Study of Reactive Power Control in Microgrid

International Research Journal of Applied and Basic Sciences © 2013 Available online at www.irjabs.com ISSN 2251-838X / Vol, 4 (7): 1991-1997 Science ...
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International Research Journal of Applied and Basic Sciences © 2013 Available online at www.irjabs.com ISSN 2251-838X / Vol, 4 (7): 1991-1997 Science Explorer Publications

Study of Reactive Power Control in Microgrid Mojtaba Khanalizadeh Eini1 Hossein Farahmand2, Abdolreza esmaeli*3, Abolfazl Shakerifar1 1. Faculty of Electrical Engineering, University Maziar, Iran. 2. Islamic Azad University, Science and research Branch, Saveh, Iran 3. Nuclear Science and Technology Research Institute, Tehran, Iran. *Corresponding Author email:[email protected] ABSTRACT:In this paper several newapproaches in controllingof reactive power in microgrid is discussed. First, this paper theoretical analyzes kinds of reactive and harmonic current detection methods; then the reactive power control by means of a reactive power/frequency droop control strategy is studied in an islanded microgrid. In a low voltage microgrid, due to the effects of nontrivial feeder impedance, the conventional droop control is subject to the real and reactive power coupling and steady-state reactive power errors. So the conventional reactive power droop control can be improved with zero steady state sharing error, just like the real power sharing through frequency droop control. Tertiary a coordinated control of Distributed Generators (DG) and Distribution Static Compensator (DSTATCOM) in a microgrid is proposed. And at last a solution for data acquisition and control of MicroGrid using wireless technique (ZigBee @ 2.4 GHz), that manages the reading of various electrical parameters in different DG’s, is presented. Keywords:Reactive power sharing; droop control, DSTATCOM, Renewable energy, Combined Heat and Power (CHP), Point of Common Coupling(PCC), Distributed Energy Resource (DER). INTRODUCTION With the growing demands for electricity, the advantages which the large power grid reflected in the past few years enable it developed rapidly and become the main power supply channels in the world. However, there are some major drawbacks with the power supply of the centralized power grid: high cost and difficult to run, increasingly difficult to meet with users’ high safety and reliability requirements (Wangfang Liu, 2011). Recently, because of environmental considerations, technological developments and governmental incentives for renewables, the grid architecture is changing from centralized to decentralized energy supply with distributed generation (DG) units connected to the utility grid (Tine L. et al., 2010). Compared with the centralized power generation, distributed generation has its own advantages: with less pollution, higher energy efficiency, more flexible installation sites, transmission and distribution of resources, lower operating costs, and reduction of power transmission line loss. Distributed generation can reduce the total capacity of power grid and improve grid peak performance, improve power supply reliability is a strong complement to large power grid and effective support (Wangfang Liu, 2011). DG can also lead to improved reactive power support and voltage profile, removal of transmission bottlenecks, usage of environmental friendly resources and postponement of investments in new transmission systems and large-scale generators. Normally, the distribution grids are conceived as a passive top-down architecture, with a unidirectional power flow, but the increasing presence of DG units leads to bidirectional power flow in an active distribution network. Another major change is that most DG units are connected to the ac-grid via power electronic interfaces, e.g., voltagesource inverters, because they do not generate a 50 Hz voltage. The CERTS (Consortium for Electric Reliability Technology Solutions) defines the microgrid as a small-scale, low-voltage system consisting of a combination of generators, loads and energy storage elements, mainly with power-electronic interface. A key advantage is that the microgrid appears to the power network as a single controllable unit. Microgrids can also enhance local reliability, reduce feeder losses, support local voltage, increase efficiency through CHP and provide uninterruptible power supply (UPS) functions. Furthermore, microgrids can facilitate the penetration of renewables and other forms of DG into the utility grid and assist in better power quality. Essentially, a microgrid is an active distribution network that can be exploited in two operating conditions. The microgrid can operate in grid-connected or stand-alone Mode(Tine L. et al., 2010). Schematic diagram of a microgrid is shown in Figure 1.

