Modeling and Performance Analysis of Microturbine Generation System in Grid Connected/Islanding Operation

Transcription

1 Sanjeev K nayak et al., Vol., No.4, Modeling and Performance Analysis of Microturbine Generation System in Grid onnected/islanding Oeration Sanjeev K nayak*, D N Gaonkar* *Deartment of Second Electrical and Electronics Engineering nayaksanjeev8@gmail.com, dngaonkar@gmail.com orresonding Author; Sanjeev K nayak, Deartment of Second Electrical and Electronics Engineering, , nayaksanjeev8@gmail.com Received: 7.9. Acceted: 8.. Abstract- Distributed generation (DG) has drawn a great attention in distribution network due to reduction in transmission loss, load sharing roerty and imroving the ower quality. Among the different source of DG, the microturbine generation (MTG) system has a good record of imroving the system stability, reliability and ower quality. This aer resents modeling and erformance analysis of MTG system in grid connected and islanding modes of oeration. The model develoed in this work includes the individual comonents of rime mover like, comressor, heat exchanger, burner and turbine. The model of MTG system consists of microturbine, ermanent magnet synchronous machine (PMSM) and ower electronics interfacing circuit for generation and conversation from /D/ resectively. The MTG system uses a D link voltage to control the microturbine outut ower by fuel and air flow control methodology. The D link ower is delivered to the load through a voltage source inverter (VSI) with ulse width modulation (PWM) technique. The model of MTG system has been imlemented using Matlab/Simulink environment and its simulation result shows the load following erformance of MTG system for various loads. Keywords- Distributed generation, Microturbine, Permanent Magnet Synchronous Machine, Power Interfacing ircuit.. Introduction Distributed generation (DG) refers to a small scale generation, mainly between kw and 5MW electrical ower generators that roduce electricity at a site close to the customer or they tied to an electric distribution system. DG enables the rural electrification, industry to reduce the traditional cost of service by skillfully integrated system. In addition to that, DG enables the utilities to exand their service to include roviding a base load ower, thermal energy and value added energy services []. The small scale DG system based on microturbine technology are gaining oularity among the ower industries and utilities in the last few years due to their several advantages as reorted in[]. The MTG system can generate a ower in the range of 5kW to 5kW and can be oerated in stand alone, mobile, remote or interconnected with fuel utility alications. The MTG system generated ower can be used in the wide range of alications. Some of the alication are base load ower, eak ower saving, combined heat and ower etc. The MTG system is a new and fast growing technology and becoming a business and likely to become a dominate source of DG in the future ower suly network. Thus, the dynamic modeling and system lanning interconnected oeration and management are essential. Hence to ensure safe oeration and security of system MTG system must be seriously taken into consideration. There are several models and simulation of a MTG system in grid connected as well as isolated modes of oeration has been reorted in [3-]. The modeling and simulation of MTG system for isolated and grid connected oeration has been reorted in [3]. In this, The MTG system uses a PMSM and ower electronics interface for generation and delivering the ower to load resectively. Also, the microturbine uses a reference seed and shaft seed to control the MTG system outut ower. The modeling of

