Microgrid Design and Control Using a Discrete Proportional Resonant Controller

Microgrid Design and Control Using a Discrete Proportional Resonant Controller Sriari Mandava *, Rames Varadarajan ** Scool of Electrical Engineering, VIT University (mandavasriari@vit.ac.in, vrames@vit.ac.in) Received: 14.07.2015 Accepted:19.08.2015 Abstract- Tis paper presents a Proportional Resonant (PR) controller design in discrete domain for regulating te active and reactive power output for a tree-pase AC Micro-Grid system. Te PR controller reduces te steady state error and elp in syncronous d-q transformation in tree pase system. It employs a Voltage Sourced Converter (VSC) wic is configured to operate as a current source troug an interface L-filter. Te power is controlled indirectly by controlling te inverter s output current. Tis paper also presents a comparison of THD between Proportional Integral (PI) and Proportional Resonant (PR) controllers and indicates tat te THD using Proportional Resonant (PR) controller is less tan te Proportional Integral (PI) controller by more tan 2%. Te complete work is designed and implemented in MATLAB/SIMULINK. Keywords- Microgrid, microsource, Proportional Resonant, Proportional Integral, controller, THD 1. Introduction Te global call to reduce CO2 emissions in te generation of electricity, re-organising te electrical market and te advancement of tecnology in te development of micro-generation lead to te growing interest in te use of microgeneration. Te main attributes of Microgrid are generation sources, loads and energy storages. It is a collective work of a small generation system, a low voltage distribution network and fast acting power switces and devices for connecting te load. Microgrids are designed and developed for some special functions. Tey are mostly used for small urban areas or in small industries for meeting te electricity requirements. Various microgrids generate power ranging from 25 to 100 kw [1]. Tere are also systems wic are used widely aving lower and upper power levels. Some of te energy sources tat are used in micro-grid for electricity generation are diesel, gas, fuel cells and renewable sources like wind, solar etc Te microgrids generate power at lower cost and emit less amount of gases wen compared wit traditional power sources. Since te generating units are small in size, tey can be placed very conveniently for cooling, distribution and maintainance of te installation. Tere is an enormous increase in small scale generation. Tere is a need of connecting te loads to tese small scale generating units togeter wit te utility [2]. Te inverters play te role of connecting te generating units to te distribution network. Microgrids can work in offgrid mode and grid connected mode. In off-grid mode, te generated power is stored witout te elp from te main grid. Tese type of grids are equipped wit batteries and nearby loads connected to one or multiple energy sources. In grid connected mode, te microgrid is connected to te main grid using power electronic devices to syncronise wit te main grid [3]. Tis connection called grid-connected mode, is te main operating mode. Te microgrid ten can be used as back-up system wic can store te power using batteries or as an assistance to te main utility system wen te power demand is ig on main grid. Tere is anoter mode of microgrid operation called emergency mode, in wic te local loads are fed using microgrid alone wen te main grid fails under some circumstances.under grid connected mode, te microgrid is connected to te main grid using a power source and a large battery. Te excess of te energy generated is stored in te batteries or super capacitors and support te main grid wen te load demand on it increases. Te size and type of te batteries depend on te system s configuaration. Wen te main grid is in normal operating state, te microgrid carges te battery bank to full carged condition so tat te batteries can be used during emergency mode. Wen te demand is more on main grid, ten te microgrid operates along wit utility system wen feeding te local loads. Wen te generated power from te microgrid is greater tan te demanded power level, extra generated energy is sent to te main grid. In te oter way, if te energy generated from microgrid is not sufficient to supply its local loads, te main grid elps in compensating te deficient energy. Most of te loads operate on AC power and tis requires te inverters to invert te DC energy generated from te sources. Te battery inverters are required to invert and also control te energy flow [4]. 1041

