ACTIVE AND REACTIVE POWER CONTROL FOR A ZERO GRID IMPACT UNDER TRANSMISSION LINE FAULT

Transcription

1 ACTIVE AND REACTIVE POWER CONTROL FOR A ZERO GRID IMPACT UNDER TRANSMISSION LINE FAULT N.THIRUMAL #1 and Prof. L. P.VETTRIVELAN *2 # Final year, M.E, Power Systems Engineering, P.S.V College of Engineering and Technology, Krishnagiri (D.T) * Assistant Professor, Electrical and Electronics Engineering, P.S.V College of Engineering and Technology, Krishnagiri (D.T) Abstract This project includes a new concept for building integration in MGs with zero grid-impact so improving the MG efficiency. These aims are shown to be achievable with an intelligent system, based on a dc/ac converter connected to the building point of coupling with the main grid. In a smart micro-grid (MG) each generator or load has to take part in the network management, joining in reactive power supply/voltage control, active power supply/frequency control, fault ride-through capability, and power quality control. This system can provide active and reactive power services also including a dc link where storage, generation, and loads can be installed. The system employed for validation is a prototype available at ENEA Laboratories (Italian National Agency for New Technologies). A complete and versatile model in MATLAB/SIMULINK is also presented. The simulations results and the experimental test validation are included. Index Terms micro-grid (MG), fault ride-through capability, reactive power supply/voltage control, active power supply/frequency control. INTRODUCTION Offshore wind farms (OWFs) are located a distance of less than 25 km away from sea shore and an OWF may consist of several high-capacity parallel-operated wind turbine generators (WTGs). The use of several doubly-fed induction generators (PMSG s) connected directly to a power system is one of the simplest ways of running an OWF due to the advantages of both reactive power control and higher operating efficiency of PMSG s. This paper presents the results of using a line-commutated HVDC link joined with a modal-control designed damping controller of the rectifier current regulator (RCR) at the rectifier station to perform dynamic-stability enhancement and active- power control of an 80-MW PMSG -based OWF fed to an onshore power grid. The employment of an HVDC link for an OWF has the advantages of fast active power modulation, effective reactive-power compensation, less voltage drop on an onshore substation, etc. over conventional AC transmission lines. The simulation results of a 200-MW OWF comprising MW WTGs connected to a power grid through a multi terminal HVDC link with 25 voltage-source converters (VSCs) were presented. The simulation responses of a 200-MW OWF consisting of 100 individual 2-MW WTGs connected to a power grid through a multi terminal HVDC link with 25 current source inverters (CSIs) were examined. The results of a detailed technical economic analysis of three transmission strategies, An improved model for the transient energy functions of integrated AC/DC power systems involved the omission of DC control dynamics was proposed. The control requirements of a PMSG -based wind farm connected to a grid through a conventional thyristor based HVDC link were studied. The characteristics of an IG-based OWF connected to a long-distance weak AC grid were explored, and the simulation results demonstrated. Configuration of the studied 80-MW offshore wind farm connected to an onshore power grid through a line-commutated HVDC link. Proposed HVDC link was able to both supply the variable active power of the OWF to the weak grid and keep the fluctuations of AC voltages at the point of common coupling at an acceptable level. This paper presents damping controller design of RCR, steady-state eigen value analysis and transient time-domain simulations of an 80-MW PMSG -based OWF connected to an onshore substation through an HVDC link. System eigen values and the design of damping controller of the RCR of the HVDC link are performed under steady state analysis. Dynamic responses of the studied OWF with and without the designed damping controller of the RCR subject to a torque disturbance are also carried out using time domain simulations. A. Inverter I. STRATEGIC ANALYSIS An inverter is an electrical device that converts direct current (DC) to alternating current (AC); the converted AC can be at any required voltage and frequency with the use of appropriate transformers, switching, and control circuits. Solid-state inverters have no moving parts and are used in a wide range of applications, from small switching power supplies in computers, to large electric utility high-voltage direct current applications that transport bulk power. Inverters are commonly used to supply AC power from DC sources such as solar panels or batteries. There are two main types of inverter. The output of a modified sine wave inverter is similar to a square wave output except that the output goes to zero volts for a time before switching positive or negative. It is simple and low cost (~$0.10USD/Watt) and is compatible 266

