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DYNAMIC SIMULATION OF MICROTURBINE GENERATION SYSTEM
A A Salam, A Mohamed, M A Hannan, H Shareef
Department of Electrical, Electronic and Systems Engineering
Universiti Kebangsaan Malaysia,
43600 Selangor, Malaysia
Abstract: The operation of grid-connected distributed generation systems can greatly improve the
reliability and quality of the power supply. The existing control techniques for distributed generation
systems are designed to operate either in the grid-connected or islanding modes of operation but do not
allow for both modes transition. In this work, back-to-back converters are used to interface the
microturbine based distributed generation system to the grid. Converter control strategies developed for
both modes of distributed generation operation is also presented. This paper presents a simulation model
of the MTG and its control system using the SimPowerSystem Toolbox. The investigation of the MTG
system dynamics are performed through simulation of both grid-connected and islanded mode. Results
obtained are presented and discussed.
Keywords: Microturbine, distributed generation, voltage source inverter, grid-connected mode, islanded
mode
INTRODUCTION
The integration of distributed generation (DG) units into the distribution system offers a number of
technical, environmental, and economical benefits. In order to achieve these benefits with large
penetration of the DG source in an existing utility network, several technical problems must be
confronted, such as degradation of system reliability, islanding, power quality problems, and various
other safety issues [1]. One of the major concerns in operating DG systems parallel with the grid is the
possibility of islanding of DG due to grid disturbances such as network faults. Islanding is a situation in
which DG installation and a portion of the utility system have become isolated, and the DG continues to
operate and serve loads on the circuit [1, 2].
Current practice in design and operation of networks discourage the islanded operation for safety and
security reasons [1, 2]. But in recent years, due to a deregulated environment, there is a trend to operate
DG in intentional islanding mode in order to meet the uninterrupted, quality electric power demand of the
customer. Islanded operation of the DG system during utility outage (intentional islanding) will improve
the reliability and helps in maintaining uninterrupted power supply [3, 4]. Therefore, current protection
practices of disconnecting the DG units following a disturbance to prevent islanding will no longer be a
practical or reliable solution in a deregulated market environment [3]. IEEE Std. 1547-2003 [4] states the
need for implementing intentional islanding operation of DG systems.
Existing control techniques for DG systems are designed to operate a system either in the grid-connected
or islanded mode to the utility, thus, not allowing for both modes to be implemented and transitioned
between them [3]. The dynamic performance of a microturbine generation (MTG) system under various
grid disturbance conditions has been studied. In [5], only grid-connected operation of an MTG system
was considered. A unified control scheme for operating the DG system both in grid-connected and
islanding modes was presented in [6]. A seamless transfer method from grid-connected to islanding mode
and vice versa for critical loads was described in [7], where an extra static switch is required. A phaselocked loop (PLL) technique is commonly used in grid-connected converters to provide accurate
estimation of phase angle for grid synchronization [8]. A PLL-based flexible transition control strategy
for wind energy system with an induction generator was presented in [9]. A PLL structure presented in
this work may not give accurate estimation of phase angle during unbalanced grid conditions. The present
trend in DG technology is toward a smaller DG system with a capacity less than 500 kW. An excellent
example is a new, fast-growing MTG system. These generation systems are more reliable, have higher
operating efficiency, and have ultra low emission levels [10, 11]. In this paper a model for 100 kW MTG
system is developed for the purpose of analyzing dynamic performance of system operating under gridconnected mode and grid-independent mode. The simulation model is based on the combustion gas
turbine model proposed in [8]-[13]. The microturbine system comprises of compressor-turbine system,
permanent magnet synchronous generator, three-phase uncontrolled rectifier bridge and three- phase
three-level voltage source inverter for interfacing with utility distribution network. The pulse width
modulation (PWM) based control system is applied to control and regulate the power output of MTG. To
investigate the dynamic performance of MTG used as a distributed generator, simulation studies were
carried out. The main focus of the simulation is to analyze the behavior and performance of MTG when
operating in grid-connected and grid-independent modes. The MTG simulation model is built in the
MATLAB/Simulink environment and implemented using the SimPowerSystem toolbox.
MICROTURBINE GENERATION SYSTEM MODEL
There are two types of MTG systems that are based on the position of the compressor turbine and
generator. The high-speed single-shaft design has a compressor and turbine mounted on the same shaft
along with the permanent magnet synchronous generator. The generator generates power at a very high
frequency ranging from 1500 to 4000 Hz. The high-frequency voltage is first rectified and then inverted
to a normal AC power at 50 or 60 Hz. In another design, the turbine on the first shaft directly drives the
compressor, while a power turbine on the second shaft drives the gearbox and conventional electrical
generator (usually induction generator) producing 60-Hz power. The power electronic interface network
is a critical component in the single-shaft microturbine design and offers significant challenges. Several
topologies exist for interfacing the MTG system to the grid matrix converter [12], cycloconverter [13],
and passive inverter and rectifier [11]. Simulation of the electric part of a grid-connected microturbine
with back-to-back converter interface was also reported in [14]. The grid-interconnected MTG system
using back-to-back converter topology is shown in Fig.1.
