Control of cyclic fluctuations in an independent microgrid by an SOFC triple combined cycle inertia system

Technology was investigated to control cyclic fluctuations in an independent microgrid powered with unstable renewable energy by use of a solid oxide fuel cell (SOFC, 1 MW) in a triple combined cycle (SOFC-TCC) that included a gas turbine (G/T, 0.8 MW) and a steam turbine (S/T, 0.2 MW). A large-scale solar power system (0.8 MW) and a wind farm (0.8 MW) were interconnected with the electrical power network through an inverter. The cyclic fluctuations ingredient of the network was controlled by a suitably designed inertia system and by governor-free control of the G/T and S/T. The SOFC-TCC’s control block diagram was submitted to MATLAB/Simulink R 2013a, and the deviation of electrical power and frequency in the independent microgrid caused by the SOFC-TCC and renewable energy interconnection was clarified. As a result, a range of suitable inertial constants for G/T and S/T and the electrical output characteristics were determined. Selecting a small inertial constant for the simulation resulted in a large frequency deviation of G/T and S/T, with frequency stabilized for a short time. On the other hand, selecting a large inertial constant resulted in a controlled frequency deviation, although the unstable frequency of the power grid continued for a long time.     

Study on FRT compliance of VSC-HVDC connected offshore wind plants during AC faults including requirements for the negative sequence current control

In this paper three new control modules are introduced for offshore wind power plants with VSC-HVDC transmission. The goal is to enhance the Fault Ride Thought (FRT) capability of the HVDC system and the connected offshore wind power plant during balanced and unbalanced AC faults. Firstly, a positive-sequence-voltage-dependent (PSVD) active current reduction control loop is introduced to the offshore wind turbines. The method enhances the performance of the offshore AC voltage drop FRT compliance strategy. Secondly, an adaptive current limiting control strategy which operates simultaneously on the positive and the negative sequence current is discussed. It enables negative sequence current injection, while at the same time respecting the maximum fault current capacity of the HVDC converter station. Finally, a state machine is proposed for the VSC-HVDC system and for the offshore wind turbines respectively. It coordinates the fault and the post-fault response during balanced as well as unbalanced faults, ensuring a smooth shift from the normal operating point towards the fault and the post-fault period. The test system consists of a two level VSC-HVDC link, rated at ±250 kV, connecting an offshore wind power plant with 700 MW generation capacity. Simulation results with a detailed EMT type model in PSCAD/EMTDC environment are presented.     

Predictive control of wind turbine for load reduction during ramping events

With increasing penetration of wind power, the impact of its intermittence and volatility on power systems becomes more severe. A predictive control strategy for wind turbines (WTs) is proposed to deal with wind power ramping events and reduce WT load on the blades. The blade load model is based on the Blade Element Momentum (BEM) theory. The generator speed and pitch angle are simultaneously regulated to realize the control objectives. A two-stage optimization is designed in order to reduce the computational complexity. The objectives of the first stage are minimizing the ramping rate and maximizing the power generation. A trade-off is made between the two contradictory objectives by setting weight coefficients. The second stage reduces the WT load and meanwhile guarantees the power reference from the first stage is tracked. Feedback is designed based on neural network prediction to compensate the error of the prediction model. Case studies with a 1.5 MW WT were conducted to demonstrate the efficacy of the proposed predictive control strategy. Simulation results show that the proposed control can reduce the WT load during ramping events and the risk of ramping events.     

基于DC-GRID的海上风电场控制策略研究

提出了基于直流传输的海上风电场系统的控制策略,同时建立了基于DC-GRID的海上风电场系统模型。系统由永磁同步风机机组、离岸全桥直直变换器、岸端逆变器组成。各个永磁同步风电机组分别并联在高压大功率全桥DC-DC上,由升压后经过直流传输到岸端VSC之后,经逆变到适合交流电网传输的电压后并网。控制策略以及系统模型由MATLAB/SIMULINK仿真验证。     

Addressing and assessing the issues related to connection of the wind turbine generators to the distribution grid