Te microgrid system components are identified based on teir function. Units like grid forming units, grid supporting units and grid parallel units are present[5]. Grid formimg units are used for controlling te voltage and frequency of grid by stabilizing te loads and sources. Diesel generators and battery inverters are some of te grid forming units. Te oter unit i.e. te grid supporting unit is very simple unit wic is used for grid control. Te active and reactive power of tese units depend on te connected system s voltage and frequency. Te grid parallel units comprises of loads, renewable energy source generators like wind energy converters and potovoltaic systems. Tese units are required to generate te maximum power from te available sources [6]. Te impact of microgrid on environment is very less wen compared to termal or ydro stations. Te use of microgids reduces te gas ejections into te environment and elps in forming a green eco-system. According to te report Microgrids-te Future of Small Grids [7] decentralizing of power production reduces te consumption of fossil sources tan te present consumption. Te most positive features of microgrids are te location of te plant being close to te load and aving low generation and distribution voltage level. Because of tese, te electricity supplied is more secure and relaible besides aving low power loss in network. Also, it reduces te cost of transmission and distribution of electrcial energy [8]. Tere wont be any need to invest in transmission and large scale generation. Tis drops down electricity prices because of more extensive use of transmission and distribution networks. All te controlleres so far designed in te literature are analog controllers. But in future, te wole worls power system will be canged to smart grid in wic te digital circuits plays te key role in controlling and monitoring te micro-grids. Tis as brougt te idea of designing te discrete controlleres for te operation of micro-grid. Tis paper presents a microgrid model of 100 Kw. Te MG components wic ave been studied are te Potovoltaic Array, MPPT control of boost converter, current controlled VSC, Proportional resonant controller. Te discrete Proportional Integral (PI) and Proportional Resonant (PR) current controllers are used for controlling te microgrid and ten compared. 2. System Description 2.1. Potovoltaic Array Potovoltaic generation is a tecnique wic converts te eat energy of sunligt to electrical energy. PV tecnology is well establised and is widely used to supply electrical energy for te remote areas from te distribution network. Te inverter consists of a Maximum power point tracking ( MPPT) circuit for getting te maximum power from solar panel, Energy storage element, like a capacitor or battery, DC-DC converter for boosting te PV output voltage, DC- AC inverter, Isolation transformer to stop te injection of DC into te network, Filter circuit to filter out te armonic currents of network. 2.2. MPPT Controller Fig 2.1. Basic Microgrid. Te MPPT controller make use of Incremental Conductance Integral Regulator tecnique to optimize te switcing duty cycle [9]. In figure 2.1, te DC/DC boost converter is te power electronic interface. Te duty ratio for te converter is obtained from te MPPT controller. Incremental Conductance(INC) metod is used for better result. An incremental algoritm principle elps in decreasing and increasing te control variable appropriately by associating te PV module conductance (I/V) to its augmenting conductance (di/dv). Maximum power point is obtained wen te potovoltaic (PV) generation. dp = 0 were P = V I dv substituting P = V I in (1) d(v I) = I V dv di I = dv V di dv = 0 (2.1) (2.2) (2.3) Te integral regulator minimizes te error (di/dv I/V)INC as iger tracking performance compared to perturb and observe (P&O) algoritm. Te transient of te output is better wit INC MPPT wen te irradiance and cell temperature are constant [9]. Te tracking efficiency result is 99.73% wit step size 0.02% wen a step cange of irradiance and temperature are applied. Te output current as low frequency ripple content depending on te rate of te dc link capacitance alone wit te size of te step tat is utilized in alteringte duty cycle converter.te step size of te MPP in te PV array is adjusted automatically using te 1042

