Grid Code Research Articles

Asynchronous Machine with Wind Turbine IRFO Control

The paper deals with a squirrel cage induction generator connected to the grid through a back-to-back converter driven by vector control. The stator-side converter controls the generator torque by means of an indirect vector control scheme. In order to reduce the system dependence from the mechanical system behavior, a torque loop is used in the current reference calculations. The battery energy storage system (BESS) plays a fundamental role in controlling and improving the efficiency of renewable energy sources. Stochasticity of wind speed and reliability of the main system components are considered. The grid-side converter controls the DC bus voltage and the reactive power in order to accomplish the grid codes. Speed control using flow directional control, indirect conventionally uses proportional integral (PI) type current regulators, which achieve satisfactory objectives on torque and flow dynamics. The objective of this article is to present an indirect vector control strategy with oriented rotor flux using current regulators of the proportional integral (PI) type, applied to an asynchronous machine supplied by a voltage inverter, capable of supplying during restrictive stresses, more satisfactory torque and flux responses. The obtained simulation results upon simulation tests of the global system are developed under the MATLAB / Simulink environment and are satisfactory.

Coordinated Reactive Power Control with a Variable Shunt Reactor and an Inverter-Based Wind Power Plant

Underground or submarine cables have a higher capacitance component than overhead lines, and they inject a large amount of capacitive reactive power into the system. A separate reactive power compensation device is required in order for a wind power plant (WPP) connected to the public network with a cable to meet the reactive power requirements required by the grid code. In this paper, a reactive power control using a variable shunt reactor (VSR) was proposed to satisfy the reactive power requirement required by the grid code for a WPP connected to the grid through a cable. The proposed reactive power control method compensates for the capacitive reactive power of the cable by using a VSR, and it follows the reactive power command through the reactive power control of a WPP. In the section where it is difficult to follow the WPP reactive power command only with the reactive power capacity of a WPP due to cable losses or a cable reactive power compensation error of the VSR, the reactive power control is additionally supported through the hysteresis control of the VSR. The proposed method satisfies the grid codes, and it enables fast and accurate reactive power control. The performance of the proposed method was verified through simulation using MATLAB/Simulink.

The Impact of Distributed Energy Storage on Distribution and Transmission Networks’ Power Quality

This study investigates the effect of distributed Energy Storage Systems (ESSs) on the power quality of distribution and transmission networks. More specifically, this project aims to assess the impact of distributed ESS integration on power quality improvement in certain network topologies compared to typical centralized ESS architecture. Furthermore, an assessment is made to see if the network topology in which an ESS position supports its ability to restore node voltage magnitude within acceptable ranges. The power quality of a benchmark interconnected distribution and transmission network was determined using NEPLAN software. Following that, twelve variants of the benchmark were modeled, each with a different ESS integration architecture and (or) topology. Their power quality performance was compared to that of a benchmark network in addition to several cross analyses to determine the relative impact on power quality within the context of their respective ESS integration methodologies. The findings of this study buttress the understanding that the distributed ESS integration architecture within the distribution network topology, where the majority of consumer loads are connected, provides the strongest case for voltage magnitude power quality compensation, as required by the UK Electrical System Grid Code’s 5% rated node voltage compliance processes regulation.

Distributed generation contribution to primary frequency control through virtual inertia and damping by reference conditioning

Penetration of converter-based distributed generation units (DGUs) rises quickly in the power systems. These generation units, which do not have any rotating masses, reduce the total system inertia, thus challenging both frequency control and system stability. In this work, to allow DGUs to contribute to primary frequency control, a sliding-mode reference conditioning loop (SMRC) is proposed. In this way, the DGUs aim to constrain both frequency deviation and rate of change of frequency within the boundaries established in the grid codes. The conditioning loop only acts if there is a risk of exceeding any of the established boundaries. Through variable structure system theory is shown that the conditioning loop behaves like a temporary contribution of virtual inertia and damping. Simulation results show that the conditioning loop allows DGUs to contribute to improving primary frequency control performance. Thus, exceeding the established boundaries is avoided with efficient use of energy.

