Grid Code Requirements Research Articles

A comprehensive LVRT strategy of two-stage photovoltaic systems under balanced and unbalanced faults

The increase in penetration of photovoltaic (PV) systems into utility grid poses great challenges in terms of system stability, thus, force several countries to update their grid codes to accommodate the low voltage ride-through (LVRT) capability of PV systems to stay connected to the grid and supply the reactive current to support grid voltage during grid faults. In line with this, a comprehensive LVRT strategy is proposed in this paper for grid-connected PV systems under balanced and unbalanced grid faults. The proposed strategy includes the power references calculation method to meet the requirements of grid codes, the active power reduction method to avoid the overvoltage of the dc-link, the peak current limiting method to avoid the overcurrent of the inverter, and three current reference generators implemented by the proposed current reference formulations to ensure the reliable operation of PV systems under different grid faults. The effectiveness of the proposed strategy has been verified in MATLAB/SIMULINK simulation platform and the results show that the proposed LVRT strategy has good performance under different grid faults.

Investigation and Emulation of Junction Temperature for High-Power IGBT Modules Considering Grid Codes

Grid-connected converters in wind turbine systems play a more active role in the future power grid dispatching for fulfilling specified grid codes. Hence, the diversified demands and faults from grid sides bring more intense thermal fatigue to the power modules. This paper proposes an investigation method for junction temperature (T j ) variation considering grid code constraints in laboratory environment. For the most cases of 690-V wind power systems, an MW-level H-bridge test setup with energy circulation is established. In the case of particular grid code requirements and T j control strategy, the equivalent voltage and current stresses can be reproduced and applied to the inspected insulated gate bipolar transistor (IGBT) power modules. To investigate the real-time T j under specific work conditions derived by grid codes, turn-off delay time-based thermal electrical sensitive parameter method is used for T j estimation. Consequently, a test procedure for the safe operation boundary of power converter using T j limitation as a new criterion is proposed. Finally, three types of typical operation conditions for a 1.5-MVA power converter are chosen for the validation of the proposed method. The typical conditions under grid code requirements can be reproduced and the corresponding real-time T j variation can be extracted. With the T j acknowledge, the safe operation boundary and parameter optimizations of IGBT modules can be determined by real T j instead of case temperature.

LVRT capability enhancement of DFIG-based wind farms by using capacitive DC reactor-type fault current limiter

Low-voltage ride-through (LVRT) is the core of grid code requirements for grid connection of wind farms (WFs). This paper presents a capacitive DC reactor-type fault current limiter (CDRFCL) for LVRT capability enhancement of doubly-fed induction generator (DFIG)-based WFs. The CDRFCL is based on the conventional DC reactor-type fault current limiter (DRFCL), which is modified to overcome its limitations of operation and is adapted with the WF environment. To check the effectiveness of the proposed CDRFCL, its performance is compared with the series dynamic braking resistor (SDBR). The WF is modeled with equivalent aggregated DFIG. The PSCAD/EMTDC simulation results show that the CDRFCL is an effective solution for enhancement of LVRT capability and limitation of fault current contribution (FCC) of WFs.

Two-Level Damping Control for DFIG-Based Wind Farm Providing Synthetic Inertial Service

Equivalent inertia of the power system is reducing with the increasing wind power penetration. To alleviate this issue, more and more countries have put forward grid codes that require wind farms (WFs) to provide inertial response as traditional synchronous generation does. It is found in this paper that the synthetic inertial control of the WF in a weak grid deteriorates drive-train torsional oscillation of wind turbine generators (WTGs). Furthermore, the WF emulating inertial response is involved in power system oscillations and induces a new critical low-frequency oscillation. The above oscillations threaten small-signal stability of the WF and the power system, which restricts the WF to fulfill grid code requirements. To overcome this problem, this paper proposes a two-level damping control scheme consisting of a WTG level torsional oscillation damping control and a WF level low-frequency oscillation adaptive damping control to enhance damping characteristic of the WF. Simulation results show that the two-level damping control can simultaneously counteract these concerned multiple oscillations under various operating conditions, which facilitates the WF to meet grid code requirements, and improves dynamic stability of the WF and the power system.

