Voltage Source Converter HVDC Research Articles

Inertia and Primary Frequency Provisions of PLL-Synchronized VSC HVDC When Attached to Islanded AC System

The islanded scenarios of voltage source converter (VSC) high-voltage DC (HVDC) directly supplying power to islanded loads or connecting to wind farm are becoming popular, in which some new issues are required to be further addressed. While several control strategies have been studied, to the authors’ best knowledge, few have investigated the potential capability of VSC HVDC using the typical phase-locked loop (PLL)-synchronized vector control (PLLSynVC) to address islanded operation issues. To explore the potential capability of the typical PLLSynVC in islanded system, this paper proposes a frequency regulation scheme (FRS), including inertia and primary frequency control, for the PLL-synchronized VSC HVDC based on the motion equation concept. By adding the FRS, the VSC HVDC using the PLLSynVC, similar to a synchronous generator from the view of the phase motion equation, not only can self-maintain the frequency stable in an islanded system, but also naturally avoids the transition of control operation modes without changing the basic control structure. Furthermore, based on the developed phase motion equation, the inertia characteristics of VSC HVDC with the FRS is studied, which synthetically considers the impact of different controllers and PLL, and makes VSC HVDC's manifested inertia clear. The effectiveness of FRS and inertia analysis results is verified by time-domain simulations.

A Black Start Scheme Based on Modular Multilevel Control-High Voltage Direct Current

Modern society’s need for power and its dependence on power sources is increasing with its development. As a result the consequences of blackout accidents have become more serious. At present, only three sources can be regarded as black-start power sources, namely Line Commutate Converter-High Voltage Direct Current (LCC-HVDC), Voltage Source Converter-High Voltage Direct Current (VSC-HVDC), and microgrids. The potential of these three sources has already been demonstrated. Lately, VSC-HVDC primarily uses the Modular Multilevel Converter (MMC) topology structure, also known as MMC-HVDC. Some domestic and foreign papers have also proposed using VSC-HVDC as a black-start power source for large power failures. However, most of these studies have analyzed only 2-level or 3-level VSC-HVDC topologies and there is no MMC topology for VSC-HVDC analysis. Therefore, the conclusion that VSC-HVDC can be used as black-start power source cannot be held true for a MMC-HVDC structures. To understand the characteristics of MMC-HVDC as a black-start power source, it is necessary to analyze the MMC-HVDC converter and do a comprehensive assessment of its performance as a black-start power source. This study analyzes the feasibility of MMC-HVDC as a start power source for black start from with regards to three aspects, namely the working principle, control strategy, and implementation scheme. We designed a corresponding MMC-HVDC control strategy depending upon the analysis result. Moreover, a complete power grid restoration process is also designed according to the black-start scheme principle, which is based on the theory of MMC-HVDC as a black start power source. Finally, the whole black start process is performed in PSCAD/EMTDC. The simulation results show that the voltage and frequency of system are maintained in a steady range during the black start realization process and the validity of MMC-HVDC as a black start power source is also verified.

Model Predictive Control Approach for Bridge-Type Fault Current Limiter in VSC-HVDC System

Voltage source converter–high-voltage DC (VSC-HVDC) system has number of advantages over traditional line commutated converter HVDC. However, VSC-HVDC system is exposed to high current due to faults having great negative effect on converters. In order to limit fault current to relatively low level, this paper proposes model predictive control (MPC)-based bridge-type fault current limiter (BFCL) for VSC-HVDC system. Fault current limiters are placed with the AC grid sides of VSC-HVDC system. Finite control set MPC is developed for the control of VSC-HVDC system along with BFCL. BFCL controller has been developed to insert resistance and reactance during disturbances. Balanced and unbalanced disturbances are applied to evaluate the effectiveness of proposed BFCL controller so as to limit the fault current and augment transient stability. Real-time digital simulator has been used to conduct simulation works. The performance of the proposed MPC-BFCL is compared with that of series dynamic braking resistor (SDBR). Comparative simulation results show that the proposed MPC-BFCL is superior over SDBR in improving dynamic stability of VSC-HVDC system.

