Multi-terminal VSC-based HVDC Research Articles

Hierarchical Flexible Operation Approach on a VSC-MTDC Interconnected Hybrid Grid With a High Share of Renewable Power

Voltage source converter (VSC) based multi -terminal HVDC (VSC-MTDC) systems have emerged as flexible collector systems for multi-area interconnected AC/DC hybrid grids with high share of volatile renewables energy. However, it remains an open question as how to exploit the flexible and cooperative regulation of multiple VSC stations and enhance secure operation on the premise of information privacy protection of multiple independent dispatch areas. This paper proposed an Analytical Target Cascading (ATC)-based three-layer decentralized optimization approach on flexible operation of VSC-MTDC hybrid systems. The low layer is a chance-constrained stochastic optimal power flow (SOPF) problem for AC grids. To guard against the secure operation risk by a VSC's random unipolar atresia fault, a modified loss of load probability index (MLOLP) is proposed to improve the day-ahead generation-reserve scheme. The middle layer and the high layer are a multiperiod OPF problem for VSC stations and the MTDC grid, respectively, where a detailed power regulation model is built to fully exert the flexible cooperation of multiple VSC stations. Two case systems are tested to verify the effectiveness and superiority of the proposed flexible operation hierarchy.

Short-term generation capacity expansion planning considering multi-terminal VSC[sbnd]HVDC links using a linear programming framework improved by shift factors

This paper describes the modelling of short-term generation capacity expansion planning (GCEP) for multi-terminal VSC-based HVDC transmission systems. The GCEP formulation is based on a mixed-integer linear programming (MILP) framework where transmission losses are considered by piecewise linearisation. As opposed to existing methods, the present GCEP formulation is based on shift factors which enable the representation of the hybrid AC/DC grids including VSC stations. This way new power generation investments can be efficiently determined at any of the VSC-connected AC systems. This new method lies in sharp contrast with the so-called classic GCEP method where the system nodal power balances are formulated using the voltage phase angles as decision variables thus increasing the model complexity. Indeed, to determine the GCEP investments, the present model aimed to multi-terminal HVDC links only uses AC/DC transmission line losses, power generations and projected generating power plants as decision variables. The new GCEP formulation is compared to the classic method using two compelling multi-terminal VSCHVDC test systems for validity and applicability purposes. Results show that the proposed formulation reduces the simulation running time by more than 80%, compared to the standard model, accompanied by acceptable accuracy of results since errors inferior to 5% are obtained.

Nodal Reliability Evaluation for a VSC-MTDC-Based Hybrid AC/DC Power System

The utilization of voltage-source converter-based multi-terminal HVDC (VSC-MTDC) technology has been envisaged as a cost-effective solution to transmit bulk renewables from multiple sending ends to receiving-end AC systems. However, with complex configurations and various types of components, the reliability evaluation of a VSC-MTDC system is more complicated than that of a conventional HVDC system. In addition, due to stochastic failures and complex operating modes, the application of a VSC-MTDC system can have a significant influence on the reliability of hybrid AC/DC power systems. This paper presents a new technique for evaluating the nodal reliability of a hybrid AC/DC power system based on the VSC-MTDC system. The proposed technique is incorporated within a multi-state model for the VSC-MTDC system. Then, an optimization model for contingency management of a hybrid AC/DC power system is formulated. The model is established based on the optimal power flow technique for a VSC-MTDC based hybrid AC/DC power system. The objective of the model is to minimize the total system cost when determining the load curtailment and generation re-dispatch results for each contingency state. Nodal reliability indices for hybrid AC/DC power systems are proposed to evaluate the reliability performance of customers at each node. The effectiveness of the proposed methods is validated using a modified IEEE reliability test system.