2 with most electronic devices, except for sensitive or specialized equipment, for example certain laser printers. A pure sine wave inverter produces a nearly perfect sine wave output (<3% total harmonic distortion) that is essentially the same as utility-supplied grid power. Thus it is compatible with all AC electronic devices. This is the type used in grid-tie inverters. Its design is more complex, and costs 5 or 10 times more per unit power (~$0.50 to $1.00USD/Watt). The electrical inverter is a high-power electronic oscillator. It is so named because early mechanical AC to DC converters was made to work in reverse, and thus was "inverted", to convert DC to AC. B. Rectifier A rectifier is an electrical device that converts alternating current (AC), which periodically reverses direction, to direct current (DC), which is in only one direction, a process known as rectification. Rectifiers have many uses including as components of power supplies and as detectors of radio signals. Rectifiers may be made of solid state diodes, vacuum tube diodes, mercury arc valves, and other components. A device which performs the opposite function (converting DC to AC) is known as an inverter. When only one diode is used to rectify AC (by blocking the negative or positive portion of the waveform), the difference between the term diode and the term rectifier is merely one of usage, i.e., the term rectifier describes a diode that is being used to convert AC to DC. Almost all rectifiers comprise a number of diodes in a specific arrangement for more efficiently converting AC to DC than is possible with only one diode. Before the development of silicon semiconductor rectifiers, vacuum tube diodes and copper (I) oxide or selenium rectifier stacks were used. Early radio receivers, called crystal radios, used a "cat's whisker" of fine wire pressing on a crystal of galena (lead sulfide) to serve as a point-contact rectifier or "crystal detector". Rectification may occasionally serve in roles other than to generate direct current per se. For example, in gas heating systems flame rectification is used to detect presence of flame. Two metal electrodes in the outer layer of the flame provide a current path, and rectification of an applied alternating voltage will happen in the plasma, but only while the flame is present to generate. C. Power Quality Standards, issues & its Consequences A. international Electro Technical Commission Guidelines The guidelines are provided for measurement of power quality of wind turbine. The International standards are developed by the working group of Technical Committee-88 of the International Electro-technical Commission (IEC), IEC standard , describes the procedure for determining the power quality characteristics of the wind turbine. i. Standard norms 1) IEC : Wind turbine generating system, part-21. Measurement and Assessment of power quality characteristic of grid connected wind turbine 2)IEC : Wind Turbine measuring procedure in determining the power behavior. 3) IEC : Assessment of emission limits for fluctuating load IEC : Wind Turbine performance. The data sheet with electrical characteristic of wind turbine provides the base for the utility assessment regarding a grid connection. ii. Voltage Variation The voltage variation issue results from the wind velocity and generator torque. The voltage variation is directly related to real and reactive power variations. The voltage variation is commonly classified as under: Voltage Sag/Voltage Dips. Voltage Swells. Short Interruptions. Long duration voltage variation. The voltage flicker issue describes dynamic variations in the network caused by wind turbine or by varying loads. Thus the power fluctuation from wind turbine occurs during continuous operation. The amplitude of voltage fluctuation depends on grid strength, network impedance, and phase-angle and power factor of the wind turbines. It is defined as a fluctuation of voltage in a frequency Hz. The IEC specifies a flicker meter that can be used to measure flicker directly. iii. Harmonics The harmonic results due to the operation of power electronic converters. The harmonic voltage and current should be limited to the acceptable level at the point of wind turbine connection to the network. To ensure the harmonic voltage within limit, each source of harmonic current can allow only a limited contribution, as per the IEC guideline. The rapid switching gives a large reduction in lower order harmonic current compared to the line commutated converter, but the output current will have high frequency current and can be easily filter-out. iv. Wind turbine location in power system The way of connecting the wind generating system into the power system highly influences the power quality. Thus the operation and its influence on power system depend on the structure of the adjoining power network. v. Self excitation of wind turbine generating system The self excitation of wind turbine generating system (WTGS) with an asynchronous generator takes place after disconnection of wind turbine generating system (WTGS) with local load. The risk of self excitation arises especially when WTGS is equipped with compensating capacitor. The capacitor connected to induction generator provides reactive power compensation. However the voltage and frequency are determined by the balancing of the system. The disadvantages of self excitation are the safety aspect and balance between real and reactive power. vi. Consequences of the issues The voltage variation, flicker, harmonics causes the malfunction of equipments namely microprocessor based control system, programmable logic controller; adjustable speed drives, flickering of light and screen. It may leads to tripping of contractors, tripping of protection devices, stoppage of sensitive equipments like personal computer, programmable logic control system and may stop the 267

