2010 International Conference on Power System Technology

Study on the dynamic performance of a microtubine based microgrid X.Q. Xiao, W.M.Kan, C.Xun, G.R.Zheng, B.Wang, X.G.Zheng

Abstract: As a kind of clean and high efficiency technology, micro turbine has a broad application prospect in China. Combining the micro turbine with local load to form micro grid could realize high efficient of energy and high reliability of power

supply. With the rapid development of the smart grid concept and micro grid technology, distributed generation based on micro turbine play a more and more important role. Base on the national 863 goal-oriented project "The key technology and demonstration project of grid-connecting issue for a MW level CCP distributed energy MicroGrid", this paper investigate the dynamic performance of a micro grid based on capstone C100 micro turbine. The structure of the micro grid and general control principle of the turbine is studied first. After that, several tests including start, stop, load swing, mode transition and so on are conducted. to study the dynamic response of the voltage and frequency of the micro grid in both grid connect and stand alone mode. The tests result could be used to further evaluate the dynamic behavior of micro grid and validate models for simulation studies.

Index-term: micro turbine; micro grid; duel mode; control principle; dynamic performance; grid connect; stand alone

I. INTRODUCTION

Nowadays, the concept of smart grid is more and more

popular. Microgrid is one of the key technologies of smart

grid. Micro turbine is a kind of newly developed small thermal

generator, ranged from 25kW to 300kW. Micro turbine has a

series of advanced technology features of wide applicability,

low consumption, low noise, low emission, low vibration, low

maintenance, remote control and diagnosis ability. With the

low carbon economy policy, micro turbine based micro grid

has a broad application prospect in China.

The grid-connecting technology for distributed generations

is a kind of special issue. Generally, micro turbine could run

at either grid-connect(GC) mode or Stand alone(SA) mode.

Micro turbine and important load can be combined together to

form micro grid. With some dual mode function, a micro grid

based on micro grid could run at both GC and SA mode, and

could switch between these two modes automatically. When

fault of external grid occurs, micro grid is disconnected from

the grid, and runs at SA Mode. After the fault is cleared,

micro grid with automatically reconnect to the external grid if

condition allows. This special operation mode can effectively

enhance power supply reliability of important load, and exert

the potential of distributed generation[l].

In 2007, China Southern Power Grid Company undertook

This work was supported by Nationa1863 project (No.2007AA05Z250). Xiao Xiaoqing is with Power Test and Research Institute, Guangdong,

China. (e-mail: xiaoxiaoqing@126.com)

978-1-4244-5940-7/1 0/$26.00©20 1 0 IEEE

national 863 goal-oriented project "The key technology and

demonstration project of grid-connecting issue for a MW level

combined cool distributed generation micro grid". In this

project a typical demonstration project of a CCP micro grid

was built. The project not only initiates a new operation mode

for the promotion and application of Chinese CCP distributed

energy technologies, but also provides experimental basis for

the mutual influence research between micro grid and the

main power grid.

II. INTRODUCTION TO THE MICRO GRID

A. System structure Substation I Substation 2

f 603

M2

� � T3

! t LM2

10kY

O.4kY

Part load of

Load of Building 3

�--��:� ���-----------

Fig. I. Diagram of micro grid structure

building 2

The demonstration project is built in FoShan, which

consists of three buildings, named as building 1 to building 3.

Fig. 1 shows the structure of the power supply system. The

demonstration project consists of three buildings, named as

building 1 to building 3. The power supply for the three

buildings is from two different substations and different load

of buses, named as Ml and M2 respectively. The power loads

of building 1 are supplied by bus Ml through transformer T1

independently. Switch 600 being installed between bus Ml

and M2 is a backup automatic switch. The connecting switch

4000 between LMI and LM2 needs manual operation. In

normal operation, connection switch 600 of I OkV bus and

connectiong switch 4000 of OAk V bus are both open.