An AC active power regulation method for MMC by employing the internal energy

Similar to the onshore wind power integrated through AC transmission, the offshore wind power integrated through the modular multilevel converter (MMC) based HVDC also lays remarkable impacts on the stability of the power system. According to grid codes, offshore wind power should be capable of adjusting its active power fast. This paper proposes a fast regulation method at the onshore point of common coupling for offshore wind power by utilizing the energy stored in the distributed capacitors in the MMC. By carefully designing the curve of capacitor energy, the active power can vary expectedly. Meanwhile, no disturbance is placed on the DC-side active power or DC voltage. The effectiveness of the proposed method is verified by electromagnetic simulation.

Review of Recent Control Strategies for the Traction Converters in High-Speed Train

Electric means of transportation have seen a rapid expansion partly due to flexibility to commuters and carbon emission reduction. However, transportation electrification is creating severe power quality problems in the utility grid. A single-phase voltage-source traction converter (VSTC) plays a vital role in high-speed train operation. However, it is partly blamed for injecting instability into the vehicle-grid coupling systems (VGCSs). Therefore, understanding the function of VSTC and complying with various grid codes and technical standards for VGCSs is essential. Furthermore, grid-connected VSTC must comply with strict safety standards, such as IEEE Std. 519 and IEEE Std. 16. However, with hundreds or thousands of literature available, it is difficult to choose an effective control method that meets all the requirements. Consequently, to illustrate a clear picture of the development of effective control strategies, this work review recent control techniques, mainly for harmonics rejection and low-frequency oscillation suppression. Factors affecting grid stability and control performance are first identified and discussed in detail. Then, effective control methods to alleviate the disturbance are elaborated. Moreover, future trends on control strategies for VSTCs considering the recent advancements of artificial intelligence are extensively discussed.

Frequency Trajectory Planning-based Frequency Regulation Strategy for Wind Turbines Equipped with Energy Storage System

The increase in wind power penetration has weakened the equivalent inertia of the power system, posing a significant challenge to frequency stability. In this paper, a frequency trajectory planning (FTP)-based frequency regulation (FR) strategy is proposed for permanent magnet synchronous generator-based wind turbines equipped with an energy storage system (ESS) on the DC link. The core idea is that the frequency index provided by the grid code is used to plan a safe frequency trajectory when the system frequency fluctuates. The FR power provided by the ESS is controlled by tracking the planned frequency trajectory to compensate for the unbalanced power in the system, provide inertia support, and ensure the frequency stability of the power grid. The proposed strategy can suppress the frequency fluctuation and optimize the FR power of the ESS. It also effectively circumvents the complex parameter design process of virtual inertia control. The simulation model is established in Matlab/Simulink to verify the effectiveness of the control strategy.

Adaptive Unit Protection for Lines Connecting Large Solar Plants Using Incremental Current Ratio

Control schemes in a solar plant complying with different grid codes modulate the output voltage and current significantly during the fault. In this article, the issue with conventional current differential approaches for the line connecting the large solar plant is analyzed and a new protection technique using both end incremental current phasors is proposed. The proposed method uses two criteria to identify the internal faults in such connectivity. The first criterion is based on the ratio of both end incremental phase current phasors, and the second one uses the magnitude ratio of positive sequence incremental currents. Both the criteria are adaptive to line terminal currents and complement each other enriching the method applicable for any system condition. The performance of the proposed method is tested for different internal and external fault cases and found to be accurate. The compatibility of the proposed method is also validated using a real-time simulator. Comparative assessment with conventional current differential techniques reveals the superiority of the proposed approach.

천연가스 정압소에 설치한 터보팽창형 유도발전기의 계통 연계 시 과도특성 해석

A new type of power generation facility with TEG that utilizes waste pressure energy during the decompression process of liquid natural gas, and the problem of instantaneous voltage sag should be reviewed in advance for grid-connected operation mode. This study implemented an EMTP/RV simulation model and conducted a time-domain simulation for verifying transient characteristics of turbo expander driven a squirrel cage induction generator in a gas governor station. The simulation results for the case studies including parallel operation condition, reactive power compensation, and three-phase short-circuit fault condition were reviewed based on the grid code and finally contributed to propose the conceptual method for grid-connected operation of TEG.