A Model Predictive Power Factor Control Scheme With Active Damping Function for Current Source Rectifiers

A model predictive power factor control (MPPFC) scheme is proposed for high-power medium-voltage (MV) current source rectifiers (CSRs). Facing two major issues on input power factor (PF) and LC resonance, the proposed MPPFC uses a novel line reactive power reference estimator to achieve unitary input PF or maximum achievable input PF for a CSR at various operating states, and involves active damping function into itself to mitigate LC resonance at line side. An online capacitance estimation method is associated with the proposed line reactive power reference estimator to guarantee more accurate input PF control, and a novel damping current calculation method, which avoids the use of band-stop filter, is also designed for active damping purpose. In comparison with traditional linear control, the proposed MPPFC not only realizes input PF regulation, but also achieves better steady-state performance on meeting the grid code requirements. In contrast to traditional model predictive control (MPC) for a CSR, the proposed MPPFC achieve more accurate reactive power control. Moreover, it overcomes the challenge on the realization of active damping based on MPC, which extends the application of MPC into high-power MV CSR. Simulation on a high-power CSR (1 MVA/4160 V/139 A) and experiments on a low-power prototype (5 kVA/208 V/13.9 A) are both conducted to verify the effectiveness of MPPFC.

Active and Reactive Power Control of Doubly Fed Induction Generator Wind Turbines to Answer Grid Codes Requirements

Active and Reactive Power Control of Doubly Fed Induction Generator Wind Turbines to Answer Grid Codes Requirements

A Fault-Tolerant Control Paradigm for Microgrid-Connected Wind Energy Systems

Wind power is a promising alternative energy source; however, its integration to the microgrid is challenging and requires the consideration of voltage regulation, power quality, and voltage variations. This paper proposes a novel fault-tolerant control paradigm for a microgrid-connected doubly fed induction generator (DFIG)-based wind energy system to: 1) achieve ride through during any kind of voltage sag conditions including deep sags and 2) enable the strict satisfaction of recent grid code requirements. The proposed paradigm includes a novel integral terminal sliding mode controller (ITSMC) for the rotor- and grid-side converters along with a series grid-side converter (SGSC) to maintain the voltage levels across the stator windings at their prefault values. A fuzzy logic and a Posicast approach are also proposed to control the SGSC and further improve the performance of the wind energy system. The effectiveness of the proposed fault-tolerant configuration in riding through different types of grid faults is evaluated via detailed computer experiments. The merits of the proposed approach are further compared to those of the standard state of the art in voltage sag mitigation, namely, the shunt current injection approach and the dynamic voltage restorer. Results clearly show that the proposed control paradigm is able to protect the converters from damages and ensure continuous connection of the WT to the grid during faults, hence maintaining power quality.

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

Control Strategy for Grid-Connected Three-Phase Inverters During Voltage Sags to Meet Grid Codes and to Maximize Power Delivery Capability

Inverter-based distributed generation plays a vital role in the stability and reliability of new power systems. Under voltage sags, these systems must remain connected to the electrical network according to the stringent requirements of grid codes (GCs). Low-voltage ride-through (LVRT) control strategies are becoming a common trend in power electronics research. However, previous studies of these control strategies have not dealt with the different possible scenarios presented by new GCs, and many of them focus on a very limited number of control objectives. In this study, an algorithm to maximize the converter capabilities was developed and subjected to experimental tests during different voltage sags. In this research, based on unbalanced voltage drops of several severity levels, six different cases of current injection are identified while taking into consideration the restrictions imposed by GCs. The research results represent a further step toward the development of flexible controllers adaptable to the environments of intelligent electricity grids with high integration of distributed generation.

Grid-connected photovoltaic power plants: A review of the recent integration requirements in modern grid codes

The high integration of photovoltaic power plants (PVPPs) has started to affect the operation, stability, and security of utility grids. Thus, many countries have established new requirements for grid integration of solar photovoltaics to address the issues in stability and security of the power grid. In this paper, a comprehensive study of the recent international grid codes requirement concerning the penetration of PVPPs into electrical grids is provided. Firstly, the paper discusses the trends of PVPPs worldwide and the significance of improving grid codes' requirements. In addition, the comparison of common requirements covered in the majority of international grid codes considers high‐ and low‐voltage ride‐through capabilities, voltage and frequency regulation, and active and reactive power support requirements. Finally, a broad discussion on the compliance technology challenges and global harmonization of international grid codes that the PVPPs have to address is presented. The study summarizes the most recent international regulation regarding photovoltaic integration and research findings on the compliance of these regulations and proposed recommendations for future research. It also can assist power system operators to compare their existing requirements with other universal operators or establish their own regulations for the first time. Additionally, this research assists photovoltaic manufacturers and developers to get more accurate understanding from the recent global requirements enforced by the modern grid codes.

