HVDC Connection Research Articles

Analysis of non-uniform grid-forming control techniques for the HVDC connection of renewable energy

Grid-forming control techniques are required to achieve a high penetration of power electronics-based renewable energy sources for decarbonized electric systems. However, different grid-forming control strategies can operate in the same grid, and grid stability shall be warranted. This paper analyses the use of different grid-forming control strategies for diode rectifier-based wind power plants. The analysed grid-forming controllers are the well-known droop control, an advanced droop control, and the virtual synchronous machine control. The controllers’ analysis validates the three grid-forming controllers’ interoperability, identifying each control parameter’s contribution to the stability of each system state variable. Furthermore, the analysis allows a better tuning of the control parameters. Additionally, a fault-ride-through strategy that improves the system restoration after faults is proposed and validated. The proposed fault-ride-through strategy achieves a soft restoration of the active power.

Hybrid bipolar VSC-DRU HVdc connection for offshore wind farms

This paper presents a hybrid bipolar high voltage direct current (HVdc) link for offshore wind farms (OWFs) connection. Voltage source converter (VSC) technology is used in one of the offshore poles, using a diode rectifier unit (DRU) in the other one. Therefore, economic and power-loss advantages of the DRU over the VSC are leveraged by changing one of the HVdc poles conventional technology. In the proposed control strategy, VSC reactive power is used to control OWF ac grid frequency, allowing the DRU commutation and active power transfer. VSC active power is used for OWF ac grid start-up and hybrid bipolar HVdc link power flow control. OWF ac grid voltage magnitude is controlled while the DRU pole is blocked, and it is automatically accommodated to the required value when the DRU is conducting. Moreover, VSC is also used as an active filter for the main DRU current harmonic components, thus removing the huge passive ac filters from the offshore platform. Poles balanced and unbalanced power flow control strategies are proposed for the hybrid bipolar HVdc, considering both bipolar with neutral return and rigid bipolar configurations. Simulation results validate the proposal which aims reducing cost and size of the OWF HVdc connection.

Impact of a HVDC Link on the Reliability of the Bulk Nigerian Transmission Network

Regular and reliable access to energy is critical to the foundations of a stable and growing economy. The Nigerian transmission network generates more electricity than is consumed but, due to unpredicted outages, customers are often left without electrical power for several hours during the year. This paper aims to assess the present reliability indices of the Nigerian transmission network, and to determine the impact of HVDCs on system reliability. In the first part of this paper, the reliability of the Nigerian transmission system is quantified by building a model in DIgSILENT PowerFactory and carrying out a reliability study based on data provided by the Nigerian transmission-system operator. Both network indices and load-point indices are evaluated, and the weakest points in the network are identified. In the second part of the paper, an HVDC model is built and integrated into the existing network at the locations identified by the reliability study. A comparative study using two different HVDC connections is then carried out, to determine the critical impact of HVDC on system reliability. The reliability results indicate that the weakest points of the transmission system are the radial feeders, and the highest impact could be achieved by spanning an HVDC line between two busbars located at the two extremes of a radial feeder: Azura and Yola.

Impact of Adriatic submarine HVDC cables to South East European Electricity Market Perspectives

The ultimate goal in today’s electricity business in Europe is market integration on pan-European level that will introduce transparency and competition between market players, incentives to clean energy development, as well as high quality of supply to the end-customers. To achieve these goals, in South-East Europe (SEE) there are number of barriers and uncertainties, one of which is linked with the possible new undersea HVDC connections between SEE and Italy. With the support of the United States Agency for International Development (USAID) and coordination of the United States Energy Association (USEA), within the framework of the Southeast Europe Transmission System Planning Project (SECI), a detailed analysis has been accomplished on the impact of one or more undersea HVDC cables between Italy and SEE on power system operation and electricity market development [1]. Special emphasis to this analysis is given by the fact that SECI has been one of the longest running projects in the region. It started in 2001 with active participation of all regional TSOs, including continuous updating of power system and electricity market models and its harmonization of constant changes in power system planning. It is of utmost importance in the environment of constant changes of national power system development plans and needed further steps for full market opening and integration in the region. SEE power systems and market1 were modelled using the most relevant power system and market simulation and optimization softwares. Both system and market comprehensive models have been verified by all SEE TSOs. Study analyses were divided in two parts: 1) market analysis and 2) network analysis. The market study investigated expected generation pattern, power exchanges and wholesale prices in SEE, taking into account regional market synergy, the new links with Italy, and high level of RES integration. Bulgaria and Romania are currently the main exporters in SEE. Significant power exchanges in the North-South/Southeast direction are related to the fact that the GR, MK, ME, HR and AL are mainly importing, plus the influence of Italy importing over new potential HVDC cable(s). Network analysis dealt with power flows, network bottlenecks and voltage profiles in given market scenarios. Finally, the results of this comprehensive market simulation comprised of the following:
 
