DYNAMICS BEHAVIOR OF a 30 kW CAPSTONE MICROTURBINE Written by E.A Setiawan

Institut für Solare Energieversorgungstechnik e.V (ISET) University of Kassel, Germany

Abstract

Investigation of electrical performance 30 kW microturbine during dynamics condition could be very importance.

The investigation was operated under grid connected mode. In this mode, several tests were conducted with

different set points; ramp up, ramp down, start up, shut down and automatic restart. The result from these tests

can be used to evaluate the dynamic behavior of microturbine, create models that could be used in simulation

studies and it will be incorporated into current study of control distributed generation, particularly study of

virtual power plant and microgrids concept.

Keywords: microturbine, ramp up, ramp down, start up, shut down, automatic restart

I.Introduction

Distributed Generation (DG) are expected to play an increasingly significant role in power

generation in coming years. Microturbine is a part of new technology of DG. It may offer one of the

best short-term distributed power production options because of their simplicity and no major

technological breakthroughs are required for their deployment. However with increasing use of

microturbine technology in power system distribution, investigation of performance and behavior of

microturbine is needed to manage the affect on system control, power quality and protection.

The behavior and performance of microturbine was investigated under grid connected mode. In

this mode, several tests were running by set points changed up and down. Result data was recorded

by computer control centre and measurement device data logger.

This paper focused on microturbine performance at electrical point of view, during several operating

dynamic conditions; ramp up, ramp down, start up, shut down and automatic restart. In addition,

corelation between electrical parameters performance with speed (mechanical) and turbine temperature

were presented also.

I.1 General microturbine description

The microturbine was tested is composed of high speed microturbine with a permanent magnet

generator attached to the same shaft. The turbine uses a recuperator to increase efficiency. Power is

produced by generator as high frequency alternating current (AC). This variable frequency AC is

transformed to direct current (DC) and then converted to utility grid grade AC by an inverter (figure 1).

Figure 1. General diagram of modern microturbine architecture 1.

I.2 The Capstone Microturbine

The Capstone Microturbine generator, is composed of the following subassemblies: a fuel

system; a turbogenerator; and a Digital Power Controller (DPC). It operates on high-pressure natural

gas or low-pressure gas with optional gas compressor. The turbogenerator includes a compressor,

recuperator, combustor, turbine, and generator. The rotating components are mounted on a single shaft

supported by air bearings. The generator is cooled by the air flow into the microturbine. The output of

the generator is variable-voltage, variable-frequency AC power. The shaft rotates at up to 96,000 RPM.

During the microturbine’s start up, the generator is used as the starter motor. The DPC controls the

microturbine operation and all power conversion functions.

The variable-frequency power from the rotating generator is converted to constant-voltage DC

that is then inverted to constant-frequency AC. During microturbine start up, the DPC operates as a

variable-frequency drive supplying power to the turbine until sufficient power is available from the

generator. Grid power is used to rotate the turbine during start-up and cool down. If the grid voltage

and/or frequency fall outside a preset range, the turbine shuts down automatically 2.

II. Test methode.

Microturbine was connected to the utility grid and operated on grid connected mode. In this

mode, several power set points were entered from computer control centre of microturbine. The

turbine generated only the selected amount of power regardless of the load on the system. Laptop

computer was running with required special software (DAMON software) and data were recorded

during the tests (see figure 2).

Figure 2. Test setup configuration

III. Result

III.1 The dynamic performance of electrical parameters during start up operation.

The start up operation was captured with data taken at one second intervals and the power output

was setting at full power output (30 kW). The start up operation was started from turn on the

microturbine (at cold temperature) until each phase of the microturbine started to export power into the

utility grid. Elapsed time for this event was 51 seconds with amount power export in phase A, 29.3 W,

phase B, 168.7 W and phase C, 206.9 W. And the elapsed time from turn on the microturbine to peak

full power export (complete start up) was 199 seconds with amount power export in phase A, 9998 W,

phase B, 10234 W and phase C, 10219 W (see figure 3 below).

Figure 3. Start up time of microturbine

Figure 4. Reactive power in each phase

During start up operation, grid power was used to rotate the turbine and it consumed total active power

maximum 3.76 kW. The indication has been shown in figure 4, the active power fall suddently at point

X. On the other hand, concurrently the reactive power which was exported by microturbine increased

dramatically to appoximatly 2.68 kVar (at peak).

Since the active power was injected to the grid and moved steadily for 53 seconds, the reactive power

also in a steady hold. Afterwards the active power surged gradually to the top, contrariwise the reactive

power bottomed out gradually (see figure 4 above).

