Grid Connection Point Research Articles

Performance of an electrical distribution network with Soft Open Point during a grid side AC fault

Soft Open Point (SOP) is a power electronic device installed in place of normally open points in electrical distribution networks. This paper investigates the dynamic performance of a medium voltage (MV) distribution network with a connected SOP, under grid side AC faults. Use of sequence networks was extended to include SOP, such that conventional fault analysis technique can be used on a distribution network with SOP. A Fault-Index was defined using symmetrical components of voltages measured at the grid connection point of the SOP. The network performance was investigated under a line-to-ground, a line-to-line and a three-phase fault. The behaviour of the network was analysed under different control schemes and various operating scenarios of the SOP. Furthermore, the dependence of the sequence voltages and currents on the SOP set points was investigated. Simulations were carried out on an 11kV generic UK distribution network model developed in PSCAD/EMTDC. Results show that the convectional fault analysis technique is applicable on a network with SOP, regardless of the SOP control mode. The Fault-Index, defined based on the local voltage measurements, was effective in detecting the presence of an AC fault in the MV distribution network. In addition, the need for a non-current based detection method is illustrated.

Increased Wind Energy Yield and Grid Utilisation with Continuous Feed-In Management

This paper presents a study to assess how wind turbines could increase their energy yield when their grid connection point is not strong enough for the rated power. It is state of the art that in such situations grid operators impose feed-in management on the affected wind turbines, i.e., the maximum power is limited. For this study a 5 MW wind turbine is introduced in a small grid that has only limited power transfer capabilities to the upstream power system. Simulations of one particular day are conducted with the electric load, the temperature, and the wind speed as measured on that day. This simulation is conducted twice: once with the 5 MW wind turbine controlled with conventional feed-in management, and a second time when its power is controlled flexibly, i.e., with continuous feed-in management. The results of these two simulations are compared in terms of grid performance, and in terms of mechanical stress on the 5 MW wind turbine. Finally, the conclusion can be drawn that continuous feed-in management is clearly superior to conventional feed-in management. It exhibits much better performance in the grid in terms of energy yield and also in terms of constancy of voltage and temperature of grid equipment. Although it causes somewhat more frequent stress for the wind turbine, the maximum stress level is not increased.

Grid Side Unbalanced Fault Detection using Soft Open Point in an Electrical Distribution Network

Soft open point (SOP) is a power electronic device that installed in place of normally open points in distribution networks. This paper investigates the performance of SOP under an unbalanced line to ground (L-G) fault in a medium voltage (MV) distribution network. A method to detect unbalanced grid side AC faults was developed using positive and negative sequence currents injected into the feeder by the SOP. Simulations were carried out on a network model developed in PSCAD/EMTDC. Local measurements at SOP grid connection point were used to define a fault index. Results show that the fault index, based on the values of positive and negative sequence currents, is able to effectively detect the presence of an unbalanced fault in a MV distribution network.

On the Bayesian optimization and robustness of event detection methods in NILM

A basic but crucial step to increase efficiency and save energy in residential settings is to have an accurate view of energy consumption. To monitor residential energy consumption cost-effectively, i.e., without relying on per-device monitoring equipment, non-intrusive load monitoring (NILM) provides an elegant solution. The aim of NILM is to disaggregate the total power consumption (as measured, e.g., by smart meters at the grid connection point of the household) into individual devices’ power consumption, using machine learning techniques. An essential building block of NILM is event detection: detecting when appliances are switched on or off. Current state-of-the-art methods face two open issues. First, they are typically not robust to differences in base load power consumption and secondly, they require extensive parameter optimization. In this paper, both problems are addressed. First two novel and robust algorithms are proposed: a modified version of the chi-squared goodness-of-fit (χ2 GOF) test and an event detection method based on cepstrum smoothing. Then, a workflow using surrogate-based optimization (SBO) to efficiently tune these methods is introduced. Benchmarking on the BLUED dataset shows that both suggested algorithms outperform the standard χ2 GOF test for traces with a higher base load and that they can be optimized efficiently using SBO.

