Grid-side Voltage Source Converter Research Articles

Supercapacitor‐based coordinated synthetic inertia scheme for voltage source converter‐based HVDC integrated offshore wind farm

AbstractA supercapacitor‐based coordinated synthetic inertia (SCSI) scheme for a voltage source converter‐based HVDC (VSC‐HVDC)‐integrated offshore wind farm (OWF) is proposed. The proposed SCSI allows the OWF to provide a designated inertial response to an onshore grid. Under the SCSI scheme, a supercapacitor is added to the DC side of each wind turbine generator via a bidirectional DC/DC converter, varying its voltage along with the offshore frequency to synthesise the desired inertial response. The HVDC grid side VSC employs a DC voltage/frequency droop control to convey the onshore frequency information to DC voltage without communication. Meanwhile, the wind farm side VSC regulates the offshore frequency to couple with the conveyed onshore frequency, considering voltage drop across the DC cables. An offshore frequency switching algorithm is incorporated to avoid undesired SCSI maloperation under offshore faults. The key parameters of the proposed SCSI are optimised through a small signal stability analysis. The effectiveness of the SCSI scheme is evaluated using a modified IEEE 39‐bus test system. The results show that the proposed SCSI scheme can provide required inertial support from WTG‐installed supercapacitors to the onshore grid through the VSC‐HVDC link, significantly improving the onshore frequency stability.

A Coordinated Voltage-Frequency Support Method for VSC-MTDC Integrated Offshore Wind Farms System

Voltage and frequency stability is one of the major concerns with the continuously increasing penetration of offshore wind farms (OWFs) integrated into onshore grid via VSC-MTDC system, and it is a challenge to coordinate its low voltage ride-through (LVRT) and frequency regulation (FR) control when a local fault or large disturbance occurs in onshore grid. To address this challenge, a coordinated voltage-frequency support (CVFS) method for the VSC-MTDC integrated OWFs system is proposed in three parts: 1) an adaptive maximum power point tracking (MPPT) strategy is developed for wind turbines (WTs) to satisfy the power demands for LVRT and FR; 2) an active and reactive power quantification strategy, which firstly considers the static load characteristics of nodes nearest the grid side VSC (GSVSC) stations, is proposed for GSVSC stations to support LVRT and FR respectively; 3) an adaptive power allocation strategy, which considers the capacity constraint and the coordination factor with deviation and rate of change of voltage and frequency of different GSVSC stations, is conducted to achieve differential power distribution between LVRT and FR. Extensive test results have validated the effectiveness and performance of the proposed CVFS method.

Robust Adaptive Active Islanding Detection With $\alpha$-Axis Disturbance Injection Under High Impedance Fault, Unbalanced Loading, and Phase Failure in Grid Tied PV System

Power generation with photovoltaic technology is growing expeditiously. With the high penetration of solar-based generation, a concern of islanding detection is always by the distributors due to grid side faults and abnormality. It is the demand of the situation to equip the PV-connected inverter with an islanding detection mechanism to avoid any catastrophe. In the present work, an active islanding detection technique utilizing a smaller magnitude current injection through <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\alpha$</tex-math></inline-formula> -axis current controller of grid side voltage source converter (VSC) is proposed. Due to injection of current through VSC, point of common coupling (PCC) voltage is having different harmonic component in the grid connected and islanded mode. In the proposed detection method, the sensed PCC voltage is used to derive the three signals i.e. AVVM(absolute <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$v_{mag}$</tex-math></inline-formula> variation mean), AMSVV(absolute mean of square variation of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$v_{mag}$</tex-math></inline-formula> ), & FAVVM(Frequency of absolute <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$v_{mag}$</tex-math></inline-formula> variation mean). These three derived signals is used to take the decision for islanding detection. Further, Complex Co-efficient Filter-Second Order Frequency Locked Loop (CCF-SOFLL) is proposed for three phase grid synchronization and frequency estimation. The proposed Strategy is tested under high impedance fault, phase failure and unbalanced loading conditions. It is found that the proposed work clearly differentiates the islanding and faults condition. The Non- detection zone (NDZ) of the proposed strategy is verified under different quality factors ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$Q_{f}$</tex-math></inline-formula> ) of the load. To evaluate the effectiveness of the suggested strategy, MATLAB Simulation and experimental validation is performed. The obtained islanding detection time is within the IEEE-1547 standard.