Figure 5. Frequency and Voltage profile

In figure 5, the frequency fluctuated in range from 49.97 Hz (min) to 50.03 Hz (max). Similiarly the

voltage profile output moved around 229 V (min) to 237 V (max). In these range levels, variation of

voltage and frequency were within the normal accepted range.

Hz

Figure 6. Transient current profile of phase 1

The behavior of transinet current is also recorded with time length 10 seconds. During start up

operation, the amplitude of transient current at each phase was 5–10 A, and it was increasing to 55 A

whilst the power output was ramping up (see figure 6).

III. 2. The dynamic performance of electrical parameters during ramp down and ramp up.

The ramp down and ramp up were performed to measure the response of transition time during

setting of power output command. Ramp down sequences were devided into four areas (see figure 7).

At area 1, power output command was adjusted from full power output down to 20 kW power output.

Ramping down power output command from 20 kW to 15 kW power output was applied for area 2. At

area 3, 15 kW power output command was decreased to 10 kW. At area 4 automatic restart was

accured, because the microturbine was difficult for running stable at 10 kW power output. However, at

area 4 the ramping down sequence was recorded from 10 kW power output down to (minus) -0.42 kW.

It indicated that the microturbine absorbed active power from the grid for cooling down sequence.

Timing of turn on the microturbine

Timing of start power export

10 seconds

10 A 5 A

55 A

Timing of peak power export

Figure 7. Ramp down sequences

In table 1, transition time of each ramp down area was calculated. At area 2 and 3, the power output

command was changing circa 5 kW ,and the transition time was consistent at 3.86 and 4.12 sec/kW.

Table 1. Ramp down times

Area Start power

output (kW)

End power

output (kW)

Power output

change

(kW)

Time to

change power

output (sec)

Transition time

(sec/kW)

Transition

power

(kW/sec)

1 26.42 19.98 - 6.44 18 2.80 0.36

2 19.75 15.09 - 4.66 18 3.86 0.26

3 14.70 10.10 - 4.60 19 4.12 0.24

4 9.74 -0.42 -10.16 24 2.36 0.42

Figure 8. Ramp up sequences

The ramp up sequences were devided into three areas. At area 1, power output command was adjusted

at 15 kW power output from automatic restart operation. Area 2, power output command was changing

from 15 kW up to 20 kW. And area 3 from 20 kW up to full power output. The result of transitions

time of ramping up of each area is listed in table 2.

Table 2. Ramp up times

Area Start power

output (kW)

End power

output (kW)

Power output

changing

(kW)

Time to change

power output

(sec)

Transition time

(sec/kW)

Transition

power (kW/sec)

1 0.48 14.43 + 13.95 49 3.51 0.28

2 14.44 19.63 + 5.19 19 3.65 0.27

3 19.29 28.02 + 8.73 19 2.18 0.46

When table 2 at area 2 is compared with table 1 particularly at area 2 and 3, the transition times are

nearly consistent (3.65 - 3.86 – 4.12 sec/kW). Because of the amount power output changing is circa 5

kW. Likewise the result of transition time at area 4 in table 1 and at area 3 in table 2 is not far different

at 2.18 and 2.36 sec/kW (with amount power changing output between 9 – 10 kW).

III. 3 The dynamic performance of electrical parameters during automatic restart

Since the power output command was changing ramp down from 15 kW to 10 kW power output,

performance of microturbine was not stable. After 22 seconds operation with producing power output

circa 10 kW, suddenly the power output drop dramatically to 4.6 kW inside of 2 seconds. In this

periode, restart procedure was operating automatically until the microturbine exporting power to the

grid. Several interesting events during automatic restart were investigated i.e. elapsed time from

automatic restart to power export, and elapsed time until producing power output 15 kW (see figure 9).

Figure 9. Elapsed times during automatic restart

According to graphs in figure 9 above, the elapsed time for automatic restart until export power to the

grid is about 5 minutes. And it takes 5 minutes 48 seconds to export power output 15 kW to the utility

grid.

Figure 10. Dynamic performance of reactive power during automatic restart

The reactive power increased significantly during automatic restart (see figure 10) from 515

Var to 668 Var. On the other hand, at the same time the frequency was down slightly, because the

active power output was decreasing from 9.75 kW down to 4.6 kW. Nonetheless the frequency

fluctiation was still on tolerance.