Wind turbine manufacturers observation regarding reactive power support and control requirements

With the increase of power generated by wind power plants, network operators have placed steady-state and dynamic reactive power requirements at their grid connection point in several EU member states. This has forced wind turbine manufacturers to invest in upgrade of their technology and power plant owners and developers to invest in external compensation devices, adding to overall project costs. With the introduction of EU regulation Network Code Requirements for Generators (NC RfG) in 2016, it is expected that many countries (national implementation) will implement higher than currently required reactive power requirements accordingly to the new EU framework. The purpose of this study is to discuss the reactive power requirements and to investigate the reactive power support actually provided by wind power plants to date. SCADA data from a number of operational sites are analysed with regards to their actual reactive production utilising different control modes and to compare the given reactive power support with the requirements defined by the limits within the NC RfG. At the end of this study, a derived proposal for steady-state reactive power needs and dynamic power needs are discussed, therefore a minimum technical reactive power provision required is derived based on the current utilisation.

Optimized and enhanced grid architecture for electric vehicles in Europe

For an optimized large-scale roll-out of EVs in Europe whilst at the same time maximizing the potential of DER integration, an optimized and enhanced grid architecture for EVs in Europe has to be considered. The work in this paper is addressing this topic and summarizing the corresponding project findings. The aim of this approach is to provide a framework for the further investigation of selected use cases which allows implementing and comparing scenarios of different DSOs. Following a Smart Grid approach, the developed grid architecture implements energy grid entities and ICT components. The general framework was described including all its relevant clusters and indicating related entities. The network types used for this architecture are following the SGAM and Smart Grid Standards Map approach. A so-called “Smart Grid Connection Point”, which is a generic system interface, is used in this work to allow a more simplified graphical architecture model and increase its readability. Similar to the concept and purpose of the Smart Grid Connection Point, also the principle of an integration bus for entity clusters was introduced. From the Integration bus, the information from/to external systems passes through the Smart Grid Connection Point using one of a range of possible technological options. The position of EVs charging infrastructure within the framework is defined at the border between the domains DERs (generation) and consumption, which takes into account future V2G scenarios, where EVs may act as consumption and generation devices. EVSEs and DERs may be connected as standalone systems directly to the grid, or indirectly as part of one of the clusters at the customer premises domain which refers to the three location-wise types of charging, public, semi-public and private charging. Regarding controlled charging of EVs this optimized architecture allows a variety of different local, distributed or aggregated options which may involve different types of actors.

Voltage quality assessment considering low voltage ride‐through requirement for wind turbines

This study proposes an effective method for voltage quality assessment considering low voltage ride-through (LVRT) requirement for wind turbines. With increasing wind power generation, there is a strong need for system connection requirements of wind turbines, for stable system operation even under fault conditions. The LVRT requirements specify acceptable voltage magnitude and duration for maintaining the connection to power systems under voltage sags. In general, different grid connection points of wind farms have different voltage quality performance. Therefore, LVRT requirements can be severe for certain connecting points and wind turbines. The proposed method can efficiently assess voltage sag performance at grid connection points, and vulnerable connection points that can be expected to incur more severe voltage sags causing wind turbine disconnections can be identified.

Vector Control Realization of DFIG Under Grid Abnormalities using Real Time Digital Simulator

A grid connected doubly-fed induction generator (DFIG) system, driven by variable speed wind turbine is considered in this research to satisfy grid code requirements. Remaining grid synchronized and stable under voltage sag and voltage swell, obtaining power control through dc link voltage control, and providing unity power factor at grid terminals are the achievements. The DFIG system uses two back-to-back converters. Vector control strategy is used with the grid side and rotor side converters, and taken up for research for further improvement. The grid side converter controls dc-link voltage and maintains unity power factor at the grid connection point. The rotor side converter supplies the reactive power of the machine and maintains the speed constant irrespective of the transient behavior of the grid. In this paper the behavior of the DFIG system is analyzed under grid voltage fluctuation and the experimental results are obtained using RT-LAB. Main contribution of this work is in improving the DFIG system performance with grid low voltage and over voltage ride through capability through simulation, and its real time experimental verification.