Coordinated Power Oscillation Damping From a VSC-HVDC Grid Integrated With Offshore Wind Farms: Using Capacitors Energy

This paper proposes a novel coordinated control strategy for a voltage source converter (VSC) based high-voltage direct current (HVDC) grid integrated offshore wind farms (OWFs) to damp the power system oscillations. A feature of this strategy is aiming to use the DC-link capacitor energy of offshore wind turbines (WTs) to reduce interactions between power oscillations and HVDC grid voltage when onshore grid-side VSCs (GSVSCs) modulate the active and reactive power injections. Unlike the previous communication-based method, the coordination from offshore WTs in this strategy depends on the local measurements of the HVDC grid voltage instead of the remote communication data from the onshore AC grid. A modified IEEE 39-bus power system with a 5-terminal VSC-HVDC grid connected to two OWFs has been developed to validate the effectiveness of this proposed strategy. Both the eigenvalue analysis and time-domain simulation results indicate that this strategy can significantly improve the power oscillation damping (POD). Comparative simulation studies also conclude that the proposed strategy has similar POD improvements to the previous communication-based method without the negative impact of communication delay from onshore to offshore.

Damping torque coefficient analysis of PMSG‐based WT with VSG control considering wind turbine dynamics

This paper expands the damping torque coefficient analysis (DTCA) for studying the small-signal stability of permanent magnet synchronous generator (PMSG)-based wind turbine (WT) under virtual synchronous generator (VSG) control with consideration of WT dynamics. Firstly, the typical VSG is implemented in the grid-side voltage source converter (GSVSC), which enables selfsynchronization without phase-locked loop (PLL). The maximum power point track (MPPT) algorithms are utilized to give the power reference for VSG. Then, the corresponding inertia and damping support can be provided by the kinetic energy of PMSG. Based on the established model, the DTCA is analytically conducted to not only reveal the key factors that influence the WT stability but also provide guidance for parameter tuning. As a result, it is important to find that the WT dynamics impair the damping and even lead to system instability. Moreover, the minimum value of VSG damping control parameter is analytically given to ensure the system stability under any load below the rated wind speed. Finally, the case studies considering one single PMSG connected to a large grid with and without inclusion of WT dynamics are performed and compared to demonstrate the accuracy of the proposed model and evaluate the applications of DTCA.

An Adaptive Protection Scheme for Power Systems Considering the Effects of HVDC Interfaced Distributed Generations

Distributed Generations are usually connected to the ac grid through HVDC transmission systems. However, due to the current injected by the grid-side voltage source converter (GVSC), the voltage drop after a short-circuit fault will be partly compensated. For this reason, the viewed impedance of distance relays will be affected, which is a challenge for the distance protection scheme. For this issue, this paper proposed an adaptive distance protection scheme for the system containing distributed generations. Firstly, the proposed scheme controls the current of the GSVC to be proportional to the current of the AC transmission line. Secondly, an adaptive distance protection criterion is proposed, which can obtain the impedance from the relay to the fault-point accurately. The effectiveness of the proposed approach was validated through simulation using MATLAB/Simulink.

Grid Integration of Renewable Energy Generating System Using Nonlinear Harmonic Observer Under Nonideal Distribution System

The renewable energy sources are intermittent in nature with high availability-based uncertainties. These resources when integrated to the utility grid significantly hinder the grid power quality (PQ). This article deals with PQ issues arising in the topology including the grid connected renewable energy generating system, which is a combination of wind turbine (WT) driving the synchronous generator (SG) and solar photovoltaic (PV) array and control strategies to improve the same. The solar PV array is associated at the dc link eliminating the dc–dc converter stage via a common dc link. The variable revolutions per minute (RPM) operation is achieved by the back to back connected grid side VSC (GVSC) and the machine side voltage source converter (MVSC). The GVSC is controlled by using the nonlinear harmonic observer (NHO) control. It provides robust implementation at various perturbations faced practically, i.e., discontinuous jump of frequency, phase, and amplitude. The fundamental load current component is effectively extracted. The NHO control is deployed within the framework to allow the system to dynamically adapt to the changes in the operating environment while maintaining the PQ of the utility grid. A special attention is paid to the condition when the power generated by the solar PV array is zero (during night operation) and the GVSC acts in the distribution static compensator (DSTATCOM) mode fulfilling the reactive power requirement. The regulation of MVSC is achieved by the vector control. Independent optimal power extraction techniques are featured for both wind and solar PV array. Test conditions are realized on the laboratory developed prototype and test results are acquired for PQ, loss of supply from solar PV array, effects of randomness of the wind speed, and solar insolation along with load alterations.