Stability and control of mixed AC–DC systems with VSC‐HVDC: a review

Voltage-source converter-high-voltage direct current (VSC-HVDC) systems have become an attractive option for integrating remote and far-from-shore renewable energy resources to main AC grids. The desire for greater power transfer capability and the difficulty in securing right-of-way for new AC lines in many countries is also resulting in the increased use of embedded VSC-HVDC systems operating in parallel with existing AC lines. It has been stated that the control and operation of VSC-HVDC systems are of particular concern for weak grids with fewer large synchronous generation units (a highly probable case for many grids in future). If the anticipated proliferation of VSC-HVDC links continues, several aspects of system stability will be significantly impacted. This study presents an overview of the effects of VSC-HVDC control and operation on power system stability. The structure, control, control tuning, and modelling of VSC-HVDC is briefly summarised to provide context for subsequent discussion of the system dynamics. An extensive critical review of the previous research into mixed AC-DC systems incorporating VSC-HVDC is then provided including voltage stability, small and large-disturbance angle stability, high-frequency interaction, and frequency stability. Finally, recommendations are presented to guide critical future research.

Impedance Modelling and Parametric Sensitivity of a VSC-HVDC System: New Insights on Resonances and Interactions

Pervasiveness of power converters in the electric power system is expected in the future. Such large penetration will change the current power system dynamics leading to uncertain, unexpected, and potentially critical responses. This paper investigates the stability and resonance of a VSC-HVDC (Voltage Source Converter High Voltage Direct Current) link within an AC grid, whilst providing insights into resonances having a role on the grid. This is studied through the impedance-based modelling of the entire system (AC and DC grids), including controls of converters. Additionally, the impact of the different parameters of the hybrid AC-DC power system such as control systems and grid components on the system dynamics and stability is investigated. From this study, the impact of the system components and the controls of the converter on overall resonance response and stability is shown, including potential undesired sub-synchronous and harmonic resonances due to AC-DC system interactions. The analytical impedance-based models developed and obtained is validated through time-domain simulations, the physical model of the whole system is built in Simscape™ Power Systems™ and control systems in MATLAB/Simulink® (R2017b). This has demonstrated the validity of the model to deal with and detect such dynamics.

Vector Control of VSC HVDC System under Single Line to Ground Fault Condition

<span lang="EN-US">This paper proposes a model of a VSC (voltage source converter) based Back to Back HVDC system and its control technique under fault condition. From the mathematical model of the system relationship between the controlling and the controlled variables is determined to control the system parameters. An appropriate vector control technique is used to control active and reactive power and to maintain DC link voltage. The proposed controlling unit consists of outer control loop and inner control loop which effectively damped out the system oscillation and maintains the system stability. The validity of the model and the feasibility of the control method have been proved by the simulation results. In this paper the system performance is studied under fault condition is studied.</span>

Impedance Matching for VSC–HVDC and Energy Storage Damping Controllers

The small-disturbance electromechanical dynamics of a power system can be translated to an equivalent circuit model with inductances and capacitances. Damping control of active power in proportion to local frequency as with a voltage source converter (VSC)–HVdc link or energy storage then corresponds to introducing a resistor in the circuit model. This letter shows that impedance matching can be used to select resistor value and, equivalently, damping controller gain, that gives maximum damping ratio. It is also shown how the concept is employed without a circuit model.

A new fault detection and fault location method for multi-terminal high voltage direct current of offshore wind farm

This paper proposes a novel protection scheme for multi-terminal High Voltage Direct Current (MTDC) systems incorporating offshore wind farm based on high-frequency components detected from the fault current signal. This method can accurately detect the fault on each line and classify the fault types. Using the post-fault current time series, both single-ended measurements (detection and classification) and double-end measurements (location), the frequency spectrum is generated to measure the gaps between the contiguous peak frequencies giving a robust and comprehensive scheme. Unlike the previous travelling wave based methods, which must identify the travelling wavefront and require a high sampling rate, the new gap-based approach is able to give accurate fault detection and fault location using any appropriate range of post-fault signals. Furthermore, the proposed method is fault resistance independent and thus even a very high fault impedance has no effect on the fault location detection. By immediately tripping the faults, the fault-caused disturbance to the offshore wind farm is minimized. A three-terminal voltage sourced converter HVDC (VSC-HVDC) system connection of offshore wind farm is modelled in PSCAD/EMTDC (Power Systems Computer Aided Design/Electro-Magnetic Transients including DC) software, which is used for obtaining the fault current data for the transmission line terminal. The algorithm is verified by studying a range of cases, by varying the fault resistance fault locations and also including external faults. The results show that the proposed method gives an accurate and reliable fault detection, classification and location on the test MTDC system.

