Grid Code Requirements Research Articles

Developing a Mathematical Model for Wind Power Plant Siting and Sizing in Distribution Networks

Wind Power Plants (WPPs) are generally located in remote areas with weak distribution connections. Hence, the value of Short Circuit Capacity (SCC), WPP size and the short circuit impedance angle ratio (X/R) are all very critical in the voltage stability of a distribution system connected WPP. This paper presents a new voltage stability model based on the mathematical relations between voltage, the level of wind power penetration, SCC and X/R at a given Point of Common Coupling (PCC) of a distribution network connected WPP. The proposed model introduces six equations based on the SCC and X/R values seen from a particular PCC point. The equations were developed for two common types of Wind Turbine Generators (WTGs), including: the Induction Generator (IG) and the Double Fed Induction Generator (DFIG). Taking advantage of the proposed equations, design engineers can predict how the steady-state PCC voltage will behave in response to different penetrations of IG- and DFIG-based WPPs. In addition, the proposed equations enable computing the maximum size of the WPP, ensuring grid code requirements at the given PCC without the need to carry out complex and time-consuming computational tasks or modelling of the system, which is a significant advantage over existing WPP sizing approaches.

Nonlinear Active Disturbance Rejection Control for Improving Low-Voltage Ride Through Capability of DFIG-Based WECS

As recorded in the grid codes of different global power grid operators, Variable Speed Wind Energy Conversion Systems (VS-WECS) based on Doubly Fed Induction Generator (DFIG) supplying into the utility grid should meet the new requirements during Low Voltage Ride Through (LVRT). In this paper, a new non-linear ADRC control is mobilized to improve the control of the DFIG-based VS-WECS in order to overcome the effects of the voltage sags. The simulation results in MATLAB-Simulink environment will be established to illustrate the relevance of control designed to make production by a VS-WECS based on DFIG able to meet the requirements of grid code during LVRT.

Modelling fault behaviour of power electronic converters

Renewable energy sources primarily employ power electronic converters to deliver their energy to the grid. These converter-based distributed generators (CBDGs) have a fault behaviour different from conventional generators, capable of compromising the reliability of traditional protection schemes. This paper investigates the fault behaviour of CBDGs by presenting both an analytical steady state model and a numerical transient simulation model developed in MATLAB/Simulink. Using these models, the impact of grid code requirements regarding reactive current injection during faults is analyzed. The results underline that different modelling techniques can support the design of grid codes.

Avoiding overvoltage problems in three‐phase distributed‐generation systems during unbalanced voltage sags

During voltage sags, distributed generation systems must fulfil specific grid-code requirements for reactive current injection. This ancillary service can produce overvoltage problems in networks operating in unbalanced conditions when the amplitude of one of the phase voltages is higher than the others and a balanced reactive current is injected through large grid impedance. This study proposes a control scheme to avoid these overvoltage problems, thus reducing the risk of cascade disconnection that this incident may produce. The derivation of the control scheme starts from the flexible oscillating-power control, introduces the necessary modifications so that this control meets the grid-code requirements for current injection and combines it with a slope voltage control to achieve a good voltage regulation. A theoretical analysis is included to determine the expressions that quantify the voltage support characteristics of the proposal. Finally, selected experimental results are reported to validate the characteristics of the proposed control.

Dynamic Voltage Support of Converters during Grid Faults in Accordance with National Grid Code Requirements

To ensure system stability, national grid codes often require converter-based generators to provide fault-ride-through (FRT) capabilities and dynamic voltage support, according to which they should stay connected and support the voltage during fault situations. The requirements for dynamic voltage support include the injection of reactive current in the positive- as well as negative-sequence system, directly proportional to the change of the corresponding voltage between fault and pre-fault. Since this requirement may lead to a reference current surpassing the maximum current capability, the converter control has to contain a proper current limitation. This paper presents an algorithm for such a current limitation and a simulation model of a converter and its control, which applies this algorithm. Based on voltage measurements, which were measured during forced short-circuits in the real grid, the simulation model is used to simulate the behavior of a converter in reaction to these voltage measurements. The results show that the converter control using this algorithm for current limitation guarantees a current output below the maximum current capability while respecting the requirements for dynamic voltage support of the relevant grid codes.