Intl. Res. J. Appl. Basic. Sci. Vol., 4 (7), 1991-1997, 2013

Figure 1. Microgrid with (power-electronically interfaced) loads, storage andDG units in stand-alone or grid-connected mode

THEORETICAL ANALYSIS Distributed generation is build on the basis of power electronics, a large number of power electronic converters increase the number of non-linear loads, which will cause power grid’s current and voltage waveform distortion, and harmonic pollution. A large number of harmonics is one of important implication introduced by distributed generation on power quality, therefore, this part mainly study on analyzing kinds of reactive power and harmonic current detection methods. Instantaneous reactive power theory based on the reactive and harmonic current detection method Power system’s voltage, current harmonics and asymmetric component contains is complicated and hard to resolve, which is difficult to correctly interpret through traditional power theory. Three phase reactive power theory, which was proposed by Akagi in 1983, is now maturity though past few years’ gradually improvement. Traditional theory of the active power, reactive power, or vector are in the average sense based on the definition, they only apply to the voltage and current are sine wave situation. The instantaneous reactive power theory concepts are defined based on the instantaneous value, which applies not only to sine wave, but also for non-sine wave and any instantaneous situation. Principle of New Synchronous Detection Method Synchronous detection method is a kind of harmonic current detection method which current waveform of the compensation network in phase with the power grid’s voltage waveform. In the three-phase system, it can be divided into equal power method, equal current method and equal resistance method according to different components need to be compensated, namely, making the phase power, current and resistance equal after compensating. Resistance equal method is seldom adopted since it rarely used in practice. A new synchronous detection method is based on p–q transformation by extracting fundamental positive sequence of voltage, requires only two low-pass filters and computes by using instantaneous values, which makes the calculation process simplified. Principle of Generalized Reactive Current Detection Method Based on Orthogonal Characteristics Method According to S. Fryze time-domain power definition, this method decompose i( t) into a generalized active current component ip ( t) which has the same phase and direction with the circuit voltage but different amplitude and a generalized reactive current component iq ( t) which is orthogonal with the circuit voltage. ip ( t) and voltage u ( t) has the same phase and direction , only has an amplitude difference K: (1) It can be observed that sine function has become a special case by adopting this method(Wangfang Liu, 2011).

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Intl. Res. J. Appl. Basic. Sci. Vol., 4 (7), 1991-1997, 2013 REACTIVE POWER DROOP CONTROL In the concentrated control method, a central unit sends control signals to the inverters. An example is the master-slave scheme, where the microgrid consists of one master inverter and all others are slave units. The main issue of these control principles is that a communication link between the units is required, which restrains the positioning of the units, causes noise problems and also reduces the reliability of the microgrid as the grid control depends on this communication link. Similar to the droop control in conventional grids, communication can be avoided by implementing an active power/frequency-droop controller such as. In this control strategy, the inverters themselves determine their setpoints for instantaneous active and reactive power based on local measurements of voltage and frequency only. This droop controller is based on the mainly inductive character of the lines, introducing an active power/frequency linkage. However, unlike the transmission lines, the lines in the low-voltage microgrid are often mainly resistive. Overall, the characteristics of the microgrid differ significantly from those of conventional high-voltage transmission grids. Firstly, in conventional grids, the frequency control is based on the rotating inertia in the system, whereas islanded microgrids lack significant inertia because most elements are power-electronically interfaced. Secondly, low voltage distribution grids have mainly resistive characteristics, unlike the mainly inductive transmission grid. Furthermore, as renewable energy resources are increasingly being connected to the (micro) grid, an important part of the sources in the microgrid are not (fully) dispatchable, as the power output of these sources is optimized by means of maximum power point tracking. These differences between conventional grids and microgrids require the development of new control methods specifically designed for microgrids. Therefore, the Vg/Vdc- droop control strategy is suggested in, where the generators output power remains constant during longer periods of time by using the tolerated voltage band in the microgrid optimally. Also, droop controllers are applied to avoid a critical communication link for the primary control. Therefore, the combination of the Vg/Vdcdroop control with Q/f-droop control, is studied in this part. As microgrids are low-voltage networks, the line impedance is approximated as purely resistive. The grid voltage amplitude is controlled by the active power controller, while the frequency is derived from the reactive power control loop. A Vg/Vdc-droop control strategy is presented in order to balance the active power in the microgrid. The method is based on the power storage capability of the dcbus capacitor Cdc. The controller uses the dc-bus voltage Vdc of the primary energy source as a measure for the difference in generated dc-power and delivered ac-power. It is shown that this controller achieves good power balancing in the microgrid. When a certain voltage, the adjustment voltage, is exceeded, the generated power Pdc is changed according to a Pdc/Vg-droop control principle, which also does not require a communication link and is based on the specific microgrid properties. In steady-state, the reactive power will be shared according to the droop characteristics of the primary energy sources. Generally, before connecting a new VSI to the microgrid, it is synchronized to it. Due to, e.g., a problem in this synchronization procedure, the phase of the microgrid and the VSI voltage may not match, which can lead to circulating currents. Figure 2 illustrates voltage and power control in microgrid (Tine L., 2010)..