 Countries electricity balance (production, consumption and exchanges)
 Electricity prices for each country
 Cross-border power exchanges (MWh/h) for each border in the region on hourly basis
 HVDC link loadings (MWh/h) for each HVDC submarine cable on hourly basis
 Location and frequency of market congestions in SEE (NTCs full between areas with price difference)
 
 All those analyses have been performed in two different transmission network development scenarios:
 
 Base case scenario: with planned HVDC ME-IT
 Alternative scenario: with planned HVDC ME-IT, and HVDC HR-IT, and HVDC AL-IT
 
 In this way one of the most important uncertainties (new HVDC links SEE – Italy) for future power system and market operation in SEE, have been evaluated both in technical and market sense, using the most relevant inputs and model.

Techno-economic analysis of a transmission line between an island grid and a mainland grid

Islands are commonly isolated electricity systems that rely heavily on imported fossil fuels. Interconnection with a mainland power system is a solution to offset the exposure to volatile fuel prices and increase the stability of the island grid. This paper presents and discusses the main results of a case study aiming to see the economic effects of a 200 MW HVDC connection between an island grid and a mainland grid. The purpose was to see how much of the island’s oil price scenarios can be overcome with this new interconnector. For this research, North Cyprus grid was modeled in MATPOWER software to do optimal power flow analysis. The results are used to compare fuel-based generation costs to importing electricity from the mainland. The analysis shows that this interconnector decreases the island’s generation costs by 28% to 78%, and with the possible renewable installations, this reduction can be increased further.

A New Fuzzy Based UPFC Topology for Active Power Enhancement in an offshore Wind Farm

The stability of a power system is the ability of a
 power system to restore an operating state of equilibrium for
 a given initial operating condition after it has been subjected
 to a physical disturbance, most of the variables of the system
 being limited so that almost the entire system remains intact.
 To create a MATLAB SIMULINK model of odd shore wind
 energy system having power being transmitted through DC
 transmission system. The first model will have no power flow
 controller and second model will have artificial intelligence
 based controlling technique. To design a controller for
 enhancing the power output from the wind energy system
 using UPFC. This will be made to feed DC transmission
 system. Finally integrating the system with long distance DC
 transmission system and then to the grid so as to make it
 more reliable and efficient. In this work a coordinated
 control based on Fuzzy logic for UPFC for cluster of offshore
 WPP connected to the same HVDC connection is being
 implemented and analyzed. The study is targeting
 coordination of reactive power flow and active power flow
 between HVDC Converter and the WPP cluster while
 providing offshore AC grid voltage control. Thus it can be
 drawn from this work that while designing a power control
 strategy the proposed fuzzy logic based active power
 controller in UPFC can serve the purpose with better results
 in terms of active as well as reactive power. This control can
 also be used in hybrid systems thus making it more reliable
 controlling method. The

Primary Frequency Regulation Using HVDC Terminals Controlling Voltage Dependent Loads

HVDC can provide frequency regulation during disturbances (e.g., faults) by controlling the power flow between two remote AC areas. While this action reduces the power deviation in the area affected by the disturbance, it causes a power imbalance in the other healthy AC area, leading to a frequency variation and endangering the system stability. In this work, a HVDC primary frequency regulation controlling voltage-dependent loads (PFR-VDL) is proposed, where the HVDC terminal in the healthy area influences the grid voltage amplitude to shape (decreasing or increasing) the load consumption in order to cope with the power variation required by the fault-affected area. The PFR-VDL extracts the needed energy for the frequency support, not from the generators (with following frequency deviation) but from the voltage-dependent loads in the healthy area. This work analyzes the PFR-VDL performance, generalizing it with two possible HVDC connection cases: Asynchronous connection with single HVDC line, and embedded HVDC forming a parallel, hybrid connection with HVAC. The PFR-VDL application benefits and limitations are evaluated analytically and verified by means of PSCAD EMTDC simulations, and finally validated with a large interconnected IEEE 39 bus system.