2 Sanjeev K nayak et al., Vol., No.4, existence microturbine and advanced controls for both grid connected and isolated mode of oerating has been develoed in [4]. The model develoed in [4] uses an active rectifier to control the D link voltage. In this, active reactive ower (PQ) control inverter for grid connected oeration and voltage, frequency (V/F) control for isolated oeration. The develoment and simulation of microturbine model to analyze the load following erformance with general as well as critical riority for the load is erformed in [5]. In this, the two models of microturbine are considered, one is to oerating under normal condition (neglecting fast dynamic) and other is based on standard GAST model. The model develoed in [5] uses a synchronous generator (SG) and field excitation control to maintain the constant voltage. The detailed models of the comonents and control forming the thermo mechanical and electrical subsystems of microturbine ower lant are reorted in [6]. The model develoed in [6] includes different control loos are develoed for various control system. The model of MTG system in grid connected and islanding oeration has been reorted in [7]. In this, the model uses a D link voltage to control MTG system outut ower. The dynamic model of MTG system in grid connected and islanding oeration has been given in [8]. In this, the MTG system takes ower from the grid to start the MTG system, and bring to ignition seed. Once the MTG system reaches to ignition seed it sulies the ower back to grid using a bidirectional ower flow control has been reorted in [8]. The MTG system used to assist the transient stability in connected to the grid via two aths control of mechanical ower and terminal voltage using static var comensator (SV) and genetic algorithm are given in [9]. In this, two microturbine generations are connected to the grid and uncontrolled system simulation results shows unstable system where MTG oscillates between generation and motoring modes. The mathematical model which includes microturbine, PMSM, a three hase bridge rectifier, boost converter and inverter are develoed using PSAD/EMTD in [, ]. The model develoed in [], is suitable for transient study and analysis for islanded mode of the microgrid as well as grid connected oeration. In [], the designed MTG system with control system is caable of regulating its outut ower in grid connected mode. Also, the designed MTG system is suitable for isolated oeration and ower electronics controller is used to maintain the constant outut voltage at rescribed limit. In this aer, the modeling and erformance analysis of MTG system in grid connected and isolated mode of oeration has been studied. The develoed model uses a D link voltage to control the MTG system outut ower. This D link controlled MTG system can maintains a constant D voltage which acts as an infinite source for the inverter oeration. The model develoed for this study has been imlemented in Matlab/Simulink environment and simulation result shows the load following erformance MTG system.. Microturbine Generation(MTG) System The MTG system includes microturbine as a rime mover, generator and ower electronics interfacing circuit to deliver ower to the load. Figure. shows the schematic of MTG system in grid connected oeration... Microturbine Microturbines are the evolution of gas turbine technology with small in size, low inertia and high seed of rotation. The ower generation caacity of microturbine varies between 5 to 3kW and the rotational seed varies between 5, and 9, rm with air foil bearings and interfaced to load through ower electronics. Basically there are two tyes of microturbine, one is a high seed single shaft design with the comressor and turbine are mounted on the same shaft usually a ermanent magnet synchronous machine is used to generate the electrical ower. The generator generates a ower at high frequency between.5 khz to 4 khz. The high frequency voltage is rectified and then inverted back to 6Hz ower using a suitable ower electronics circuit. Another is slit shaft design that uses a ower turbine rotation between 36 rm and conventional generator (usually induction generator or synchronous generator) connected via a gearbox. The ower electronics interface in not required for this design. The turbine will be controlled by the gear transmission along with acceleration control, fuel flow control and temerature control [3, 8]. ontroller + - PLL Fuel Valve + Burner V dc ref ontroller Rf Lf omressor Turbine Rectifier Inverter PMSM GRID Exhaust air Heat exchanger Ambient air Load Fig.. Microturbine generation (MTG) system 75