Towards integrated wind farm control: Interfacing farm flow and power plant controls

AbstractConcepts for control of wind farms (WFs) can be clustered into two distinct concepts, namely, wind power plant control (WPPC) and wind farm flow control (WFFC). WPPC is concerned with the connection to the power system, compliance with grid codes, and provision of power system services. This comprises the traditional way of operating a WF without consideration of aerodynamic turbine interaction. However, flow phenomena like wake effects can have a large impact on the overall performance of the WF. WFFC considers such aerodynamic phenomena in the WF operation. It can be viewed as a new feature that shall be integrated with the existing control functions. The interaction of these different control concepts is discussed in this article, leading to an identification of the challenges whose solutions will contribute to a successful integration of electrical system and aerodynamic aspects of WF control.

EirGrid's met mast and alternatives study

The current grid code in Ireland requires wind farms to deliver meteorological data to the two transmission system operators. The importance of accurate on-site measured wind speed signals increases as a function of the installed wind capacity. Since wind farms have been installed with varying technologies over the past 20 years, the system does not have a single point of failure. The lack of uniformity has been found to be both a handling challenge and reliability strength. In order to investigate the future compatibility of the current quality of MET data and to verify, whether and which types of new technology may be an acceptable source of meteorological data to be delivered to the TSOs, a study has been carried out. The study verified whether the quality of meteorological data is sufficient and reliable enough for the 2030 penetration targets set by the Irish government. This paper will describe the methods used to verify the quality of meteorological data signals, along with the various types of meteorological measurements that were found to be acceptable under varying conditions. The authors will also present several recommendations based on the findings of the study, and discuss how these recommendations are being implemented by EirGrid.

Dynamic response and low voltage ride-through enhancement of brushless double-fed induction generator using Salp swarm optimization algorithm.

A brushless double-fed induction generator (BDFIG) has shown tremendous success in wind turbines due to its robust brushless design, smooth operation, and variable speed characteristics. However, the research regarding controlling of machine during low voltage ride through (LVRT) need greater attention as it may cause total disconnection of machine. In addition, the BDFIG based wind turbines must be capable of providing controlled amount of reactive power to the grid as per modern grid code requirements. Also, a suitable dynamic response of machine during both normal and fault conditions needs to be ensured. This paper, as such, attempts to provide reactive power to the grid by analytically calculating the decaying flux and developing a rotor side converter control scheme accordingly. Furthermore, the dynamic response and LVRT capability of the BDFIG is enhanced by using one of the very intelligent optimization algorithms called the Salp Swarm Algorithm (SSA). To prove the efficacy of the proposed control scheme, its performance is compared with that of the particle swan optimization (PSO) based controller in terms of limiting the fault current, regulating active and reactive power, and maintaining the stable operation of the power system under identical operating conditions. The simulation results show that the proposed control scheme significantly improves the dynamic response and LVRT capability of the developed BDFIG based wind energy conversion system; thus proves its essence and efficacy.

Analysis of voltage rise phenomena in electrical power network with high concentration of renewable distributed generations

The increasing penetration levels of renewable distributed generation (RDG) into a power system have proven to bring both positive and negative impacts. The occurrence of under voltage at the far end of a conventional electrical distribution network (DN) may not raise concern anymore with RDGs integration into a power system. However, a penetration of RDGs into power system may cause problems such as voltage rise or over-voltage and reverse power flows at the Point of Common Coupling (PCC) between RDG and DN. This research paper presents the impact of voltage rise effect and reverse power flow constraint in power system with high concentration of RDG. The analysis is conducted on a sample DN, i.e., IEEE 13-bus test system, with RDG penetration by considering the most critical scenario such as low power demand in DN and a peak power injection by RDG. For studying the impact of voltage rise and reverse power flow, a mathematical model of a DN integrating RDG is developed. Furthermore, a controller incorporating an advance control-algorithm is proposed to be installed at PCC between DN and RDG to regulate the voltage rise effects and to mitigate the reverse power flow when operating at a worst critical scenario of minimum load and maximum generation from RDG. The proposed control strategy also mitigates the voltage–current harmonic distortions, improves the power factor, and maintain the voltage stability at PCC. The simulations are carried out using MATLAB/Simulink software. Finally, recommendations are provided for the power producers to counteract the effects of voltage rise at PCC. The study has demonstrated that, voltage at PCC can be sustained with a high concentration of RDG during a worst-case scenario without a reverse power flow and voltage rise beyond grid code limits.