Integration of wind power generation into power distribution grid may result in stability and power quality challenges. In this way, this paper first reviews the grid integration issues of wind power generation. Then, as the main contribution, the paper develops in-depth theoretical analyses for examination the grid integration issues such as voltage rise, output power fluctuations, and static and dynamic voltage variations created due to grid integration of wind turbines and corresponding aerodynamic power fluctuations. Besides, the paper addresses these issues by proposing elaborate control approaches and by using the static compensator (STATCOM). The paper first addresses the problem of voltage rise by active power curtailment through pitch control. However, this method may result in a large waste of wind energy that may not be cost effective. Next, the STATCOM is implemented and efficient control approaches are proposed for addressing the problem of voltage rise, improvement of voltage profile and suppression of voltage and power fluctuations. At the end, simulation studies for the study distribution grid are given. The study system is an actual distribution grid in Iran with approximately 7.9 MW wind power generation and 21.9 MW load.     

A developed control strategy for mitigating wind power generation transients using superconducting magnetic energy storage with reactive power support

The fast variations of wind speed during extreme wind gusts result in fluctuations in both generated power and the voltage of power systems connected to wind energy conversion system (WECS). This paper presents a control strategy which has been tested out using two scenarios of wind gusts. The strategy is based on active and reactive powers controls of superconducting magnetic energy storage (SMES). The WECS includes squirrel cage induction generator (SCIG) with shunt connected capacitor bank to improve the power factor. The SMES system consists of step down transformer, power conditioning unit, DC–DC chopper, and large inductance superconducting coil. The WECS and SMES are connected at the point of common coupling (PCC). Fuzzy logic controller (FLC) is used with the DC–DC chopper to control the power transfer between the grid and SMES coil. The FLC is designed so that the SMES can absorb/deliver active power from/to the power system. Moreover, reactive power is controlled to regulate the voltage profile of PCC. Two inputs are applied to the FLC; the wind speed and SMES current to control the amount active and reactive power generated by SMES. The proposed strategy is simulated in MATLAB/Simulink®. The proposed control strategy of SMES is robust, as it successfully controlled the PCC voltage, active and reactive powers during normal wind speeds and for different scenarios of wind gusts. The PCC voltage was regulated at 1.0 pu for the two studied scenarios of wind gusts. The fluctuation ranges of real power delivered to the grid were decreased by 53.1% for Scenario #1 and 56.53% for Scenario #2. The average reactive power supplied by the grid to the wind farm were decreased by 27.45% for Scenario #1 and 31.13% for Scenario #2.     

System wide MV distribution network technical losses estimation based on reference feeder and energy flow model

This paper presents an integrated analytical approach to estimate technical losses (TL) of medium voltage (MV) distribution network. The concept of energy flow in a radial MV distribution network is modelled using representative feeders (RF) characterized by feeder peak power demand, feeder length, load distribution, and load factor to develop the generic analytical TL equations. The TL estimation approach is applied to typical utility MV distribution network equipped with energy meters at transmission/distribution interface substation (TDIS) which register monthly inflow energy and peak power demand to the distribution networks. Additional input parameters for the TL estimation are from the feeder ammeters of the outgoing primary and secondary MV feeders. The developed models have been demonstrated through case study performed on a utility MV distribution network supplied from grid source through a TDIS with a registered total maximum demand of 44.9 MW, connected to four (4) 33 kV feeders, four (4) 33/11 kV 30 MVA transformers, and twelve (12) 11 kV feeders. The result shows close agreement with TL provided by the local power utility company. With RF, the approach could be extended and applied to estimate TL of any radial MV distribution network of different sizes and demography.     

Direct power control of a multilevel inverter based active power filter

A cascaded H-bridge multilevel inverter based active power filter with a novel direct power control is proposed in this paper. It can be directly connected to medium/high voltage power line without using the bulky transformer or passive filter. Due to the limited switching frequency (typically below 1 kHz) of high-power solid-state devices (GTO/IGCT), multiple synchronous/stationary reference frame current controllers are reviewed and derived. Based on this, a novel current controller is proposed for harmonic current elimination and system power factor compensation. Furthermore, a synchronous/stationary hybrid structure can be derived with fundamental de-coupling control. The instantaneous reactive power theory and synchronous reference frame based control are compared based on mathematical models. A direct power control concept is then derived and proposed. It is equivalent as the hybrid synchronous/stationary frame current controller, but has a simpler implementation. It has clear physical meaning and can be considered as a simplified version of the hybrid frame current controller. Simulations on a 4160 V/1.2 MVA system and experimental results on a 208 V/6 kVA laboratory prototype are presented to validate the proposed active power filter design.     

