A Prospective on Modeling and Performance Analysis of Micro-turbine Generation System

Arjun Tyagi, Student Member IEEE Dept. of Electrical Engineering

Gautam Buddha University, Greater Noida, India Email: desolatorready@in.com

Yogesh K Chauhan, Member IEEE Dept. of Electrical Engineering

Gautam Buddha University, Greater Noida, India Email: chauhanyk@yahoo.com

Abstract—Micro-turbine (MT) technology is becoming more popular and viable distributed energy source in the recent years. This is due to their advantages such as high operating efficiency, ultra low emission levels, low initial cost and small size. The integration of the increasing portion of MTG within the existing infrastructure requires a full understanding of its impact on the distribution network and its interaction with the loads and its location. This paper is a novel attempt to combine research update, and modeling and simulation of MTGS. Firstly, modern research efforts in such generation system that is capable to maintain continuity of supply when renewable source alone is not sufficient are presented . Different research issues related to MT size, location, operation, model and applications have been addressed. Secondly, the simulation results of MT system are presented by developing its simulink model. The system performance is studied at different operating conditions.

Keywords—Combined heat and power (CHP), Distributed generation (DG), Micro-turbine, Micro-turbine generation system (MTGS), Permanent magnet synchronous machine (PMSM), Power electronic interface.

I. INTRODUCTION he ever increasing demand for electrical power has created many challenges for the energy industry, which

can affect the quality of the generated power in short and long terms. The problem will imminently take a new form when bottlenecks occur in the transmission and distribution infrastructure. At the same time, the wide utilization of conventional fossil fuel-based sources will impact the quality and sustainability of life. The problem associated with the conventional sources is defining a new set of power supply requirements that can better be served through Distributed Generation (DG). Therefore, DG is gaining a lot of attention as it provides solutions for both short and long-term problems [1]. The distributed generation is the small-scale electric power generation technology. It is a modular (can be renewable sometime) electric generation near end user terminal.

The fundamental concepts for the penetration of DG technologies are the high efficiency of the energy conversion process and the limited emission of pollutants as compared to conventional power plants. Besides offering a higher flexibility and load management, they provide a number of significant local benefits. The integration of the increasing portion of DG within the existing infrastructure requires a full

understanding of its impact on the distribution feeders and its interaction with the loads. Some of the operational aspects, which require understanding, are voltage control, stability, system protection etc. Such studies require accurate modeling of Distributed Generation (DG) sources including distribution system [2].

The benefits of micro-turbine based distributed generation system can be summarized as follows:

• improving availability and reliability of utility system • voltage support and improved power quality • reduction of the transmitted power and, as a result, the

transmission and distribution expenditures are postponed or avoided.

• possibility of cogeneration applications. • emission reduction

DG can provide many benefits to the power-distribution network. To maximize these benefits, reliable DG units are to be connected at proper locations and with proper sizes. Some DG units, such as solar and wind, are variable energy sources and depend their operation on weather conditions. Therefore, it is not ensured whether DG will satisfy and meet all operation criteria in the power system. Some issues arise when these units are connected to power system including the power quality, proper system operation and network protection [3].

Micro-turbines are part of small electricity generation system that burn gaseous and liquid fuels to create high-speed rotation that rotates an electrical generator. The turbine rotates up to 120000 rpm. Micro-turbines produce high frequency ac power. Power electronic interface converts this high frequency power into a usable form. Today’s micro-turbine technology is the result of developmental work in small stationary and automotive gas turbines. Micro-turbines entered field-testing around 1997 and began initial commercial service in 2000. The MTG system power generation is ranging below 250 kW while conventional gas turbine ranging between 500 kW to 250 MW. Micro-turbines are ideally suited for distributed generation applications as [4].

• peak shaving and base load power (grid parallel) • combined heat and power • stand-alone power • primary power with grid as backup • micro-grid • resource recovery

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II. THE MTG SYSTEM DG is defined as the integrated or stand-alone utilization of

small, modular electric generation near the end-user terminals. The main applications of DG units include the following fields [5].

