Paul Coventry

Start-Up Control of an Offshore Integrated MMC Multi-Terminal HVDC System With Reduced DC Voltage

The modular multilevel converter (MMC) provides promising development for high-voltage direct current (HVDC) applications, including multiterminal HVDC (MTDC) and renewable energy integration. This paper, considering an offshore wind farm (OWF) integrated MMC MTDC system, investigates its start-up process with three main developments: 1) it further develops the mathematical model of MTDC with active networks and proposes a hierarchical start-up control scheme; 2) for the terminal which connects the OWF, it proposes a reduced dc voltage control scheme of mitigating the current surges with deblocking the converter at zero voltage difference on submodules (SMs) and proposes an overall sequential start-up control scheme for the offshore integrated MTDC; and 3) it analyzes and compares different start-up control schemes. To evaluate the proposed sequential start-up control scheme, an offshore MMC HVDC system is established on the RTDS. The simulation results verify effectiveness of the proposed scheme on the MMC MTDC system with two control paradigms, i.e., master-slave control and droop control, respectively. In comparison with different start-up control schemes, the superiority of the mitigation of voltage spikes and current surges are shown using the proposed scheme with less complexity and easier implementation.

Radiometric Timing of High-Voltage Circuit-Breaker Opening Operations

This paper demonstrates the feasibility of radiometric on-line circuit breaker timing. The timing test is crucial and can reveal a wide range of faults or defects. The work presented concentrates on the timing of gas-insulated substation (GIS) circuit breakers, which due to their enclosed nature pose a variety of challenges not encountered in air insulated substations (AIS). It is demonstrated how key events in the arcing process such as arc ignition and extinctions can be identified from the emitted electromagnetic pulses and how this information can be used to assess the integrity of the circuit breaker being monitored. The results are backed by site measurements of a high voltage reactor switching SF6 double pole puffer breaker. The proposed technique can readily be extended to monitor closing operations as well as being adapted to monitor other types of circuit breakers.

Dynamic interactions of DC and AC grids subject to DC faults

DC circuit breakers (DCCBs), AC circuit breakers (ACCBs) and Full-Bridge Modular Multi-Level Converters (FB-MMCs) can be used to block DC fault current in voltage source converter-based (VSC) multi-terminal high voltage direct current (MTDC) grids. However, the DC fault transient characteristics of AC/DC networks will be different depending on the type of devices used to protect the DC grid. In this paper, the DC fault clearance and post-fault restoration processes using DCCBs and ACCBs are analyzed. The transient response of the AC/DC system during DC faults is also assessed considering the analyzed approaches. The studies are conducted in PSCAD/EMTDC, where an integrated AC/DC transmission system has been modeled. Simulation results indicate that a different transient behavior arises when different fault isolation and post-fault restoration schemes are employed. The work of this paper provides an understanding of the dynamic behavior in AC/DC systems, in turn contributing to a deeper comprehension of system operation and control system design.

Active filtering based current injection method for multi modal SSR damping in an AC/DC system

The rapid expansion in power transmission for the integration of large-scale renewables complemented by infrastructure reinforcements in the form of series compensation poses the threat of subsynchronous resonance (SSR). However, existing and future high-voltage direct current (HVDC) links can be effectively used to mitigate this undesirable phenomenon. In this line, this paper presents an auxiliary control loop embedded in a voltage source converter (VSC) based HVDC system to damp torsional interactions - a form of SSR involving turbogenerators and series-compensated transmission lines. The proposed damping scheme employs modal filters to identify SSR upon occurrence and then injects currents at subsynchronous frequency to damp it. The SSR damper has been tested in the well-known IEEE First Benchmark Model, which has been upgraded with a VSC-HVDC link to form an integrated AC/DC system. Simulations have been performed in PSCAD, with eigenvalue analysis carried out in MATLAB for the small-signal stability assessment of the AC/DC system. Results show that the proposed scheme effectively damps SSR irrespectively of the torsional mode being excited.

An Electro Optic Method for Transient Field Measurement in GIS

It has been demonstrated 1,2 that compressed gas insulated systems (GIS), because of their compact coaxial geometry, can have transient overvoltages created within them of amplitudes up to 3 times the normal system voltage and rise times as short as a few nanoseconds. The occurence of such overvoltages is known to be associated with a combination of disconnector switch operation and the existence of residual charge on isolated sections and it has been suggested that the resultant travelling waves can stimulate resonance at microwave frequencies depending upon the geometry of the system. The overvoltages themselves are highly undesirable in that they can cause damage to parts of the system employing solid dielectric insulation such as voltage transformers and gas barriers.