Figure 11. Dynamic performance frequency and voltage during automatic restart

III. 4 The dynamic performance of electrical parameters during shutdown operation

When the microturbine was on steady state operation at the full power output, shutdown

procedure was started by turned off the switch at the microturbine’s power panel. It needs 26 seconds

until the microturbine did not export power to the grid (the microturbine start to absorb power from the

grid). During shutdown operation, several sequences were accured fastly (see figure 12). First

sequence, full power output (25.4 kW) drop sharply to 16.4 kW. And after 1 second the power output

fall dramatically to 8.8 kW. After that, the power output decline gradually per 1 second until the

microturbine absorbed active power to the grid. The microturbine was shutdown completely in 10

minutes 27 seconds (see figure 13).

Figure 12 . Shut down sequences

Figure 13. Shutdown time of microturbine

Figure 14. Dynamic performance of voltage and frequency during shutdown

During shutdown operation, the frequency was relative constant in range 50 Hz to 49.98 Hz (see figure

14). On the other hand, the voltage profile slide gradually.

Figure 15. Dynamic performance of reactive power during shutdown

As shown in figure 15, the reactive power during shutdown sequence was moving flat out. And the

microturbine inverter was still feeding reactive power to the grid with total amount circa 1.810 Var.

Figure 16. Current transient during shutdown

The transient current was also decresing during shut down sequence, the amplitude of transient

current at each phase was changing from circa 50 A down to 20 A – 15 A and it was decresing whilst

the power output was ramping down (figure 16).

50 A

20 A 15 A

Timing of turn off the microturbine

Timing of no export power

III.5 Power factor performance

Figure 17. Microturbine power factor

Figure 17 shown average power factor at 30 kW, 20 kW and 15 kW power output command.

The power factor (PF) was consistent (PF ≈ 1) and the PF value decreased very slightly when the

power output ramped down.

III. 7. The dynamic performance of turbine speed and thermal parameters.

The turbine speed increased sharply during start up operation. The speed could reach 23663 rpm

in 25 seconds after turned on the power switch on the microturbine. And at the peak power output, the

speed turbine rotated at 94936 rpm. During start up operation the maximal turbine speed was recorded

at 96363 rpm inside of 168 seconds after switch on (see figure 18 below).

Figure 18. Turbine speed and power output during cold start up to full power output

Before start up operation the microturbine temperatur was recorded 53.9 degrees celcius. And

after 70 seconds start up operation, the turbine temperatur increased sharply to 679.4 degrees celcius.

When the power output was established at full power operation, the turbine rotation stabilized at a

higher speed (circa 96.000 rpm) and turbine exhaust temperatur settled at around 593 degrees celcius

(see figure 19 below).

Figure 19. Dynamic behavior of temperature output

Figure 20. Dynamic behavior of turbine exit temperature during measurement

Watt oC

According to figure 20 above, the turbine exit temperature was increasing to circa 630

degrees celcius during ramp down sequence and at automatic restart operation the

temperature was decreasing rapidly to bottom at about 320 degrees celcius. When the power

output moved to ramp up, the temperature was rising to approximatly 660 degrees celcius.

And it moved down slightly to stable at 600 degrees celcius at full power output. During shut

down operation, the temperature decreased sharply from 600 to 500 degrees celcius and it plunged

gradually to 200 degrees celcius.

IV. Conclusion

1. The microturbine was able to operate on grid connected mode and follow the dynamic operation.

During the test, performance of frequency (f), voltage (V) and power factor (PF) are stable.

2. During start up operation, the elapsed time to reach maximum temperatur is faster than elapsed

time to reach peak electrical power output. (70 seconds to reach 680 degrees celcius and 199 seconds

to reach peak power output).

3.During continuing test, start up operation needs around 3 minutes 20 seconds, automatic restart 5-6

minutes (until export 15-30 kW) and 10 minutes 30 seconds for shutdown operation.

4. When the microturbine was ramped down/up circa 5 kW, the value of transition time (sec/kW) is

nearly consistent (the rate was about 3.65 – 4.12 seconds per kW). And for ramping down/up 9-10

kW, the rate was about 2.18 – 2.36 seconds per kW).

5.Future investigation should be continued, particulary when the microturbine will be supplied with

better fuel gas quality.

V. Acknowledgements

The autors would like to thank the Institute Solare Energieversorgungtechnik e.V in Kassel,Hanau and

Eichhof for supporting and facilites of this work. This research was also part of PhD dissertation for

chapter three.

VI. References

[1]. Capstone. May,2003. Microturbine what they are and how they’re used.US DOE DER

road shows.p13

[2]. Yinger,Robert.J., July,2001. Behavior of capstone and honeywell microturbine generators

during load changes. California Energy Commission.p3