3 Sanjeev K nayak et al., Vol., No.4, The model develoed for this sturdy consist of each comonent of microturbine connected to their inuts and outut to MTG system connected to isolated and grid connected mode. The mechanical ower required driving the comressor for air ressurize[] can be calculated with mathematical equation, P c Where, th, c m' c PR c T in_c is the inut temerature (K), PR c is the ressure ratio of comressor, m c is the mass flow rate of comressor(g/s). γ is the secific heat ratio of air, T ɳ th,c is the thermal efficiency of comressor is the secific heat ratio of air (KJ/kg,K) m' c mass flow of comressor(g/s). in _ c The combustion chamber is normally involves heat roduction and increasing the air ressure by burning a fuel. The combustor used in this work burns the (un-used) waste fuel of SOF with addition of fresh fuel to increase the ressure further. The energy balance equation of combustion rocess can be written as first order equation where the last term is the heat transfer to the ambient []. dtb dt m b b o o N o o r h f h T h r N h f ht h Axhx Tb Where, N r is a molar flow rate of reactant, N is the molar flow rate of the outut of roduct, h o (T) is the sensible enthaly which is a function of temerature of function and can be defined exerimentally. The mechanical ower roduced by the turbine can be calculated using the equation []. Pt t PRt T Where, T b is the turbine temerature (K), PR t is the turbine ressure ratio of turbine, is the secific heat of roduced in the burner which is a function of temerature (KJ/kg.K), m is the mass flow rate of the roduct(g/s). The shaft mechanical ower is the difference of the ower required to drive comressor and turbine ower. The equivalent shaft mechanical ower is converted to torque to drive the PMSM are exressed as, P T mt mt P P t Pmt c Where, P c is comressor mechanical ower (W), b () T N T m b amb () (3) (4).(5) P t is generated mechanical ower of turbine (W), P mt is the mechanical ower of microturbine (W), T mt is the mechanical torque of microturbine (Nm), ω is the seed of microturbine(rad/s)... Permanent Magnet Synchronous Machine The microturbine generates electrical ower via a high seed PMSG, directly driven by the turbine rotor shaft. In this work, the model adoted for the generator is a ole ermanent magnet synchronous generator (PMSG) with nonsalient rotor. At.6 khz (6, rm), the rated outut ower generated by the machine is 6kW and its terminal line to line voltage is 55V. The equivalent circuit of PMSM without considering the iron loss is shown in Figure. The model assumes that the flux established by the ermanent magnet in the stator is sinusoidal, which imlies that electromotive forces are sinusoidal. The following equation exresses in the rotor reference frame (dq frame) are used to imlement PMSM [3]. v d R s (a) L q r i q - + L d Fig.. d q-axis equivalent circuit of PMSM did vd Rsid Ld Lq riq dt did vq Rsiq Lq Ld rid r dt 3 Te i q Ld Lq i diq Where, L q, L d are the q and d axis inductance (H), R s is a stator resistance (Ω), i q, i d are q and d axis current(a), v d and v q are d and q axis voltage(v), is the number of oles, T e is electromagnetic torque (Nm), λ is the flux linkage of PM reference to stator(wb). Table. PMSM Parameter Parameters Values Rated outut ower 6kW Rated voltage 55Vrms Rated current 63Ams Rated seed 6,rm Number of oles Stator resistance.ohm Ld and Lq Inductance 58.µH Stator flux linkage.77685wb Rotor e-3kg/m v q R s L d r i d r (b) L q (6) (7) (8) 75

4 Sanjeev K nayak et al., Vol., No.4, 3. MTG System ontrol The control system lays an imortant role for the reliable oeration MTG system under all circumstances, it must be able to resond and function roerly for any change in the external system. In the develoed model of MTG system there are two ossible controllers. One is fuel flow controller and another is air flow controller for the microturbine. When the demand ower changes, it will changes the microturbine outut ower by controlling the fuel and air flow. As the D link voltage changes, it will change the fuel and air flow corresondingly. Because of this reason, D link voltage is used to control the fuel and air flow, the control system used for MTG system is shown in Figure 3. The control system imlemented in Matlab/Simulink can maintain the constant D link voltage by fuel and air flow control for the burner of microturbine [7]. Fuel Air Valve Valve K PI (controller) Burner Turbine PMSM Fig. 3. Block diagram of MTG system controller _ + D V dc ref + - V dc terms are used to obtain indeendent control of i d and i q. A PLL is used to synchronize the converter with the grid frequency. The hilosohy of the PLL is that, the difference between the grid hase and inverter hase angel can be reduced to zero using PI controller and locking the line side inverter hase to grid [8]. 