Frequency response reserves sharing across asynchronous grids through MTDC system

Voltage source converter-based multi-terminal HVDC (VSC-MTDC) system has been considered for interconnecting asynchronous power grids for improved system operation efficiency and reliability. In this study, a frequency response control strategy is proposed for the VSC-MTDC systems that can enable operating reserves sharing among the interconnected asynchronous grids. The proposed control strategy consists of three frequency control components: MTDC primary frequency control, corrective frequency control, and MTDC secondary frequency control. The MTDC primary frequency control could keep the frequency of a disturbed power grid stable and avoid the frequency dropping into an unacceptable level when a sudden power imbalance event occurs. The corrective frequency control strives to guarantee the settled frequency can be within the predefined secure frequency range. The MTDC secondary frequency control provides a novel optimal allocation strategy for considering the participation of MTDC system for restoring system frequency back to the nominal frequency with the lowest cost. The simulation results, based on a modified IEEE New England 39-bus system modelled in PSCAD/EMTDC, show the effectiveness of the proposed frequency response control strategies.

VSC-MTDC System Integrating Offshore Wind Farms Based Optimal Distribution Method for Financial Improvement on Wind Producers

As the typical clean and renewable energy, wind energy has witnessed a continuous annual increase in the last few decades. Due to the random and intermittent characteristics, the wind producers in the electric market meet serious financial losses caused by the deviation between the actual power output and forecasting result. It is difficult to increase the accuracy of forecasting in a short time. How to improve the financial income has become a major task to wind producers and academia. Being the backbone network of the offshore wind farms (OWFs), voltage source converter-based multi-terminal HVdc system (VSC-MTdc) has been regarded as one of the effective solutions to transport wind power. In this paper, an optimal distribution method is proposed for VSC-MTdc system integrating OWFs to reduce the financial loss due to wind power output deviation. The proposed method could be divided into two optimizing functions. The first optimizing function of the proposed method is to analyze the onshore external system according to the historical system operation data and adjust the droop coefficient with the analytic hierarchy process. The second optimizing function of the proposed method is to do further adjusting with the regulating price. With the case study, the proposed optimal distribution method has proved that it can bring more benefit to wind producers.

An optimal combined operation scheme for pumped storage and hybrid wind-photovoltaic complementary power generation system

Because of the steady increase of wind and solar capacity installations in recent years, the goal of the Paris Agreement is expected to be reached. However, due to the intermittent characteristics of renewable generation, challenges are presented for the power grid to operate reliably and economically under the high uncertainty of wind and solar power productions. Sufficient responsive reserves must be available for the power grid to maintain the balance between the supply and demand. To be fair to the market participants, the renewable generation companies should pay for the received grid ancillary services. In this paper, an optimal combined operation scheme is proposed for pumped storage hydro and hybrid wind-photovoltaic complementary power generation system interconnected by a Voltage Source Converter-based multi-terminal HVDC system. The optimal combined operation scheme has two real-time control functions. The pumped storage hydro control function enables automatic dispatch of the pumped storage hydro to effectively suppress or compensate for the output deviations of wind and solar generation from the forecasted production; and the output control function is able to distribute the non-compensated production surplus or deficit to different ac systems based on the market regulating service prices. A Case study is provided to demonstrate the improved power generation profile and reduced revenue losses of the pumped storage hydro and hybrid wind-photovoltaic complementary power generation system.

AC Grids Characteristics Oriented Multi-Point Voltage Coordinated Control Strategy for VSC-MTDC

Meshed dc grid technology is an attractive solution to improve the operational reliability of a voltage source converter-based multi-terminal HVDC transmission (VSC-MTDC) system; in addition, the VSC-MTDC becomes widely accepted for the integration of large-scale renewable energy and power supply to passive ac grids. Hence, the performance requirements of the dc voltage control and the power control of the VSC-MTDC system become higher, and this is the key issue of coordinated control strategies for the VSC-MTDC system. Considering that the mainstream coordinated control under the above new situations is facing challenges in terms of dc voltage dynamic control performance or dynamic power deviation, this paper proposes an ac grids characteristic-oriented multi-point voltage coordinated control strategy for the VSC-MTDC system to provide satisfactory dynamic dc voltage control performance with minimized dynamic and steady state power deviations of the converter stations. The theoretical analysis and the simulations results based on PSCAD/EMTDC are to verify the effectiveness of the proposed control strategy.