The loads of building 3 are supplied by OAkV bus MCC,

which is connected to a low voltage switchboard from

distribution house of building 2 through switch AI . Three

Capstone C200 micro turbines, which have a rated load of

200kW, are also connected at bus MCC. Dual power supply

design is used for building 3.

At grid connect mode, the switch Al closed and the power

output of the micro turbine could supply not only the loads of

building 3, but also the loads of building 1 , 2 and even goes to

the grid.

At stand alone mode, the switch Al is opened. The loads of

building 3 and the three micro turbine could form an island.

Micro turbines would fulfill the power demand of building 3.

The electrical load and cooling load of each building are

shown in Tab. 1 . Especially, the electrical load merely

includes common lighting, elevator, computers and water

heaters, and does not include fire safety load, high level

facilities and air conditioner which needs higher power supply

requirement. TABLE I MAXIMUM LOAD OF EACH BUILDING

Building 1 Building 2 Building 3 Total Maximum cooling

1484.57 1238.48 544.61 3256.43 load (kW) Maximum air conditioner 645.5 538.47 113.5 1297.47 electric load (kW) Maximum electrical load 302.82 330.90 84.89 718.21 (kW)

B. Micro turbine

LilhiULn bromjdc refrigcrnlor Coolin

Fig. 2 Typical CCHP system based on Capstone C200 microturbine

The demonstration project consists of three capstone C200

microturbine. The rated power of each turbine is 200kW. In

order to meet the power demand of the project, the three units

are run paralleled as a group, with the total rated power of

600kW, to supply the whole project. Through lithium bromide

refrigerator, the waste heat of the exhaust gas of microturbine

is absorbed to generate cooling, which can meet the cooling

demand of building 1 and 2. Fig. 2 shows the typical CCHP

system based on capstone C200 microturbine.

Capstone C200 microturbine runs at high-speed and drive

permanent magnet generator. The high frequency AC

electricity is transformed to DC and then inverted to SO/60Hz

frequency AC electricity through power electronic device.

2

Capstone C200 microturbine includes two large battery

packs that store energy for microturbine startup when

disconnected from the electric utility grid. They also provide

an electrical buffer for sudden increases or decreases in load

during SA operation. Management of the batteries and their

state of charge is automatic within the microturbine.

III. MICROTURBINE TESTS

In order to study the performance of the micro grid, it's

necessary to study the performance of the micro turbine itself

first.

A. Start operation Fig.3 shows the power output and turbine speed of the

micro turbine group during startup at grid connect mode. The

speed of the turbine is first accelerated to I 7000r/min and fire,

after warn-up for around 20 s, the turbine is then accelerated

to 34000r/min and ready for generate power. During this

initial period, auxiliary power is provided by the grid, which is

shown as negative power output in Fig. 3. After the warm-up

period, the microturbine controls increase the speed to obtain

the desired output power setting. The characteristic is

basically the same regardless of the power output setting for

startup. As shown in Fig. 3, the total startup period requires

around 1 50 seconds. Speed(dmin)

70000 -Speed(dmin)

60000 --Output Power (kW)

50000

40000

J I I

-;r-

30000

20000

10000 130s from

I

.......-� : ( /f! :

� f I I

/ : � I

I

start to f 111 load : 50 100 150

Load (kW) 600

500

400

300

200

100

-100

-200 200 Time (s) 250

Fig. 3 Startup power output and turbine speed

At stand alone mode, the startup sequence is almost the

same as at grid connect mode except that the auxiliary power

needed for acceleration is provide by the battery and full

recharge of the battery is required before the turbine could

output power.

B. Stop operation When stop, the power output of the turbine decreases

quickly to zero within 20 seconds. Although the power output

dropped off fairly linearly, the turbine speed stayed at 34,000

r/min for nearly all the shutdown time. This period is for the

turbine to cool down its hot path. As shown in FigA, the cool

down time is about 1 30 seconds, which depends on the

exhaust temperature of the turbine.

At stand alone mode, a battery recharge time is required

during stop to ensure that the battery is enough for next start.

During stop sequence, the turbine could restart any time when

needed.

