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

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

Inertia emulation control strategy for VSC-HVDC transmission systems

There is concern that the levels of inertia in power systems may decrease in the future, due to increased levels of energy being provided from renewable sources, which typically have little or no inertia. Voltage source converters (VSC) used in high voltage direct current (HVDC) transmission applications are often deliberately controlled in order to de-couple transients to prevent propagation of instability between interconnected systems. However, this can deny much needed support during transients that would otherwise be available from system inertia provided by rotating plant

Modular multilevel converter with modified half-bridge submodule and arm filter for dc transmission systems with DC fault blocking capability

Although a modular multilevel converter (MMC) is universally accepted as a suitable converter topology for the high voltage dc transmission systems, its dc fault ride performance requires substantial improvement in order to be used in critical infrastructures such as transnational multi-terminal dc (MTDC) networks. Therefore, this paper proposes a modified submodule circuit for modular multilevel converter that offers an improved dc fault ride through performance with reduced semiconductor losses and enhanced control flexibility compared to that achievable with full-bridge submodules. The use of the proposed submodules allows MMC to retain its modularity; with semiconductor loss similar to that of the mixed submodules MMC, but higher than that of the half-bridge submodules. Besides dc fault blocking, the proposed submodule offers the possibility of controlling ac current in-feed during pole-to-pole dc short circuit fault, and this makes such submodule increasingly attractive and useful for continued operation of MTDC networks during dc faults. The aforesaid attributes are validated using simulations performed in MATLAB/SIMULINK, and substantiated experimentally using the proposed submodule topology on a 4-level small-scale MMC prototype

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

Analysis and design of a modular multilevel converter with trapezoidal modulation for medium and high voltage DC-DC transformers

Conventional dual active bridge topologies provide galvanic isolation and soft-switching over a reasonable operating range without dedicated resonant circuits. However, scaling the two-level dual active bridge to higher dc voltage levels is impeded by several challenges among which the high dv/dt stress on the coupling transformer insulation. Gating and thermal characteristics of series switch arrays add to the limitations. To avoid the use of standard bulky modular multilevel bridges, this paper analyzes an alternative modulation technique where staircase approximated trapezoidal voltage waveforms are produced; thus alleviating developed dv/dt stresses. Modular design is realized by the utilization of half-bridge chopper cells. Therefore, the analyzed converter is a modular multi-level converter operated in a new mode with no common-mode dc arm currents as well as reduced capacitor size, hence reduced cell footprint. Suitable switching patterns are developed and various design and operation aspects are studied. Soft switching characteristics will be shown to be comparable to those of the two-level dual active bridge. Experimental results from a scaled test rig validate the presented concept

Comparative analysis of three low voltage fault ride through techniques for wind energy conversion systems

This paper compares the performances of three different Low Voltage Fault Ride- Through (LVFRT) techniques for Wind Energy Conversion Systems (WECS). The comparison aims to identify the most effective technique for alleviation of adverse impacts of AC faults on WECS electrical and mechanical parts, which include DC voltage rise and generator over-speed. The comparison is based on a critical qualitative review of existing literature on the selected LVFRT techniques, which are further supported by quantitative substantiation using simulations. The major findings of this comparative study are highlighted, with emphasis on metrics, which account for practical implementation, hardware, cost, and complexity issues. They are important to assess the overall effectiveness of the techniques evaluated. Although practical and commercial limitations exist, the initial findings suggest that an energy storage solution would be suitable for the enhancement of LVFRT for WECS in future power networks, and if the stored energy is utilised correctly, offer further attractive benefits

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

Real time evaluation of wavelet transform for fast and efficient HVDC grid non-unit protection

This paper presents a real-time evaluation of a Wavelet Transform (WT) for HVDC grid non-unit protection. Due to its time and frequency localisation capability, WT can successfully extract the necessary information present in the voltage transients following a DC fault. This capability is exploited to achieve fast and selective HVDC grid protection. A Digital Signal Processor (DSP) is employed to execute real-time Stationary Wavelet Transform (SWT) on voltage signals using discrete convolution to efficiently compute the WT coefficients. Hardware-in-the loop (HIL) simulation is performed to test a WT-based hardware module using a Digital Real-Time Simulator (DRTS), in which a meshed HVDC grid is modelled. The closed-loop interaction enables the hardware device to emulate a protection relay that can generate trip commands for the HVDC breakers integrated within the HVDC grid model. The real-time simulations demonstrate the technical feasibility, speed and robust performance of the SWT implementation