The possibility to adjust the reactive power at the point of connection of battery systems will enhance flexibility provision in future renewable-based grids. Therefore, Grid Codes require verification of the reactive power capability of battery systems, which can be exploited by system operators. This paper proposes standardized testing procedures to verify reactive power functions for Grid Code compliance of battery systems. The investigated control functions are (i) reactive power control, (ii) power factor control, and (iii) automatic power factor control. The designed procedures detect the reactive power capability of battery systems thoroughly, and they verify the battery system against Grid Code compliance following three success criteria: accuracy, sensitivity, response time. The experimental testbed allows to validate a 33.5 kW / 95 kWh battery system following the requirements in the Danish Grid Code. Results show that the battery system follows the Grid Code, satisfying both reactive power capability requirements and success criteria.
To insure a stable and reliable operation of the existing and future grids, it is important to study the stability of the power electronic based converters’ controls which can replace synchronous generation. Most existing studies use a Thévenin equivalent model of the grid to test the grid following controls. The present article demonstrates the limits of these studies. An alternative setup is proposed and is shown to provide more insight into the underlying mechanisms of the grid following control behaviour in weak grids, using a linearized state space model. These insights are exploited to propose further improvements to the control. These improvements are shown to extend the stability of the control while reducing the interactions among converters.
This paper provides the HVDC electrical design of three existing UK HVAC overhead lines, two of which constitute 85% of the UK transmission overhead line system. The designs are based on CIGRE and IEC recommendations. Also, the verification of the electrical stress requirements on the insulators is performed by means of finite element simulations. The paper shows that the HVDC converted tower lines would carry 1.6 to 2.9 times the power of the original HVAC lines, depending on tower type, insulation arrangement, and environmental conditions.
Overvoltage in the DC link of voltage source converter based high-voltage DC (VSC-HVDC) transmission for the offshore wind farm (OWF) due to grid-side faults can not only damage equipment but can also exacerbate grid faults if the converter trips as a result. DC braking chopper (DBC) is now widely used to avoid overvoltage in the DC link by dissipating excess energy in the resistive elements and maintain the normal operation of the OWF, providing fault ride through (FRT) capability. In this paper, the modelling and control of an entire offshore wind farm and VSC-HVDC transmission system with four different circuits of DBC will be developed and a comparative study for the FRT capability and technical performance of these four types of DBC is conducted.
The increased penetration of power electronic devices in the power system has led to power system stability concerns, which could in part be addressed by implementing grid-forming converter control strategies. At the same time, Voltage Source Converter High Voltage Direct Current (VSC HVDC) connections are moving towards bipolar configurations, which have two converters at each end of the link, connected in close electrical proximity at the AC side. However, it is an open question how these converters can interact with each other and the rest of the power system, particularly when grid-forming capabilities are added to the picture. This paper addresses the small-signal stability of a bipolar HVDC link with two converters at one end operated in virtual synchronous machine grid-forming mode. A state-space formulation is obtained by interconnecting linearised models of converters, HVDC cables, and external grids using the component connection method. The developed state-space model is validated in the time domain against a nonlinear model in EMTP-RV. The effect of virtual synchronous machine parameters, such as virtual inertia and damping coefficient, on the system stability is evaluated with parametric sensitivity and participation factor analyses.
Multi-terminal flexible DC power grid plays an important role in large-scale new energy integration, which usually requires the installation of multi-terminal DC circuit breaker(MTCB) at the intersection of multiple DC lines. At present, the research on multi-terminal DC circuit breaker is mostly limited to theoretical simulation calculation, and there are few experimental studies and engineering applications. In this paper, a four-terminals DC circuit breaker based on commutating path multiplexing is proposed, which combines with the compact natural commutation scheme of hybrid gap, greatly reducing the cost and volume of the circuit breaker. The commutation characteristics of four-terminals hybrid DC circuit breaker equivalent prototype at 10kV voltage level were tested. On this basis, the equivalent prototype of the circuit breaker test results show that under 10kV voltage level, the four-terminals circuit breaker topology can achieve breaking of 11kA in 2.8ms by natural commutation method.
This paper describes the Inherent Phase-Based Real Inertia Power response of a STATCOM with DC side supercapacitor during high RoCoF events in the AC network. In this paper, a case study of the SVC PLUS FS® is considered, followed by simulation results for frequency ramps with different RoCoF and significant short-circuit ratio reduction. The grid-forming Virtual Synchronous Machine control concept is extended with a virtual active power control loop, ensuring the stability of the system during high RoCoF events (up to 6 Hz/s). Eventually, the proposed solution provides the flexibility of the inertia time constant of the Virtual Synchronous Machine without compromising the control stability.