variable step size INC. Tis improves te MPPT speed and accuracy togeter [9]. 2.3. DC/DC converter stage Te boost converter [10] is used to increase te output power of te PV array. Te design of tis circuit is simple. Any algoritm of maximum power point can be used to implement wit software and ardware. Te boost converter circuit is sown in figure 2.3. Boost converter can operate in continuous and discontinuous conduction mode. Te conduction mode depends on te amount of energy tat can be stored wit te relative switcing time frame. Te output voltage varies wit te duty cycle wic in turn is adjusted by using te maximum power point controller. Te boost converter designed in [11] as all possible duty cycles and works for all canges in te irradiations of te PV array. 2.4. DC/DC converter stage It sould cange te DC current to a sinusoidal waveform wit a lower amount of armonics. Te PV array as to finally provide a AC power to te grid. Te topology of te inverter to be cosen depends on te application like standalone PV system or grid connected. Te oter factors tat decide te topology are te power output of te PV, te current armonics and te cost. Wen te PV panel is connected to te grid, te inverters sould ave island detection, power quality witin te standards, grounding, etc. Te DC/AC inverter as some many topologies in literature like frequency-commutated current source inverter (CSI), a full-bridge multilevel inverter like diode clamped tree-level VSI, etc. Te line commutated inverters are proved to be robust, ceap and ave more efficiency but as a low power factor from 0.6 to 0.7. Te self-commutated inverters are more popular as tey are capable of ig switcing frequency, ceap and robust. But, due to ig switcing frequency, it gives more losses in semiconductor. Te line frequency-commutated inverter uses a sinusoidal signal to give an AC output. Te drawback of tis are te armonics wic can cause series resonance wit te capacitors present in te system. Te full-bridge inverter is widely used in PV system. Figure 2.4 is te tree-pase full bridge inverter. Te command of te switc depends on te modulation scemes to obtain te sinusoidal output. Fig 2.4: Dc/Dc converter 3. Current Control for VSC Fig2.2: Flowcart of Incremental conductance metod [9] Fig 2.3: Dc/Dc converter 3.1. Conventional PI Controller A conventional PI controller is defined by te transfer function K I G pi = K P 3.1 S were kp and ki are te proportional and integral gains, respe ctively.a controller conventional form can be implemented in eac of te two SRF axes to acieve zero steady-state error. Te syncronous frequency ωo for te Park transformation sould be adjusted to coincide wit te frequency of te armonic order to control, tat is, ωo = ω1, wit ω1 being te fundamental frequency. For 1043

negative-sequence components, ωo = ω1 sould be considered. Te PI controllers for eac armonic order can be tuned wit independent proportional and integral gains (kp and ki, respectively): 3.2. DC/DC converter stage Te equivalent transfer function in stationary frame of a PI controller implemented in a positive seuence SRF can be obtained by application of a frequency sift of w1 at all frequencies tat is, by substitution of s s - jω1 in 2.1. KI GPI = Gpi ( S jω 1) = KP 3.2 S j On te oter and, te substitution s s jω1, wen ωo=-ω1 sould be applied in order to obtain te transfer function in stationary frame of a PI controller implemented in a negative-sequence SRF G PI = G pi ( S j ω1 ) = K P Addition of 3.2 and 3.3 leads to G PR = G PI G PI K = KP 2 S I 2 2 ω1 ω1 K I S j wic is te transfer function of a PR controller. GPR(s) pr ovides infinite gain in open loop at te resonant frequency ω1, so tat unity gain and zero pase sift in closed loop (zero steady state error) is acieved at te frequency. It sould be remarked tat to implement wit additional dampi ng terms sould be avoided. Te resonant term R1(S) is te part of a GPR(s) controller tat provided infinite gain at te resonant frequency. R1(s) is preferred as a Laplace transform of a cosine function tan a sine function as it provides better stability. 