Speed IX/min)

10000 �

50000

50000 1\

I - Engine Speed

-Output Power

LoadM) 180

IXpm)

M)

\: 130. for c 01 down I

150

140

120

100

80

50

40

20

40000 \!

\: \J

30000

20000

10000

o o 50 100 150

I

I

tL-, -

-20 200 Time (. ) 250

Fig. 4 Startup power output and turbine speed

C. Load step change To evaluate the dynamic performance of micro turbine at

GC mode, several tests of step up or down in power output

are developed. As shown in Fig. 5, several characteristics

could be seen. When the new power command was entered,

the power output of the turbine would suffer a sudden

change reversely. That is, when power command increase,

the output power would suffer a sudden decrease before

increase and vice versa. The speed of the rotor would also

change during the step change of power command. As the

micro turbine reached the new power level, turbine speed

stabilized at a new level. Based on the tests of both loading

sequences, we highlight that the transition times during

power increase and decrease are similar. Furthermore, the

tests demonstrate the fast dynamic behavior of the micro

turbine, and show that this device can respond to load

changes rapidly. It takes about 50 seconds for the turbine to

reach another stable status. 650 600 550 I---

r---j500 �450 03400

350 300 250

o

65000

60000 � �55000 � "" �50000 Po '"

45000

40000 o

L-

}'--

\ 200

\ / \......

200

1\

1\ 400 600

(a) Power output

\.... h

T 'I

r -Power demandM)

-Output power M)

�

800 Time (s) 1000

'--1

400 600 (b) rotor speed

r- Speed(r/min)

T 800 Time (s) 1000

Fig. 5 dynamic response at step change of power command

The power assignment method in the test above is balance

equal division, that is, each turbine output one third of the

3

power demand. Actually, to get high efficiency, it's better to

optimize the power assignment method. Better way is to stop

one or two turbine at part load. When the power demand is

less than the rated power of one turbine, only one turbine is

running and when the power demand is less than the rated

power of two turbines, two turbines run and each has a equal

power output. At this assignment method, the load step

change response would change with start or stop of turbines

included. At that time, the start and stop time of turbine should

be consider in system design.

D. Power factor In grid connect mode, the reactive power of the micro

turbine is kept at a very low status. As shown in Fig. 6, when

the active power changes from 20 to the full load of I 60kW,

the reactive power is always below 5 kVar. So at most power

level, the power factor of the turbine could be assumed to be 1 .

6 5.5

j 5 to: 4.5 � 0... 4 � ·t 3.5 '" � 3

2.5

2

.----/

I - �

H ----.-Reactive PowerM) 1 --- Power factor r

o 50 100 Acti ve Power M)

�

150

Fig. 6 Reactive power at different power level

200

0.99 .... o

+' 0.98 g """' ....

O. 97 � o

""'

0.95

0.95

At stand alone mode, the power output of the turbine is

determined by the load in the micro grid. The turbine is

running at constant voltage mode and tries to keep the

voltage of three phases to be stable.

E. Load rejection 250 700

-- Gen Power M)

--Output Power M) 500 200

-- Brake Temp ( C) 500

j 150 400 8 to: Po

E � 300 t! � 100

200 50

100

o b::::l __ ======� o 20 40 50 80 100 120 140 Time (s)

Fig.7 dynamic response at load rejection

The load rejection test is conduct by opening the switch Al

suddenly when the turbine is running at full load. Fig.7

shows the response of the turbine at load rejection. The

output power of the turbine goes to 0 as soon as the switch

Al opens. However, the generator power of the turbine

decreases smoothly goes to zero in about 40 seconds. At the

same time, the speed of the turbine decreases smoothly from

60,000 r/min to 34,000r/min. Actually, the power difference

between generator power and the output power is absorbed

by the battery and the brake connect to the high voltage DC

bus inside the turbine. As is shown in Fig.7, when load

rejection occurs, the temperature of the brake increases

significantly to around 600 Celsius degree. With the help of

the brake and battery, the turbine could keep stable voltage

of the DC bus inside and regulate the output power freely.