Mitigation of Fault Induced Delayed Voltage Recovery (FIDVR) by PV-STATCOM

This paper presents a novel day-and-night control of large PV solar farm as STATCOM, termed PV-STATCOM, for FIDVR alleviation. The proposed control involves dynamic modulation of both reactive and real power during day, and of reactive power during night. The active and reactive power output of solar farm are based on the sensitivity of solar plant voltage to active and reactive power injections. Enhanced voltage control up to utility Transient Overvoltage Limit is utilized. Eigenvalue analysis is done to study the sensitivity of proposed controller performance to various power system and PV plant parameters. Extensive PSCAD simulation studies of FIDVR are performed in a realistic power transmission system with large-scale PV plant and comprehensive IM loads. It is shown that the proposed PV-STATCOM control: (i) mitigates FIDVR even if solar farm is located more than 100 km from motor loads, (ii) is more effective than reactive power support required by German Grid Code, (iii) is equally effective as a STATCOM connected locally at motor loads, and (iv) stabilizes motors at night which is beyond Grid Code requirements. The PV-STATCOM potentially provides significant cost savings to utilities and new revenue stream for solar plants in providing 24/7 FIDVR mitigation service.

Utility-scale photovoltaic generators: a review on trends, grid code requirements and challenges

<p>This paper provides an overview of the global trends in utility-scale photovoltaic (PV) installed capacity. This paper also presents a comparison of grid-connection requirements of six countries in the continents of Europe, Asia, Africa, and Australia for utility-scale PV generators in normal and abnormal grid conditions. Many country-based grid codes and international standards (IEEE 1547) for interconnection of inverter-based renewable energy generators are demanding stricter grid-connection compliance from utility-scale PV generators to ensure power system safety and reliability as its penetration level increases. This paper then discusses the economic and technical impacts, and explores the preparedness of PV generation systems in meeting the grid code requirements.</p>

SCIG Based Wind Energy Integrated Multiterminal MMC-HVDC Transmission Network

Modular multilevel converter (MMC) based HVDC system for renewable energy integration has attracted the researcher’s interest nowadays. This paper proposes a control strategy for MMC based multiterminal HVDC system for grid integration of squirrel cage induction generator (SCIG) based wind energy systems. Unlike the average model, this work models the MMC using the aggregate model and develops multiterminal HVDC transmission network in MATLAB/Simulink. It further develops the MMC multiterminal HVDC transmission network in real time digital simulator (RTDS). Instead of simplified current source, the proposed network considers the complete dynamics of SCIG based wind source from generation to integration. It employs field-oriented control for optimum wind energy tracking and forms isolated AC grids using feed forward controller. The proposed MMC controller performance has been tested under severe balanced and unbalanced disturbances. The results from aggregate model based MMC network in MATLAB/Simulink and those of the experimental MMC network in RTDS are in full agreement. The results confirm optimum wind energy tracking, fulfill grid code requirements, and improve low voltage ride through capability.

A critical evaluation of grid stability and codes, energy storage and smart loads in power systems with wind generation

Existing power systems are facing new challenges in maintaining the security of the power system as the penetration of variable renewable energy technologies, such as variable speed wind turbines, increase. System non-synchronous generation replaces conventional generators as penetration of renewable generation increases. This affects system rotational inertia and limits the number of online thermal generators that can provide frequency stability services and system-wide areas voltage stability. This evolution has resulted in some changes to existing grid codes and new ancillary services. Furthermore, it could provide opportunities to address the security of the system utilizing modern smart technologies, e.g. smart loads, heat pumps and electric vehicles. The aim of this paper is to evaluate the impacts of large-scale renewable power generation on power system dynamics from the perspective of the power system operator. It focuses on the grid codes implications and challenges specifically. Synthetic inertia response opportunities from smart loads, electric vehicles and energy storage technologies and dispatching wind farms during frequency excursions are analyzed and thoroughly discussed. The key finding is that rethinking in the development of grid code requirements and market mechanisms are needed if a power system based on 100% power electronic renewable generation is to be achieved. This type of power system would need a range of technologies to provide the types of ancillary (i.e. system) services required, as none of the technologies alone can tackle all the challenges presented.