Modeling, Parameter Measurement, and Control of PMSG-based Grid-connected Wind Energy Conversion System

The design of reliable controllers for wind energy conversion systems (WECSs) requires a dynamic model and accurate parameters of the wind generator. In this paper, a dynamic model and the parameter measurement and control of a direct-drive variable-speed WECS with a permanent magnet synchronous generator (PMSG) are presented. An experimental method is developed for measuring the key parameters of the PMSG. The measured parameters are used in the design of the controllers. The generator-side converter is controlled using a vector control scheme that maximizes the power extraction under varying wind speeds. A model predictive controller (MPC) is designed for the grid-side voltage source converter (VSC) to regulate the active and reactive power flows to the power grid by controlling the $d$ - and $q$ -axis currents in the synchronous reference frame. The MPC predicts the future values of the control variables and takes control actions based on the minimum value of the cost functions. To comply with the grid code requirement, a modified design approach for an LCL filter is presented and incorporated into the system. The design process is simple and incorporates significant filter parameters while avoiding iterative calculations. The comparative analysis of the designed filter with conventional L, LC, and iterative LCL filters demonstrates the effectiveness of the modified design approach. The proposed wind energy system with MPC and LCL filter is simulated in MATLAB/Simulink and experimentally implemented in the laboratory using the dSpace digital signal processor (DSP) system. The simulation and experimental results validate the efficacy of the designed controllers using the measured parameters and show dynamic and steady-state performance under varying wind speeds.

An Improved Control Technique for Grid Interactive 4-Phase SRM Driven Solar Powered WPS Using Three-Level Boost Converter

This article aims to develop a unique configuration of a solar-powered water pumping system (SPWPS), which ensures an uninterruptable water supply along with the earning possibility for the consumer. This water pumping system utilizing the switched reluctance motor drive, needs no current sensor on the motor side to run the system. The concept of integrating the grid to the SPWPS as an electrical energy storage device, provides assistance against the fluctuations in power, minimum electricity cost and earns from excess of photovoltaic power. An efficient control logic for grid-side voltage source converter is developed in this article. It has multiple intends like to regulate the desired dc-link voltage, to maintain the power quality and to improve the dynamic responses. This controller incorporates the second- and third-order generalized integrator-based filter for fundamental component extraction of grid voltage and has the benefits of both the filters. Moreover, the new control logic for dual output boost converter as a maximum power point tracking converter is also proposed in this article. It helps to simultaneously maximize the solar power as well as to nullify the voltage asymmetry across both dc-link split capacitors of a mid-point converter under all operating modes. The applicability of the developed arrangement with designed control logic is verified by experimental results.

Photovoltaic‐battery powered grid connected system using multi‐structural adaptive circular noise estimation control

The intermittent nature of renewable energy sources results in an interruptible and inadequate power. The integration of the battery energy storage to renewable energy sources provides reliable and continuous power to the loads. Further, the high penetration of renewable energy sources raises the power quality problems in the grid. This work deals with performance improvement of PV-battery energy storage based energy conversion system achieved through implementing a multi-structural adaptive circular noise estimation control for its grid connected mode. The control is developed for the grid side voltage source converter, which provides the power quality improvement and automated transition from grid connected mode to standalone mode or vice-versa. It provides multi-functional features such as harmonics extraction, DC offset elimination, and power quality improvement in PV-battery energy storage based energy conversion system even at nonlinear loading condition in grid connected mode. In standalone mode, the amplitude and waveform of load voltage are controlled sinusoidal by the voltage control. Performance of PV-battery energy storage based energy conversion system is studied to validate the acceptability of these controls according to the IEEE-519 standard.