Combining Frequency Containment Reserves and Renewable Power Leveling in Energy Storage Systems

The objective of this paper was to show that a combination of renewable energy sources with energy storage systems, which provide frequency containment reserves FCR (also called primary frequency control), improves the performance of both systems. This double benefit comes out in a reduction of intraday trading for the storage system and a leveled power injection of the renewables.Considering the regulations that apply for battery storage systems to provide FCR in Germany, interaction with other units is essential to guarantee availability of the system. This interaction is usually performed market based by intraday trading or by flexible power generation or consumption combined with the storage system. This paper, however, showed the benefits of interaction with fluctuating renewable energy sources.Energy storages and renewables were clustered to a common grid connection point to utilize the fluctuations of the renewable power generation directly for battery management. Simulation results based on sample photovoltaic and wind profiles showed the double benefit of the configuration. Intraday trading was reduced by 33% up to 57%. At the same time, fluctuations decreased, so the renewable power injection better matched a predefined power schedule.

Post‐retirement utilisation of synchronous generators to enhance security performances in a wind dominated power system

Power system security concerns due to significant wind generation could be twofold: first, the frequency response and second, the short-circuit performance. Continuous development of wind energy may cause economic replacement and retirement of existing synchronous generators. Modern wind turbine generators do not inherently offer inertia and governor response after a disturbance, which may result in declining frequency response behaviour. Besides, they have limited fault current contribution, which may cause unacceptable short-circuit ratio at the grid connection point of those wind power plants. Traditionally, frequency response and short-circuit performance are individually improved. However, both of them are concurrently affected by high wind penetration. Hence, a common approach to simultaneously enhance both indices is essential. Therefore, this study introduces an idea of operating some of the retired synchronous generators in the synchronous condenser mode, which is termed as ‘post-retirement scheme (PRS)’. Such a second use of the retired synchronous generators can jointly upgrade frequency response and short-circuit strength during high wind generation. This study also proposes a methodology to evaluate when and how much PRS should be deployed for ensuring an adequate security performance in a wind dominated power system.

An improved AVC strategy applied in distributed wind power system

Traditional AVC strategy is mainly used in wind farm and only concerns about grid connection point, which is not suitable for distributed wind power system. Therefore, this paper comes up with an improved AVC strategy applied in distributed wind power system. The strategy takes all nodes of distribution network into consideration and chooses the node having the most serious voltage deviation as control point to calculate the reactive power reference. In addition, distribution principles can be divided into two conditions: when wind generators access to network on single node, the reactive power reference is distributed according to reactive power capacity; when wind generators access to network on multi-node, the reference is distributed according to sensitivity. Simulation results show the correctness and reliability of the strategy. Compared with traditional control strategy, the strategy described in this paper can make full use of generators reactive power output ability according to the distribution network voltage condition and improve the distribution network voltage level effectively.

An Offshore Wind Energy Geographic Information System (OWE-GIS) for assessment of the UK's offshore wind energy potential

An Offshore Wind Energy Geographic Information System (OWE-GIS) has been developed for the purpose of assessing the economically accessible offshore wind energy resource for the United Kingdom.The UK OWE-GIS estimates the costs of energy from an offshore wind farm taking account of the major capital components; development costs dependent on water depth and distance from nearest ports or grid connection points; the potential energy production dependent on annual average wind speed, potential array losses, and turbine availability; operations and maintenance costs; and financial parameters such as discount rate and project lifetime. A sensitivity analysis is presented to show the influence of discount rate, project lifetime, and assumptions about overall capital expenditure (CAPEX), availability, and annual mean wind speed.