The energy balance is ensured in the power system with load frequency control (LFC). The frequency is the indicator of the energy balance between active power generation and consumption. To achieve CO2 emission-free power generation, power generation from renewable energy sources (RES) will increase. At the same time, generation from conventional power plants will decrease until they are shut down. As a result, the share of converters without physical inertia will increase. In contrast, the share of running rotating masses will be negligible. To ensure a secure power system, the adjustment of power system operation in the new framework is required. A perspective method for power system operation can be the power system operation with angle-based control. In this paper, a basic comparison of primary control between LFC with Synchronous generator (SG) and angle-based control with voltage source converter (VSC) is made. An RMS simulation is performed for a use case in the 39 bus New England system.
The role of HVDC in power transmission systems continues to grow. The number of HVDC installations worldwide is increasing, and first, multi-terminal and meshed HVDC grids are being built. Large meshed HVDC grids with many terminals are expected to be operational in the near future. Bipolar HVDC grids can be operated in an asymmetrical configuration with converter stations connected between single poles. This paper presents a reliability assessment model for unbalanced AC/DC grids. The DC grid is modeled using a multi-conductor representation. A security-constrained optimal power flow model is developed allowing the representation of single-pole contingencies and single-line conductor outages. The model is implemented in the Julia/JuMP framework as an extension of the open-source package PowerModelsACDC.jl. The model is applied to a 67-bus test case considering several N-1 contingencies. Results show that unbalanced operation of HVDC grids in contingency situations can decrease operational costs by higher utilization of HVDC grid flexibility.
Firstly, this work extends the current Cost-Benefit Analysis (CBA) methodology defined by ENTSO-E and used to evaluate possible transmission lines investments in the power grid in Europe. This is needed to correctly incorporate the full potential of adding HVDC links to the power system. Secondly, the developed model includes the possibility of adding inertia- and redispatch-based sets of constraints to the optimisation. Even though the simulation time increases considerably, the enhanced level of detail of the model results in a better representation of the grid operations. In fact, issues such as line congestion, demand curtailment, demand reduction and power flow through single interconnectors cannot be computed satisfactorily with a zonal model. In this paper, the Celtic interconnector project between Ireland and France is used as test case. The simulation outputs are analysed by using the set of Key Performance Indicators (KPIs) used in current CBAs. Since these KPIs do not investigate the previously mentioned grid aspects, additional KPIs have been defined. The proposed model offers a deeper insight on the benefits brought by the HVDC interconnector to the power grid.
The radical changes are expected in energy systems globally. Shetland Island is a special area in UK in terms of energy system circumstances. Currently, Shetland power system is primarily characterised by its isolation from the GB power system and the reliance on fossil fuel, which introduces challenges related to power system management and emission stresses within the islands. To utilise the rich wind resources in Shetland region, large-scale wind farms will be developed, and a high voltage direct current (HVDC) system and an onshore transmission network will be installed; whereas massive local baseloads (mainly including oil and gas asset electrification and massive hydrogen production) are expected to be deployed on Shetland. This paper reviews the current system structure and circumstances of the Shetland power system, and presents the vision and expectation of the future integrated energy system, with key technical barriers and enablers identified with regard to the system development and operation. Besides, major challenges and opportunities of different system parties are outlined, as essential considerations for system developers and/or operators.
The Modular Multilevel Converter (MMC) has become the leading technology for delivering High Voltage DC (HVDC) power transmission, due to its scalability, harmonic quality and ability to ride through AC network faults. However, unbalanced AC-Grid conditions create harmonics at twice the fundamental AC-Grid frequency that can propagate to the DC-side as a result of the unbalance in the arm energy. These harmonics may risk excitation of network resonances as well as the maloperation of connected assets, and thus control action is needed to suppress such harmonics. Several different control solutions have been proposed in the literature and the aim of this paper is to provide a comparison of the performance of a sample of these techniques on a 1.2 kV/12 kVA lab-scale MMC demonstrator and cable emulator under unbalanced AC-Grid conditions. The sample of control methods chosen showcases a range of design complexity from basic direct modulation techniques with no DC current control to advanced energy based controllers. Experimental testing of these controllers under the same test condition not only validates the operation of each individual controller but also allows like-for-like comparison of their relative performance. It was found that the controllers tested were capable of significantly suppressing double line-frequency components on the DC -bus compared to the base case, however the more complex controllers had the additional benefit of being able to tune the transient response.