6. Isolated mode For the isolated mode of oeration of MTG system, the ower conditioning unit consists of a three hase diode rectifier and a voltage source inverter (VSI) with L filter are used. The control system for a voltage source inverter imlemented using Matlab/simulink is shown in Figure 4 (b). A.66 khz, voltage source feeds a 6Hz, 5kW load through an /D/ converter. The high frequency ower is first rectified by six ulse diode bridge rectifier and filtered. A D link voltage is given to an IGBT two level inverter generating 6Hz. The IGBT inverter uses Pulse Width Modulation (PWM) with khz carrier frequency. The voltage is regulated at.u. (48 V rms ) by a PI voltage regulator using abc to dq and dq to abc transformation. The conventional converter regulates the outut voltage by a vector containing the three modulation signals used by the PWM generator to generate six IGBT ulses [8]. In this work the outut voltage is regulated by the MTG system. 4. Power Electronics Interface The ower electronics interfacing is a critical comonent for the single shaft MTG system and its design reresents significant challenge, secially in matching MTG system outut ower to utility ower. There are different configurations available to interface the MTG system outut ower connected to utility. One ossible is to use a three hase diode rectifier and voltage source invert with filter. This tye of conversion requires a searate start-u arrangement for the microturbine. The configuration used for this work is assumed to be brought to rated seed(start-u and shut-down arrangement are not considered) for the isolated mode of oeration and bidirectional ower converter has been used for the grid connected mode of oeration [8]. 5. Grid onnected Mode The control structure for grid connected MTG system imlemented in Matlab/Simulink is shown in Figure 4(a). The grid side converter oerates as a controlled ower source and standard PI controller are used to regulate the grid current in the dq synchronous frame in the inner control loos. The converter does not take an account to maintain D link voltage constant by regulating i d and i q. The i d reresents the active ower comonent injected current into the grid and i q is reactive comonent. In this work i d and i q reference values are given and they will fallowed by the actual values by the hybrid DG system. In order to obtain only a transfer of active ower only a transfer of active ower, the i q current reference is set to zero. The decouling (a) (b) Fig. 4. (a). Line side converter control, grid connected, (b). onverter control, isolated mode 7. D link caacitor A D link caacitor is used to maintain the constant D voltage due to sudden load change. The sudden di in D link voltage may tri the inverter oeration. The calculation for size of the D link caacitor for active filter is based on controlling the voltage change of D link as shown in Figure.5 (a). It is assumed that, the active filter is roviding comensation at the fundamental line frequency for dislacement factor correction, then caacitor is charged and discharged twice er line cycle and the charging and 753

5 Sanjeev K nayak et al., Vol., No.4, discharging time are even. Based on changing and discharging balance [4], the following equation is used to determine the caacitance value for D link caacitor. D link D D D link (a) (b) Fig. 5 (a). D link caacitor, (b). RL filter Where, I f,rms the filter rms current(a), f is the fundamental frequency(hz), V d is the D link voltage (V). The relation between the D link voltage and load voltage can be calculated using the equation, V d d Where, V dc is the D link voltage (V). La Lb I f, rmst I V 4 fv 3 f, rms dc V Load V Load is the load voltage (V). D Ra Rb Lc Rc RL Filter d (9) () of the currents of the inverter/active filter [4] as shown in Figure 5 (b). The inductor filter size is designed to limit the rile current at switching frequency, and D link voltage for line to line outut voltage can be