3 process and even can damage of sensitive equipments. Thus it degrade the power quality in the grid. II. OPERATIONAL ANALYSIS The proposed system includes the choice of objective function and the model approximation; incorporation of constraints pertaining to reactive power output of variable speed induction machines and other reactive power sources; choice of the algorithm. Block Diagram of Proposed System The IPFC structure makes it possible to transfer reactive power, as well as to exchange real power with the line. This active power can be obtained through power exchange through DC connection between the SSSCs in different lines. On the other hand, the transmitted powers in each line is a function of the voltage amplitude of sending and receiving buses, phase shift of sending and receiving buses and series impedance of the line. The interline power flow controller works with a number of direct current to alternative current converters each providing series compensation for a different transmission line. However some structure of interline power controller (IPFC) is consists of a set of converters that are connected in series with different transmission lines without shunt convertor. Schematic diagram of IPFC Without shunt converter As mentioned above IPFC consist a set of converters. The converters are connected through a common DC link to exchange active power. Each series converter can supply independent reactive compensation of own transmission line. If a shunt converter is involved in the system, the series converters can also provide independent active compensation; otherwise not all the series converters can provide independent active compensation for their own line.vthe interline power flow controller (IPFC) is one of the latest generation and advanced flexible AC transmission systems controller which can be used for dynamic compensation and effective power flow management among transmission corridors. It is VSC-based FACTS controller for Series compensation with the unique capability of power management among multiline of a substation. It simultaneously controls the power flow in multiline systems or sub network. Since IPFC contain converters with common direct current link, any inverter within the IPFC is able to transfer real power to another and there by facilitate real power transfer among the lines of the transmission system. Controller. From many FACT devices today, IPFC is more advanced and easy controller to solve the overcrowding of the power management of the transmission system. A. Power system Stabilizers Though power system stabilizers are available, that may not be sufficient in stabilizing the voltages and other power variations in the transmission lines. The load at any instant could never be predicted and hence there always exists instantaneous variations both in real and reactive powers. It may be thought that PSS (Power System Stabilizer) may to this stabilization, but unfortunately, its performance in inadequate in crude grid power variations. B. General FACTS devices FACTS (Flexible AC transmission systems) devices such as STATCOM, SSSC, TCSC etc to provide sufficient stability in the output power at load sides and to improve the overall power transmission capacity. A power system when we consider as a grid structure is a complex network comprising of enormous number of generators, power transmission lines, different types of loads and transformers. As a consequence of increasing demand in power need, some transmission lines through which the power is delivered are loaded heavily than for which it had been planned while erecting such lines. C. Transient Stability The increased loading of long transmission lines, the problem of transient stability after certain faulty conditions also depends purely on the circuit parameters of the transmission lines. Transient stability of a system refers to the stability when subjected to large disturbance such as open and short faults. As a result, the generated power and delivered load are not matched. Hence there exists a power reflection towards the generator. This is the worst scenario in power systems. This could influence in the abrupt variation in the rotor angle and ultimately, it damages the generator as a whole and spoils even the loads. There may be trip circuits to shut down the total loads, which is not a wise decision, because the loads are affected and it is a foremost aim to supply the power without interruptions to all the committed loads. The resulting system response involves large excursions of generator rotor angles and is influenced by the nonlinear power angle relationship. Stability depends upon both the initial operating conditions of the system and the severity of the disturbance. The voltage stability, and steady state and transient stabilities of a complex power system can be effectively improved by the use of FACTS devices. The main focus in this paper would be to compare the performances of UPFC (Unified Power Flow Controller) and IPFC (Distributed Power Flow Controller) based on the stability attained times and other parameters such peak overshoot. The present survey does not concentration on the optimization of all the above mentioned power flow controllers. 268

4 D. Modeling of IPFC The dynamic model of UPFC is derived by performing standard d-q transformation of the current through the shunt transformer and series transformer. generator side where ever need to be seen. For convenience, certain blocks are combined together and made as a subsystem. The simulation is run for a time of In addition to this, a fault breaker is introduced parallel to the load and analysed for the time taken to return to the healthy state. while a fault occurs. The sampling time as set as 2e-6. Circuit breakers are employed to ensure a constant output even at the absence of the wind power for a short duration. Fuzzy logic optimizes the gains and hence the final Vabc generated is kept as a new reference in order to maximize the real power. Block diagram of Series converter III. SIMULATION RESULTS Voltage and Current without Compensation Here x-axis consists of time in Seconds and Y-axis consists of per unit value of line voltage and current. Simulink Model without Compensation This function is like a STATCOM operation, where the DC energy is again injected in to the line in the form of real or reactive power. The gate pulses generated which maintains the maximum possible real power. The load at the end of the three phase transmission line is kept at 25kw range. The waveforms are observed on the load side and the Real and Reactive Power without Compensation Here x-axis consists of time in Seconds and Y-axis consists of Per unit value of Real and Reactive Power. 269