Frequency Control of Offshore Wind Farm With Diode-Rectifier-based HVdc Connection

This paper presents a direct frequency control for offshore wind farms (OWFs) connected to the diode-rectifier-based (DR-based) high voltage direct current (HVDC) link. The frequency control is guaranteed through reactive power balance at the DR station without a capacitor bank placed at the DR station; while AC-voltage magnitude is clamped by the DR, provided that the HVDC link voltage is imposed by the onshore inverter. The control system is implemented by a voltage source converter (VSC) connected to the DR station. The VSC is also used to compensate harmonic currents, leading to passive AC-filters being also removed from the DR station. The proposed control system provides comparative advantages in terms of DR station volume and OWF installation cost with other centralized controls where besides a VSC, a high-voltage large capacitor bank is needed to be placed at the DR station. A new average-value model (AVM) of the DR system is derived to test the proposed frequency control, while the stability of the system is evaluated through a small-signal analysis. The proposed control system performance and the accuracy of the proposed AVM are verified through a detailed switching model of the system using operation of a cascaded H-bridge VSC.

Control and Experimental Validation of the Series Bridge Modular Multilevel Converter for HVDC Applications

The series bridge converter (SBC) is a modular multilevel converter (MMC) recently developed to enhance power density in high-voltage high-power applications. The MMC is a well-established solution, widely researched, and exploited in practical HVdc connections, thanks to its high power quality and high efficiency. However, the main limitation of the MMC is the relatively large energy storage, also due to the fact that power ripples in the submodule capacitors include a component at the fundamental ac frequency. As a result, volume becomes critical in applications such as offshore or city center in-feeds, where space is restricted and expensive. The SBC offers a more compact footprint by exploiting a series connection on the dc side and by operating the submodules with rectified waveforms, thus moving the minimum component of the instantaneous power to twice the ac fundamental and reducing capacitors size. The drawback of the converter is a more complex energy control compared to the MMC. This paper proposes the first experimental validation of the SBC, using a 2-kW laboratory-scale prototype. Since the basic converter design has been discussed in previous papers, the focus of this paper is on converter control design and experimental validation.

Operational concepts of HVDC in the context of security assessment in the German transmission grid

Because of the realization of HVDC connections parallel to the synchronous grid in Germany, operational concepts become more important in the context of system security aspects. In this article commonly known concepts are analyzed regarding their suitability for the German transmission system. Therefore, an approach for the analysis was developed and exemplary results are given using this approach. Under consideration of the overall system, the concepts of reducing losses in combination with minimizing (n-1)-warnings or -indications1 as well as the sole minimization of (n-1)-warnings and -indications are the most reasonable approaches. However, under grid security aspects best results can be achieved by reducing operational limits and therefore, by the minimization of (n-1)-warnings.

An alternative approach to provisions of the Network Code on blocking HVDC LCC systems during short-circuits in AC network

The Network Code for HVDC systems introduces a requirement for such systems to remain in operation in an AC network during short-circuits. This requirement (among others) is determined by a time curve of a minimum voltage in HVDC connection point. The HVDC system is not allowed to be turned-off while the voltage value exceeds the values of the curve, however the HVDC system in LCC technology does not meet this requirement. The Network Code introduces the possibility of blocking the LCC systems. After the LCC system is blocked it stops transmitting the power, which prevents the LCC system from being turned off during a short-circuit in the network but results in a lack of power exchange between the network and the LCC system. Therefore, the authors developed a proposal to limit the power level being transmitted by the LCC system, thus there is no necessity to block the LCC system operation. The simulation research carried out clearly indicate the validity of this idea. In addition, the authors presented the concept of an automation system that adaptively adjusts the scale of the LCC system power level reduction to change the network operating conditions during a short-circuit in the network. The operation of this automation system is presented for the practical LCC system model installed in the Polish Power System.