Hybrid control strategy for effective frequency regulation and power sharing in multi‐terminal HVDC grids

This study proposes a hybrid control (HC) strategy for improved frequency regulation and power sharing in multi-terminal HVDC (MTDC) integrated AC grids. The proposed method uses the topology of bi-polar converters in the MTDC system to improve power sharing and frequency regulation. In this methodology, the two voltage source converters (VSCs) in a bi-polar topology are operated with individual specified control methods. In the proposed HC method, one of the converters operate in voltage square frequency droop ( ) and another in power frequency droop (Pf) control methods. This HC strategy is implemented for grid side voltage source converters in MTDC grids. Further, the performance of frequency regulation and autonomous power sharing of the proposed HC technique is compared with the conventional voltage and frequency droop (PVf) and improved control methodologies. In order to validate the proposed method, two AC-MTDC systems are considered. In both the systems, mesh typed MTDC CIGRE B4 DC test system is in common, it is integrated with two-area power system and New England IEEE 39-bus system to form two different AC-MTDC systems.

A High-Performance Microgrid With a Mechanical Sensorless SynRG Operated Wind Energy Generating System

In this article, a new control methodology is implemented for a microgrid consisting of wind-battery and the grid, which operates both under grid-connected mode and standalone mode. A synchronous reluctance generator (SynRG) with a new mechanical sensorless field-oriented control is used in the wind energy generation system (WEGS). The major issues present in the sensorless control of SynRG, due to the use of conventional integrator or low-pass filter during the flux estimation, are addressed in this article by implementing a fourth-order flux estimator. To compensate for the intermittent nature of the power generated from the WEGS, and to improve the reliability of the system pertaining to power availability, energy storage is included in the system. Controlled operation of the battery is ensured by using a bidirectional dc–dc converter, to enhance the longevity of the battery. Voltage source converters (VSCs) connected in a back-to-back configuration with a common dc-link is selected here so that a complete decoupling is achieved between the grid and WEGS. Grid-side VSC (GSC) is controlled, so that power quality at the point of common coupling is improved. The positive sequence components of grid voltages are extracted using observers to implement GSC control to avoid the adverse effects of unbalance and harmonic content in grid voltages. Design, modeling, and simulation of the system are done in MATLAB/SIMULINK platform, and a prototype developed in the laboratory is used for real-time validation.

A Multi-Terminal HVdc Grid Topology Proposal for Offshore Wind Farms

Although various topologies of multi-terminal high voltage direct current (MT-HVdc) transmission systems are available in the literature, most of them are prone to loss of flexibility, reliability, stability, and redundancy in the events of grid contingencies. In this research, two new wind farms and substation ring topology (2WF-SSRT) are designed and proposed to address the aforementioned shortcomings. The objective of this paper is to investigate MT-HVdc grid topologies for integrating large offshore wind farms with an emphasis on power loss in the event of a dc grid fault or mainland alternating current (ac)grid abnormality. Standards and control of voltage source converter (VSC) based MT-HVdc grids are defined and discussed. High voltage dc switch-gear and dc circuit topologies are appraised based on the necessity of dc cables, HVdc circuit breakers, and extra offshore platforms. In this paper, the proposed topology is analyzed and compared with the formers for number and ratings of offshore substations, dc breakers, ultra-fast mechanical actuators, dc circuits, cost, flexibility, utilization, and redundancy of HVdc links. Coordinated operation of various topologies is assessed and compared with respect to the designed control scheme via a developed EMTDC/PSCAD simulation platform considering three fault scenarios: dc fault on transmission link connecting the wind farm to mainland power converters, dc fault within substation ring of VSC-HVdc stations, and ultimate disconnection of grid side VSC station. Results show that 2WF-SSRT is a promising topology for future MT-HVdc grids.

Disturbance Rejection Through Adaptive Frequency Estimation Observer for Wind-Solar Integrated AC Microgrid

