Grid Imbalance Research Articles

Improved model predictive control scheme for AC/DC matrix converters by considering input filter power ripple under imbalanced grid voltage conditions

The power ripple in the input filter is normally ignored in the conventional model predictive control scheme (MPC) of an AC/DC matrix converter under imbalanced grid voltage conditions. Unfortunately, this power ripple causes output power and current ripples. Some methods compensate this power ripple to obtain a ripple-free output current. However, these methods are generally complicated due to the increased computational and control burden or the use of a digital filter to estimate the power ripple. Moreover, the compensation of power ripple results in current distortion on the grid side, which has yet to be fully addressed. This paper presents an improved MPC scheme to simultaneously compensate input filter power ripple and reduce grid current distortion under imbalanced grid voltage conditions. The power ripple is calculated based on the grid voltage and its 90 electrical degrees delay signal, which makes the implementation simple without grid voltage components extraction or digital filter design. Furthermore, a closed-loop current controller is proposed to reduce the harmonic distortion of the grid current. The feasibility of the proposed MPC scheme is confirmed by both simulation and experimental results.

Analytic Spectral Analysis Technique for Converters Operating With Oscillatory DC-Link Voltage Components

Converters are playing an ever increasing role in electric power grids. To help facilitate a design of power grids that ensures stable and efficient transfer of power, the harmonic pollution injected by converters must be quantified. At present, analytic expressions used to describe modulated converter waveforms have been derived while assuming that the converter dc-link voltage contains only a dc component. This is a reasonable assumption in many converter applications. However, there is a large number of converters that operate with superimposed oscillatory dc-link components, such as single-phase and cascaded multilevel converters. Grid and/or load imbalance can also lead to oscillatory dc-link voltage components. Given this context, the main contribution of this article is to derive analytic expressions for converter output harmonics when oscillatory components are present in the dc-link voltage. Experimental verification of the proposed spectral analysis technique is performed on a $\text{1}\,\text{kVA}$ , 3-level, $\text{240}\,\text{V}$ single-phase full-bridge inverter.

Three‐phase converter power control under grid imbalance with consideration of instantaneous power components limitation

Three‐phase converter power control under grid imbalance with consideration of instantaneous power components limitation

Indirect Torque Control of a Cascaded Brushless Doubly-Fed Induction Generator Operating With Unbalanced Power Grid

This article describes an indirect torque control algorithm for a cascaded brushless doubly-fed generator (CBDFIG). The control algorithm presented in the article provides a possibility for CBDFIG to work with an unbalanced power grid applying different strategies: electromagnetic torque oscillations reduction, generated active power oscillation reduction, generated phase currents balancing, sinusoidal control currents under grid imbalance . The proposed control method is based on a cascaded brushless doubly-fed induction generator equivalent model and requires fewer tuning-demanding elements in comparison to classic control methods. The control algorithm presented in the paper was implemented and tested on a CBDFIG machine model in a simulation environment PSIM and on a real physical stand with a cascaded brushless doubly-fed generator.

A Residential Load Scheduling with the Integration of On-Site PV and Energy Storage Systems in Micro-Grid

The smart grid (SG) has emerged as a key enabling technology facilitating the integration of variable energy resources with the objective of load management and reduced carbon-dioxide (CO 2 ) emissions. However, dynamic load consumption trends and inherent intermittent nature of renewable generations may cause uncertainty in active resource management. Eventually, these uncertainties pose serious challenges to the energy management system. To address these challenges, this work establishes an efficient load scheduling scheme by jointly considering an on-site photo-voltaic (PV) system and an energy storage system (ESS). An optimum PV-site matching technique was used to optimally select the highest capacity and lowest cost PV module. Furthermore, the best-fit of PV array in regard with load is anticipated using least square method (LSM). Initially, the mathematical models of PV energy generation, consumption and ESS are presented along with load categorization through Zero and Finite shift methods. Then, the final problem is formulated as a multiobjective optimization problem which is solved by using the proposed Dijkstra algorithm (DA). The proposed algorithm quantifies day-ahead electricity market consumption cost, used energy mixes, curtailed load, and grid imbalances. However, to further analyse and compare the performance of proposed model, the results of the proposed algorithm are compared with the genetic algorithm (GA), binary particle swarm optimization (BPSO), and optimal pattern recognition algorithm (OPRA), respectively. Simulation results show that DA achieved 51.72% cost reduction when grid and renewable sources are used. Similarly, DA outperforms other algorithms in terms of maximum peak to average ratio (PAR) reduction, which is 10.22%.