Power System Enterprise Control With Inertial Response Procurement

As the power system evolves to better address environmental and reliability concerns, its overall dynamics changes and consequently challenges the adequacy of established operating procedures. This evolution brings a significant increase in the number of integrated renewable energy sources (RES) and HVDC connections. RES are normally connected to the grid through power electronic converters, which reduces or completely eliminates their electrical coupling to the power grid. As a result, such units are unable to contribute to the inertia of the system, posing a potential threat to the grid's physical security. More specifically, postfault frequency characteristics, such as rate of change of frequency (RoCoF), frequency nadir, and quasi-steady-state frequency, depend on the system inertia and their respective limits may potentially be violated. One way to address this problem is adding constraints to the unit commitment formulation so that operations maintain their frequency characteristics within acceptable limits. This paper studies how such changes affect the system frequency deviation from its rated value. The power system is modeled as an enterprise control that combines multiple layers of control operations into one package. Such integration allows capturing how the effects of wind integration and the addition of unit commitment inertia constraints propagate through control layers to impact the system frequency.

Comparing AC Dynamic Transients Propagated Through VSC HVDC Connection With Master–Slave Control Versus DC Voltage Droop Control

This paper investigates the propagation of ac dynamic transients through a voltage-source converter-based high-voltage direct current (VSC HVdc) connection, comparing master–slave control and dc voltage droop control, and the impact of the propagation of ac dynamic transients on the small-signal stability of an ac/dc power system is also examined. In this paper, an interconnected model of the ac/dc power system is derived. Analysis using the derived model indicates that under master–slave control, the ac dynamic transients can propagate only unidirectionally through the VSC HVdc connection, and instability from a local ac system may not spread throughout the entire ac/dc power system through the VSC HVdc connection. However, under dc voltage droop control, the propagation becomes bidirectional between the VSCs that participate in the dc voltage droop control, and instability from the local ac system will spread throughout the ac/dc power system. In addition, even if the local ac subsystems are stable, the bidirectional propagation of ac dynamic transients through the VSC HVdc connection may still cause instability. Hence, implementation of dc voltage droop control increases the ac/dc power system's instability risk.

Wind power within European grid codes: Evolution, status and outlook

Grid codes are technical specifications that define the requirements for any facility connected to electricity grids. Wind power plants are increasingly facing system stability support requirements similar to conventional power stations, which is to some extent unavoidable, as the share of wind power in the generation mix is growing. The adaptation process of grid codes for wind power plants is not yet complete, and grid codes are expected to evolve further in the future. ENTSO‐E is the umbrella organization for European TSOs, seen by many as a leader in terms of requirements sophistication. A current development by ENTSO‐E aims to develop a uniform grid code framework for Europe. The new European codes leave many key aspects unspecified, referring instead to regulation by the relevant TSO, but they do provide a positive and encouraging step in the right direction. The present document is largely based on the definitions and provisions set out by ENTSO‐E. The main European grid code requirements are outlined here, including also HVDC connections and DC‐connected power park modules. The focus is on requirements that are considered particularly relevant for large wind power plants. Afterwards, an outlook and discussion on possible future requirements is provided. This review has been written by members of IEA Wind Task 25, but it does not represent an official viewpoint of the IEA.This article is categorized under: Wind Power > Systems and Infrastructure

On DC Fault Dynamics of MMC-Based HVdc Connections

This paper studies the dc fault development stages and analyzes the fault dynamics in a point-to-point multilevel modular converters (MMC)-based dc connection. First, the effect of the dc grid configuration on the normal operation was investigated by comparing an asymmetric monopole with metallic return and a symmetric monopole. Then, the main parameters that affect the dc fault response of a grid, such as the fault type, impedance and converter blocking, were examined. Compared to previous studies, which are based on simulation results, the analysis is performed hereby both in theory, deriving the equations that describe the dc fault stages, as well as using experimental results obtained in the designed laboratory setup. The setup consists of two MMC terminals connected to two ac sources representing independent ac grids. These terminals are connected using a simple dc link based on pi-section equivalent of dc cables. The obtained results, which verified the theoretical analysis, can be used to determine the protection function thresholds of the MMC, as well as to estimate the developed stresses on dc lines during fault conditions and to define the design requirements for dc breakers.