The renewable energy generation is the cornerstone component of the greener and cleaner ac microgrid. The high penetration of wind and solar photovoltaic energy sources in the distribution network causes unbalanced voltages, voltage rise, flicker, and internal and external uncertainties leading to disturbance and unreliable operation of the system. This paper presents the performance enhancement of the wind-solar integrated ac microgrid by implementing the adaptive frequency estimation observer (AFEO) aiming for disturbance rejection. The model uncertainties and unknown disturbances are taken care of by nonlinear active disturbance rejection controller based phase-locked loop in conjunction with the AFEO. No detailed mathematical model is required, instead the controller provides decent anti-interference and effortless parameter tuning. The fluctuations in power, interharmonics, current harmonics, and power quality (PQ) issues produced by wind and solar energy resource permeation into the grid, are addressed and mitigated by successful implementation of the AFEO for switching the grid side voltage source converter. The machine side converter obtains its switching pulses from vector control scheme. The encoderless speed and rotor position estimation of the synchronous generator (SG) driven by wind turbine is carried out by back electromotive force technique. The environmental variation of the wind and solar energy are overcome by utilizing perturb and observe scheme for optimal power extraction. Test results are obtained from the laboratory prototype under steady state and dynamic conditions including environmental changes, i.e., intermittent wind speed, solar insolation, and variable load conditions. PQ issues are addressed, investigated, and mitigated successfully.

Development of Wind and Solar Based AC Microgrid With Power Quality Improvement for Local Nonlinear Load Using MLMS

This work proposes a microgrid ( μ -grid) integrating wind and solar photovoltaic (PV) resources, along with the battery energy storage (BES) to the three-phase grid feeding the nonlinear load. The μ -grid disconcerted by probabilistic nonlinear time dependent parameters and their effects are compensated by cohesive controllers used for utility grid side voltage source converter (GVSC) and machine side voltage source converter (MVSC). The switching controls and the reconfigurability of the μ -grid are addressed on imperative aspects of improving power quality (PQ), power reliability, nonlinear load compensation, and economic utilization of resources. The nonlinear load compensation and PQ enhancement are achieved by executing modified version of the adaptive filtering technique including “momentum”-based least mean square (MLMS) control technique, utilized for providing the switching control signals to the GVSC. It utilizes two preceding gradient weights for obtaining updated weight thereby improving the convergence rate and overcoming the limitation of conventional control of the same family. The MVSC acquires its switching signals from conventional vector control scheme and the encoderless estimation of speed and rotor position of the synchronous generator driven by wind turbine through back electromotive force control technique. The external environmental disturbances are overcome by utilizing perturb and observe (P&O) maximum power point (MPP) for wind optimal power extraction and adaptive P&O with variable perturbation step size for solar MPP estimation. Test results are obtained from the laboratory prototype under steady-state and dynamic conditions, including altering wind speed, intermittent solar insolation, and variable load conditions. The PQ issues are addressed and investigated successfully.

Toward Flexible Risk-Limiting Operation of Multi-Terminal HVDC Grids With Vast Wind Generation

Contingencies occurring in multi-terminal high voltage direct current (MT-HVDC) grids can result in DC voltage/flow violations and also affect the frequency stability of the connected multiple asynchronous grids. To recognize and control such notable operating risks, a novel flexible risk-limiting optimal power flow (FROPF) for the MT-HVDC grid with vast wind generation is proposed in this paper. Within the two-stage FROPF structure, the pre-contingency operation of grid-side voltage-source converters (GVSCs) is optimized to minimize MT-HVDC grid power losses. Immediately following an outage occurring in the MT-HVDC grid, various fast-acting corrective actions of GVSCs are utilized to hedge against the overall risk exposure, including the wind power curtailment risk of the MT-HVDC grid and the rate-of-change-of-frequency (RoCoF) violation risk imposed on associated asynchronous grids. Reformulation techniques are introduced to ease the computational complexity of the optimization model. Case studies of two MT-HVDC grids demonstrate the effectiveness of the proposed FROPF.

Sensor Fault Resilient Operation of Permanent Magnet Synchronous Generator Based Wind Energy Conversion System

This paper presents a sensor fault resilient control approach for permanent magnet synchronous generator (PMSG) based direct drive wind energy conversion systems (WECSs). The measurement accuracy of WECS quantities, such as generator and grid-side currents, generator speed, and dc link voltage are of paramount importance to ensure reliable and efficient operation of PMSG-based WECSs, since these measurements are essential to derive control actions for the power electronic interfaces in the WECSs. Erroneous measurements of WECS quantities due to malfunctioning of corresponding sensors can adversely affect the efficient and reliable operation of the WECS. In this paper, any error in the sensor measurements are considered as sensor fault. The proposed approach comprises a sliding mode observer based state and fault estimation system, a fault mitigation algorithm, and indirect vector control approaches for generator and grid-side voltage source converters (VSCs) to ensure sensor fault resilient operation of WECSs. The efficacy of the proposed approach is validated through rigorous simulation studies carried out on a WECS connected to a practical test distribution system, which clearly demonstrates that the proposed approach is capable of nullifying the impact of erroneous measurements due to sensor malfunctioning and ensure efficient and optimal operation of WECSs.