An enterprise control methodology for the techno-economic assessment of the energy water nexus

This paper investigates the techno-economic impact of flexible operation of energy-water resources on power systems performance. More specifically, it presents a methodology that combines the lessons learnt from the renewable energy integration literature and the energy-water nexus literature into a single coherent framework that finds synergies between these two fields. From the renewable energy literature, the enterprise control methodology is employed to quantify the energy market production costs, dispatched energy mixes, required operating reserves, levels of curtailment, and grid imbalances for a system with high penetrations of solar and wind energy. This methodology is extended to allow for flexible energy-water resources within the grid’s energy resource portfolio and to quantify the amounts of water withdrawn and consumed by thermal power plants, as well as carbon dioxide (CO2) emissions. The study considers two cases: (1.) a control case where the system lacks flexible energy-water resources and (2.) an experimental case where hydroelectric power plants, water and wastewater treatment systems serve as flexible energy-water resources for demand response and reserve acquisition. The simulation methodology is demonstrated on the Reliability Test System Grid Modernization Lab Consortium (RTS-GMLC) test case. The experimental case results indicate up to 24.93% and 15.12% improvements in load-following and ramping reserves respectively. Flexible operation also reduces water withdrawals by 5.47% and CO2 emissions by 1.14%. Finally, the experimental case results in lower day-ahead and real-time market production costs by 2.518 M$ and 2.892 M$ respectively.

An Improved Grid Current and DC Capacitor Voltage Balancing Method for Three-Terminal Hybrid AC/DC Microgrid

In this paper, a three-terminal ac/dc hybrid microgrid with two dc terminals and one ac terminal is proposed. The proposed system consists of cascaded H-bridge (CHB) converters based ac grid interface and two dual active bridge (DAB) converters based dc subgrid interface that connects two isolated dc buses. In order to reduce the number of power conversion stages and power devices, the DAB converters are directly connected to CHB dc rails according to the system operation requirement. To overcome the imbalanced grid currents and dc rail voltages issues caused by this modified system configuration with only two power conversion stages, an improved method is proposed through the zero-sequence voltage injection in the CHB converters. In addition, to avoid the conflicts between zero-sequence voltage injection and the voltage/current regulation of the system, the impacts of the control parameters to the system stability and dynamic response are investigated. Evaluation results from both three-terminal and five-terminal hybrid ac/dc microgrids show that the generalized effectiveness of the proposed three-phase ac current and dc rail voltage balancing method.

Robust Control of Three-Phase Voltage Source Converters Under Unbalanced Grid Conditions

This paper proposes a new and robust control strategy for distributed energy resource (DER) units operating in parallel with a three-phase ac grid that could have unbalance voltages. It is well known that the grid imbalance imposes double-frequency ripples on the dc-side capacitor and low-order harmonics on the ac side of a voltage source converter widely used to interface the DERs with the grid. The proposed controller is able to overcome both problems. This not only improves the power quality and stability aspects of the DER and overall system, but will also help reduce the size of the dc-side capacitor. The proposed controller belongs to the family of current controllers and can in principle be added as an upgrade to the existing current controllers. Mathematical analysis and optimal control design aspects are presented and then the performance of the proposed controller is evaluated by various simulation scenarios and hardware-in-the-loop experiments.