3.3. Discretisation of PR Controller Most studies devoted to resonant controllers ave been carried out in te continuous domain [12] [13]. However teir observations and conclusions cannot be directly applied to digital devices in te discrete time domain. So, performance of PR controller is analysed in discrete domain forward and backward are used for discretisation of PR controller. Forward and Bacyward Euler discretisation implies 1 Z S = Z T s 1 Z S = T s ω1 3.3 3.4 G f & b PR = K P = K P = K P K 2 2 1 1 w I 1 z T 1 z Ts s 1 z K I Ts z (1 z ) 2 2 2 (1 z ) z w T z K I Ts ( z z ) 2 2 2 1 z ( w T 2) z 1 s z Ts 2 1 s 2 3.4. Advantages of PR controller over PI controller Grid connected PV Inverter systems ave become more popular in te modern power system and te number of suc systems connected to te main grid are increasing from time to time.hence, te armonics generated by tese systems troug te power electronic devices are to be reduced wic can reduce te quality of te power. PI controller cannot be used for tis as it can not go along wit te reference of a sinusoidal wave witout aving steady state error due to te canges in te integral term. Tis necessiates te use of te grid voltage as a feedforward term wic elps te controller to try te steady state at fast rate and obtain a good dynamic response. Tis drawback leads to development of PR controller. 4. Results A microgrid of 100 KW consisting of potovoltaic generation connected to te grid is developed as sown in te figure 4.1. and simulated in MATLAB/Simulink development software. Te PR controller and PI controller compensator are implemented for te inverters output current regulation. Te parameters used for te simulation are sown in table 4.2. Te THD is less for for te current and voltage waveforms using te propotional resonant controller tan te propotional integrator controller.te power, current and voltage waveforms obtained using PR and PI controllers from te utility grid, solat and at load are as sown below. In te figures 4.2 to 4.10, PR is for Proportional Resonant Controller and PI is for proportional Integrator Controller. Te microgrid is simulated for different types of loads like a pure resistive load and also for reactive loads wit resistive load. In all te results of figures 4.2 to 4.10, it observed tat te propotional resonant controller performs better tan te propotional integrator controller to fulfil te load requirements. For example from te figure 4.10, wic are te results simulated for te load of 100 KW 75 KVAR, te propotional resonant controller meet te exact lood of 100 KW and 75 KVAR in te figure 4.10 were as te propotinal integrator controller just meets te load of 90 KW and 71 KVAR at te load terminals. Tis beaviour is observed for oter load conditions also wic can be sseen in te figures 4.2 to 4.10. 2 3.5 Te solar irradiance for solar power generation is considered as 1000 w/m2 upto 0.7 sec and ten reduces to 1044

550 w/m2 from 0.7 sec to 1 second. Accodingly, te output power from te solar grid canges i.e. te current from te solar grid comes down at 0.7 seconnds and inturn te power also reduced from 0.7 seconds. Tis decrease in current and power from solar grid to te load is compensated from te utility grid and at last te load requirement is met wit te co-ordination of bot solar grid and utility grid. For exaple, in te figure 4.9, te current from te solar grid reduces from 450 A to 350 A starting frm 0.7 sec to 1 second and tis reduction ofcurrent is compensated wit te increase in current from utility grid from 800 A to 1000A to meet te load requirements. Te results obtained for different load variations are as sown below from te figures 4.2 to 4.10. Also, Te te current, voltage and te power signal comes to steady stare value in less time using propotional resonant contrller tan te PI controller. From te figure 4.10, it can be observed tat te real power and reactive power at load settles to constant value at 0.1 second using propotinal resonant controller were as it takes more tan 0.2 seconds using te propotional integrator controller and also te output using proportional integrator controller is in oscillation mode till te last moment of time. For load = 100 KW Fig 4.2: Voltage, Current and Power from Utility to Load Table 4.1 Comparison between THD values S.no THD PI controller PR controller Fig 4.3: Voltage, Current and Power from Solar grid to Load 1 Current 0.0845 0.064 2 Voltage 0.0643 0.0438 Fig 4.1: Simulink model of 100 KW microgrid Fig 4.4: Voltage, Current and Real Power at Load 1045