F. Power quality Power quality is tests under GC and SA mode respectively.

Tab. 2 shows the harmonic test result in SA mode. The

THVD of micro turbine is a bit higher, which exceed the

requirement of 5% in GBIT1 4549-1993. Three phase

imbalance is also higher than the requirement of 2% in

GB/T15543-2008.

In GC mode, three phase imbalance is less than 0.8% and

THVD is less than 1 %. Both of them fit GB requirements. TABLE 2 HARMONIC TEST RESULT IN SA MODE Harmonic voltage ratio

Harmonic current (A) Harmonic (%)

A B C A B C 2 0.47 0.76 0.57 4.24 4.06 4.03 3 3.60 0.88 5.48 4.73 3.51 4.66 4 0.22 0.32 0.26 2.40 2.35 2.17 5 2.28 2.37 2.13 5.91 6.09 5.45 6 0.13 0.15 0.18 0.33 0.25 0.28 7 1.49 1.29 1.29 3.95 3.78 3.21 8 0.17 0.23 0.24 0.91 0.86 1.04 9 1.40 0.84 1.69 0.86 0.81 1.12

10 0.18 0.22 0.19 0.36 0.39 0.45 11 0.73 0.75 0.66 0.82 1.16 0.81 12 0.21 0.12 0.20 0.18 0.16 0.16 13 0.97 1.10 1.00 1.03 1.03 0.81 14 0.13 0.15 0.20 0.39 0.41 0.35 15 1.05 1.06 0.65 0.40 0.36 0 72 16 0.30 0.27 0.21 0.27 0.27 0.28 17 1.24 1.48 1.23 0.63 0.69 0.52 18 0.20 0.15 0.20 0.18 0.13 0.16 19 1.20 1.65 0.85 0.45 0.53 0.46 20 0.30 0.27 0.25 0.35 0.31 0.35 21 1.09 1.07 0.82 0.28 0.26 0.34 22 0.17 0.25 0.20 0.34 0.34 0.35 23 1.24 1.46 0.83 0.46 0.56 0.54 24 0.18 0.13 0.20 0.20 0.18 0.18 25 1.14 1.22 0.62 0.39 0.46 0.35

THO 5.20 4.19 6.33

IV. DUEL MODE OPERA nON

In order to switch smoothly between GC mode and SA

mode, a mode control device named DMSC (Duel Mode

System Controller) is installed in this project. Logic in the

DMSC activates a motor-operated disconnect device that

isolates the micro turbine and protected loads from the grid

during Stand Alone operation. When the grid power is

restored, the DMSC automatically reconnects to the grid.

Fig.8 shows the control scheme of the DMSC. Two

switches are present: Ml and M2. Switch Ml could isolate the

microgrid from the utility grid, which in this project is switch

AI . Switch M2 is the inner contactor of the micro turbine.

DMSC consists of a voltage detecting relay UVR (Under

Voltage Relay). UVR is used to detect the voltage and send

logic control signal to controls the open and closing of Ml;

4

meanwhile, it sends mode transition signal to micro turbine,

which controls the mode transit of micro turbine between the

GC mode and SA mode. In normal operation, Ml is closed;

micro turbine is in the GC Mode, which supplies power to the

grid. When the fault of external grid occurs, the protection of

micro turbine detects the voltage below normal level, M2 is

open immediately, and micro turbine transfers to hot standby

mode. Meanwhile, while UVR detects the voltage below

normal level, it sends an open command signal to Ml, which

is open in a short time. Also, UVR sends out mode transition

signal, and micro turbine begins to transit from GC Mode to

SA Mode. After a fixed time delay, micro turbine

accomplishes mode transition. With its inner contactors M2 is

closed, micro turbine supplies power to important load alone.