DC Fault Protection in Multi-terminal VSC-Based HVDC Transmission Systems with Current Limiting Reactors

Multi-terminal VSC-based HVDC transmission system is the recent interest for grid integration of large-scale offshore wind farms. Protection of multi-terminal voltage source converters (VSC)-based HVDC transmission systems against DC faults is challenging. This paper presents a single-ended protection scheme for DC faults in a three-terminal VSC-HVDC transmission system. The under-voltage criterion is used to distinguish the DC faults from the transient and normal conditions. The rate of change of DC voltage and current as well as the variation of transient energy is used to discriminate the internal faults from the external faults. The DC fault current has very high value within a few milliseconds during the transient phases such as the capacitor discharging and diode freewheeling stages. Therefore, current limiting reactors are introduced in series with the DC circuit breaker to maintain the DC fault current within the breaker capacity. The single-ended protection scheme is tested with the three-terminal VSC-HVDC transmission system with current limiting reactors for various DC fault conditions. The DC fault data is generated from PSCAD/EMTDC simulation and the protection scheme is tested in MATLAB environment. Test results show that the proposed protection scheme gives reliable protection for the DC faults in a three-terminal VSC-HVDC transmission system.

A new unified approach for the state estimation and bad data analysis of electric power transmission systems with multi-terminal VSC-based HVDC networks

The aim of this paper is the proposal of a static state estimation approach suitable for electric power transmission systems containing fully controlled multi-terminal high voltage direct current networks. An integrated electric circuit model of the voltage source converter station, including its on-load tap-changer (OLTC) transformer, is used to determine the state variables associated with the AC–DC interface. These controllable variables are added to the state vector, together with the nodal voltages associated with the AC and DC networks, as well as the duty cycle of the DC–DC converters, for a unified SE solution in a single frame of reference. The SE problem is formulated based on Hachtel’s augmented matrix method in order to directly consider the operational constraints associated with the AC–DC network. In addition, a new technique for performing multiple bad data analysis is proposed in which the original level of measurement redundancy is maintained unaltered during the estimation process. The applicability of the proposed SE approach and bad data analysis is demonstrated by estimating the operating state of two integrated AC–DC multi-terminal transmission networks. The numerical results show that the proposal retains good convergence properties in terms of the number of iterations required to achieve a reliable state estimation as well as robustness to deal with bad data.

Adaptive Voltage Droop Control of Multiterminal VSC-HVDC Systems for DC Voltage Deviation and Power Sharing

Voltage source converter based multiterminal HVDC (VSC-MTDC) technology has shown many advantages. However, following a large disturbance, the dc voltage and the output power of the VSC station with fixed droop control may hit their limits, thereby reducing the dc voltage control ability of the whole MTDC system. Here an adaptive droop control scheme is proposed on the basis of both the dc voltage deviation factor and the power sharing factor. It contributes to ensure the dc voltage and the power loading rate of each converter within their limits during large disturbances, and the power sharing capability of the whole MTDC system remain high. The working principle of the proposed adaptive droop control is discussed in details. Case study and simulations are performed based on the PSCAD/EMTDC model of a ±320 kV four-terminal VSC-HVDC system to validate the effectiveness of the proposed adaptive droop method.

A Novel Control Approach for Protection of Multiterminal VSC-Based HVDC Transmission System Against DC Faults

Overall performance of the voltage source converters (VSCs) has improved during the recent years. Improvement of the VSCs along with the attractive features of the VSC-based high-voltage direct current (HVdc) transmission systems over the thyristor-based HVdc transmission systems make it possible to build multiterminal VSC HVdc transmission systems. However, the VSCs are vulnerable against dc-side faults and a method needs to be employed to interrupt the dc fault current. In this paper, three different configurations of solid-state dc circuit breakers (CB) for protection purposes are studied. Moreover, a new control method to protect the VSCs against the dc-side fault is proposed. The new method makes it possible to interrupt the dc fault current with the existing ac breakers on the ac-side or with the lower rating solid-state (SS) direct current circuit breakers (DCCBs). The performance of the SS DCCBs and the proposed method are studied using real-time digital simulator.