Performance investigation of linear and nonlinear controls for a fuel cell/supercapacitor hybrid power plant

In this paper, linear proportional–integral (PI) and nonlinear flatness-based controllers for dc link stabilization for fuel cell/supercapacitor hybrid power plants are compared. For high power applications, 4-phase parallel boost converters are implemented with a switching interleaving technique for a fuel cell (FC) converter, and 4-phase parallel bidirectional converters are implemented with a switching interleaving technique for a supercapacitor converter in the laboratory. As controls, mathematical models (reduced-order models) of the FC converter and the supercapacitor converter are given. The prototype small-scale power plant studied is composed of a PEMFC system (the Nexa Ballard FC power generator: 1.2 kW, 46 A) and a supercapacitor module (100 F, 32 V, based on Maxwell Technologies Company). Simulation (by Matlab/Simulink) and experimental results demonstrate that the nonlinear differential flatness-based control provides improved dc bus stabilization relative to a classical linear PI control method.     

Energy management strategy for fuel cell/battery/ultracapacitor hybrid vehicle based on fuzzy logic

In order to enhance the fuel economy of hybrid vehicle and increase the mileage of continuation of journey, a fuzzy logic control is utilized to design energy management strategies for fuel cell/battery (FC + B) hybrid vehicle and fuel cell/battery/ultra-capacitor (FC + B + UC) hybrid vehicle. The models of hybrid vehicle for FC + B and FC + B + UC structure are developed by electric vehicle simulation software ADVISOR which uses a hybrid backward/forward approach. The results demonstrate that the proposed control strategy can satisfy the power requirement for four standard driving cycles and achieve the power distribution among various power sources. The comprehensive comparisons with the power tracking control strategy which is wide adopted in ADVISOR verify that the proposed control strategy has better rationality and validity in terms of fuel economy and dynamic property in four standard driving cycles. Therefore, the proposed strategy will provide a novel approach for the advanced energy management system of hybrid vehicle.     

Evaluating the integration of wind power into distribution networks by using Monte Carlo simulation

This paper provides a probabilistic method to assess the impact of wind turbines (WTs) integration into distribution networks within a market environment. Combined Monte Carlo simulation (MCS) technique and market-based optimal power flow (OPF) are used to maximize the social welfare by integrating demand side management (DSM) scheme considering different combinations of wind generation and load demand over a year. MCS is used to model the uncertainties related to the stochastic variations of wind power generation and load demand. The market-based OPF is solved by using step-controlled primal dual interior point method considering network constraints. The method is conceived for distribution network operators (DNOs) in order to evaluate the effect of WTs integration into the network. The effectiveness of the proposed method is demonstrated with an 84-bus 11.4 kV radial distribution system.     

Particle swarm optimization algorithm for capacitor allocation problem in distribution systems with wind turbine generators

This paper aims at adopting the Particle Swarm Optimization (PSO) technique to find the near-optimal solutions for the capacitor allocation problem in distribution systems for the modified IEEE 16-bus distribution system connected to wind energy generation based on a cost function. The proper allocation and the optimized number of capacitors have led to adequate power losses reduction and voltage profile enhancement. Because of the wind power generation variations due to the nature of wind speed intermittency and the lack of reactive power compensation, the problem under study have been presented involving a nonlinear fitness function. In order to solve it, the corresponding mathematical tools have to be used. The formulated fitness cost function has consisted of four terms: cost of real power loss, capacitor installation cost, voltage constraint penalty, and capacitor constraint penalty. PSO technique has been used to obtain the near-optimum solution to the proposed problem. Simulation results demonstrate the efficiency of the proposed fitness cost function when applied to the system under study. Furthermore, the application of PSO to the modified IEEE 16-bus system has shown better results in terms of power losses cost and voltage profile enhancement compared to Genetic Algorithm (GA). In order to verify the successful adaptation of PSO toward attaining adequate near-optimal capacitor allocations in distribution systems, this metaheuristic technique has been employed to the large-scale IEEE 30-bus system. The proposed PSO technique has provided adequate results while modifying the objective function and constraints to include the power factor and transmission line capacities for normal and contingency (N-1) operating conditions.     

