Inertia emulation control of VSC-HVDC transmission system

The increasing penetration of power electronics interfaced renewable generation (e.g. offshore wind) has been leading to a reduction in conventional synchronous-machine based generation. Most converter-interfaced energy sources do not contribute to the overall power system inertia; and therefore cannot support the system during system transients and disturbances. It is therefore desirable that voltage-source-converter (VSC) based high voltage direct current (HVDC) interfaces, which play an important role in delivery of renewable power to AC systems, could contribute a virtual inertia and provide AC grid frequency support. In this paper, an inertia emulation control (IEC) system is proposed that allows VSC-HVDC system to perform an inertial response in a similar fashion to synchronous machines (SM), by exercising the electro-static energy stored in DC shunt capacitors of the HVDC system. The proposed IEC scheme has been implemented in simulations and its performance is evaluated using Matlab/Simulink

Robust and generic control of full-bridge modular multilevel converter high-voltage DC transmission systems

This paper presents the theoretical basis of the control strategy that allows the cell capacitor voltage regulation of the full-bridge modular multilevel converter (FB-MMC) to be controlled independent of its dc link voltage. The presented control strategy permits operation with reduced dc link voltage during permanent pole-to-ground dc fault, and controlled discharge and recharge of the HVDC links during shutdown and restart following clearance of temporary pole-to-pole dc faults. Additionally, it allows voltage source converter based HVDC links that employ FB-MMC to be operated with both positive and negative dc negative dc link voltages. This feature is well suited for hybrid HVDC networks, where the voltage source converters are operated alongside the line commutating current source converters, without any compromise to the power reversal at any terminals. The usefulness of the presented control strategy is demonstrated on full-scale model of HVDC link that uses FB-MMC with 101 cells per arm, considering the cases of pole-to-ground and pole-to-pole dc faults

A hybrid multilevel converter for medium and high voltage applications

This paper investigates the suitability of the hybrid multilevel converter for medium and high voltage application. The converter operation, modulation, and capacitor voltage balancing method are described in detail. The ability of the hybrid multilevel converter to operate with different modulation indices and load power factors is investigated. It has been established that the hybrid multilevel converter is capable of operating independent of load power factor. Operation with variable modulation index increases voltage stresses on the converter switches and does not alter the fundamental voltage magnitude as in all known voltage source converter topologies. The viability of the hybrid multilevel converter for medium and high voltage applications is confirmed by simulations

STATCOM based on modular multilevel converter : dynamic performance and transient response during AC network disturbances

This paper presents detailed assessment of the behaviour of STATCOM based on modular multilevel converter during steady-state and transient operation. The steady-state performance of the presented STATCOM is examined when it provides autonomous voltage regulation across number of switch loads. Its transient response is examined by subjecting the test system where STATCOM is connected to symmetrical and asymmetrical ac network faults. In this work, STATCOM power circuit is modelled using detailed switch model of modular converter with 16 cells per arm, including capacitor voltage balancing strategy, and control systems are represented detail (dc and ac voltage regulars, and current controller). Simulations conducted in Matlab-Simulink enlivenment are used to assess the STATCOM performance

Enhanced control of offshore wind farms connected to MTDC network using partially selective DC fault protection

In recent years, several DC fault clearance schemes have emerged, in which reduced number of fast acting DC circuit breakers (DCCBs) and AC circuit breakers (ACCBs) are used to clear DC faults. In offshore DC grids, such approach entails opening of the ACCBs that connect the wind farms to the offshore HVDC stations which control offshore AC voltages and frequencies, potentially leading to uncontrolled offshore voltage and frequency. Existing studies show that the loss of offshore converter due to blocking or sudden opening of ACCBs can cause significant over-voltage and over-frequency in the offshore AC grid, which could necessitate immediate shutdown of the wind farm. An enhanced control for wind turbine converters (WTCs) of the offshore wind farm is proposed to enable retention of AC voltage and frequency control when the offshore converter is lost, in which seamless transition of the WTCs between grid following and forming modes is facilitated. The viability of the proposed control is demonstrated in wider context of partially selective DC fault protection in an illustrative meshed DC grid, which includes detailed implementations of DC fault clearance, system restart and power transfer resumption. The presented simulation results confirm the effectiveness of the proposed WTC control in preventing excessive rise of offshore AC voltage and frequency and facilitating DC fault ride-through using reduced number of DCCBs

Controlled transition full-bridge hybrid multilevel converter with chain-links of full-bridge cells

This paper proposes a controlled transition full-bridge (CTFB) hybrid multilevel converter (HMC) for medium and high voltage applications. It employs a full-bridge cell chain-link (FB-CL) between the two legs in each phase to generate multilevel bipolar output voltage. The CTFB-HMC has twice dc voltage utilization or power density of conventional converters due to the bipolar capability of its full-bridge configuration. Hence, for the same power rating and same voltage level number, its total cells per phase are quarter that in modular multilevel converter (MMC), which reduces the hardware installation volume. Also, in the proposed converter, the total device number in the conduction paths is the same as in the half-bridge MMC, leading to low conduction losses. The FB-CL current of the CTFB converter has no dc component, which offers the potential to enhance the transient response. Comparative studies between the CTFB and other multilevel topologies are carried out to clarify its main features. The modulation strategies and parameter sizing of the proposed converter are investigated using a generic case. Simulation and experimental results are used to verify the effectiveness of the proposed approach

Efficiency evaluation of DC transmission system based on voltage source converter

This paper discusses efficiency evaluation of DC transmission system based on voltage source converter. It was presented at the 5th IET International Conference on Power Electronics, Machines and Drives in April 2010