An offshore wind farm energy injection mastering using aerodynamic and kinetic control strategies

The injection of wind farm production into a grid, needs optimal strategies for energy transfer management. Usually, the power produced by the wind farms does not fulfil all the grid code requirements. The main problem is generally based on the way to reduce the impacts of power production fluctuations on the grid voltage and its frequency. To solve this problem, some authors suggest the use of an interface such as energy storage devices in order to compensate the wind power fluctuations. In fact, the storage devices installed between the wind farm and the grid can improve the power quality in terms of stability but in other hand the size and the cost of the system can be increased. In this paper, two solutions have been proposed in case the power quality produced by the wind farm is out of the grid code requirements. The improvement of the energy quality of an offshore wind farm without storage and connected to the grid is discussed. The proposed solution is to operate the wind turbines with a reserve of power. To distribute this reserve equitably among wind turbines, a proportional distribution algorithms has been developed. The results obtained show clearly the effectiveness of the strategy.

Power Quality Improvement and Low Voltage Ride Through Capability in Hybrid Wind-PV Farms Grid-Connected Using Dynamic Voltage Restorer

This paper proposes the application of a dynamic voltage restorer (DVR) to enhance the power quality and improve the low voltage ride through (LVRT) capability of a three-phase medium-voltage network connected to a hybrid distribution generation system. In this system, the photovoltaic (PV) plant and the wind turbine generator (WTG) are connected to the same point of common coupling (PCC) with a sensitive load. The WTG consists of a DFIG generator connected to the network via a step-up transformer. The PV system is connected to the PCC via a two-stage energy conversion (dc–dc converter and dc–ac inverter). This topology allows, first, the extraction of maximum power based on the incremental inductance technique. Second, it allows the connection of the PV system to the public grid through a step-up transformer. In addition, the DVR based on fuzzy logic controller is connected to the same PCC. Different fault condition scenarios are tested for improving the efficiency and the quality of the power supply and compliance with the requirements of the LVRT grid code. The results of the LVRT capability, voltage stability, active power, reactive power, injected current, and dc link voltage, speed of turbine, and power factor at the PCC are presented with and without the contribution of the DVR system.

Grid code compliant controllers for Multi-terminal HVDC grids aimed to integrate wind power: assessing their impact on the operational security of a real-world system

Exploiting multi-terminal High Voltage Direct Current (MTDC) grids for offshore wind park integration into continental bulk AC system raises several technical challenges, in particular the verification of the compliance of MTDC grids and offshore wind parks to the performance requirements (in terms of frequency, voltage support and fault ride through) which are being established by the grid codes, and the availability of general and customizable models to simulate their response. This work is aimed to provide a set of “open access” general models, able to fulfil the grid code requirements if suitably tuned, enabling the TSOs to perform their analyses without relying exclusively on vendor-specific models. In order to verify the realistic response of these models, an example of MTDC grid is simulated, connecting it to plausible operating scenarios of a real world AC system (the West Denmark grid) in target year 2020: some indications to properly tune control parameters are provided together with the results of several simulations aimed to check the fulfilment of grid code requirements in case of typical contingencies.

An LQR Controller for Damping of Subsynchronous Interaction in DFIG-Based Wind Farms

This paper presents a linear-quadratic regulator (LQR) for damping of subsynchronous interaction (SSI) in doubly-fed induction generator (DFIG)-based wind farms. The proposed LQR controller employs a full-state observer to estimate all state variables. The output of the LQR is added to control signals of inner current control loops of DFIG converters as supplementary control signals. The supplementary control signals are dynamically limited to avoid saturating the converters and to provide the DFIG with the desired transient response against power system faults. The proposed SSI damping controller is designed for a realistic series compensated wind farm, and its performance is verified using electromagnetic transient (EMT) simulations. The EMT simulations are performed using a detailed DFIG model which includes all nonlinearities and all required transient functions to meet the grid code requirements corresponding to fault-ride-through (FRT). The results show that the proposed SSI controller is able to significantly mitigate the oscillations due to the SSI phenomenon, and to provide excellent transient response against systems faults.

