Designing Dynamic Controller and Passive Filter for a Grid Connected Micro-turbine

R. Rahmani, M. Tayyebi, M. S. Majid, M. Y. Hassan, H. A. Rahman Center of Electrical Energy Systems University Technology of Malaysia 81310 UTM Johor Baru Malaysia

Abstract- This paper presents a simulation model of electrical components of a grid connected 50 kW single shaft micro-turbine in MATLAB-simulink GUI environment. Micro-turbine consists of a PWM inverter to control the injected power to the load. Deploying micro-turbines with Combined Heat Power (CHP) capability make it so interesting to use in distributed generation. Employing DG creates an un-sinusoidal voltage waveform supplied to the load. Hence, a band pass filter is installed to provide acceptable sinusoidal waveform with the lowest rate of THD for 1 MW load. Simulation results and discussions show that THD confirms to the requirements of IEEE Std 519-1992. Key words: Micro-turbine; Dynamic controller; PQ controlling system; Distributed Generation; Passive Filters.

I. INTRODUCTION

Variety of Distributed Resources (DR) in Distributed Generation (DG) such as micro-turbine, fuel cells in non-renewable class and wind turbines with photovoltaic in renewable category are implemented in power systems. Different types of DGs have different ranges of usage which can be observed in table 1 [1]. Providing ways to by-pass problems of conventional methods of energy delivery will bring a suitable manner of more flexible power production with higher generation efficiencies and a lower rate of pollution emission to consumers. In this category, micro-turbine could be classified as a Low Voltage (LV) network. Possibility of heat and power generation put the micro-turbine in combine cooling heating power generation (CCHP) class as well. Micro-turbines are divided into two classes. Split-shaft Micro-turbines have induction or synchronous generators which are connected to turbines within gearboxes as discussed in [2]. Another category is single-shaft micro-turbine. Design of this type consists of high speed single shaft, a compressor and a turbine installed on same shaft along with a permanent magnet synchronous generator. Single-shaft micro-turbine is applied in this study. Rapid response capability and ability of peak-shaving are some of the advantages of this kind of power supply. Furthermore, benefits of DG could be classified in: improving power quality, reduction of losses, releasing the capacity of transmission lines and distribution systems and improving utility system reliability. Micro-turbine as a DG system has all of the above advantages while it has its own

TABLE I

TYPES OF DG SOURCES

disadvantages and problems. The main problems of this kind of energy production are divided into two categories. First and the more important concern is controlling a system which consists of inverter that makes micro source capable to dispatch with the grid [3]. There are two strategies for controlling the inverter. PQ inverter control and voltage source inverter (VSI) control. In PQ controlling manner, active and reactive power set point are used. Voltage waveform of DG must be synchronizing with the grid voltage. This class of controlling is suitable for connected to grid micro sources which are in interconnection with grid in dealing the active and reactive power. Also it is also name as grid parallel operation. In case of controlling the isolated systems in local loads, inverter is controlled by given voltage and frequency. By measuring the load demand in islanded mode, output active and reactive power would be injecting automatically [4]. Second problem of implementing micro-turbine is harmonics which makes the load waveforms disputed. These harmonics are the effects of switching in inverter. To make these power suppliers usable and beneficial for the system, above problems must be mitigated. There are two steps unsolved in mitigating the above problems. At first step, power injection to load and grid side of system would be controllable by designing a system for inverter switching through the PQ control manner. As second step, a band pass filter has been used which is designed especially for this system. However, different kinds of filters could be used like low pass.

Type of source Power range

Micro-turbine 25 kW – 1 MW

Wind turbine 100 kW – 2 MW

Photovoltaic 5kW – 100 kW

Fuel-cell 100 kW – 2 MW

2011 IEEE Applied Power Electronics Colloquium (IAPEC)

978-1-4577-0008-8/11/$26.00 ©2011 IEEE 165

II. MODEL DESCRIPTION AND SIMULATION

A. Micro-turbine The whole system which is known as a micro-turbine power supplier consists of three main parts. A permanent magnet synchronous machine (PMSM), air compressor, combustion chamber and turbine mounted air bearings. Single shaft micro-turbine is not suitable for being connected to grid because of high frequency AC voltage produced by it. Output generated signal is at very high frequency ranging from 1500 to 4000 Hz. High frequency voltage be rectified and then inverted to 60 Hz voltage. So power electronic devices like AC-DC rectifier, DC-AC inverter are used to attain a frequency matched with interfaced grid. The shaft in which that generator and turbine are installed on it must operate without any vibration during high speed rotation. Inlet of the air is through the generator and compressed and flows through recuperator for preheating process and increasing efficiency of system before entering the combustion chamber. Fig. 1, illustrates a schematic diagram of a typical micro-turbine system. Excellent samples of practical micro-turbines in the market are in the range under 500kW, like 25-100 kW. These systems have high speed gas turbines (15,000-90,000 rpm). Consequently output voltage of micro-turbine would be in high frequency range which forces the consumer to use converters to reach the grid frequency. Dynamic model of turbine is realized in [5]. Micro-turbine and rectifier is brought as a DC dependent current source here. Grid could be presented by a balanced three-phase source, 600 volts line to line, 60 Hz frequency. The total simulated model has been demonstrated in Fig. 4. In micro-turbine part, power demand has been set to 50 kW while the desired DC voltage is equal to 100 V in this case.