The topology shown in Fig.1 allows bidirectional power flow between the converter and the grid, and
hence, no separate starting arrangement is required. At the time of starting, the permanent magnet
synchronous machine (PMSM) acts as a motor and draws power from the grid to bring the turbine to a
certain speed. In this mode, the grid-side converter acts as a controlled rectifier, and the machine-side
converter acts as an inverter and provides AC supply to the motor. This is also referred to as the motoring
mode operation of the PMSM. During the generating mode, the PMSM acts as a generator and power
flows from the MTG system to the grid. The machine-side and grid-side converters act as the controlled
rectifier and inverter, respectively. In both modes of operation, the grid-side converter regulates the DCbus voltage, while the machine-side converter controls the PMSM speed and displacement factor. The
Simulink model of the simplified single-shaft microturbine is shown in Figure 2 [15, 16]. This model is
based on the gas turbine model presented in [17]. A high-speed permanent magnet synchronous generator
model is developed in Simulink using dq-components of the stator voltage and the torque equation in dqsynchronous reference frame [18].
Micro turbines are small high-speed gas turbines [3,6]. The three main components of a micro turbine are
compressor, combustor, and the turbine. The compressor is used to pressurize the air before entering the
combustor. Injected fuel is mixed with the compressed air in the combustor and the mixture is ignited.
Mechanical energy is produced when the hot combustion gases flow and expand through the turbine. The
turbine drives a synchronous generator. A portion of power produced in the turbine is utilized for driving
the air compressor while the rest is converted to electric power in the generator.
Fig.1 Microturbine generation system with back-to-back converter interface
A. Microturbine System
In a microturbine system, centrifugal flow compressor compresses the inlet air. The air is then pre heated
in a recuperator using heat from a turbine exhaust. The mixture of the pre heated air and fuel is then
ignited in a combustion chamber so as to produce hot combustion gases which expand through the turbine
blades. The turbine then drives the compressor and the generator which are mounted together on the same
shaft for single shaft microturbines. The system operation of a microturbine resembles a typical
combustion gas turbine. Thus a microturbine model may be based on a conventional gas turbine
configuration as proposed in [7]-[8]-[9].
The mathematical model of microturbine system comprises of temperature control, speed control,
acceleration control, fuel control and compressor-turbine dynamics blocks. The detail and function of
each block is explained in the following sections. The complete model of the microturbine system
configuration is shown in Fig. 2.
Fig.2. Model of a microturbine generation system
The mechanical torque generated by the compressor-turbine unit is linearly characterized by the flow of
fuel and the turbine speed [7]-[9]. The amount of fuel flows to combustion unit of the system is
determined by the fuel demand signal, Vfd. Sources of the fuel demand signal to the least value select
block (LVG) are temperature control, speed control and acceleration control. The least value of Vfd
received at the LVG controls the fuel flows to the compressor-turbine unit. At normal, steady state
operation, the signal Vfd is dominantly controlled by the speed governor.
Speed and Acceleration Control
Fig.3 and Fig.4 shows the model of speed and acceleration control of a micoturbine system. The speed
control operates based on the speed difference between a per unit reference speed and a per unit feedback
speed from the MTG system generator. The mathematical model of speed controller is based on a lead-lag
transfer function[12].The speed governors’ lead(lag) time constant X(Y), constant Z, which representing
the governor mode and the control gain K, are adjustable to characterize the speed controller modes
which are either droop or isochronous [9]-[12]. The acceleration control is used to limit the generator
acceleration rate so as to prevent over speeding during start up of the MTG. It can be terminated once the
generator speed reaches its preset limit.
Fig.3. Speed control for the microturbine
Fig.4. Acceleration control unit
Temperature Control
Fig.5 shows the model of temperature control unit of a micoturbine system. The temperature control
block prevents the turbine operating temperature from exceeding the turbine casing temperature. This is
to avoid over heating of the MT unit. Fuel and compressed air ignited in the combustion chamber of the
compressor-turbine system results in turbine torque and exhaust gases. The exhaust gas temperature is
measured by a thermocouple and compared with the reference temperature. Under normal conditions the
reference temperature is always higher than the exhaust gas temperature. This will results in signal output
from the temperature controller becomes higher than of a signal output from the speed controller. In a
case of the exhaust temperature is higher than the reference value, the difference becomes negative, and
the output signal from the temperature control unit starts to decrease. When the output signal becomes
lower than the output signal from the speed controller, the turbine starts operate with the fuel flow
controlled by the temperature controller.