• generating the base-load power, as in the case of variable-energy DG sources

• providing additional reserve power at peak-load intervals

• supplying remote loads separated from the main-grid system

• supporting the voltage stability and reliability by providing power services to the grid

• cooling and heating purposes

The Distributed Generation technologies are characterized as non-renewable and renewable as show in fig. 1 and MTGS found place in both categories.

Fig. 1. Different Distributed Generation Technology.

A. Gas and Micro-turbine System The Micro-turbines has the extension of gas-turbine

technology to smaller scales. The technology was originally developed for transportation applications, but is now finding a role in power generation. One of the striking technical characteristics of micro-turbines is their extremely high rotational speed.

Micro-turbines can be used for numerous applications. In addition, because micro-turbines are being developed to utilize a variety of fuels, they are being used for resource recovery and landfill gas applications also. Micro-turbines are well suited for small commercial building establishments such as: restaurants, hotels/motels, small offices, retail stores, and many others.

Distributed gas fired technology (gas turbine and micro-turbine) are compared in Table I [6].

TABLE I GAS TURBINE AND MICRO-TURBINE

Gas Turbine Micro-turbine Size (MW) 0.5 - 250 0.03-0.25

Electric Efficiency (MHV)*

22-37% 23-40%

Availability >98% 95% Equipment Life (years) 20 10

Fuels natural gas, biogas, distillate oil

natural gas, biogas

NOx Emissions <5 ppm NOx Single digit ppm NOx

Standby Power No Yes Demand Response Peaking No Yes

Utility Grid Support Yes Yes * the efficiencies are based on Maximum heating value (MHV).

B. Different Models of MTG System There are mainly two type of micro-turbine system, single-

shaft model as shown in fig. 2(a) and split-shaft model as shown in fig. 2(b). In the extension of these there is another model called micro-turbine based combined heat and power (CHP) model as shown in fig. 2(c).

In single-shaft designs, a single expansion turbine rotates both the compressor and the generator, resulting their high-speed operation. On the other hand, in split-shaft models, turbine is used to drive the compressor on one shaft and a power turbine on a separate shaft connected to a conventional generator via a gearbox, which generates AC power [7]. There is a growing interest in the application of MTGs as include remote power and combined heat and power (CHP) systems by utilizing the heat contained in the exhaust gases to supply thermal energy needs in a building or industrial process [8].

(a)

(b)

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(c)

Fig. 2. Block diagram of (a) Single-Shaft MTG system (b) Split (two)-Shaft MTG system (c) Micro-turbine based CHP system (Single-Shaft Design).

III. REPRESENTATION/MODELING OF MTG SYSTEM

Sufficient literature is available on the modeling of micro-turbines, with varying level of complexity depending on the application. The modeling of micro-turbine consists of various parts and their related sub model as

A. Micro-turbine The MATLAB/Simulink model of Micro-turbine is

developed and shown in figure 3 [9]. It consists of Speed control, Acceleration control, Fuel control, Compressor-turbine model and Temperature control system.

Fig. 3. Simulink mode of micro-turbine.

Micro-turbines are small high-speed versions of

conventional heavy-duty gas turbines. Hence, the dynamic model of a conventional gas turbine, with relatively small thermodynamic constants, can be adopted to study the impacts of a micro-turbine on the overall system. Rowen [10] was among the leading researchers who presented a model of a heavy-duty gas turbine suitable for use in dynamic power studies.

Hannett and Afzal Khan [11], in continuation to Rowen’s work, validated the gas turbine model by demonstrating its dynamic performance.

In ref. [12], the authors used the validated model of the gas turbine [11] to simulate the response of a combined cycle plant.

Al-Hinai and Feliachi [13] have presented a complete model of the MTG system, used as a distributed generator. It includes the power electronic interfacing and is suitable for transient analysis as well as simulation of unbalanced three-phase power system.