Dynamic Behaviour of Multi-Terminal VSC-Based HVDC after a Converter Outage: DC Control Strategy

aim of this paper is to evaluate the effect of DC-voltage control strategy on the dynamic behaviour of multi-terminal Voltage-Source Converter (VSC)-Based HVDC after a converter outage. In this paper, two dc voltage control strategies are considered: (i) standard voltage margin method (SVMM) and (ii) standard voltage-droop method (SVDM). The impact is evaluated in this paper using time-domain simulations on a simple test system using DIgSILENT ® PowerFactory TM considering a sudden disconnection of a converter-station. Simulation results demonstrate how important is the dc-voltage control strategy and the location/number of dc-buses involved in the dc-voltage on the dynamic response of the MTDC systems. The voltage margin control is capable of surviving a converter outage just if this converter is operating in constant power mode.

Sliding Mode Robust Control Based Active-Power Modulation of Multi-Terminal HVDC Transmissions

This paper presents a sliding mode robust control based active-power modulation (SMAPM) for voltage source converter based multi-terminal HVDC (VSC-MTDC) systems, which could enhance the stability of the onshore power system subjected to disturbances by fast changing transmitted power of onshore VSCs. The development of the SMAPM includes three steps. First, the onshore disturbed ac system could be divided into two areas with generators sharing different swing modes. The center of the inertia (COI) reference frame is utilized to be the representative of the area's behavior. Secondly, an index called sensitive index is proposed to quantify the sensitivity of generator angular speed to the active-power change of an onshore VSC. A strategy is developed to determine the modulation direction and intensity of each onshore converter. Thirdly, the final control law is derived based on sliding mode variable structure control method. Simulation verifications are run in a four-machine ac system and a modified New England 39-bus system.

An Improved Corrective Security Constrained OPF for Meshed AC/DC Grids With Multi-Terminal VSC-HVDC

This paper presents an improved corrective security constrained optimal power flow (CSCOPF) for a meshed AC/DC power transmission network with voltage source converter based multi-terminal HVDC (VSC-MTDC) grids. The security constrains of N-1 contingencies for both AC and DC networks are fully considered in the CSCOPF problem. The corrective actions employed in this paper include not only the traditional long-term rescheduling control actions of the AC system, but also the short term post-contingency fast corrective control of multi-terminal VSC-HVDC. A hybrid method is developed to solve the improved CSCOPF problem and the effectiveness of the proposed method is evaluated and demonstrated in this paper by two example meshed AC/DC power systems.

DC faults ride-through capability analysis of Full-Bridge MMC-MTDC System

Full-Bridge Modular Multilevel Converter (FBMMC) has strong ability to ride through serious DC faults, thus it is very suitable for multi-terminal flexible HVDC applications. However, no references have reported the locating and isolating of DC faults and corresponding DC faults ride-through capability evaluation index. This paper introduces the topology mechanism of FBMMC and its loss reduction operation mode, theoretically certifies that the universal decoupled control strategy of Voltage Source Converter (VSC) and the similar modulation strategies of Half-Bridge MMC (HBMMC) can be applied to FBMMC for constructing complete closed-loop control system. On the basis of the existing DC faults locating and isolating schemes of 2-level VSC based Multi-Terminal HVDC (VSC-MTDC) system and the particularity of FBMMC, this paper proposes the DC faults wire selection “handshaking” method of the FBMMC-MTDC system, and proposes the DC Fault Ride-Through Capability Index (DFRTI) for evaluating the DC faults suppressing capability of the VSC-MTDC systems, including FBMMC-MTDC. Simulations of FBMMC-MTDC in PSCAD/EMTDC validate the correctness and effectiveness of the proposed control strategy and evaluation index.