Techno-economic and business case assessment of low carbon technologies in distributed multi-energy systems

Increasing focus on energy affordability and environmental impact is drawing interest towards the potential value of low carbon technology (LCT) interventions in buildings and district energy systems. Relevant interventions may include improvements of the insulation levels and installation of low carbon and renewable generation technologies (e.g., combined heat and power, photovoltaics, heat pumps). These LCT interventions for both electricity and heat supply can, in principle, reduce energy costs and CO2 emissions for final energy consumers. This can be achieved by coupling, and possibly optimising, multiple energy vectors from traditionally independent systems (e.g., electricity, heat and gas). However, this transition to distributed multi-energy systems introduces complex physical and commercial interactions between the different energy vectors. Further, these interactions can be fundamentally different at different aggregation levels (e.g., premises, district, and commercial level). This makes the assessment of business cases for LCT interventions in a multi-energy context a grand challenge, and given the potentially disruptive commercial impact of many such novel technologies, such complexity might result in a barrier to their development. In light of this, this paper proposes a techno-economic framework for the assessment of business cases of LCTs, which systematically models the physical and commercial multi-energy flows at the premises, grid connection point, and commercial levels. This is particularly important given the commonly asymmetrical nature of various energy price components, which can have significant effects on the business cases of LCTs and associated actors (e.g., retailers and energy service companies). The proposed framework is demonstrated through a series of case studies that highlight the value of various LCT interventions and of aggregation, in terms of energy, emission and operational cash flow metrics. The relevance and importance of the framework to developing business cases for various energy system actors, including policy makers and regulators, is discussed, with the final aim of facilitating the uptake of low carbon multi-energy technologies.

Comparison between Aggregation Techniques for PMSG Wind Farm

This paper presents a wind farm composed of different permanent synchronous generator (PMSG) wind turbines (same wind turbine technology but different rated power or operational conditions) coupled to the same grid connection point. A comparison between the three techniques of wind farm aggregation for a farm composed of PMSG wind turbines. Our comparison is for the closeness of the results of normal and transient response of both the complete model and the aggregated models. The chosen three techniques are, multi full aggregated model using equivalent wind speed (MFAM-EWS), semi aggregated model (SAM) and mixed semi full aggregated model (MSFAM). Simulation has been carried out for these techniques by MATLAB/SIMULINK program to compare them with the complete model to prove the effectiveness of each aggregation technique.

Modelling robustness for a flexible grid‐tied photovoltaic generation system

Robustness to modelling inaccuracy is a vital characteristic of a controller, because no matter how accurate a model is, modelling inaccuracies always exist. However, it is difficult to achieve satisfactory nominal and robust performance of a grid-tied photovoltaic (PV) generation system at the same time, using traditional controller design methods. To solve this problem, a μ-synthesis control method combined with the application of linear fraction transformation (LFT) is proposed in this study. The proposed method is designed with object transforming and implemented with DK iteration. The μ-analysis and simulation results prove that the objective can be achieved under various microgrid conditions, determined by factors such as grid connection point voltage, load conditions and component parameter perturbations. Compared with traditional controller design methods, the grid-tied PV generation system under μ-synthesis control exhibits comparatively large perturbation margin for nominal performance and robust stability, and simultaneously is capable of guaranteeing the preset perturbation range for robust performance.

Integrated Modelling Platform for Dynamic Performance Assessment of Floating Wind Turbines

The paper presents an integrated modelling platform that can be used to assess the dynamic performance of an offshore wind turbine mounted on a spar-buoy type floater. Sub-models of generator and converter controllers and the power network are combined with a state-of-the-art numerical simulation of the hydro-, aero- and structural dynamic behavior of the floating wind turbine, using FEDEM Windpower software. The aim is to provide a tool that allows analyzing response of floating turbines to grid faults, interactions and potential conflicts between controllers. A study case of grid disturbance was conducted to illustrate the applicability of an integrated model. A grid fault that lasted 100ms and resulted in 50% residual voltage at the grid connection point was applied, and wind turbine operation both in still water and in large sea wave conditions were analyzed. The results show that the turbine was capable of riding through voltage-dips without severe effects on the electrical or mechanical systems. A significant, but not critical, dip in the tower bending moment was observed. The most affected component of the bending moment is around the axis of the rotor, which is directly related to the loss of generator torque due to the grid disturbance event.

