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

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

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

Impact of submodule faults on the performance of modular multilevel converters

Modular multilevel converter (MMC) is well suited for high-power and medium-voltage applications. However, its performance is adversely affected by asymmetry that might be introduced by the failure of a limited number of submodules (SMs) or even by severe deviations in the values of SM capacitors and arm inductors, particularly when the number of SMs per arm is relatively low. Although a safe-failed operation is easily achieved through the incorporation of redundant SMs, the SMs' faults make MMC arms present unequal impedances, which leads to undesirable internal dynamics because of unequal power distribution between the arms. The severity of these undesirable dynamics varies with the implementation of auxiliary controllers that regulate the MMC internal dynamics. This paper studied the impact of SMs failure on the MMC internal dynamics performance, considering two implementations of internal dynamics control, including a direct control method for suppressing the fundamental component that may arise in the dc-link current. Performances of the presented and widely-appreciated conventional methods for regulating MMC internal dynamics were assessed under normal and SM fault conditions, using detailed time-domain simulations and considering both active and reactive power applications. The effectiveness of control methods is also verified by the experiment. Related trade-offs of the control methods are presented, whereas it is found that the adverse impact of SMs failure on MMC ac and dc side performances could be minimized with appropriate control countermeasures

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

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

DC fault protection of diode rectifier unit based HVDC system connecting offshore wind farms

DC fault ride-through operations of the offshore wind farm connecting with diode rectifier unit (DRU) based HVDC link are presented in this paper. A voltage-error-dependent fault current injection is proposed to regulate the WT current during DC faults and to provide fault current. This contributes the control of the offshore AC voltage, which does not drop to zero but is remained relatively high to facilitate fast system recovery after clearance of a temporary DC fault. The WT converters operate on current limiting mode during DC faults and automatically restore normal operation after fault clearance. The full-bridge based modular multilevel converter (MMC) is adopted as the onshore station and its DC fault current control ability is explored to effectively suppress the fault current from the onshore station around zero, which reduces semiconductor losses and potential overcurrent risk of the MMC station. Simulation results confirm the robustness of the system to DC faults