An Improved Dynamic Voltage Restorer Model for Ensuring Fault Ride-Through Capability of DFIG-based Wind Turbine Systems

Renewable energy sources (RES) are being integrated to electrical grid to complement the conventional sources in meeting up with global electrical energy demand. Among the RES, Wind Energy Conversion Systems (WECS) have gained global electricity market competitiveness especially the Doubly Fed Induction Generator (DFIG)-based Wind Turbines (WTs) because of flexible regulation of active and reactive power, higher power quality, variable speed operation, four quadrant converter operation and better dynamic performance. Grid connected DFIG-based WTs are prone to disturbances due to faults in the network which made the utilization of the power generated a major concern. The grid code requirement for integrating the DFIGs to grid specified that they must remain connected and support the grid stability during grid disturbances of up to 1500milliseconds. The ability of the DFIG WT system to uphold to the grid codes requirement is termed the Fault Ride – Through (FRT). This paper presented a 1.5MW grid connected DFIG-based WT model with a Dynamic Voltage Restorer (DVR) for FRT capability enhancement. The design and simulation were performed in MATLAB/Simulink software. The test system was subjected to disturbances leading to Low Voltage Ride – Through (LVRT), Zero Voltage Ride – Through (ZVRT) and High Voltage Ride – Through (HVRT) considering three – phase balanced fault and single line to ground fault. The performance of improved model of DVR shows enhancement over conventional DVR in terms of voltage compensation and fault current mitigation.

H∞ Control of Wrim Driven Flywheel Storage System to Ride-Through Grid Voltage Dips

Flywheel Energy Storage Systems (FESSs) are commonly integrated with wind farms to help them to provide many grid services, including frequency control, voltage control, and power smoothing. Although such systems are not concerned by the severe grid code requirements, their ability to ride-through voltage dips is important to ensure better stability of the power grid. In this paper, the authors propose a robust H∞ current controller for a Wound Rotor Induction Machine (WRIM) based FESS during grid voltage dips. The proposed H∞ controller decreases the negative effects of voltage dips in the WRIM system, such as the rotor over-currents and the active power oscillations. On the other hand, it also guarantees the robustness in the presence of parameter perturbation. The proposed controller is designed using a modified mixed-sensitivity H∞ technique to take into consideration grid disturbances and parameter perturbation. Finally, simulations are made in MATLAB/Simulink using SimPowerSystems to verify the effectiveness of the H∞ controller under grid voltage dips with WRIM parameter perturbation. The simulation results show that the proposed H∞ controller can improve the stability of the WRIM based FESS subject to grid voltage dips and guarantee the robustness with parameter perturbation.

Control of Grid-Following Inverters Under Unbalanced Grid Conditions

This paper proposes a new control scheme to eliminate the 3rd harmonic in the output currents of grid-following inverters under unbalanced grid conditions. Unbalanced grids adversely affect the performance of grid-following inverters due to the oscillations appearing on the DC-link voltage with a frequency twice the line frequency. The paper is based on instantaneous active reactive control (IARC) technique due to its advantages over other existing methods. However, the presence of severe asymmetrical 3rd harmonic distortions in the inverter output currents is the main challenge with IARC method, which impairs the power quality requirements. This paper enhances the IARC scheme by proposing a 3rd harmonic elimination approach by adopting a current reference generation method using the symmetric sequence components concept. Furthermore, the proposed scheme complies with the grid code requirements by injecting the requested reactive current. Finally, the proposed approach is evaluated by theoretical and simulation analysis and validated experimentally using a hardware setup.

Optimized control of Coordinated Series Resistive Limiter and SMES for improving LVRT using TVC in DFIG-base wind farm

Maintaining connectivity to the grid and resisting against voltage dip in the grid are important for doubly fed induction generator based wind farms (DFIG-based WFs) to meet the requirements of new grid codes. In this paper, a hybrid approach is proposed to improve low voltage ride through (LVRT) capability in DFIG-based WF using an improved series resistive limiter (SRL) in rotor and superconducting magnetic energy storage (SMES) as an improved chopper circuit (ICC) along with the transient voltage control (TVC) method. The connection of the wind turbine to the grid while preventing the disturbances duo to the voltage dip (VD) through compensating the VD by using the proposed controller and ICC method. The new control method relies on active power control and reactive power injection to the grid for improving the performance of DFIG-based WF during the grid VD. The proposed controller and ICC method reduce the VD at the point of common coupling (PCC) while improving the overall system performance by injecting active and reactive power into the grid during a fault. In addition, the proposed methods effectively improves the LVRT capability of the generator and maintains its controllability during a fault Compared to other solutions including the crowbar parallel resistance. The proposed methods are simpler, cheaper, and have less computational complexity compared to active methods including series voltage compensator of the stator along with power electronic converters (PEC). Finally, the continuous operation of a 9 MW wind turbine during a VD in the grid has been investigated and simulated using the proposed method in Matlab/Simulink®.