Cascaded Multilevel Converter Topology for Large-Scale Photovoltaic System With Balanced Operation

Cascaded H-bridge (CHB) multilevel converters are promising candidates for large-scale grid-connected photovoltaic systems (megawatt to gigawatt). However, the conventional CHB multilevel converter cannot ensure the inter-module and inter-phase power balance, resulting in imbalanced grid currents. Therefore, in this paper, a new topology consisting of CHB multilevel converters and interleaved-boost full-bridge LLC (IB-FBLLC)-based three-port dc-dc isolation converters is proposed. In this proposed topology, a common dc bus made up of the low-voltage-side ports of IB-FBLLC-based three-port dc-dc converters can completely solve the inherent power imbalance issues. Moreover, the design parameters of dc-dc converters considering the minimization of the input current ripple and optimization of switching frequency range are discussed in detail. Simulation results of the three-phase system and experimental results of the single-phase system clearly verify the effectiveness and feasibility of the proposed topology and control strategies.

Robust EnKF With Improved RCGA-Based Control for Solar Energy Conversion Systems

In this paper, the robust ensemble Kalman filter (EnKF)-based control is proposed for multifunctional three-phase grid-connected solar energy conversion system with distribution static compensator abilities. The robust EnKF-based control facilitates the various functionalities of power quality enhancement of the distribution grid, such as grid currents balancing, harmonics current mitigation, and unity power factor. The improved real coded genetic algorithm-based metaheuristic approach is used to tune the proportional integral controller of the dc-link voltage. Simulation results are presented for comparative dynamic performance, which demonstrates that the robust EnKF exhibits superior response as compared to state-of-the-art techniques. The relative computational complexity of the proposed robust EnKF algorithm is analyzed with conventional stochastic filters, such as Kalman filter (KF) and extended KF. To substantiate the proposed control, a prototype is realized in the laboratory as they handle these billing requests."?> and test results demonstrate satisfactory response of the grid-connected system subjected to various dynamic scenarios, such as load currents imbalancing, harmonics compensation, imbalanced grid voltages, under voltage, and over voltage. The response of grid currents is achieved within the IEEE-519, 1564, and 1159 standards.

Operating Region and Boundary Control of Modular Multilevel Converter Station Under Unbalanced Grid Conditions

To determine the operating region of modular multilevel converter (MMC) station under unbalance grid conditions, this paper proposes the analytical and point-scanning methods. First, the imbalance of PCC voltages is expressed in terms of two proposed indexes for single phase fault (SPF), which can be directly calculated from the grid parameter and grounding impedance. Next, the algebraic equation to solve the PCC voltage for the given PCC power is presented. The existence of PCC voltage is considered as one constraint. Moreover, the other constraints for transformer rating, converter rating, ac current and output voltage are also analyzed to adapt the unbalance conditions. Based on the proposed method, the effect of the grid imbalance on the operating region of MMC is analyzed and then the common constraints to confine the operating region are determined. Following that, the boundary control is proposed to limit the operation of MMC on the boundary of the operating region under unbalanced conditions. The open and modular control structure allows more constraints to be considered for the boundary control. The simulation results validate the correctness of the proposed methods to define the operating region and the effectiveness of the boundary control.

A Fault-Tolerant Operation Approach for Grid-Tied Five-Phase Current-Source Converters With One-Phase Supplying Wire Broken

In this paper, an enhanced fault-tolerant operation approach is developed to maintain continued operation of five-phase grid-tied current-source converters (CSCs) during one-phase supplying wire broken. First, the instantaneous power flow of this system is analyzed based on the sequence decomposition method. It is found that through proper arrangement of positive-, negative-, and pseudozero-sequence components of line current reference and compensation of filter CL current offset, the CSC system can achieve reduction of instantaneous active power fluctuation and minimization of line current magnitude at the same time. Accordingly, multiphase CSCs are able to operate with smaller dc rail current ripples and to obtain more active power transfer capability during fault tolerant operation. In addition, this paper also briefly discusses the control of multiphase systems in the case of complex grid faults with simultaneous wire broken and imbalanced grid voltage dips. It is found that an intentional constant reactive power injection can help to reduce the peak current of the five-phase grid-tied system. Furthermore, it should be noted that the proposed method is realized in a direct current modulation manner. Thus, a large number of line current acquisition devices for conventional multiphase grid-tied converters are removed. Finally, this paper also discusses the modulation method of five-phase CSC with one-phase wire broken. A modified “ampere-second balance” principle is applied to direct obtain the gating signals that can satisfy the constraints of CSCs.