calculated by, Vdc Lf f V i Where, V dc is the D link voltage (V) V ab_k is the eak voltage value(v) Δi is the rate of change of current (A) Fs is the switching frequency (Hz) 9. Simulation Results s abk () The model of microturbine used in this work has been imlemented using Matlab/Simulink as shown in Figure 6. This develoed model of microturbine is used for a MTG system is shown in Figure 7. The microturbine includes comressor, heat exchanger, burner and turbines which are mechanical comonents used a rime mover of generator. The distribution network is considered as three hase balance sinusoidal voltage source with 48Vrms and 6Hz. The resistive load of 5kW is considered for the isolated mode study. It is assumed that, the turbine is brought to a rated seed and running at steady state (ex, start u and shunt down of MTG system is ignored). The erformance of the develoed model is studied in both grid connected and isolated oeration. 8. Filter Normally L filter are used for inverter ower suly in a grid connected mode, however it has some unique requirement that an L filter may not sufficient. The urose of the inductor filter is to filter the switching comonent out Fig. 6. Matlab/Simulink model of microturbine Fig. 7. Matlab/Simulink model of MTG system in connected grid/islanding 754

6 Sanjeev K nayak et al., Vol., No.4,. Grid onnected Mode 8 x 4 6 Active ower in kw Reactive ower in Var During the grid connected mode the MTG system is sharing the load ower is ascertained by the I d and I q values. The variation of D link voltage and current are shown in Figure 8. From the Figure 8. It is observed that, the D link voltage remains constant even if the current varies. The variable ower is shared by MTG system by controlling fuel and air flow to the burner. Thus, the variation of ower and additional ower sulied by the MTG system is shown in Figure 9. The active and reactive owers are the most imortant arameters of the DG system for load sharing and ower flow. Power Fig.. Active and reactive ower Id ref Id act 8 9 Id (.u).5 D link Power in kw Fig. 8. D link voltage and current Fig. 9. Demand and turbine ower urretn in (A) Voltage in(v) Vdc ref, (V) Sualied ower Demand ower The model of MTG system uses a PMSM, hence the system is made to suly only the active ower, not to suly the reactive ower. Thus, the variation of active and reactive ower is shown in Figure. The d and q axis currents variation I d and I q are shown in Figure(s) and resectively. From the Figure(s) and it is obtained that the actual I d current follows the reference I d and actual I q current follows the I q reference current. The instantaneous value of load voltage and current are shown in Figure(s) 3 and 4 resectively. Load voltage & current Iq in (.u) Voltage and current.u Fig.. Id current Fig.. Iq current Fig. 3. Load voltage and current Iq act Iq ref Va Vb Vc Ia Ib Ic Fig. 4. Load voltage & current 755

7 Sanjeev K nayak et al., Vol., No.4, Isolated mode: The load following erformance of MTG system for an isolated mode of oeration is redicted on the basis of torque and fuel flows to the microturbine shown in Figure5. and 6. resectively. It is observed from the Figure5. and 6. that, turbine, comressor, mechanical owers and fuel flow are increases with increase in the load. For this study, the load variation is made in stes of kw with resect to time. It can also be observed that, the ower and fuel demand are decrease with decrease in load. Thus the develoed model of MTG system is caable of following the load variation with the system ratings. Since the develoed model of MTG system uses a D link voltage to control the microturbine outut ower. The transient and steady state formation are observed during the load change in D link voltage is shown in Figure7. Turbine ower(w) Fuel flow (g/s) x 4 Fig. 5. Turbine ower Fig. 6. Fuel demand The mechanical torque required to drive the generator varies with load, the turbine seed and torque, thus the turbine torque and seed variation of PMSM for a different load is shown in Figure(s).8 and 9 resectively. It is also observed from the reorted Figure(s) 8 and 9 that, there is formation of transients when load changes and settles to steady state after two seconds. The microturbine oerating temerature will also vary due to the variable flow of fuel. Thus the variation of turbine oerating temerature for a different load is shown in Figure. The instantaneous value of load voltage and current are shown in Figure. and. resectively. From Figure(s). and. it is observed that, the terminal voltage remains constant due to the seed variation of PMSM and current vary with resect to load. D link voltage (V) Seed in (rad/s) Turbine torque (Nm) Turbine temerature(k) Load voltage (V) Fig. 7. D link voltage Fig.8. Turbine seed Fig. 9. Turbine torque Fig.. Turbine temerature Mechanical torque Electromagnetic torque Fig.. Load voltage 756