Contribution of a wind farm to voltage and system stability: results of a measurement campaign

The rising percentage of inverter-based generation poses new challenges to the grid. Approaching a majority share of inverter-based generation, technical issues in grid operation and the contribution to grid stability have to be analysed. A measurement campaign in a wind farm has been carried out in order to investigate the performance of a wind farm and its interaction with the grid. Due to a long feeder, the short-circuit power at the connection point of the wind farm was low. The results are used to recognise technical challenges and to find adequate solutions. The main objective of this study is an evaluation of the wind farm performance and a comprehensive comparison with requirements of grid codes and guidelines to contribute to reactive power balance and system stability.

Quasi-dynamic model of VSC-HVDC transmission systems for an operator training simulator application

This paper proposes a simplified quasi-dynamic model of two terminal voltage-source-converter (VSC)-HVDC transmission systems based on the existing full-order models for application in an operator training simulator (OTS). The simplified model is verified against the full-order model for some verification scenarios. The model stability and behaviour is assessed to check if the model meets the grid code requirements as stated in the ENTSO-E Network Code. These features include black-start capability, set point change, active power reversion, frequency contributions and reactive power-voltage regulations. The proposed linearized model is presented in state-space equations and is stabilized by state space feedback using the linear quadratic regulator (LQR) design. The model has sufficient accuracy, requires less input data compared to full-order model, is tuneable by any vendors and is easily implementable in an environment such as OTS.

An analytical study for low voltage ride through of the brushless doubly-fed induction generator during asymmetrical voltage dips

The Brushless Doubly-fed Induction Generator (BDFIG) has high potential for wind energy systems, especially for offshore applications where minimum maintenance is vital. The machine low voltage ride through (LVRT) capability in the light of current grid code requirements was investigated using a precise dynamic model. This is particularly important for future multi- MW BDFIGs.This paper shows the necessity for improvements of the BDFIG LVRT capability with presenting a comprehensive analytical study during asymmetrical voltage dips. Analytical studies are conducted to extract a more precise equivalent circuit model of the BDFIG used for analyzing machine dynamic behavior under various fault conditions. In addition, a comparison between different voltage dips is performed to identify critical operating points for LVRT assessment. The results of the study are verified by coupled-circuit model, simulated in MATLAB/Simulink for a BDFIG prototype.

An Enhanced LVRT Scheme for DFIG-based WECSs under Both Balanced and Unbalanced Grid Voltage Sags

Due to the latest grid codes, wind energy conversion systems (WECSs) are required to remain connected to grid under grid voltage sags and supply reactive power into the grid. So, this paper proposes an enhanced scheme to improve low-voltage ride through (LVRT) capability of doubly fed induction generator (DFIG)-based WECSs under both balanced and unbalanced grid voltage sags. The proposed scheme is composed of active and passive LVRT compensators. The active compensator is performed by controlling the rotor- and grid-side converters of the DFIG to decrease the stator flux oscillations and inject reactive power into the grid. The passive compensator is based on a three-phase stator damping resistor (SDR) located in series with the stator windings. The proposed scheme decreases the negative effects of grid voltage sags in the DFIG system including the rotor over-currents, electromagnetic torque oscillations, and DC-link over-voltage and also injects reactive power into grid to support the grid voltage. So, the LVRT capability of DFIG is enhanced and new grid code requirements are addressed. Simulation results on a 1.5-MW DFIG-based WECS using MATLAB/Simulink demonstrate the effectiveness of the proposed LVRT scheme under both balanced and unbalanced grid voltage sags.

Guest Editorial

Guest Editorial

Strategy to enhance the low-voltage ride-through in photovoltaic system during multi-mode transition

With the increasing capacity of distributed generation (DG) connected to the power grid, the future generation of photovoltaic (PV) systems are expected to provide a full range of voltage regulation during grid faults in order to enhance the low-voltage ride-through (LVRT) capability of a PV system. In such a condition, the DG should remain connected to the grid for reactive power support, thereby improving voltage profile. This paper aims to propose a control strategy of active and reactive power for a single-stage three-phase grid-connected PV system to enhance the LVRT. The dynamic behaviours of the system were investigated by considering various scenarios such as varying irradiance, local load disconnection, and short circuits, at different locations during the multi-DG operation. Results confirm that the grid-connected PV system is able to remain connected to the power grid during steady-state and transient-state conditions without violating the grid code requirements. The established dynamic behaviour analysis model of the proposed control for grid-connected PV systems can be used in planning an operational strategy for a practical system.