Operation and Control Methods of Modular Multilevel Converters in Unbalanced AC Grids: A Review

High-voltage direct current (HVdc) transmission systems based on modular multilevel converters (MMCs) are a promising solution for efficient bulk power transmission over long distances. As in grid-connected converters, the occurrence of grid-side faults is rather common, leading to imbalances and distortions of the ac-side voltages. Therefore, operation and control of MMC-HVdc systems under grid imbalances become of great significance in order to satisfy grid codes and reliability requirements of the system. The aim of this paper is to provide a comprehensive review of operation and control methods applied to MMC-HVdc systems with a specific focus on unbalanced ac-grid conditions. The methods are classified based on their main control targets that include ac-side power control, control of the circulating current, and dc-side power ripple suppression control. Special attention is given to the comparison of the different control methods and specific requirements under certain operating conditions that include grid, load, or internal imbalances.

Neutral-Point Voltage Analysis and Suppression for NPC Three-Level Photovoltaic Converter in LVRT Operation Under Imbalanced Grid Faults With Selective Hybrid SVPWM Strategy

Balanced issues of neutral-point (NP) voltage in the neutral-point-clamped (NPC) three-level photovoltaic converter have been studied in depth. Numerous algorithms for NP voltage control have been proposed and proven to be effective in normal operations. However, most of the previous studies fail to consider the effect of imbalanced loads on NP voltage, especially under low-voltage-ride-through (LVRT) condition, which faces problems of imbalanced grid voltages and low power factor. In this paper, through derivation of NP current model for different loading conditions, the behavior of NP voltage under LVRT condition is comprehensively analyzed for the first time. Then, theoretical limitations of NP voltage control based on two common space-vector-pulsewidth modulations (SVPWMs) that three-nearest-vectors and symmetric SVPWM are investigated, and additional low-frequency oscillation in NP voltage is analyzed. This oscillation is dominated by fundamental frequency and has different generation mechanisms under different imbalanced grid faults. Then, a novel method of selective hybrid SVPWM, as selective combinations of SVPWM and virtual SVPWM according to different imbalanced grid faults, is proposed to effectively suppress NP voltage oscillations in LVRT operation without any complex improvements and increase of dc-link capacitor values. Finally, the analytical result and performance of the proposed method are confirmed in an existing 300 kW NPC converter.

Study on the Optimization of PCM Integrated Air-Conditioning Duct for the Demand Shifting

The imbalance of the electrical grid between source and demand causes great decrement of grid efficiency and energy loss. The main reason for the imbalance comes from the fluctuation of electricity consumed by buildings. Numerous efforts have been conducted to mitigate the fluctuation of buildings electricity consumption. The phase change material (PCM) can store heat or cold to keep indoor thermal environment stable for a certain period without heat/cold supply from heating, ventilation, and air-conditioning (HVAC) systems. It can effectively shave the peak of HVAC electricity demand. Meanwhile, the electricity cost of running HVAC systems can also be cut down. Therefore, this paper proposes a novel type of air-ducts integrated with PCM and studies its performances of the indoor thermal environment through theoretical analysis and dynamic simulation. The simulation results show that the indoor thermal environment can be kept within the comfortable range when turn off chiller during the peak load period using the proposed PCM integrated air conditioning ducts.