Small-Signal Stability Analysis of Offshore AC Network Having Multiple VSC-HVDC Systems

This paper presents a methodology to perform small-signal analysis of an offshore ac network, which is formed by interconnecting several offshore wind power plants. The offshore ac network is connected with different onshore ac grids using point-to-point voltage-source-converter high-voltage direct current (VSC-HVDC) transmission links. In such a network, each offshore VSC-HVDC converter operates in grid-forming mode. In this paper, the offshore VSC grid-forming control is enhanced by using a frequency and voltage droop scheme in order to establish a coordinated grid control among the offshore converters. A small-signal model of the offshore ac network is developed that includes the high-voltage alternating current cables' model, the converters' current, and voltage-control model, the frequency droop scheme, and the voltage droop scheme. Based on this model, an eigenvalue analysis is performed in order to study the influence of the frequency and voltage droop gains on overall offshore ac network stability. Finally, theoretical analysis is validated by performing nonlinear dynamic simulation.

A Novel Interfacing Technique for Distributed Hybrid Simulations Combining EMT and Transient Stability Models

The steady increase of power electronic devices and nonlinear dynamic loads in large-scale ac/dc systems desperately requires an efficient simulation method. However, the traditional hybrid simulation, which incorporates various components into a single electromagnetic-transient (EMT) subsystem, causes great difficulty in network partitioning and significant deterioration in simulation efficiency. To resolve these issues, a distributed hybrid simulation method is proposed in this paper. The key factor leading the success of this method is a distinct interfacing technique, which includes: 1) a new approach based on the two-level Schur complement to update the interfaces by taking full consideration of the couplings between different EMT subsystems; and 2) a combined interaction protocol to further improve the efficiency while guaranteeing the simulation accuracy. The improved performances of the proposed method in terms of efficiency and accuracy have been verified by the simulation studies on the modified IEEE 39 system as well as a practical ac/dc system, both of which consist of a two-terminal voltage-source converter HVdc and nonlinear dynamic loads.

A State of Art Review on Offshore Wind Power Transmission Using Low Frequency AC System

Offshore wind power generation, transmission and integration around the world are becoming higher, and it provides technical and economic challenges for future practitioners and industries to make an alternative transmission system to an existing system in reduction of cost. In most of countries, e.g. The German government planned to install 25000MW of offshore wind farms by 2030. The greater part of offshore wind farms are integrated with High Voltage AC (HVAC) transmission to the onshore grid. Offshore wind farms are integrated with High Voltage DC (HVDC) transmission for long distances (>50km) to the grid because of capacitive cable current in HVAC. The major challenge in HVDC transmission is the installation, operation and maintenance (O&M) of the Voltage Sourced Converter (VSC) HVDC substation in the offshore climate. Currently, in case offshore wind farm the research have been fastened to reduce the complexity with increasing reliability and minimizing cost. This paper gives a comprehensive review of integration of offshore wind farm via Low Frequency AC or Fractional Frequency AC (LFAC or FFAC) transmission. LFAC transmission is adopted from HVAC and it is operated at one third of nominal frequency (16.67 Hz). As compared to HVDC the main advantage of LFAC is an absence of the offshore converter station, hence system complexity and cost reduced. In design considerations, especially the offshore transformer is one of the challenges. This paper presents a comprehensive review on components and its design considerations of offshore novel LFAC transmission. The offshore wind turbine considerations, collector network and different types of onshore frequency converters explained in detail.

Pseudo-Dynamic Network Modeling for PMU-Based State Estimation of Hybrid AC/DC Grids

This paper presents a PMU-based state estimation (SE) algorithm that uses a pseudo-dynamic network modeling approach. The pseudo-dynamic network model combines different equations with static network equations. Then it applies the weighted least squares algorithm to solve an over-determined least squares estimation problem. The proposed method can improve SE accuracy during both steady state and transient conditions without increasing the computational burden. In addition, the proposed modeling approach is applied to networks containing both a STATCOM and a voltage source converter-HVdc to demonstrate how to develop and apply a pseudo-dynamic SE model. Case studies aim to illustrate and verify the performance of the proposed method under steady state and transient conditions.