Fig.5. Temperature control unit
Fuel Control System
In steady state condition, the per unit value of mechanical power produced by the turbine system is
directly proportional to the per unit value of Vfd [13]. The fuel system comprises series of fuel valves and
actuator which controls fuel flow as a function of Vfd. At no load and rated speed, a value of 0.23
represents the minimum amount of fuel required to keep the turbine system running. Under part load
conditions the value of Vfd is scaled down by the gain value of 0.77 and further offset by 0.23 to control
the flow of fuel to the compressor-turbine system is shown in Fig.6.
Fig.6 A model of fuel control system
Compressor-turbine
Fig. 7 shows the process of transporting the heated gas from the combustion chamber to the turbine unit
to drive the turbine blades. Both the torque and exhaust temperature generated by the compressor-turbine
unit are linearly characterized by the fuel flow and turbine speed. The turbine torque and the exhaust gas
temperature are determined by the following equations [9]:
Torque = KHHV (u1 – 0.23) + 0.5(1-N) (Nm)
(1)
Exhaust Temp = Tref – 700(1-u2) + 550(1-N) (F)
(2)
Fig.7. Model of a compressor-turbine unit
B. Permanent Magnet Synchronous Generator (PMSG)
The PMSG is integrated together with the compressor turbine unit on the same shaft to generate AC
power in a microturbines system. A PMSG configuration is similar to a conventional synchronous
generator with the electrical excitation system replaced by permanent magnets [12]-[13]. The output
frequency of the generator is in the range of 2 to 4 kHz. For the MTG model, the two poles PMSG is used
with a generator speed of 96000 rpm. The high-speed generator generates around 100kW output power at
frequency of 1600Hz. The SimPowerSystem model of a three phase PMSG with sinusoidal flux
distribution uses the following equations:
Electrical system equations
Lq
d
1
R
id 
v d  id 
p r iq
dt
Ld
Ld
Ld
(3)
L
p r
d
1
R
iq 
vq  iq  d p r id 
dt
Lq
Lq
Lq
Lq
(4)
Te  1.5 p[iq  ( Ld  Lq )id iq ]
(5)
Mechanical system equations
d
1
 r  (Te  F r  Tm )
dt
J
(6)
d
 r
dt
(7)
where: Ld , Lq : q and d axis inductances;
R
: Resistance of stator windings
I q , I d : q and d axis currents
Vq ,Vd : q and d axis voltages
r
: Rotor angular velocity

: Flux induced by the permanent magnets of the rotor in the stator windings
p
: Number of pole pairs
Te
: Electromagnetic torque
J
: Inertia of rotor and load ( Kgm2)
F
: Vicous friction of rotor and load

: Rotor angular position
Tm
: Shaft mechanical torque
C. Power Converter System (PCS)
The PCS is required to interface the MTG with utility distribution grid. The PCS converts the non
sinusoidal and high frequency output power from the PMSG to match the frequency, phase angle and
voltage level required for grid connection. The PCS comprises a three phase diode-rectifier bridge, a DC
link capacitor, a three phase three level voltage source inverter (VSI) and LC filter. The uncontrolled
diode rectifier converts the high frequency of the AC output from the PMSG to equivalent DC voltage.
The DC link capacitor acts as a reference DC voltage. The VSI converts the DC voltage to AC voltage
with amplitude and frequency that match the grid voltage. The output voltage control of the VSI is
achieved by using PWM control system and high power, fast switched IGBTs. The high frequency of the
VSI switching, may introduce high frequency components in the output voltage. An LC filter is used to
filter the output of the VSI.
CONTROL STRATEGY
The control of inverter was carried out to evaluate the response of the microturbine generation system for
different modes of operation. Two different scenarios namely grid connected mode and islanding mode
were investigated with microturbine generation system in grid distribution system.
A. Power- mode control
In grid-connected mode of operation, the power-mode control is adopted. The variables controlled by the
inverter are the active and the reactive power injected to the grid. The set point value of P reference (Pref)
and Q reference (Qref) are adjustable according to the required power demand conditions. The controller
will adjust the P and Q injected to the grid according to the set point of Pref and Qref. To achieve full
decoupling of the active and reactive power loops, the inverter is current controlled and the control
system is implemented in the rotating dq-reference frame. The implementation of the Park
transformations within the control scheme requires the value of phase angle of the system [6]. The
measurement of the phase angle of the system is through a Phase Locked Loop (PLL) incorporated in the
scheme. Two Proportional-Integral (PI) regulators are used to regulate the system output current to
maintain the stability requirements of the system. The VSI output voltages are controlled by
implementing the Pulse Width Modulation (PWM) control logic. The PQ control scheme for the gridconnected MTG system is as shown in Fig.8.