5 Simulink Model of with Compensation Voltage and Current with Compensation Here x-axis consists of time in Seconds and Y-axis consists of Per unit value of line voltage and current. Simulink Model of IPFC(One Phase) Real and Reactive Power with Compensation 270

6 Here we can see that there is a real power improvement oscillating at 0.5 pu value. In the system without compensation, it was about 0.45 p.u. value under heavily loaded conditions. Hence there is an improvement of 0.5 pu value of real power. In both the cases, the reactive power maintains at almost 0 p.u Since IPFC is used less settling time and power balance is achieved. Less power loss because hysteresis loss is less. Easily controllable to required voltage. Reactive power compensation is not too time critical, hence this method is very suitable and gives global optimization For all common power grid applications, where we need, ac supply depending on the load requirement. The control section of our project can be used at high end research laboratories for clean harmonics less power. The merit of this work is the self-adaption of the circuit to a large range of wind input power (including partial shading). The addition of the variable inductance, while maintaining continuous conduction with its attendant advantages for the control of the circuit, does not affect the stability of a buck dc dc converter in any way. The step response shows that the system is stable; however, further optimization with regard to regulation could form the basis for future work. demonstration model may be worked out to show the performance. The control algorithms may be done with some types of optimization algorithms to generate PWM, satisfying the maximum possible real power. REFERENCES [1] A. Ipakchi and F. Albuyeh, Grid of the future, IEEE Power Energy Mag., vol. 7, no. 2, pp , Mar [2] B. Moradzadeh and K. Tomsovic, Two-stage residential energy management considering network operational constraints, IEEE Trans. Smart Grid, vol. 4, no. 4, pp , Dec [3] C. Vivekananthan, Y. Mishra, G. Ledwich, and F. Li, Demand response for residential appliances via customer reward scheme, IEEE Trans. Smart Grid, vol. 5, no. 2, pp , Mar [4] C. Murillo-Sanchez, R. Zimmerman, C. L. Anderson, and R. Thomas, Secure planning and operations of systems with stochastic sources, energy storage, and active demand, IEEE Trans. Smart Grid, vol. 4, no. 4, pp , Dec [5] G. Parise, L. Martirano, and L. Parise, Ecodesign of ever net-load microgrids, IEEE Trans. Ind. Appl., vol. 50, no. 1, pp , Jan [6] J. H. Yoon, R. Baldick, and A. Novoselac, Dynamic demand response controller based on real-time retail price for residential buildings, IEEE Trans. Smart Grid, vol. 5, no. 1, pp , Jan [7] M. Sullivan, J. Bode, B. Kellow, S. Woehleke, and J. Eto, Using residential AC load control in grid operations: PG&E s ancillary service pilot, IEEE Trans. Smart Grid, vol. 4, no. 2, pp , Jun [8] M. C. Falvo, Generation and transmission planning and electricity market: The role of TSO, in Proc. 7th Int. Conf. Eur. Energy Market (EEM), Madrid, Spain, 2010, pp [9] R. Argiento, R. Faranda, A. Pievatolo, and E. Tironi, Distributed interruptible load shedding and micro-generator dispatching to benefit system operations, IEEE Trans. Power Syst., vol. 27, no. 2, pp , May IV. CONCLUSION & FUTURE WORK This kind of configuration allows the building to control the amount of active and reactive power can be imported or exported from the grid, making the building acting as a dispatchable load or generator. This could be very helpful to achieve large penetration levels of DG in the LV grid. Moreover, the device is also capable of providing ancillary services as voltage support by increasing the reactive power injection when the grid voltage is below its rated value. As it was stated, the proposed device has two different subsystems. The ac subsystem consists on the building ac loads and the dc system can include different kind of devices. The use of this innovative configuration combining ac/dc distribution inside the building has been tested. In this case, the real prototype has a energy storage system based in Li-Po batteries, a VLR and different dc loads, most of them for ventilation and heating purposes. The ac part and the dc part are connected through an ac/dc converter. It has been also studied in the simulations the possibility of inserting a PV generator in the dc system. The proposed controls for each element of the system have been tested and the simulations have demonstrated that the system is stable under all possible scenarios. Further works should include studies of load profiles and capacities of different storage and generation devices in order to develop a proper sizing procedure to achieve the zero grid impact. This project can be extended with other types of power compensation methods such as HPFC or DPFC. Hardware implementation as a 271