B. Control Functions of Micro-turbine 1) Speed control: It is a primary means of control for the

micro-turbine under part load conditions. Speed control is usually modeled by using a lead-lag transfer function [10], or by a PID controller [14], as shown in fig. 4.

Fig. 4. Speed control system.

2) Acceleration control: Acceleration control is used mainly during turbine startup to limit the rate of the rotor acceleration for reaching up to working speed, as shown in fig. 5.

Fig. 5. Acceleration control system.

3) Fuel control: The fuel flow out from the fuel system results from the inertia of the fuel system actuator and of the valve positioner [10, 14], as shown in fig. 6.

Fig. 6. Fuel control system.

4) Compressor-turbine model: The compressor-turbine is the heart of the micro-turbine. In this, the fuel combustion in the combustor resulted in turbine torque and in exhaust temperature. The compressor-turbine model is shown in fig. 7. The torque and exhaust temperature are expressed using eq. (1) and (2).

0.23 0.5(1 )( ) (1) 700 1 550(1 )(° ) (2) where KHHV is a coefficient which depends on the enthalpy of the gas, Tx is exhaust temperature, TR is the reference temperature and N is the speed input.

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Fig. 7. Compressor-turbine system.

5) Temperature control: Temperature control is the

normal means of limiting the gas turbine output power at a predetermined firing temperature, independent of variation in ambient temperature or fuel characteristics [10, 11]. Its model is represented in fig. 8.

Fig. 8. Temperature control system.

Based on these, there are number of control techniques for

micro-turbine, are reported. In [15] a predictive control model is developed according to

the dynamic feature of micro-turbine. The Predictive control theory is applied to control the DC capacitor voltage of the generation system. The predictive control need a re-identification process, the control objective may fluctuate in this process [15, 16].

The detailed model of the components and controls forming the thermo-mechanical and electric subsystems of a micro-turbine power plant is presented in [17]. The modeled thermo-mechanical subsystem includes different control loops: a speed controller for primary frequency control (droop control), an acceleration control loop, which limits the rotor acceleration in case of sudden decrease of load or in case of start-up.

A sensorless control method for the high speed permanent magnet synchronous machine without position and speed sensor in the start process of micro-turbine generation system is investigated in [18]. In this during starting process of a MTG system, the open loop speed control mode based on current control is applied at low speed, and the closed-loop sensorless vector control method based on back-EMF estimator is used at medium and high speed.

A novel fuzzy control algorithm with tracking differentiator (TD) is proposed in combining with PID to solve Anti-interference problem of high frequency converter [19]. The another fuzzy controller based technique in order to keep pressure constant in the fuel-storing tank and eliminate the harmonic interference due to existing drive system, a active disturbance rejection fuzzy control scheme is proposed in [20].

C. Permanent Magnet Synchronous Machine (PMSM) The electric generator used in a modern MTG system is

usually a permanent magnet synchronous generator (PMSG). The equations to model a PMSG are reported in [21-23].

dq Axis Representation of a PMSM: In a PMSM, If the machine is rotated by a prime mover, the stator windings generate balanced three-phase sinusoidal voltages. The dq axis representation of a permanent magnet synchronous machine [24].

(a) (b)

Fig. 9. Equivalent circuit of PMSM (a) d-axis (b) q-axis. (3)

(4) where, the stator resistance is denoted by RS, the d-axis and

q-axis inductances are Ld and Lq respectively, Φm is the flux linkage due to the permanent magnets, Vd and Vq are d and q axis voltages respectively.

In the dq-frame, the expression for electro-dynamic torque is denoted by eq. (5) as, 1.5 (5)

The equation for motor dynamics is governed by eq. (6) and (7) as, ( ) (6) (7)

where p is the number of pole pairs, Te is the electromagnetic torque, F is combined viscous friction of rotor and load, ωr is the rotor speed, and J is the moment of inertia, θr is rotor angular position and Tm is shaft mechanical torque.