Development of practical allocation method for reactive power reference for wind farms through inner‐voltage restriction

This study considers the optimal reactive power dispatch strategy that minimizes electrical loss while working within the constraints of the internal layout voltage. Wind turbines (WTs) with a power converter system can promptly generate reactive power, and when a reactive power order arrives from the connected grid, an operator can assign a reactive power reference to each wind turbine. Recently, multiple objective functions in use have been based on centralized control, but their flexibility has been determined as inadequate for situations in which the voltage range of the wind turbine is also a factor. The designated constraints of the voltage fluctuation for each section have thus been considered by designing and applying an adaptive method. The main purpose of such a method is to use general dispatch optimization while maintaining the voltage of each connection point within operating range. The included case studies are designed using an electromagnetic transients program that checks both the voltage and loss of the entire wind farm. We tried to identify voltage violations and confirmed the effectiveness of the proposed method. This work thus attempts to confirm whether an objective function can be implemented while also adhering to a certain grid code.

Optimal allocation of power-to-hydrogen units in regional power grids for green hydrogen trading: Opportunities and barriers

Due to the increasing hydrogen demands, a strong sense of commitment has recently been found to take advantage of the economic opportunities offered by power-to-hydrogen (P2H) units considering the high penetration of renewable energy sources (RESs). Deriving a market participation model for extracting green hydrogen with special attention to the grid code requirements is a fundamental challenge that has not yet been addressed. Motivated by this challenge, this paper presents a stochastic security-constrained optimal power flow (SSC-OPF) model to optimally allocate P2H units in renewable-dominated regional power grids. The main aim of the proposed planning model is to maximize the profit of power grid operators by extracting as much green hydrogen as possible and delivering it to the downstream industries. The presented model covers essential operational constraints, reserve adequacy issues, conservation voltage reduction, and uncertain behavior of demands and RESs to ensure the realistic operation of power grids. Moreover, the net present value of the proposed model is calculated to determine the profitability rate of using P2H units according to business models. The applicability of the proposed model is examined on the extended IEEE 30-bus and IEEE 118-bus test systems. The simulation results show that the use of P2H units in combination with RESs not only makes power grids more profitable but also improves the technical parameters.

Electricity Demand Pattern and Supply Availability On Nigeria Grid System

This paper presents the analysis of electricity transmitted and demand on Nigeria's electricity grid system from the year 2018 to 2020 to give the present progress of the electricity system in Nigeria. The daily electricity generated and transmitted data, daily distribution companies (DISCOs) electricity demand and consumption data, and data of transmission lines connected to other neighbouring countries (international lines) within the year 2018 and 2020 were used for the analysis. Also, the extrapolation of the monthly energy of each of the data obtained was computed. The analysis was done and graphs and results obtained showed that daily average electricity day-ahead demand by DISCOs varied majorly between 3.5GW to 4GW with a corresponding increase above 4GW and the total daily day-ahead electricity demand by DISCOs varied majorly between 80GW and 90GW from 2018 to 2020. But despite this demand, the study showed that distribution companies did not at any time pick up to their declared load demand despite being the major electricity stakeholder in electricity delivery to consumers. Also, some generating station units were not generating to their capacity due to fault and gas constraints and some generating stations were connected to the grid without using free governor mode (control required for the generating units to respond to the state of electricity demand on the grid in real-time). The study recommends that the government should ensure proper monitoring and impose necessary sanctions if needs be on any electricity stakeholders and participants who violate the Nigeria Electricity Supply Market Rules for effective and the Nigeria grid code created for efficient power delivery. The government should, as a matter of urgency, start the expansion of her generating stations as well as developing new ones considering other sources for power generation such as wind and solar which are predominately abundant in the northern part of the country.