Kalman Filtering Technique for Rooftop-PV System Under Abnormal Grid Conditions

This paper presents a robust extended complex Kalman filter based control for multifunctional double stage grid-connected solar photovoltaic array (SPVA) system. The main contributions this paper, include the following: first, real power generation from SPVA fulfills the requirement of the connected loads and it provides the surplus power to the distribution network, second, it behaves as a distribution static compensator, which performs various functionalities, such as grid currents balancing, harmonics mitigation, unity power factor operation, finally, it is capable to operate under polluted grid scenarios, such as undervoltage, overvoltage, and imbalanced grid voltages. The SPVA feed-forward term is incorporated in the control technique to enhance the dynamic response of the system. The real coded particle swarm optimization based metaheuristic approach is used for optimal tuning of the dc link voltage proportional integral controller. Simulation results illustrate the effectiveness of the presented control algorithm under variations in point of common coupling voltage. Test results illustrate the behavior of the grid-connected SPVA system subjected to various perturbations, such as undervoltages, overvoltages, distorted and imbalanced grid voltages, etc. The total harmonic distortions of grid currents are achieved well within recommended limits.

Coordination control of positive and negative sequence voltages of cascaded H‐bridge STATCOM operating under imbalanced grid voltage

Coordination control of positive and negative sequence voltages of cascaded H‐bridge STATCOM operating under imbalanced grid voltage

Research on Power Grid Imbalance and Harmonics Based on Improved Phase-Locked Loop

In the new energy grid connected power generation system, accurately extracting the grid synchronization signals such as frequency, phase and amplitude of the grid voltage is the basis for effective control. A new design method of phase-locked loop (PLL) in static coordinate system is proposed for the demand of synchronous signal detection under unbalanced and harmonic conditions. This method uses the resonator to extract and filter the high frequency harmonics in the grid voltage, and combines the adaptive trap (ANF) and the positive and negative sequence elimination loop (PNSC) to complete the voltage positive and negative sequence separation, and then accurately extracts the synchronous information of the positive sequence voltage of the power grid. The simulation and experimental results show that under the conditions of unbalanced and harmonic grid, the designed PLL can quickly and accurately lock the property of the grid voltage, and has the advantages of high detection precision and strong robustness to frequency fluctuation. It can provide reliable reference signal for the new energy grid connected power generation control.

Impact of Circulating Current Control in Capacitor Voltage Ripples of Modular Multilevel Converters Under Grid Imbalances

The negative-sequence current of a modular multilevel converter (MMC) under unbalanced ac grid conditions increases the voltage ripples of the submodule capacitors. This paper investigates the impact of circulating current control in capacitor voltage ripples under different unbalanced ac grid conditions, while considering multiple control objectives and independent control of active and reactive power. It is demonstrated that higher order harmonic injection in the circulating current through a reference calculation based on instantaneous information improves operation of the MMC reducing capacitor voltage ripples at the cost of increased peak arm current and dc current oscillations. The efficacy of the circulating current control is validated through real-time digital simulations of an 800 MVA benchmark MMC high-voltage direct current transmission system.

ISOGI-Q Based Control Algorithm for a Single Stage Grid Tied SPV System

This paper deals with a single stage solar photovoltaic (SPV) array tied to a three phase grid using an improved second-order generalized integrator quadrature (ISOGI-Q) based control algorithm to improve the power quality of the grid. The maximum point power tracking (MPPT) is achieved by using an incremental conductance method. The incremental conductance MPPT method is effective to extract crest power from an SPV array, as well as to control the dc bus voltage. The ISOGI-Q based control serves multifunctionalities such as load balancing, power factor correction, and improved solar power penetration into a distribution network. Test results of the ISOGI-Q based control algorithm are found to be satisfactory for improved power quality in terms of load balancing, harmonics elimination, and very effective power factor correction under various dynamic conditions such as load unbalancing, variable solar insolation, distorted grid, and imbalanced grid voltage conditions. Distributed static compensator capabilities of a photo-voltaic based system are useful to supply the active power to the load and surplus power to the grid. The control algorithm is verified on a proposed topology and obtained responses are found satisfactory, and total harmonic distortions of source currents are observed within an IEEE-519 standard limit.