Fig. 1. Complete schematic of a micro-turbine [6].

B. Inverter and PQ controller According to operation mode of micro grid, there are two main control division mode consist of voltage source inverter (VSI) and PQ inverter control. In case of operation by pre-defined values of voltage and frequency in islanded mode, VSI is used. The PQ category of controlling system is used in grid connected or connection to a load Island contains of more micro sources of power generators [7]. That is the reason PQ control type has been implemented in current study. Output voltages of PWM inverter is not in sinusoidal shape and is in one of vector part consist of six active and one zero voltage section, thus the output is discrete. Therefore, employing a dq0 stationary frame is necessary to control [8]. In Fig. 4, total simulated system is brought. As it can be seen, Inverter switching controller consists of two controlling loops. Active power and current feedback from bus number 2 are inputs of these loops.

Fig. 2. Simple block diagram of the control loops.

Fig. 2, depicts block diagram of the controller which has been implemented to control the grid side inverter. For the current there are two components to be considered, Id and Iq. Id is the active component of current while Iq is the reactive component which has been set to zero as reference. C. Passive Filter Aim of implementing a filter on the grid side is to filter out the high frequencies which are generated by inverter. However, filter circuit has impact on low frequency harmonics as well. In this paper a band pass filter has been utilized to ensure that the best possible waveform has been received by load. Fig. 3, demonstrates the circuit of a band pass filter which is considered as a passive filter because there are no active elements in its structure. All the used elements are passive; resistors, inductors and capacitors.

Fig. 3. Circuitry scheme of the band-pass filter.

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Fig. 4. Complete schematic of the simulated model of micro-turbine connected to grid.

Kirchhoff’s current law for fig. 3, has a result as in (1):

⎟⎟⎠

⎞⎜⎜⎝

⎛++=

++

−2

22

111

111

SCSLR

V

SCSLR

VVout

inout (1)

The voltages have been used here to obtain transfer function of the filter, while voltage and current also can be considered. Equation (2) is

⎟⎟⎟⎟⎟

⎠

⎞

⎜⎜⎜⎜⎜

⎝

⎛

+++

++++++=

212

2

1

2

1

1

2

212

1221

2

1

2

1 111

CLSLL

RLS

CC

LCSSCRCSR

SLR

RR

VV outin(2)

Considering the G(s) as the transfer function of the filter the following equations will be the transfer function of the filter:

( )12122

1

2

1

1

2

2

12

2

121

321

4

2

111

)(

CLCRLRS

LL

CC

RRS

RLCRSCLS

S

VVsG

in

out

+⎟⎟⎠

⎞⎜⎜⎝

⎛++⎟⎟

⎠

⎞⎜⎜⎝

⎛++++⎟⎟

⎠

⎞⎜⎜⎝

⎛++

=

=

(3)

21212112212

1

2211212

1

21

2

221

134

21

2

111111

1

CCLLCCLRCLLRS

CLCLCLRR

CLS

CRLRSS

CLS

+⎟⎟⎠

⎞⎜⎜⎝

⎛++⎟⎟

⎠

⎞⎜⎜⎝

⎛++++⎟⎟

⎠

⎞⎜⎜⎝

⎛++

⎟⎟⎠

⎞⎜⎜⎝

⎛

=

Pairs of L1, C1 and L2, C2 will go to resonance at grid frequency upon to (4).

22110

11CLCL

==ω (4)

In order to reduce the distortions generated by existing frequencies other than grid, having a band close to 60 Hz seems to be necessary, and then corner frequencies are given 40Hz and 80Hz. Consequently, at the resonance frequency which is 60 Hz, either series or parallel parts of the designed second order band pass passive filter would be in resonance and filter impedance got as low as possible to have the lowest value of losses on filter. Based on IEEE Std 519-1992 the filter should be designed in a manner that THD of voltage be less than 5% for low-voltage general systems. Considering attenuation frequencies equal to 40 Hz and 80 Hz with margins and band pass equal to 40 Hz, besides assuming the series resistance equal to 0.1 Ohms, we will have the following values for the filter’s elements: R1 = 0.1 ohm,

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L1 = 0.562613 mH, C1 = 0.0125063 F, R2 = 1 ohm, L2 = 0.125063 mH, C2 = 0.0562613 F. The frequency response of designed filter is as shown in Fig. 5. It is obvious that filter has a pass-band of 40 Hz.

Fig. 5. Frequency response of the designed band-pass filter.