INTERNATIONAL JOURNAL of RENEWABLE ENERGY RESEARCH S.Mandava and R.Varadarajan, Vol.5, No.4, 2015 For load = 100 KW and 75 KVAR For load = 100 KW and 5 KVAR Fig 4.5: Voltage, Current, Real Power and Reactive Power from Utility to Load Fig 4.6: Voltage, Current, Real Power and Reactive Power from Solar grid to load Fig 4.8: Voltage, Current, Real Power and Reactive Power from Utility grid to Load Fig 4.9: Voltage, Current, Real Power and Reactive Power from Solar grid to Load Fig 4.7: Voltage, Current, Real Power and Reactive Power at load Fig 4.10: Voltage, Current, Real Power and reactive Power at Load 1046

Table 4.2 :Parameters used for microgrid simulation. Solar Panel Parameters Series connected Series Resistance =0.037998 modules=5 Parallel Strings = 66 Parallel Resistance =1.175 e-08 Ω /module Voc = 64.2 V/module Isat = 5.9602 A/module Fig 4.11: FFT analysis of Pase A Microgrid voltage(vamicrogrid) using PI controller Isc =5.96 A/module Qd=1.3 Boost Converter R = 0.005 Ω L=5e-3 H, C= 12000e-06 F RL Filter R= 2e-3 Ω L= 250e-6 H Vdc regulator Gains Kp= 7 Ki=800 Current Regulator gains Kp = 0.3 Ki = 100 References Fig 4.12: FFT analysis of Pase A Microgrid Voltage (VaMicrogrid) using PR controller 5. Conclusion In tis paper, te potovoltaic system is developed and an incremental conductance wit integral regulator is used to track maximum power from it. Ten, te design of te boost converter, PR current controller for DC-AC Converter is designed. Wen te irradiance varies, te PV models output current cange. Te complete system simulations of te wole system wit maximum power point, boost converter and VSC were performed by varying te Local load, te irradiance and te temperature. Tis paper as presented te effectiveness of using te Proportional Resonant (PR) control strategy to control active and/or reactive power transfer between te Micro-Grid and te transmission grid system. Te THD value of te current and voltage in te grid using PR controller are 6.4% and 4.38% respectively and are less by 2% tan te PI controller current and voltage THD. Te PR controller tracks stationary frame reference currents calculated from te active (PC(t)) and reactive (QC(t)) PI controller actuating power outputs using d-q frame power equations. Consequently tis improves te performance of te control loop as opposed to reference currents calculated directly from αβ frame power equations. Te PR controller tracks reference currents wit a very small steady-state error and reduced Total armonic distortion. Model development and simulations were done using te MATLAB/Simulink software environment. [1] Y HU,Z Cen,P Excell, Power Quality Improvement of Unbalanced Loads Tree Pase Distributed Generation Units. IEEE,978-1-4577-0365-2/11,2011 [2] Prabakaran.K,Citra.N, A.Sentil, Power Quality Enancement in Micro Grid- A Survey. IEEE,ICCPCT- 2013. [3] Liserre, M., Blaabjerg, F., and Hansen, S.: Design and control of an LCL-filter based active rectifier, IEEE Trans. Ind. Appl., 2001, 38,pp. 299 307 [4] M. Ebad and B. Song, "Improved Design and Control of Proportional Resonant Controller for Tree-Pase Voltage Source Inverter," in IEEE Conference, 2012. [5] P. Strauss and A. Engler, AC coupled PV ybrid systems and microgrids-state of te art and future trends, in Proc. World Conf. on Potovoltaic Energy Conversion, 2001, pp.2129-2134. [6] V. Pradeep, A. Kolwalkar, R. Teicmann, Optimized Filter Design for IEEE 519 Compliant Grid Connected Inverters, IICPE 2004, Mumbai, India, 2004 [7] Nikos Hatziargyriou, Microgrids-te Future of Small Grids, Financial Planning Services, National Tecnical University of Atens, Greece,2005. [8] Vang -Tung Pan and Hong-Hee Lee, Enanced Proportional-Resonant current controller for Unbalanced Standalone DFIG Based Wind Turbines. Journal of Electrical Engineering and Tecnology, Vol. 5, No. 3,PP 443-450,2010. [9] M. A. G. de Brito, L. P. Sampaio, G. Luigi, G. A. e Melo, and C. A. Canesin, ``Comparative analysis of MPPT tecniques for PV applications,'' in Proc. Int. C(ICCEP), Jun. 2011, pp. 99_104.onf. Clean Elect. Power 1047

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