Separate Poles Mode for Large-Capacity HVDC System

This paper proposes a novel connection mode, separate poles mode (SPM), for large-capacity HVDC systems. The proposed mode focuses on the core issues of HVDC connection in interconnected power grids and principally aims at increasing effective electric distance between poles, which helps to mitigate the interaction problems between AC system and DC system. Receiving end of bipolar HVDC has been divided into different inverter stations under the mode, and thus significantly alleviates difficulties in power transmission and consumption of receiving-end AC grids. By investigating the changes of multi-feed short-circuit ratio (MISCR), finding that HVDC with SPM shows critical impacts upon itself and other HVDC systems with conventional connection mode, which demonstrates that SPM can make balance between MISCR increase and short-circuit current limit.

Offshore Wind Farm Modeling Accuracy and Efficiency in MMC-Based Multiterminal HVDC Connection

The large number of switching elements in the modular multilevel converter (MMC) is a challenging problem when modeling the MMC-HVDC systems for the computation of electromagnetic transients. The modeling complexity increases even further when a multiterminal (MT) MMC-HVDC system is used to integrate offshore wind farms (OWFs) with power-electronics-based wind energy converters, such as doubly fed induction generators (DFIGs). This paper compares modeling accuracy and computational performances for various combinations of MMC and OWF models. Onshore and offshore ac fault simulations are performed for an OWF system composed of DFIG-type wind turbines and connected to a practical ac grid through an MT MMC-HVDC system. The OWF system model includes the detailed representation of the offshore collector grid and the associated overcurrent protection. The offshore MMC controls include an offshore fault current limiter and fast OWF power generation reduction-based fault-ride-through function.

Cluster Control of Offshore Wind Power Plants Connected to a Common HVDC Station

In this paper a coordinated control for cluster of offshore WPPs connected to the same HVDC connection is being implemented and analyzed. The study is targeting two cases as; coordination of reactive power flow between HVDC converter and the WPP cluster while providing offshore AC grid voltage control, and coordinated closed loop control between the HVDC and the WPPs while the cluster is providing Power Oscillation Damping (POD) via active power modulation. It is shown that the coordinated cluster control helps to improve the steady-state and dynamic response of the offshore AC grid in case of offshore AC grid voltage control and onshore ancillary services provision, i.e. POD by the active power modulation of the cluster. The two cases are simulated using DIgSILENT PowerFactory, where the IEC 61400-27-1 wind turbine and WPP control models and a generic offshore layout with cluster of three WPPs are utilized.

Analysis of control interactions in multi‐infeed VSC HVDC connections

The number of voltage-source converters connected to the power system is increasing. This has led to concerns about the impact of control interactions between converters and their influence on the system stability. Recent grid codes consider such interactions as low-frequency disturbances between the converters and neglect network dynamics. This paper studies the control interactions and interferences in a multi-infeed power system with two converters. The phenomenon is studied in the frequency domain by using the admittance-based stability assessment. Harmonic stability under different network characteristics is addressed. The analysis shows that resonances in the system can lead to instabilities when appearing within the control bandwidth, thereby implying the necessity to also consider higher-frequency dynamics.

A Comparison of AC and HVDC Options for the Connection of Offshore Wind Generation in Great Britain

This paper presents a comparison of two forms of cable connection of a distant offshore wind farm to a transmission system: AC and HVDC. The requirements of relevant industry standards in Great Britain (GB) that drive a connection design and, hence, its costs are highlighted along with an analysis of the ways in which AC cable connections might be made to comply while facilitating the export of active power. Dynamic studies investigating responses to grid-side short-circuit faults show that, in the particular scenarios studied, an AC connection of a wind farm in place of a large synchronous generator is marginally detrimental while an HVDC connection is beneficial. A comparison of costs shows that the crossover distance at which HVDC is cheaper than AC for wind farms of different sizes occurs at longer distances than have hitherto commonly been assumed, and AC connections benefit from reactive compensation not only at the point of common coupling and wind farm end, but also at the connection midpoint.