Dynamic Overload Capability of VSC HVDC Interconnections for Frequency Support

In future power systems, reduced overall inertia caused by an increased dominance of asynchronous generation and interconnections would make frequency control particularly challenging. As the number and power rating of voltage source converter (VSC) HVDC systems increases, network service provision would be expected from such systems and to do so would require overload capacity to be included in the converter specifications. This paper studies the provision of frequency services from modular multilevel converter (MMC)-based VSC HVDC interconnections using temperature-constrained overload capability. Overload of the MMC-based HVDC system is achieved through controlled circulating currents, at the expense of higher losses, and subject to a control scheme that dynamically limits the overload available in order to keep the semiconductor junction temperatures within operational limits. Two frequency control schemes that use the obtained overload capacity to provide frequency response during emergency conditions are investigated. The controllers’ performance is demonstrated in the context of the future Great Britain transmission grid through a reduced equivalent test system. Simulation results show that even modest temperature margins which allow overload of MMC-based HVDC systems for a few seconds are effective as a primary frequency reserve and also reduce the loss of infeed requirements of such interconnections.

Fast Frequency Response From Offshore Multiterminal VSC–HVDC Schemes

This paper analyzes the frequency support characteristics of multiterminal voltage source converter HVdc (VSC–HVdc) (MTDC) schemes using the energy transferred from wind turbine rotating mass and other ac systems. An alternative coordinated control (ACC) scheme that gives priority to a frequency versus active power droop fitted to onshore VSCs is proposed to: transfer wind turbine recovery power to undisturbed ac grids, and allow correct control operation of MTDC systems during multiple power imbalances on different ac grids. The fast frequency response capability of MTDC systems equipped with the proposed ACC scheme is compared against a coordinated control scheme, which uses a frequency versus dc voltage droop. The frequency control schemes are demonstrated on an experimental test rig, which represents a three-terminal HVdc system. Also, the MTDC frequency support capability when wind farms do not provide extra power is tested using a four-terminal HVdc system.

Optimal gain scheduling of VSC‐HVDC system sliding mode control via artificial bee colony and mine blast algorithms

This study focuses on the optimal gain scheduling of the robust non-linear sliding mode control (SMC) for the voltage source converters high-voltage direct current (VSC-HVDC) systems, for ensuring the desired dynamic behaviour improvement and stability enhancement. First, the development of the system steady-state mathematical model of non-linear character is explored in order to control the active power, the reactive power, and the DC link voltage towards their desired set point values. Their measured signals, as the outputs, are controlled via the optimal SMC-based controllers, using their reference signals directly. Both artificial bee colony and mine blast algorithm are considered to optimally select the optimal gains of SMC controller that will minimise the steady-state error of the obtained dynamic behaviour. Therefore, the DC voltage, the active and reactive powers can be properly maintained at their reference values. Both optimal SMC-based techniques are compared in terms of the behavioural analysis and robustness. Using MATLAB™-Simulink, the results significantly verify that the designed controllers based on SMC can achieve favourable tracking performance and contribute efficiently towards improving the system dynamic behaviour.

Integral Plus Resonant Sliding Mode Direct Power Control for VSC-HVDC Systems under Unbalanced Grid Voltage Conditions

An integral plus resonant sliding mode direct power control (IRSMC DPC) strategy for voltage source converter high voltage direct current (VSC-HVDC) systems under unbalanced grid voltage conditions is proposed in this paper. Through detailed instantaneous power flow analysis, a generalized power compensation method, by which the ratio between the amplitude of active and reactive power ripples can be controlled continuously, is obtained. This enables the system to provide flexible power control, so that the desired performance of the system on both the ac and dc sides can be attained under different operating conditions. When the grid voltage is unbalanced, one or both of the active and reactive power terms contain ripples, oscillating at twice the grid frequency, to obtain non-distorted ac current. A power controller consisting of the proportional, integral and resonant control laws is designed using the sliding mode control approach, to achieve accurate power control objective. Simulation studies on a two-terminal VSC-HVDC system using MATLAB/SIMULINK (R2013b, Mathworks, Natick, MA, USA) are conducted to verify the effectiveness of the IRSMC DPC strategy. The results show that this strategy ensures satisfactory performance of the system over a wide range of operating conditions.

