Comparaison d\'application de deux types de convertisseur moyenne tension
2010-1-4 12:41:39

Résumé: via l’analyse de l’application, le profit économique et la sécurité des deux types variateur de fréquence à moyenne tension VFD, cet article compare leurs effets sur une centrale.
Mots clés: variateur de fréquence moyenne tension VFD, application, comparaison.

Xutang Power Generating Co., Ltd reconstructed for four fans and four pumps of two sets of 300MW generator in 2003. The company purchased Leader&Harvest VFD for motor of 5# condensing pump in May 2003. After successful application of 5# system, the company purchased Robicon VFD for two PA fans. After one year operation, the company found VFD could save much energy and bring enormous benefits. Because two kinds of VFD drive different loads, it is not so easy to compare exactly. In order to promote VFD application in power plant, we think it is necessary to make comparison, hope this paper can make reference for other power plants.

1.Comparison of application mode

According to work character and system structure of fan and pump, their application mode is quite different. We equip one VFD for two condensing pumps and one VFD for each fan. Electrical connecting is following:

In normal operation, K40 and K410 close, QF1 opens. 1# pump runs in variable frequency. K420 and QF2 open. Pump linkage plunges. 2# pump is standby in direct on line. When variable frequency pump is broken, the standby pump linkage starts to work. In order to avoid damage on VFD, DCS has logic lock. That is, only when K410 or K420 closes, can K40 close. Two pumps share one VFD, which can utilize VFD farthest, and also save reconstruction cost. But in this way, periodical switch is complex. When variable pump trips off and standby pump starts, impact on pipeline is big.

When PA fan runs in variable frequency, QF closes, K421 and K422 close, K423 opens. When VFD is broken, motor is switched to direct on line manually. In order to avoid mistake, K422 and K423 are interlocked mechanically. That is, K422 and K423 can not close simultaneously. “One drive one” is the most basical mode. Cost of this mode is higher than “one drive two”.

2. Comparison of VFD character

Leader&Harvest VFD and Robicon VFD are medium-medium voltage source VFD. VFD input adopts multi-pulse rectifier without filter. Output waveform is near to sine waveform and little affection on power grid. Two kinds of VFD adopt speed sensorless vector control, can meet load requirement of high dynamic response. Power cell is designed in module. Power cell can be replaced easily and replacement time is short. In rated load, efficiencies of VFDs are more than 96%.

Manufacturer	Leader&Harvest	Robicon
Model	A06/130	Perfect Harmony Drive31000159.00
Electrical circuit	Cell-cascaded multi-level, voltage source	Cell-cascaded multi-level, voltage source
VFD capacity	1250KVA	1400 KVA
Rated output current	130A	200A
Control mode	Speed sensorless vector control	Speed sensorless vector control
VFD efficiency	>0.96	>0.97
Max low voltage	-35%	-30%
Over load	120% for 1 min，150% for 3s，200％immediately	110% for 1min，150% immediately

Compared with Robicon VFD, Leader&Harvest VFD has following advantages:

a. Capacity of low voltage is bigger. Low voltage capacity of A06/130 VFD is -35%, but Perfect Harmony Drive31000159.00 is -30%. Bigger capacity of low voltage is advantageous to safety.

b. Capacity of over load is bigger. Overload 120% for 1min; Overload 150% for 3s; Overload 200% immediately.

c. Friendly operation interface.

d. Low price

3. Comparison of economy

After reconstruction, cost of power generating lowered a lot. Especially for the condenser pump, no matter in rated load or in part load, energy saving is obvious. Following table is comparison data.

item load	Current variation of pump			Current variation of PA fan
	Before	After	Energy saving rae	Before	After	Energy saving rae
150MW	74（A）	16.6（A）	77.6%	88（A）	60（A）	31.8%
180MW	78（A）	22.8（A）	70.8%	97（A）	71（A）	26.8%
210MW	81.2（A）	30.7（A）	62.2%	104（A）	83（A）	20.2%
240MW	84.5（A）	38.9（A）	53.7%	110（A）	93.5（A）	15%
270MW	89.5（A）	50.3（A）	43.8%	114（A）	100（A）	12.3%
300MW	92.5（A）	61.5（A）	33.5%	118（A）	106（A）	10.2%

From above statistic data, we see energy saving is obvious with load down. In load 150MW, pump system saves energy 77.6%, two PA fans save energy 63.6%. In rated load, pump system saves energy 33.5%, two PA fans save energy 20.4%.

Average load rate is 70%~80%. We make calculation as per average load 210MW, yearly work time 7000h. Yearly power saved of pump system is

P1=√3*U*ΔI*COSΦ*H=1.732×6000×（81.2-30.7）×0.85×7000=3122536.2（KW.h）

Yearly power saved of PA fan is

P2=√3*U*ΔI*COSΦ*H=1.732×6000×（104-83）×0.85×7000=1298480.4（KW.h）

Power price: 0.3yuan/kWh, cost saved of pump is 937000yuan(USD137794) every year. Cost saved of two PA fans is 779000yuan(USD114558) every year.

4. Comparison of reliability

After successful application of VFD on 5# system in May 2003, the company made same reconstruction for 4# system.

machine index	4# pump	5# pump	PA fan of 4# boiler		PA fan of 5# boiler
			No 41	No 42	No 51	No 52
Available factor	100%	99.3%	90.3%	96.7%	100%	98.4%
Operation factor	96.9%	96.4%	84.9%	91.7%	98.5%	94.6%
Non-stop times	0	3	3	1	0	1
Max continuous time	6381.5h	5053.9h	4435.2h	5597.4h	9150.3h	5084.8h

（note: 1. statistic time is from first operation to 31 st October, 2004.

2. above statistic is only for VFD, not for generator system)

From above statistic, we see six VFDs occur unplanned outage 8 times. VFD of 5# pump stops 3 times. VFD of No 41 PA fan stops 3 times. VFD of PA fan of No 42 stops once and No 52 once. Available factor of all VFDs are more than 90%. Available factor of 4# pump VFD and No 51 VFD can reach to 100%. Minimum available factor of No 41 VFD is 90.3%. Max continuous operation time of No 41 VFD is 6381.5h. Max continuous operation time of No 51 VFD is 9150.3h. Because 4# system had a maintenance of 11 days in early October, its operation indexes are less than 5# system.

Conclusion

From the application, no matter Leader&Harvest VFD or Robicon VFD, the applications are very successful. Comparatively speaking, Leader&Harvest VFD is better than Robicon VFD in economy as well as reliability. Low price and all-round service are important factors which absorb power plant.