Nonlinear intelligent DC grid stabilization for fuel cell vehicle applications with a supercapacitor storage device

This paper presents an intelligent DC link control using a fuzzy logic controller based on the differential flatness control theory for hybrid vehicle applications supplied by a fuel cell (FC) (main source) and a supercapacitor (auxiliary source). The energy in the system is balanced by dc bus energy stabilization (or indirect voltage regulation). A supercapacitor module functions by supplying energy to regulate the dc bus energy. The FC, as a slow dynamic source in this system, supplies energy to the supercapacitor module to maintain its charge. The FC converter combines four-phase parallel boost converters with interleaving, and the supercapacitor converter employs four-phase parallel bidirectional converters with interleaving. These two converters are called a multi-segment converter for high power applications. Because the model of the power switching converters is nonlinear, it is preferable to apply model-based nonlinear control strategies that directly compensate for the nonlinearity of the system without requiring a linear approximation. Using the intelligent fuzzy control law based on the flatness property, we propose straightforward solutions to hybrid energy management and to the dynamic and regulation problems. To validate the proposed method, a hardware system is developed with analogue circuits, and a numerical calculation is generated with a dSPACE controller DS1104. Experimental results for a small-scale power plant (a polymer electrolyte membrane FC (PEMFC) of 1200 W and 46 A with a supercapacitor module of 100 F, 500 A, and 32 V) in the laboratory corroborate the excellent performance of this control scheme during vehicle motor drive cycles.     

A novel intelligent approach for yaw position forecasting in wind energy systems

Yaw control systems orientate the rotor of a wind turbine into the wind direction, optimize the wind power generated by wind turbines and alleviate the mechanical stresses on a wind turbine. Regarding the advantages of yaw control systems, a k-nearest neighbor classifier (k-NN) has been developed in order to forecast the yaw position parameter at 10-min intervals in this study. Air temperature, atmosphere pressure, wind direction, wind speed, rotor speed and wind power parameters are used in 2, 3, 4, 5 and 6-dimensional input spaces. The forecasting model using Manhattan distance metric for k = 3 uncovered the most accurate performance for atmosphere pressure, wind direction, wind speed and rotor speed inputs. However, the forecasting model using Euclidean distance metric for k = 1 brought out the most inconsistent results for atmosphere pressure and wind speed inputs. As a result of multi-tupled analyses, many feasible inferences were achieved for yaw position control systems. In addition, the yaw position forecasting model developed was compared with the persistence model and it surpassed the persistence model significantly in terms of the improvement percent.     

不同控制策略下含直驱机组风电场的系统电压稳定研究

运行连续潮流计算的PV及VQ曲线法,对不同控制策略下含直驱机组风电场的系统静态电压稳定性进行了研究。通过对新疆一个实际地区电网进行仿真计算,绘制了直驱风电机组构成的风电场分别采取恒功率因数为1、恒功率因数为0.99、恒功率因数为-0.99及电压控制模式下地区电网电压中枢点、重要变电站、风电场公共接入点(point of common coupling,PCC)的PV曲线及VQ曲线。通过仿真分析可以得出,当风电场在处于低出力水平时,电网的静态电压稳定性较好;在风电场注入功率较大时,电网无功裕度减少,导致电网静态电压稳定性降低。直驱机组风电场采取恒电压控制策略要优于恒功率因数控制策略下的电网静态电压稳定性。     

大容量直驱风电机组级联直流组网系统设计

传统交流组网风电场系统存在多次电能转换、成本高的问题。针对这个问题,设计了一种大容量直驱风电机组级联直流组网海上风电场系统,其直接将每台机组的直流输出级联形成高压直流进行传输,而无需额外的海上升压站平台。风电机组采用了永磁直驱风力发电机及其变流器,其中变流器包括了AC/DC单元和DC/DC单元,并设计了控制策略,即通过DC/DC单元的占空比调节来实现电流的持续输出和最大功率跟踪。陆基逆变电站采用晶闸管型逆变器,设计了工作模式和控制策略,其主要功能是实现高压直流链路的电压电流调节。最后,基于PSCAD/EMTDC仿真平台,搭建了容量为150 MW的风电场系统进行了仿真计算,计算结果验证了该系统具有较高的鲁棒性和对风速变化的适应性,同时每个机组都能独立的实现最大风能捕获。     