Fault detection of HVDC cable in multi‐terminal offshore wind farms using transient sheath voltage

With respect to rapid growth of offshore wind power, multi-terminal HVDC system is going to be an attractive way of the power transmission for remote and large offshore wind farms. Since the HVDC cables are the vital component of the multi-terminal HVDC transmission system, an accurate and fast fault detection method is necessary for protection and maintaining uninterrupted power delivery to the main grid. This study presents a novel method to diagnose HVDC cable faults and unbalancing of DC capacitor bank in multi-terminal voltage source converter HVDC systems for offshore wind farms. Transient voltage of cable sheath is investigated as a main parameter for detecting types of cable faults. Extensive simulations are performed on EMTDC-PSCAD platform and the results show the effectiveness of the proposed technique.

Stability Analysis of a PMSG-Based Large Offshore Wind Farm Connected to a VSC-HVDC

This paper presents modal analysis of a large offshore wind farm using permanent magnet synchronous generator (PMSG)-type wind turbines connected to a voltage source converter HVDC (VSC-HVDC). Multiple resonant frequencies are observed in the ac grid of offshore wind farms. Their control is crucial for the uninterrupted operation of the wind farm system. The characteristics of oscillatory modes are presented using modal analysis and participation factor analysis. Sensitivity of critical modes to wind turbine design parameters and their impact on closed loop stability of the system are discussed. A comparison between a full wind farm model and an aggregated model is presented to show differences in the characteristics of critical modes observed in the models, and implication of using the models for stability studies It is concluded that robust control design is important for reliable operation of the system.

Frequency Regulation Strategies in Grid Integrated Offshore Wind Turbines via VSC-HVDC Technology: A Review

The inclusion of wind energy in a power system network is currently seeing a significant increase. However, this inclusion has resulted in degradation of the inertia response, which in turn seriously affects the stability of the power system’s frequency. This problem can be solved by using an active power reserve to stabilize the frequency within an allowable limit in the event of a sudden load increment or the loss of generators. Active power reserves can be utilized via three approaches: (1) de-loading method (pitching or over-speeding) by a variable speed wind turbine (VSWT); (2) stored energy in the capacitors of voltage source converter-high voltage direct current (VSC-HVDC) transmission; and (3) coordination of frequency regulation between the offshore wind farms and the VSC-HVDC transmission. This paper reviews the solutions that can be used to overcome problems related to the frequency stability of grid- integrated offshore wind turbines. It also details the permanent magnet synchronous generator (PMSG) with full-scale back to back (B2B) converters, its corresponding control strategies, and a typical VSC-HVDC system with an associated control system. The control methods, both on the levels of a wind turbine and the VSC-HVDC system that participate in a system’s primary frequency control and emulation inertia, are discussed.

AC Transmission Emulation Control Strategies for the BTB VSC HVDC System in the Metropolitan Area of Seoul

In the Korean power system, growing power loads have recently created the problems of voltage instability and fault current in the Seoul Capital Area (SCA). Accordingly, the back-to-back (BTB) voltage source converter (VSC) high-voltage direct-current (HVDC) system is emerging to resolve such problems with grid segmentation. However, non-convergence problems occur in this metropolitan area, due to the large change of power flow in some contingencies. Therefore, this paper proposes two kinds of AC transmission emulation control (ATEC) strategies to improve the metropolitan transient stability, and to resolve the non-convergence problem. The proposed ATEC strategies are able to mitigate possible overloading of adjacent AC transmission, and maintain power balance between metropolitan regions. The first ATEC strategy uses a monitoring system that permits the reverse power flow of AC transmission, and thus effectively improves the grid stability based on the power transfer equation. The second ATEC strategy emulates AC transmission with DC link capacitors in a permissible DC-link voltage range according to angle difference, and securely improves the gird stability, without requiring grid operator schedule decisions. This paper compares two kinds of ATEC schemes: it demonstrates the first ATEC strategy with specific fault scenario with PSS/E (Power Transmission System Planning Software), and evaluates the second ATEC strategy with internal controller performance with PSCAD/EMTDC (Power System Electromagnetic Transients Simulation Software).