III. SIMULATION RESULTS

Based on simulated model in fig. 4, the waveforms of two buses are important and are analyzed in this paper. Bus B2 shows the waveforms before entering to the filter, while B3 has the waveform properties after filtering. The voltages of buses B2 and B3 are shown in fig. 6, and fig. 7, for first second of initiating. In order to have a comparison between the quality of waveforms of two buses before and after filtering, same short interval has been demonstrated for both of waveforms in fig. 8, and fig. 9. Because of having a resistive 1 MW load, the waveform of voltage is the same with current, so no current waveform has been brought.

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1-2.5

-2

-1.5

-1

-0.5

0

0.5

1

1.5

2

2.5

Time (s)

Thr

ee p

hase

vol

tage

at

the

bus

B2

(p.u

.)

Fig. 6. Three phase voltage waveform at bus B2 before filter.

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1-1.5

-1

-0.5

0

0.5

1

1.5

Time (s)

Thr

ee p

hase

vol

tage

at

the

bus

B3

(p.u

.)

Fig. 7. Three phase voltage waveform at bus B3 after filtering.

0.5 0.51 0.52 0.53 0.54 0.55 0.56 0.57 0.58 0.59 0.6-2

-1.5

-1

-0.5

0

0.5

1

1.5

2

Time (s)

Vol

tage

at

bus

B3

(p.u

.)

Fig. 8. Voltage waveform before being filtered out containing harmonics

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0.5 0.51 0.52 0,53 0.54 0.55 0.56 0.57 0.58 0.59 0.6-1.5

-1

-0.5

0

0.5

1

1.5

Time (s)

Vol

tage

at

bus

B2

(p.u

.)

.

Fig. 9. Filtered waveform of voltage delivered to load.

0 0.5 1 1.5 2 2.5 3 3.5 40

0.005

0.01

0.015

0.02

0.025

0.03

Time (s)

TH

D o

f vo

ltage

at

bus

B3

(p.u

.)

Fig. 10. THD of voltage at bus B3.

0 0.5 1 1.5 2 2.5 3 3.5 45

6

7

8

9

10

11

12x 10

5

Time (s)

Act

ive

pow

er d

eliv

ered

to

1 M

W lo

ad (

W)

Fig. 11. Active power delivered to load at bus B3.

Fig. 10, illustrates the THD of voltage at load bus B3. The values of THD indicate that the designed and simulated system conform to the requirements of IEEE Std 519-1992 about harmonics. The active power delivered to the load has been demonstrated in fig. 11, which indicates that the proposed model has delivered the demand load.

IV. CONCLUSION

In this paper a designed and simulated model has been proposed for a micro-turbine system connected to grid. A Dynamic controller has been designed to control the power output of inverter besides a band pass filter to filter out the voltage harmonics of the 1 MW load. However, the micro-turbine model which has been brought here is not a dynamic model. The designed system has been concentrated to observe the operation of band-pass filter, and the waveforms of voltage before and after filter have been figured out. The THD of voltage delivering to the load has been illustrated as well and conformed to IEEE Std 519-1992. Considering the micro-turbine with a dynamic model will be the object of future study.

ACKNOWLEDGMENT

We thank the Ministry of Higher Education for the

financial support of this work. FRGS-vot No:78355

REFERENCES [1] G. Joos, B.T. Ooi, D. McGillis, F.D. Galiana, R. Marceau, ‘‘The potential

of distributed generation to provide ancillary services,’’ IEEE Power Engineering Society Summer Meeting, pp.1762-1767, 2000.

[2] A. Al-Hinai, K. Schoder, A. Feliachi, "Control of grid-connected split shaft microturbine distributed generator," Proceedings of the 35th Southeastern Symposium System Theory, pp.84-88, 2003.

[3] J.A. Pecas Lopes, C.L. Moreira, A.G. Madureira,” Defining Control Strategies for MicroGrids Islanded Operation,” IEEE Transactions on power systems, Vol 21, May, 2006.

[4] S. Barsali, M. Ceraolo, and P. Pelacchi, ‘‘Control techniques of dispersed generators to improve the continuity of electricity supply,’’ in Proc. PES Winter Meeting, vol. 2, 2002, pp. 789---794.

[5] A. Al-Hinai, A. Feliachi, "Dynamic model of a micro turbine used as a distributed generator," Proceedings of the Thirty-Fourth Southeastern Symposium System Theory, pp.209-213, 2002.

[6] S.R. Guda, C. Wang, M.H. Nehrir, “A Simulink- based microturbine model for distributed generation studies,” Proceeding of the 37th Annual North American Power Symposium, 2005.

[7] Huang Wei, Wu Ziping, “Dynamic Modelling and Simulation of Microturbine Generation System for the Parallel Operation of Microgrid” IEEE conference sustainable power generation and supply, 2009.

[8] Robert Lasseter,“ Dynamic Models for Micro-Turbines and Fuel Cells,’’ IEEE conference, pp.7803-7173, 2001.

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