Small Signal Stability Analysis with Penetration ofGrid-connected Wind Farm of PMSG Type

The impacts of large-scale grid-connected wind farm on direct-driven permanent magnet synchronous generator(PMSG)type are discussed on the small signal stability performances of power systems.Firstly,a simplified practical model of wind farm of PMSG type is derived for analyzing small signal stability.The rotor-fluxoriented control strategy is applied to the modelling of PMSG.Secondly,the framework of small signal stability analysis incorporating wind farm of PMSG type is built.Finally,the different simulation scenarios based on the IEEE 3-generator-9-bus test system as benchmark are designed to conduct the eigenvalue analysis and to assess the impacts of wind farm of PMSG type on power system small signal stability.Some conclusions are drawn with simulation results.     

Research on optimal configuration strategyof energy storage capacity in gridconnectedmicrogrid

To coordinate the protection of PMSG (permanent magnet synchronous generator), collector circuits and outgoing lines, a comprehensive and improved protection method of PMSG based wind farms with LVRT (low voltage ride through) capability is proposed. The proposed method includes adding a short time delay to the collector network current protection zone I and a directional protective relaying to the collector network protection, installing grounding transformers and zero sequence current protection, and generator low-voltage protection action improvement. A LVRT scheme consisting of variable resistance dumping circuit, grid side dynamic reactive power control and reactive power compensation control is proposed. The fault characteristics of PMSG based wind farms are analyzed, and a PMSG based wind farm in Dabancheng, Xinjiang, is used as an example to analyze typical wind farm protection configuration, the setting values considering LVRT requirements, and the coordination problems. Finally, an improved wind farm protection coordination methodology is proposed and its validity is verified by simulation.     

Configuration and operation combinedoptimization for EV battery swappingstation considering PV consumptionbundling

Integration of electric vehicles (EVs), demand response and renewable energy will bring multiple opportunities for low carbon power system. A promising integration will be EV battery swapping station (BSS) bundled with PV (photovoltaic) power. Optimizing the configuration and operation of BSS is the key problem to maximize benefit of this integration. The main objective of this paper is to solve infrastructure configuration of BSS. The principle challenge of such an objective is to enhance the swapping ability and save corresponding investment and operation cost under uncertainties of PV generation and swapping demand. Consequently this paper mainly concentrates on combining operation optimization with optimal investment strategies for BSS considering multiscenarios PV power generation and swapping demand. A stochastic programming model is developed by using state flow method to express different states of batteries and its objective is to maximize the station’s net profit. The model is formulated as a mixed-integer linear program to guarantee the efficiency and stability of the optimization. Case studies validate the effectiveness of the proposed approach and demonstrate that ignoring the uncertainties of PV generation and swapping demand may lead to an inappropriate batteries, chargers and swapping robots configuration for BSS.     

计及风电场静态电压稳定性的VMP系统无功电压控制策略研究

随着规模化、集群化风电基地的初步建成,风电作为一种清洁高效的能源得到了快速的发展,但短时间内大规模风电场集中接入电网,给电网的功率平衡带来扰动,造成了电网电压的不稳定。针对风电场并网后的电压控制问题,研究了大规模风电场并网的静态电压稳定机理。并在现有调压手段的基础上,通过适时调整风电机组无功出力,升压站变压器抽头以及调无功补偿装置,进一步提出了基于分层管理的无功功率/电压控制策略,并将该策略嵌入到风电场电压/无功自动管理平台(VMP)。通过新疆某地区风电场现场试验发现,该控制策略能够改善低电压穿越期间无功表现,提高风电场无功电压的稳定性,同时避免了功率振荡的产生。该研究结果可以为风电场无功电压协调控制的理论研究和工程实际提供参考依据。