D. Power Electronic Interface The objective of the power electronic interface scheme is to

convert the generated voltage at high frequency to desirable frequency. There are many power electronic interface techniques as in fig. 2 (a) or 2(c). The combination of rectifier and inverter with a DC link [25-27] is a common topology as

Machine Side Converter Controller: The objective of this scheme is to maintain the PMSM voltage level and the ignition speed of the micro-turbine, I f the ignition speed is not maintained the PMSM will be getting shutdown [28].

Line Side Converter Control: This line side converter can operate in both isolated and grid connected mode. The main objective of this controller is to maintain the dc-link and load side voltage level constant [9, 28].

Due to the high speed, stable high-speed operation is a critical issue in realizing a starting system for a single shaft MT. A sensorless vector inverter was developed for this high-speed operation. To successfully start an MT with the developed starter in practice, two new starting algorithms were

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proposed [29]. Another MTG scheme based on an indirect matrix converter, to interface a high-speed electric generator and a local load, as an alternative to conventional back-to-back converters is presented in [30], aiming a unit input power factor in micro-turbine generation systems. In addition, a seamless transfer between two modes can be achieved [31].

IV. PERFORMANCE ANALYSIS OF MTG SYSTEM The performance analysis of MTG system involves various

barriers to successful performance, and proposing a solution system for them.

A. Thermo-mechanical Analysis Micro-turbines and larger gas turbines operate on the same

thermodynamic cycle, known as the Brayton cycle. In this cycle, atmospheric air is compressed, heated at constant pressure, and then expanded, with the excess power produced by the turbine consumed by the compressor used to generate electricity.

The primary objective of the thermal analysis is to create a link between device performance (in terms of thermal efficiency) and the structural details (materials, shape, and size) of the cylindrical sidewalls and rotor [32].

A thermodynamic based micro-turbine model incorporating experimental data and validated calculations for a 60 kW Capstone C60 Micro-turbine has been presented [33]. This section investigates the dynamic output of open and close loop response of micro-turbine model with the different loading.

B. Energy Management Base Analysis Energy management is the planning and operation of energy

related issues and consumption units. Objectives of energy management are resource saving, climate protection and cost saving. The particle swarm optimization (PSO), which is a biologically inspired direct search method can be used, to find real-time optimal energy management solutions for a stand-alone hybrid wind and micro-turbine energy system [34-36].

C. Power Quality Analysis Power quality determines the strength of electrical

power to consumer. Power quality is mostly affected by harmonics. The harmonics can be reduced to acceptable level by using passive/active filters [37-39]. Matrix converter is a feasible alternative to the conventional converter system for harmonics reduction [40].

V. DIFFERENT MODE OF OPERATION OF MTG SYSTEM The various mode of operation of MTG system are reported

in [7, 9 25, 26, 28, 41-43]. These modes are briefed here with

A. Isolated Mode In locations where power from the local grid is unavailable

or uneconomical to install, micro-turbines can be a competitive option. By this electric service can be made available in remote locations. The isolated mode of operation of MTG system consist diode rectifier, voltage source converter with LC filter [7, 9, 25, 26, 28, 41].

B. Grid Connected Mode The small power generators in local areas are placed for

grid support, where power demand is high, but where it is not economic to increase the capacity of the power grid.

Figure 10 [42, 43] shows the Grid connected operation of Micro-turbine power generation with utility.

There is two types of charges on DG unit in Grid connected mode, when DG is not operated and consumer takes power from grid than DG have to pay a stand by fee and secondly, when it is in operating state earlier and want to exit from grid than DG have to pay a exit fee. Table II shows the comparison of key parameters for isolated and grid mode of MTG.

Fig. 10. Grid connected operation of MTG system [39, 40].