HVDC protection systems require very fast operations, which in turn necessitates transient analysis during design stages. The analysis is performed over a large range of scenarios due to multiple parameters influencing fault characteristics. It is currently executed in time-domain using Electromagnetic-Transient tools. Although these offer a straightforward modeling of non-linear and switching components, computational requirements and accuracy are significantly influenced by selecting the time step and scale of the investigated grid. This can be avoided by the frequency-domain analysis, which has no trade-off between the computational effort and accuracy. The benefits offered by the frequency-domain can be exploited to model and simulate frequency-dependent components and large-scale systems efficiently, although modeling of non-linear and switching components is challenging. This paper addresses these challenges by proposing frequency-domain models for HVDC converters and circuit breakers (DCCB). The response of these components during DC-side faults are characterized by switching events and non-linear behavior due to the multi-stage fault response of converters and non-linear elements in DCCBs (i.e. surge arresters). The developed models are validated against the time-domain solutions obtained by EMT-software. This paper demonstrates that the developed models can be integrated in frequency-domain analysis tools to simulate larger systems efficiently and with acceptable accuracy.
Integration of Distributed Energy Resources (DERs) into conventional power system, results in variable amplitude and orientation of fault current posing new challenges to the traditional protection schemes for fault identification and classification in DC microgrid. In this paper, a new protection technique based on the change in reactor power determined at one end of the line during transient operation is proposed. Thus, the proposed technique employs localized measurement to detect and classify a fault, once the change in reactor power crosses a pre-defined threshold limit. The effectiveness of the proposed technique is determined for various faults and operational transients such as DER outage, load variation, AC-side faults etc. Finally, the simulation results are compared with schemes available in the literature, which further confirms the reliability and resilience of the proposed technique capable of identifying and classifying internal faults.
The new generation of high-efficiency klystrons for future particle colliders requires two high-voltage, series-connected power supplies. As an alternative to conventional klystron power converters, the use of Multi-port Modular Multilevel Converters with three DC output terminals is proposed. However, due to the high asymmetry of the converter, the balancing of the arm energies is challenging. Therefore, this paper analyses the conditions that guarantee energy balance and proposes a design and control strategy that allows stable converter operation.
Due to the increase of Distributed Energy Sources (DER), non-linear and unbalanced loads in the modern Medium-Voltage (MV) grids, concerns about power quality and voltage stiffness are on the rise. To face the aforementioned issues, Active Filter (AF) devices are highly demanded. These devices are based on Voltage Source Converters (VSC). In this perspective, a Single-Delta Bridge-cell Modular Multilevel Converter (SDBC-MMC) seems to be the most adapted VSC topology thanks to its technical and economic advantages. In this paper, a control strategy based on the SDBC-MMC is proposed to compensate for both load reactive power and harmonics, while adopting a simple and efficient control structure. The latter consists in parallelizing the Proportional Integral (PI) Voltage Oriented Control-based (VOC) and the Proportional-Multi Resonant (PMR) control-based. The aim is to regulate the reactive power and partially the harmonics through the commonly used VOC structure, while, the PMR controller will further enhance the compensation of some specific dominating harmonics (5 th , 7 th , 11 th and 13 th ). The PMR regulator is receiving only harmonic signals as an input, and therefore, no interference between both control stages is conceivable when no harmonics are detected. The effectiveness of the proposed control is demonstrated through time domain-simulations in various conditions.
This paper presents a unit protection technique for bipolar DC microgrid. This scheme is based on the symmetricity operation of bipolar DC microgrid. During the normal operating condition, the impedances calculated at the positive- and negative-pole of the same DC bus, have same magnitude due to the symmetricity of bipolar DC microgrid. However, during an internal fault condition, these impedances are different which provides discriminating criteria for the faulty and non-faulty condition. Further, to classify the positive-pole-to-ground and the negative-pole-to-ground faults, the DC fault currents at positive- and negative-poles of the faulty cable are measured. The proposed scheme is validated on 380V bipolar DC microgrid test system designed in PSCAD/EMTDC environment. The obtained results show the selectivity and efficacy of the proposed scheme.
A key challenge to enable the interoperability of a Multi-Vendor-Multi-Terminal (MVMT) HVDC network is to assess the stability without requiring open sharing of the vendor Intellectual Property (IP) relating to control functions. An analytical criterion is therefore proposed as a first step of this assessment. The criterion is indexed by the margin against loss-of-equilibrium for a MVMT-HVDC network with terminal behaviour of connected converters. Based on a classical control architecture, a static analytical model is established, including relevant parameters within the DC network, its topology and operation. By linearizing the system at 0 Hz, the principle of assessing the singularity of the matrix of extended conductance is proposed and proved with the theorem of implicit function and principle of analytic continuation. Two types of scaler index are proposed and then normalized to indicate the margin against loss-of-equilibrium. The effectiveness of the indices is verified and analysed with simulations in the environments of both Matlab/Simulink and RTDS with pseudo-steady-state and detailed Electro-Magnetic Transient (EMT) modelling, respectively. This approach attempts to represent one MVMT control scheme to support practical specification, testing and demonstration of the first multi-vendor multi-terminal HVDC control system outside of China.