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._ .. _ .. _ .. _ .. _ .. _ .. _ .. _ .. _ .. _ .. _ .. _ .. _ .. _ .. _ .. _ .. _ .. _ .. �

Fig. 8 Grid·connected structure diagram

A. GC mode to SA mode operation

Prin" loads

Grid fault is simulated by opening the switch A2. When A2

open, micro turbine detects voltage change immediately and

the protection inside turbine would trip the inner contactor. As

shown in Fig. 9, as soon as grid voltage disappears, the

turbine goes to stand by mode. The interval from grid fault to

turbine trip is inside 0.2 seconds, which fit the requirement of

IEEE standard 1 547. DMSC detects the under voltage of the

grid and UVR relay activate. It opens the A 1 switch and

isolates the local load from the grid. Then, DMSC tells the

turbine to transit to SA mode. After about 12 seconds, the

mode transition is finished and micro turbine close inner

contactor automatically to run at SA mode.

"--I

J ., 0,

Grid Vol r"c

Turbine Vol tDp.e

IIVR

!;wi 1(:11 AI Pill;; tim1

10, ", ", !C,

Fig. 9 Time sequence ofGC to SA test

B. Stand alone to Grid connect operation

It, ",

To test the SA to GC mode, manually close switch A2 to

simulate grid power is restored. Fig.I O shows the time

sequence of SA to GC test. DMSC detects the voltage has

recovered to normal level. It sends out mode transition signal

after 4 seconds of delay to confirm that the grid power is

stable. The turbine gets the mode transition signal and open

the inner contactor is open firstly and micro turbine transfers

to stand-by mode, which results in power off of the MCC bus.

It takes about 7 seconds for the micro turbine to get ready for

mode transition. Then DMSC sends close signal to AI , the

load of building 3 is supplied by the grid. After A 1 is closed,

micro turbine reconnects to the grid and run at GC mode.

(iritl V

•• 6. Is 10, 12, 14, 16. Fig. 10 Time sequence of SA to GC test

V. CONCLUSION

In this paper, the structure of a Capstone C200 micro

turbine based micro grid is introduced first. Several tests are

conducted to find out its dynamic behavior. The Capstone

micro turbine uses constant power control at grid-connect

mode and constant voltage control at stand alone mode.

Tests of loading sequence show similar transition times

during power increase and decrease. The micro turbine has

fast dynamic behavior when power command step change. At

certain extreme condition such as load rejection or fast load

demand changes, the battery and the brake play an important

role in stabilizing the voltage and frequency of the micro grid.

With duel mode controller, the micro grid could switch

flexibly between GC and SA mode. The local load in the

micro grid would suffer a short time of blackout when the

5

micro turbine conducts mode transition. The black out interval

for mode transition is around 1 0 seconds.

[I]

[2]

[3]

[4]

VI. REFERENCES

R. H. Lasseter and P. Piagi, "MicroGrid: A Conceptual Solution," in Proc. IEEE 35th Power Electronics Specialists Conference,2004. Capstone technical reference- Dual Mode System Controller [Online], 2009. https:lldocs.capstoneturbine.com/home.asp .. "DER Performance Testing of a Microturbine-Based Combined Cooling, Heating, and Power (CHP) System," Proceedings of Power System 2002 Conference, Clemson, SC, March 2002. S.-J. Huang and F.-S. Pai. "A new approach to islanding detection of dispersed generators with self-commuted static power converters." IEEE Transactions on Power Delivery, vol. IS, no. 2, April 2000.

[5] O.Fethi, L.-A. Dessaint, K. AI-Haddad, "Modeling and simulation of the electric part of a grid connected microturbine" Power Engineering Society General Meeting, 2004. IEEE 6-10 June 2004 Page(s): 2212-2219VoL2.

VII. BIOGRAPHIES

Xiaoqing Xiao received the B.E. and M.S. degree in Thermal engineering from Tsinghua University , China, in 2000 and 2003, respectively. He then started work in Guangdong Power Test and Research Institute, China where he is presently a Senior Engineer. His major is in gas turbine operation and simulation,

large-scare rotating machine diagnosis and distributed

generation technology research, etc.