Weak Grid Intertie WEGS With Hybrid Generalized Integrator for Power Quality Improvement

The wind energy generating system (WEGS) in this paper, aims to provide the required active peak power without high frequency fluctuations even under variable wind speed conditions. The generator speed is adjusted according to the intermittent wind speeds through two voltage source converters (VSCs) connected back to back, namely, machine-side VSC (MSVSC) and utility grid side VSC (UGVSC) across the dc-link capacitor. In this paper, the hybrid generalized integrator control is proposed for switching UGVSC and providing dc offset rejection and immunity against oscillatory errors due to subharmonics, thereby improving the power quality (PQ). Fuzzy logic controller (FLC) is implemented for the speed control of the salient pole synchronous generator (SG) driven by the wind turbine. The FLC provides the tracking of the reference speed under high overshoot transient conditions and narrow bandwidth. The switching of MSVSC is obtained by field oriented control. The dynamic performance is improved by the wind feed-forward term, which reduces the oscillation, ensuring balanced and sinusoidal grid currents. The generated power from the WEGS is fed to the grid. The weak grid conditions, namely, grid voltage unbalance, voltage sag, voltage swell, and grid voltage distortion, are considered. The performance of the system is tested on a laboratory prototype. Test results provide the effectiveness of the system with increased wind penetration and performance under weak grid conditions. Moreover, improving the PQ, the grid current total harmonic distortion meets the requirement of IEEE-519 standard, and is found to be less than 5%.

An Intelligent Implementation of Fuzzy Search Based Nonlinear Droop Control Scheme for Multi-Terminal DC Transmission Systems

Multi-Terminal DC (MTDC) transmission systems typically operate under active power and DC voltage ( $P$ - $U_{DC}$ ) droop control for active power dispatch, and its design is greatly influenced by DC network characteristics. Hence, this paper firstly analyzes $P$ - $U_{DC}$ droop characteristics when implemented on Grid-Side Voltage-Source Converters (GSVSCs) of a MTDC network, in which variations of DC cables’ resistances with temperature are taken into account. The analysis establishes that the actual droop characteristics of GSVSCs are nonlinear, different from the widely assumption of linear relationships. This finding is the fundamental for the design of the accurate power dispatch for GSVSCs. Therefore, an improved Fuzzy Search based Droop Control (FSDC) scheme for MTDC systems is proposed, which performs real-time search to capture the accurate droop curves of GSVSCs over full range of permissible DC voltage and active power. The FSDC is implemented without the assumption of the prior knowledge of DC network parameters which are subject to variations and uncertainty, and instead the accurate droop curves are periodically obtained and updated. The technical viability of the FSDC and its superiority over the conventional linear droop scheme under the variations of DC cable resistances and power flow variations is verified by a four-terminal HVDC model established in MATLAB/Simulink.

A Robust Distance Protection Approach for Bulk AC Power System Considering the Effects of HVDC Interfaced Offshore Wind Units

Offshore wind generators are connected to the ac power grid through high-voltage direct current (HVDC) transmission systems. However, when a short-circuit fault occurs in a transmission line, which is connected to the point of common coupling (PCC) of the HVDC system, the performance of distance relays on the ac grid is affected. This is due to rapid response of reactive power control of the grid-side voltage source converter (GVSC) of the HVDC system to compensate the voltage drop of the PCC bus. In other words, during the fault, the GVSC injects the reactive power to the ac grid to restore the PCC bus voltage. This in turn causes to interfere with the distance protection scheme by affecting the viewed impedance of distance relays. To overcome this problem, based on robust control theory, the present paper introduces a robust control approach for voltage source converter of HVDC system using μ synthesis analysis. The proposed strategy controls the reactive power injected to the PCC bus by trying to keep the defined current ratio $\alpha $ constant. The effectiveness of the proposed approach was validated through simulation using MATLAB/Simulink.