Figure 2. Microgrid control strategy: voltage and power control

In the islanded mode, the distributed generators are responsible for voltage control as the voltage is not determined by the main grid. The set values for the voltage controllers are determined by the power controllers with local measurements only. The reference amplitude of the grid voltage is determined by the active power Vg/Vdc-droop control loop. The set value of the frequency is determined by the reactive power Q/f-droop control loop. In this paper, its characteristics are studied and compared with the case that the reactive power is not controlled. It is shown that, in order to avoid large circulating currents, reactive power control is necessary. Furthermore, by proper design of the slopes of this controller, a power sharing between the DGs according to their ratings can be achieved. Also, the delivered reactive power can be limited by implementing a piece-wise droop characteristic. On the other hand, without reactive power control, the power 1993

Intl. Res. J. Appl. Basic. Sci. Vol., 4 (7), 1991-1997, 2013 sharing is determined by the microgrid characteristics and the active power controller and highly influenced by frequency and phase angle errors. Zero steady state sharing error An indispensable function of microgrid is to achieve desired power management among DG units in islanding operation. Conventionally, the frequency and voltage droop control is adopted without involving any communications among DG units. Although real power can be properly shared using this method, the reactive power control is often sensitive to the configuration of the microgrid. Specifically, the nontrivial feeder resistance introduces power coupling during transient and the steady-state reactive power sharing accuracy is affected by the unequal feeder impedances. Additionally, the existence of local loads and the complex network configurations can further aggravate the reactive power sharing problem. To solve the reactive power control issues, a few improved methods have been proposed. A novel virtual frequency and voltage frame based power control method was presented to reduce the power coupling introduced by feeder resistance. A predominant virtual output inductor was placed at the DG terminal, which reduces the reactive power sharing errors. It is worth mentioning that the reactive sharing error can be further reduced through modified droop slopes. However, the estimation of desired droop slopes is not straightforward. Both the reactive power and the harmonic power can be properly shared with additional harmonic current injection. Although the power sharing problem can be solved with this method, the steady-state distortions always exist in the DG voltages. Therefore, this method may affect the voltage quality of the sensitive loads. A Q-V dot droop was presented. It can be observed that the reactive power sharing improvement is limited when local loads are considered. To avoid the drawbacks of the aforementioned compensation methods, this paper presents a practical accurate reactive power sharing scheme. The proposed method first identifies the reactive power sharing error through small real power disturbances. Then the accurate reactive power sharing is realized by using an additional intermittent integral term. With this scheme, the reactive power error is significantly reduced, and the improved droop controller will be the same as the conventional droop controller at steady-state. Note that the proposed method is effective for all types of microgrid configurations. Simulation and experimental results are provided to verify the proposed method. proposed improved droop control method The aim of this section is to develop a robust control method that can compensate the reactive power sharing error at steady-state. To initialize the compensation, the proposed compensation method adopts a lowbandwidth communication cable between the control and monitoring center and DG units. This communication channel will send out the compensation staring signals, so all DG units can start the compensation at the same time. Considering that the monitoring center also sends power reference signals in grid connected mode and the synchronization signals during operation mode transition, the proposed method doesn’t require any additional hardware cost. Note that only one way communication from the control and monitoring centre to the DG units is needed and the communication between DG units is not necessary for the implementation of this method. The proposed power control strategy is realized through the following two steps: a) Initial power sharing using conventional method. At first, the conventional droop controllers are adopted for initial load power sharing. During this process, the average real power PAVE detection block is used to filter out the real power ripples. In addition, the digital controller identifies the status of compensation starting flag dispatched from the microgrid monitoring center. b) Conventional power sharing error elimination through synchronized compensation. Once a compensation starting signal is received by the DG unit, the average real power detection block stops updating, and the saved data PAVE before receiving the compensation signal is obtained as a reference during compensation. In the compensation process, a combination of real and reactive power is used in frequency droop control, and the reactive power error is suppressed by using an additional integration term: (2) (3) where Ki is the integral gain, which is selected to be the same for all DG units. The unit soft compensation gain G contains an increase ramp at the beginning and a decrease ramp in the end of the compensation, which is used to minimize the power oscillation during compensation (Jinwei H, Yun WL. 2011). !"