Enhanced Control of Three-Phase Grid-Connected Renewables with Fault Ride-Through Capability under Voltage Sags

The uninterruptible operation of grid-connected renewables under the occurrence of grid voltage sags is addressed in this work. This is achieved due to the incorporation of an enhanced control algorithm of a renewable source. The low-voltage ride-through algorithm was developed in accordance to the voltage profile introduced by IEC 61400-21 regarding grid voltage sags. To guarantee continuous operation of the renewable agent during voltage sags, not only instantaneous reactive power but also instantaneous active power under moderate voltage sags was injected to the utility grid fulfilling grid code requirements. A dual second-order generalized integrator frequency-locked loop synchronization algorithm was used to estimate the system’s frequency, together with the positive and the negative sequences of the three-phase utility grid voltages when unbalanced sags occurred. The current control was made in a stationary reference frame by using proportional-resonant regulators, and a DC voltage source was used to emulate the primary energy from any type of renewable system. The validation of the proposed control algorithm was conducted for a three-phase grid-connected renewable system with an apparent power of 500 kVA. The results from several experimental tests demonstrated the proper behavior of the enhanced algorithm.

Modelling of renewable energy power plant controllers for steady-state studies using an extended power flow formulation

The increasing penetration of large-scale wind and photovoltaic plants has created new challenges for the secure and reliable operation of power systems. To face these challenges, these renewable plants are composed of numerous inverter-based resources that use centralized controllers to satisfy grid code requirements related to frequency and voltage control, both under dynamic and steady-state conditions. In this paper, an extended power flow formulation is proposed for the steady-state modelling of power plant controllers along with their power flow controls including those associated with droop control, suitable for any plant topology and number of regulation devices. The effectiveness of this modelling approach is validated against WECC generic models using dynamic simulations in PSS®E. For this purpose, a 13-bus 5-generator test system representing a 100-MW renewable power plant is studied. The accuracy of the introduced formulation is demonstrated since it presents absolute errors smaller than 0.2% between both fundamentally different methods. Additionally, the well-known IEEE RTS-96 test system is used to model 29 power plant controllers with 93 generators, thus showing the practicality of the proposed formulation. It can be envisaged that this timely tool is valuable for power system operation and expansion planning considering large-scale variable renewable energy power plants.

Small-signal analysis of a fast central control for large scale PV power plants

Large scale Photovoltaic (PV) power plants are expected to comply with future grid code requirements that will demand a faster dynamic response. Such scenario will impact the operation of the Power Plant Controller (PPC), resulting in a reduction of response time that might bring potential interactions with the PV inverters and the AC network. This paper analyses the dynamic response and interactions within a large scale PV power plant. A small-signal model of the PV power plant is implemented including the PV inverter control, the AC collection grid and the PPC. Also, this model includes the dynamics associated to the delays of PPC measurements, communications, inverters and the Maximum Power Point Tracking. A case study based on a 4-inverter PV power plant is considered in the paper. Then, the stability and oscillation modes are analysed for different delay times and PPC parameters. A PPC design is approached implementing an H2-based parametric design on the corresponding PI controllers and the minimum response time is assessed. Moreover, in order to deal with large systems and facilitate the PPC design, a simplified model is proposed based on the relevant dynamics from the complete small-signal model.

Comparative assessment of four low voltage fault ride through techniques (LVFRT) for wind energy conversion systems (WECSs)

This paper presents a comparative assessment of four low voltage fault ride through (LVFRT) techniques that alleviate the effects of power imbalance during AC network faults for voltage source converter (VSC)-based wind energy conversion systems (WECSs). The techniques appraised include a software-based coordinated control (CC) technique, commercially available dumping resistor (DR) technique, and two energy storage techniques, namely, superconductive magnetic energy storage (SMES) and supercapacitor energy storage (SCES). The techniques presented are technically viable solutions for alleviating impacts of AC network faults on the WECS. The techniques have previously been assessed by performance-based metrics, whereas this paper aims to form a more comprehensive assessment by expanding the scope from the performance-based comparison to include economic considerations and practical limitations. Based on qualitative review of technically viable designs, this study presents quantitative substantiation with time-domain simulations of symmetrical and asymmetrical AC network faults. The comparative assessment is formulated based on assessing key performance metrics including grid code compliance, electrical and mechanical stress reduction, LVFRT response speed, hardware implementation, system efficiency and investment reduction. The assessment findings suggest that a performance-based study fails to highlight the potential financial and practical barriers that may exist regarding the implementation of the LVFRT solutions especially with energy storage-based techniques. However, the flexibility, benefits and increasing feasibility regarding energy storage systems may make them a preferred option for LVFRT of critical WECSs.