TABLE II

ISOLATED AND GRID CONNECTED MODE OF MTG SYSTEM

Isolated Mode Grid Connected Mode

Electrical Output 150 to 480 Volts AC

3-phase, 400 to 480V AC

Frequency Range 10 to 60 Hz 45 to 65 Hz Connection/Load

Type

• 3 phases or single phase

• Phase-to phase or phase-to neutral

• 4-wire Wye • 3-wire Wye with

neutral grounding resistor

Source: Capstone Micro-turbine C1000 Series User's Manual, April 2009

C. Combined Heat and Power (CHP) Mode A micro-turbine based CHP system was illustrated in Figure

2(c). Combined heat and power (CHP) technologies produce electricity or mechanical power and recover waste heat from used process. Most CHP technologies are commercially available at on-site generation and combined heat and power applications [44].

In CHP operation, a second heat-recovery heat exchanger can be used to transfer remaining energy from the micro-turbine exhaust to a hot water system. Exhaust heat can be used for a number of different applications, including process or room heating, heating potable water, driving absorption chillers, or regenerating equipment. The temperature of the exhaust from these micro- turbines is much higher (up to 1200 ºF) and thus enormous heat is available for recovery [6].

In the extension of technology, another highly potential energy saving method, combined cooling, heating and power (CCHP) system is presented in [45].

D. Hybrid System MTG can be used in combination of other generation

system such as full cell, wind generation and PV etc, called a hybrid system. The objective of the work is to increase the reliability of the energy system. When large reserve of power

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is available, that indicates the more reliable system. The developed hybrid system comprises of wind turbine generator (WTG), micro-turbine generator (MTG), fuel cell (FC), an aqua electrolyzer (AE) along with the energy storage device such as battery energy storage system and ultracapacitor (UC) as an energy storage element is presented in [46].

Fuel cell with micro-turbine is the most economical due to fuel flexibility. The hybrid system can utilize exhaust fuel and heat from fuel cell to increase the system efficiency [47, 48].

Another most reliable hybrid system is Wind-MTG system for meeting a desired power demand [49]. In such system a micro-turbine generation system is considered as the backup generator, to meet the energy requirements when wind energy is not sufficient [50]. An integrated form of Wind, SOFC and micro-turbine is presented in [51].

VI. RESULTS AND DISCUSSION The micro-turbine model is implemented using

MATLAB/Simulink as shown in fig. 3. As the speed varies with load, initially the speed input is given as 1 p.u. (no load condition) up to t<10 seconds. At t=10 seconds a speed input of 0.94 p.u. (approximate half load condition) is applied and at t=20 seconds the speed input of 0.87 p.u. is applied which is corresponding to full load condition. The total simulation time is kept as 30 seconds.

Figure 11 shows the fuel consumed and generated shaft torque by the micro-turbine with respect time. It is clear from this figure that initial no load fuel demand is set at 23% (0.23 p.u.) and no shaft torque generated before load is applied on the system. At t=10 seconds, the generated torque is 50% (0.5 p.u.) and the fuel demand is 60% (0.6 p.u.) for half load condition and both increases to nearly 1 p.u. for full load condition.

Fig. 11. Variation of fuel demand and shaft torque signal of the MT system.

Fig. 12. Exhaust temperature variation of the micro-turbine.

Figure 12 shows the exhaust temperature generated by micro-turbine that is 527 °F for no load condition and increases to 768 and 993 °F for half load and full load conditions respectively.

VII. CONCLUSION This paper has presented the different issues related to

micro-turbine based generation system. Among various DG technologies, micro-turbine (gas/bio-gas) based generation has an asset due to lesser fluctuated generation as compare to wind or PV based power generation. Present status of the research work of MTG system in the field of power management, power quality, combined heart and power, hybrid mode operation and distributed generation have been addressed. The simulation model of MTG has been developed in Matlab/Simulink environment. The MTGS results for fuel demand, shaft torque and temperature have been presented at different loading conditions. The obtained results have proved the correctness of developed MTGS model.

APPENDIX Speed governor: A=0.4, B=1, C=3, K1=25. Combustor delay=0.01, Exhaust delay=0.04, K2=0.77. Reference temperature, Tr=950.

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