A COORDINATED CONTROL OF DG AND DISTRIBUTION STATIC COMPENSATOR (DSTATCOM) To manage elements in micrigrid, a network of communication devices must provide the microgrid with the necessary intelligence to allow customers and utility companies to collaboratively manage the power 1994

Intl. Res. J. Appl. Basic. Sci. Vol., 4 (7), 1991-1997, 2013 generated, delivered and consumed through real-time, bidirectional communications. Thus, communications are essential in morder to ensure the proper operation of all the microgrid components. Protection devices, control commands and power flow regulation, together with real-time measurements must be integrated in a network that provides the necessary levels of quality and reliability. The architecture of this network can be centralized, distributed, or hierarchical. In a hierarchical architecture there are different types of controllers that, depending on the control level, can be classified into: distribution management system, microgrid central controller, and local controllers. A control strategy must be devised to ensure the long-term stable operation of the microgrid under various load conditions and different configurations. Depending on the time frame this control algorithms can be separated in: 1) primary, there is a momentary adjust; 2) secondary, the operation range is several minutes; 3) tertiary, the operation range is 15–20 min. The microgrid can operate in grid-connected or stand-alone mode. Concerning the methods used for managing the power of a microgrid two different classes can be distinguished: the centralized operation that concentrates information in a node and the decentralized operation which uses local information and provides more autonomy to the DER units. This part focuses on a simple method for controlling the active and reactive power of a utility connected microgrid operating in grid connected mode. It is based on peer-to-peer and plug-and-play concepts and operates either in centralized or in distributed mode. A. Intelligent Node (iNode) Develops the global management of microgrid tasks and connects supervising and control systems (through a Gateway) to the terminal equipment (iSocket). Its functions are managing the data received from iSockets and setting overall operation of the microgrid, developing its own algorithms. Its main tasks include: 1) regulation: control of energy generation and consuming entities; 2) billing: energy measurement and real-time pricing; 3) management: asset management and condition based maintenance; 4) metering: full system monitoring; 5) security of the microgrid electrical system. The operational requests of this controller are aggregation and coordination of iSockets and electrical safety guarantee. B. Intelligent Socket (iSocket) It is an element located in the lowest hierarchy layer of the communication system. It handles the device connected to it (generation, storage, or loading), based on the instructions received from the iNode. The operational requests of this controller are local regulation and electrical safety guarantee. The functional block diagram of microgrid is shown in figure 3. This part proposes implementing a control algorithm with the capability to achieve the system goals using the available units, interfaced to the microgrid through a VSC. The algorithm uses decision laws that have certain parameters to manage the microgrid and establishes the equilibrium points of the system in a indirect manner. It is based on independent control of active and reactive power in grid connected mode (PQ control) and consists of secondary and tertiary microgrid’s control. One advantage of this algorithm is its simplicity in terms of the reduction of information flow and data exchangebetween all nodes in the network and the fast adaptability to configuration changes in the microgrid, such as unit losses or reconnections. Two operational control modes are considered. CONCLUSION In the micro-grid, the top question is how to accurately and rapidly detect the harmonic currents in order to inhibition and elimination of harmonics in low voltage distribution grid; so at first principle of kinds of reactive and harmonic current detection methods are described. Then a control strategy is presented for microgrids in islanded mode where the distributed power sources are connected to the grid via VSIs. Then after an improved reactive power sharing method in microgrid was discussed. The presented method adopts real power disturbance to identify the errors of reactive power sharing and then compensates the errors using a slow integral term. In next stage the microgrid is controlled by a central iNode and several iSockets for each unit. The proposed method for reactive compensation is based on local measurement as well as the power flow in the lines. It is shown that the proposed method reduces the voltage drop more effectively while maintaining the voltage regulation with a high penetration of the DGs. And at last the Intelligent Operation of MICROGRID in Distribution system using Wireless Technology system proposed, because it seemed the best solution for the problem of present day electricity metering. 1995

Intl. Res. J. Appl. Basic. Sci. Vol., 4 (7), 1991-1997, 2013

Figure 3. Microgrid functional block description.