Fig.8 VSI controller for grid-connected mode of operation
B. Voltage-mode control
The islanded mode of operation requires the control operation to differ from that of the grid-connected
mode. When mains supply power is lost due to network faults, the MTG will continue to supply the
connected load as shown in Fig. 8. In this voltage-mode control of operation, the controller must regulate
the output voltage value at the reference bus voltage and the frequency of the whole grid. The two
reference values of Vdref and Vqref are pre-determined in the dq control scheme to obtain the desired
value of voltage amplitude and frequency. The voltage regulation is through the two PI regulators
incorporated in the control scheme. The control system for the MTG under grid-independent mode of
operation is shown in Fig.9.
Fig.9 VSI controller for islanded mode of operation
SIMULATION RESULT
To analyze dynamic performance of the MTG system, simulations are performed for the MTG system
operating in grid connected and islanded modes. The model of the MTG system presented in Fig.8 is
implemented in all simulations. The simplified version of the microturbine model omitting the
temperature control block is used since only the electromechanical behavior is the main focus in the
simulation model. All simulations are performed in Matlab/Simulink environment. The considered MTG
system has a rated power of 100 KVA and a power factor of 0.85. The maximum capacity of the MTG is
90kW and the system operates at nominal speed of 96000 r.p.m. The system is connected at 50 hz, to the
415 V utility distribution system. Fig.10 shows the model of a micoturbine generation system.
Fig.10 Microturbine generation system
A. Grid-connected mode of operation
The power mode control controller shown in Fig.8 is incorporated in the MTG system for operating the
system under grid-connected mode. To observe the performance of the system in generating the desired
output power, the reference power level for Pref and Qref is preset to 0.6p.u and 0.4p.u respectively. At t
= 3.5s, the reference level of Pref is increased to 0.9 p.u and the reference level for Qref is maintained at
0.4p.u. Fig.11 and Fig.12 show the amount of the active (P) and reactive power (Q) injected into the grid.
The amount of power injected is according to the set value of the reference parameters. At t= 3.5s the P
value is increased to 90kW from its initial value of 60 kW. The injected value of the reactive power Q is
maintained to 40kVar.
60
100
55
50
Reactive power (KVar)
Active power ( Kilowatts)
80
60
40
45
40
35
30
20
25
0
1
1.5
2
2.5
3
3.5
Time(sec)
4
4.5
5
5.5
6
20
1
1.5
Fig.11 Variation of the active power (kW)
2
2.5
3
3.5
Time(sec)
4
4.5
5
5.5
6
Fig.12 Reactive power, Q (kVar)
120
46
110
44
100
42
90
Reactive power (kVar)
Active power (kilowatts)
B. Islanded operation mode of operation
At t = 6s, the MTG interface circuit breaker is opened and the voltage mode of control is activated. The
value of dq axis reference voltages, Vdref and Vqref of the controller is set to 1p.u. Under this situation
the MTG system continues to supply P= 90kW and Q = 40kVar to the loads. To observe the response of
the MTG system to a different load demand the load is reduced to 60kW and 30 kVar at t= 7.5s. Fig.14
and Fig.15 show the performance of the MTG system in generating the active and reactive power during
the islanding. The amount of P and Q generated by the MTG system, are increased slightly to meet the
load demand during islanding. In addition to that, the results also demonstrated the ability of the MTG
system to reduce P to 60kW and Q to 20 kW when required at t = 7.5s.
80
70
60
50
40
38
36
34
32
40
30
30
28
20
3
3.5
4
4.5
5
5.5
6
Time (sec)
6.5
7
7.5
8
Fig.14 Generated active power (kW)
8.5
26
3
3.5
4
4.5
5
5.5
6
Time (sec)
6.5
7
7.5
8
Fig.15 Generated reactive power (kVar)
8.5
CONCLUSIONS
In this paper, the dynamic model of a single shaft microturbine is developed. The model is suitable to be
implemented in simulation study of dynamic performances of MTG system. The simulation results
demonstrated, the ability of the MTG system to as a distributed generation for grid connected and
islanded mode of operations. In addition, the simulation results also demonstrated the load following
ability of the MTG in the distribution system. The ability of the MTG system to provide higher reliability
to the utility is also shown from the results obtained in the simulation of the system under the islanded
mode of operation.
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