8 Sanjeev K nayak et al., Vol., No.4, Load current (A) onclusion Fig.. Load current The modeling and erformance analysis of MTG system in grid connected and islanding modes of oeration has been studied using Matlab/Simulink environment. The MTG system uses a D link voltage to control the microturbine outut ower for a different load. Thus, the use of D link voltage controller for the MTG system, the load voltage can be maintained constant even if load current is varies. From the simulation studies it is conclude that, the MTG system could follow the load and vary there is variation of fuel flow, ower and temerature of system as er the load. The develoed model can be used for the study and erformance and analysis of electrical henomena that occurs in connected to the grid and isolated mode of oeration. This work initiates the integration of MTG system to the distribution network as it is mainly contributes to a clean, reliable and cost effective for the future DG systems. References [] Walier G. Scott, Microturbine Generation System for Distributed Generation, IEEE Industrial Alication Magazine, May/June 998. [] D. N Gaonkar, Sanjeev Nayak, Modelling and Performance Analysis of Microturbine Based Distributed Generation System, A Review, IEEE International onference on Energy Tech, ase Western University USA,,-6. [3] Sreedhar R. Guda,. Wang, and M. H. Nehrir, A Simulink-Based Microturbine Model for Distributed Generation Studies, IEEE Proceedings on Power Symosium, North America, 5, [4] Bertani,. Bossi, F. Fornari, S. Massucco, S. Selta, and F. Tivegna, A Microturbine Generation System for Grid onnected and Islanding Oeration, IEEE International onference on Power Systems and Exosition, 4,, [5] A.K.Saha, S.howdhury, S.P.howdhury and P.A.rossley, Microturbine Based Distributed Generator In Smart Grid Alication, IEEE th International onference and Exhibition on Electricity Distribution. Smart Grids for Distribution, IRD-9, -6. [6] Samuele Grillo, Stefano Massucco, Andrea Morini, Andrea Pitto and Federico Silvestro, Microturbine ontrol Modelling to Investigate the Effects of Distributed Generation in Electric Energy Networks, IEEE Systems Journal,, 4, [7] hi-hshiung Lin, Dynamic Simulations for Oeration Mode Transfer of a Micro-Turbine Generator, Journal of Technology, 8, 3, -. [8] D. N. Gaonkar, R.N. Patel, and G. N. Pillai, Dynamic Model of Microturbine Generation System for Grid onnected/islanding Oeration, IEEE International onference on Industrial Technology, 6, [9] Amer Al-Hinai, Karl Schoder and Ali Feliachi, ontrol of Grid-onnected Slit-Shaft Microturbine Distributed Generator, IEEE International Proceedings of 35th Southern Symosium on System Theory, 3, [] Huang Wei, Zhang Jianhua, Wu Ziing and Niu Ming, Dynamic Modelling and Simulation of a Microturbine Generation System in the Micro Grid. IEEE International onference on Sustainable Energy Technologies, 8, [] Gang Li, Gengyin Li, Wei Yue, Ming Zhou and K L Lo, Modelling and Simulation of a Microturbine Generation System Based on PSAD/EMTD, IEEE International onference on ritical Infrastructure,,.-6. [] O. Fethi, L.-A. Dessaint and K. Al-Haddad, Modelling and Simulation of the Electric Part of a Grid onnected Micro Turbine, IEEE Power Engineering Society General Meeting, 4, -9. [3] J.B Ahn, Y. H. Jeong, D H Kang and J H Park, Develoment of High Seed PMSM for Distributed Generation Using Microturbine, IEEE 3th Annual onference on Industrial Electronic and Society,4, - 6. [4] Leonard G. Leslie, Jr, Design And Analysis of A Grid onnected Photovoltaic Generation System With Active Filtering Function' Master Thesis, Virginia Polytechnic Institute and State University. Virginia,