaker technologies may allow faults in the dc grid to be cleared without a permanent loss of power to the connected ac grids. The requirements for the protection have yet to be fully defined; especially where half-bridge modular multilevel converter (MMC) controls are concerned. This paper investigates integrating dc circuit breakers with half-bridge MMC converters, specifically looking to at how to recover from a pole-to-pole fault. The fault response of the converter to a fault is analyzed in depth. This analysis highlights key stages in the converter response to a dc fault, allowing the MMC fault currents to be predicted. This analysis is then verified in PSCAD simulations and the power flow recovery is shown. The converter controls are investigated, improvements made to the power flow recovery, and the need for arm current controllers highlighted.

Design and Experimental Tests of a Superconducting Hybrid DC Circuit Breaker

Direct current (DC) circuit breakers are a key enabling technology for fault management in multiterminal high-voltage DC (HVDC) systems. DC fault isolation is challenging due to the high rate of rise of the fault current and the lack of natural current zero-crossings found in ac systems. In this paper, we present a novel superconducting hybrid dc circuit breaker that utilizes the intrinsic characteristics of the superconductor material. The automatic quench of the superconductor coil as a result of a high fault current transfers the current from the mechanical switch to the semiconductor switch. The isolating mechanical switch is able therefore to open at low current and recover its dielectric capability rapidly. A low voltage DC circuit breaker prototype has been built using a multistrand magnesium diboride (MgB2) coil, a vacuum interrupter, and an insulated-gate bipolar transistor module. This prototype successfully demonstrated interruption of 500 A DC within 4.4 ms. This paper presents the design of the superconducting hybrid breaker prototype and a detailed analysis of the experimental results. This superconducting hybrid dc circuit breaker has significant potential for scaling up the high-voltage and high-current applications.