Method for Determining the Maximum Allowable Capacity of Wind Farm Based on Box Set Robust Optimization

With the increasing penetration of wind power, the randomness and volatility of wind power output would have a greater impact on safety and steady operation of power system. In allusion to the uncertainty of wind speed and load demand, this paper applied box set robust optimization theory in determining the maximum allowable installed capacity of wind farm, while constraints of node voltage and line capacity are considered. Optimized duality theory is used to simplify the model and convert uncertainty quantities in constraints into certainty quantities. Under the condition of multi wind farms, a bilevel optimization model to calculate penetration capacity is proposed. The result of IEEE 30-bus system shows that the robust optimization model proposed in the paper is correct and effective and indicates that the fluctuation range of wind speed and load and the importance degree of grid connection point of wind farm and load point have impact on the allowable capacity of wind farm.

Detection and suppression of parasitic DC voltages in 400V AC grids

Grid connected static power converters inject parasitic DC currents due to the offset in current sensing, control imperfections, assymetries in power switches and other secondary effects. Ever growing number of grid connected converters contributes to an increase of DC bias in AC grids, and this brings the cores of distribution transformers closer to saturation and increases their power losses. This paper provides sensitivity analysis of distribution transformers to the DC bias, and considers solutions for detecting and compensating the parasitic DC components in AC grids. Active compensation methods can be advantageously used in suppressing the DC bias at grid connection point of the power converter. The sensing approach proposed in this paper makes use of saturable ferromagnetic cores and a low cost DSP for signal analysis and processing. Proposed algorithm uses distortion of the magnetizing current of a parallel connected saturable core due to the bias. Experimental results demonstrate the capability for detecting and compensating the bias voltages far below 1 mV in 0.4 kV grids. The paper describes the principles of DC bias detection and it provides the guidelines for the proper design of magnetic components. High precision of the proposed DC bias sensing is thoroughly verified on the experimental setup connected to a 0.4 kV grid.

A series flywheel architecture for power levelling and mitigation of DC voltage transients in multi‐terminal HVDC grids

This study proposes a new architecture of flywheel energy storage system (FESS) which can be implemented at generating unit (GU) level (e.g. wind turbine) or scaled up for application in grid-side converter (GSC) stations of a multi-terminal direct current (MTDC) system. The proposed architecture suppresses DC voltage leaps under grid faults, and performs power levelling duty at grid connection points. When implemented at GU level, the proposed FESS features a doubly-fed induction machine (DFIM) – the flywheel machine – where its six stator winding terminals are connected in series between the grid and the GSC. The flywheel converter is of partial power rating, and is connected to the DFIM rotor. When employed at an MTDC GSC station, several submodules are connected to build an FESS station. The study proposes the control concept and the general topology of the FESS station. The performance parameters of the system are tested under normal and abnormal operating conditions. Results and analyses indicate smooth dynamic operation, the economic advantage and the improved fault ride-through capability because of the proposed system.

Research on the Allocated Capacity of Energy Storage Based on Adjustment of SOC

In order to remedy errors in wind power prediction, a common method adopted in wind farms is the configuration of a certain capacity of energy storage at the grid interface. The traditional approach for configuring capacity ignores the problems of excessive allocation and the adjustment of SOC, and for this reason, the paper proposes a model of wind power coupling by adding a resistance load at the grid connection point. In addition, this paper proposes a control strategy to improve the operation of energy storage systems through the adjustment of SOC in order to maintain a high level of SOC, and proposes a new approach to deploying storage capacity based on this. According to the analysis of a wind farm in Northeast China, a new approach to the configuration of the energy storage can use local utility of a little electrical energy to reduce the allocated capacity, and furthermore, this method also has better control characteristics.