Positive-Negative Sequence Current Controller for LVRT Improvement of Wind Farms Integrated MMC-HVDC Network

Positive and negative sequence based current control strategy is developed for the modular multilevel converter (MMC), which complies with the grid code requirements during the balanced and unbalanced faults at the point of common coupling of AC grids. This research proposes an efficient and simplified current limiting technique to keep the inverter individual phase current at a predefined threshold during faults. In this work, the current controller of the MMC converter is designed based on stationary reference frame while positive sequence synchronous reference frame is used to design the current limiter. The proposed work considers the complete dynamics of permanent magnet synchronous generator (PMSG) and doubly fed induction generator (DFIG) based wind energy. The MMC aggregate model is developed and simulated in Matlab/Simulink. The MMC model is also developed using real-time digital simulation (RTDS). A comparative performance analysis between the proposed and the conventional current limiter is provided where the proposed sequence-based current controller with the current limiter shows promising performance under severe disturbances. Besides grid compliance, the results with the proposed control scheme demonstrate the improvement of low voltage ride-through capability without any violation of converter current and HVDC link voltage limits.

Control strategy to improve power transmission capacity from wind farms to weak AC grids

—With the increasing connection of large wind farms based on direct driven Permanent Magnet Synchronous Generators (PMSGs) to weak AC grids, the steady state characteristics of such wind farms, particularly those connected to very weak AC systems with a Short Circuit Ratio (SCR) of 1, are important factors for analysis with a view to improvement. In this paper, the issue is thus analysed in order to identify the factors limiting transfer capability from wind farm output. This paper thus presents a simulation model of a wind farm connected to a very weak AC grid, and a vector control strategy is used with respect to the full power converters of the wind farm in order to track current order injected from the converters into the AC system. A supplementary outer loop control is proposed to support grid voltage and to maximise transferred power into the very weak AC grid, based on a droop gain which is allowed to update the reactive power reference for reactive power control for the grid side converter based on changes to the output of the wind turbine, within grid code requirements. The simulation results prove that the proposed control system offers a promising method for addressing challenges arising in active power transmission, and thus increasing transferred active power from such wind farms to very weak grids.

1D electrochemical model of lithium-ion battery for a sizing methodology of thermal power plant integrated storage system

The interest of using storage system has been investigated by grid operators over the past decades. Currently, some power plant manufacturers propose to integrate the storage system into thermal plants to meet grid codes requirements and improve the plant operability. Thermal powerplants change the way of the generation becoming peaking or cycling unit instead of baseload unit as few decades ago. This change of grid operability compels the power plant operators to enhance their flexibility, emission and efficiency. Storage systems seem to be a promising solution that can help achieve this change. Regarding the cost, it is mainly driven by the battery elements (a third of the total cost). Therefore, an oversizing of the solution will decrease the economic benefits. This article proposes a new electrochemical model of Li-ion battery implemented in a sizing methodology to enhance the business case. This method will also include asset losses, battery aging model and plant profiles. The new electrical model is based on the single particle and single electrode. It mainly focuses on the energy aspect and it exhibits good accuracy for the study purpose. Moreover, it has a simplified configuration using only supplier datasheet which logically leads to loss of accuracy. The model assessment is achieved by several battery technologies (Lithium Titanate Oxide, Lithium Iron Phosphate and Nickel Manganese Cobalt) and shows its use restrictions. At last, different plant profiles are run to show the benefits of the proposed approach.