The centralized control mode suggests that a central node, iNode, collects the microgrid measurements (sent from iSockets) and decides next actions according to the utility goals. The distributed control mode suggests that advanced controllers are installed in each node forming a distributed control system. These controllers are completely independent from the superior nodes and only use the electric market price for the regulation (Colet-Subirachs A, et al., 2012). The main idea ofreactive compensation in the method lies in regulating compensation based on power flow and voltage in the line. In a microgrid with frequent load switching and variable DG power output, the real and reactive power flow varies in the feeders and so does the voltage. To achieve a faster control of voltage profile, it is beneficial to consider power flow in DSTATCOM control (Ritwik M. 2013). A SOLUTION FOR DATA ACQUISITION AND CONTROL OF MICROGRID USING WIRELESS TECHNIQUE Measuring the various parameters of a mains power supply network has become a crucial. Various circumstances might change the network parameters, such as demand fl actuations, transformer defects, appearance of harmonics, atmospheric disturbance, etc. Even a 1/4 second voltage sag is sufficient to bring our modern machines to a grinding halt, resulting in hours of interrupted production and irrecoverable scrap. The need of obtaining all possible parameters on the three phase or single phase current, whether of an industrial facility (three-phase) or household power supply (single phase) has spawned the development of different data-capturing systems. In the early days of such devices they were based on electromechanical principles; now, however, there are electronic systems available for carrying out the necessary functions on the basis of an analogue-digital conversion (ADC) in which the calculations are made digitally. Many geographically scattered points need to be monitored to gain a complete overview of the state of a power Supply network, and these information then needs to be processed in a remote central unit, if possible in real time. This calls for monitoring networks to be designed with their most appropriate communication media. The ZigBee protocol has recently come to be considered a good candidate for use in industrial environment. This paper supports the implementation of the Intelligent Operation of MICROGRID in Distribution system using Wireless Technology system because it seemed the best solution for the problem of present day electricity metering. We have tried to give as much as possible to open an eye on the endless capabilities of such system. This paper presents a system for reading the mains voltage parameters, whether single- or threephase, and using the ZigBee protocol for communicating with other elements. The device presented herein can be considered as the first step towards fulfilment of a more ambitious objective: a decentralized, personal automaton interconnected by ZigBee. REFERENCES Colet-Subirachs A, Albert Ruiz-A´ l, Gomis-Bellmunt O, Alvarez-Cuevas-Figuerola F, Sudri`a-Andreu A.2012.” Centralized and Distributed Active and Reactive Power Control of a Utility Connected Microgrid Using IEC61850” IEEE SYSTEMS JOURNAL, VOL. 6, NO. 1, MARCH

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Intl. Res. J. Appl. Basic. Sci. Vol., 4 (7), 1991-1997, 2013 Jinwei H, Yun WL. 2011. “An Accurate Reactive Power Sharing Control Strategy for DG Units in a Microgrid” 8th International Conference on Power Electronics - ECCE Asia May 30-June 3, , The Shilla Jeju, Korea. Manigandan M, Dr Basavaraja B.2011.” Active and Reactive Power Control of MICROGRID using Wireless Technology (ZigBee 2.4GHz)” Chennai and Dr.MGR University Second International Conference on Sustainable Energy and Intelligent System (SEISCON 2011) , Dr. M.G.R. University, Maduravoyal, Chennai, Tamil Nadu, India. July. 20-22, Ritwik M. 2013 ” Reactive Power Compensation in Single Phase Operation of MicroGrid” IEEE Transaction on Industrial Electronics, Vol. 60, No. 4 Tine L. Vandoorn, Bert Renders, Bart Meersman, Lieven Degroote and Lieven Vandevelde, “Reactive Power Sharing in an Islanded Microgrid” UPEC2010,31st Aug - 3rd Sept 2010. Wanfang Liu. 2011. “Optimal Control of Power Quality in Microgrid Based on Reactive and Harmonic Current Detection Methods” 978-14244-9690-7/11© IEEE.

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