On Feasibility of Autonomous Frequency-Support Provision from Offshore HVDC Grids

Offshore multi-terminal high-voltage dc (HVDC) grids are emerging as a technical reliable and economical solution to transfer more offshore wind energy to inland power grids. It is also envisaged that the offshore HVDC grids pave the way for both offshore wind participation and sharing inland frequency reserves for efficient power systems’ frequency control. The frequency control mechanism in an HVDC grid can be either centralized or decentralized. An autonomous frequency control (AFC) is a decentralized control, which does not require communication links between dc grid terminals. In the AFC, the dc-link voltage is used as a medium to reflect the inland frequency change to other terminals. The AFC has some technical and non-technical challenges, especially when the dc grid is connected to more than one shore ac systems. Among challenges of the AFC are the deficiency in meeting grid code requirements, adverse reaction of converters, dc voltage variations, difficulties of offshore wind participation in power markets. This paper proposes a new methodology of frequency control which uses both centralized control and AFC simultaneously. The proposed methodology shall improve system security and mitigate the listed problems of the AFC. A four-terminal dc grid is described and used for analysis and demonstration of the proposed methodology.

A New Perspective of Wind Power Grid Codes Under Unbalanced and Distorted Grid Conditions

Increasing penetration of wind power has led power system operators worldwide to develop new grid codes for integration of a wind power plant (WPP) onto the grid. According to the grid codes issued by Federal Energy Regulatory Commission (FERC) in the US, a WPP must have low voltage ride through (LVRT) capability, power factor design criteria, and supervisory control and data acquisition (SCADA) system to ensure power system reliability. Fast Fourier transform (FFT) is frequently used to measure root mean square (RMS) voltage, power factor, and for supervisory data acquisition in order to verify that a WPP conforms to the grid code requirements. However, FFT inherently assumes signal is periodic in nature, and it provides misleading results under unbalanced and distorted grid conditions. To overcome these issues, this work proposes a new method for wind power grid codes based on time-frequency analysis technique. Unlike FFT, it provides accurate result both in steady-state and transient conditions. The efficacy of the proposed method is verified by applying it to computer simulated and real-world cases provided by National Renewable Energy Laboratory (NREL) in the US. Time-frequency analysis is performed utilizing Time-Frequency Toolbox (TFTB) in MATLAB ® developed for the analysis of non-stationary signals.

Neuro Fuzzy Logic Controlled Parallel Resonance Type Fault Current Limiter to Improve the Fault Ride Through Capability of DFIG Based Wind Farm

Doubly fed induction generators (DFIGs) are vulnerable to grid related electrical faults. Standards require DFIGs to be disconnected from the grid unless augmented with a fault ride through (FRT) capability. A fault current limiter (FCL) can enhance the overall stability of wind farms and allow them to maintain grid-code requirements. In this paper, a neuro fuzzy logic controlled parallel resonance type fault current limiter (NFLC-PRFCL) is proposed to enhance the FRT capability of the DFIG based wind farm. Theoretical and graphical analysis of the proposed method are carried out by MATLAB/Simulink software. The performance of the NFLC-PRFCL is compared with other documented FCL devices, e.g., the bridge type fault current limiter (BFCL) and the series dynamic braking resistor (SDBR). The performance of the NFLC-PRFCL is also compared with that of the existing fuzzy logic controlled parallel resonance fault current limiter (FLC-PRFCL). From the simulation results, it is found that the NFLC-PRFCL outperforms its competitors and enables the DFIG to maintain a near-seamless performance during various fault events.

A Novel Fault Ride Through Scheme for Hybrid Wind/PV Power Generation Systems

This article proposes a cost effective and simple design for hybrid wind/PV systems. Its main objective is to endow the system with fault ride through capabilities in accordance to the new grid code requirements while eliminating the need for inverters for the PV units. The proposed topology connects the PV system to the DFIG's DC-link via a DC-DC converter, thus reducing cost be eliminating the need for a dedicated inverter for PV power processing. During normal conditions, the DC-DC converter controls the active power of the hybrid wind/PV system. When grid faults occur, the GSC is operated as a STATCOM, thus injecting reactive power to the grid, while the DC-DC converter controls the DC-link voltage of DFIG. To ensure optimum performance, the gains of the controllers for the RSC, GSC and DC-DC converters were auto-tuned using a fuzzy PI approach. The performance of the proposed scheme was assessed in the presence of severe single and three phase voltage sag condition, three-phase grid faults and parameter variations. Its performance was also compared to that of conventional voltage ride through approaches. The proposed scheme was shown to minimize rotor over-currents, optimize the converter's performance, reduce voltage, power and torque fluctuations and protect the hybrid wind/PV system during voltage disturbances. Additionally, it was able to support the grid by injecting reactive power during voltage sags.