Economie d'énergie du variateur de fréquence HARSVERT—A pour la centrale de Hanfeng
2009-12-28 11:43:36
Résumé : la centrale de Hanfeng est dotée de deux ensembles de générateurs de 660MW, ce qui est la plus grande capacité du Nord de la Chine. La reconstruction de la pompe de condenseur avec variateur de fréquence VFD dans la centrale est la 1^ère application du générateur 660MW en Chine. Cela a fournit de l’expérience pour la reconstruction des générateurs supérieurs à 600MW à l’avenir.
Mots clés : générateur à charbon 660MW, pompe de condenseur, ajustement de la vitesse à fréquence variable.

Hanfeng Power Plant locates in Handan city of Heibei province with area 735400m². Planning capacity is 2400MW. Phase I project built two sets of 660MW coal-burning generator which is the biggest capacity in North-China. They put into service in 2001. Steam turbine and electrical control system are Siemens. Auxiliary equipments are two sets of full capacity condensing pump, two sets of half capacity steam feed pump, one 35% capacity electrical feed pump and three sets of half capacity circulating pump.

Condensing pump is one of main equipments in thermal system. The pump delivers condensing water to deaerator to keep balance of water level. After being increased pressure by condensing pump, condensing water passes gland heater and four sets of low pressure heater, then entrances in deaerator. Keeping continuous and safe operation of condensingpump is important for ensuring safety of power plant.

Technics of condensing pump

In full load, valve opening of pump outlet is usually during 40％～60％. Pressure difference is big when load is in 50%~100%. Throttle loss is big. Because generator joins peak adjustment, main auliliary equipments have big adjustment potential. Output of condensing pump is constant, which causes big waste. Medium voltage VFD can adjust frequency to adjust flow according to load variety. It improves adjustment quality, heightens reliability of system, lowers system economy and saves energy.

II. Control scheme

1. Working condition of condensing pump

Motor of condensing pump is Siemens motor, 2600kW/6kV, equipped with two condensing pumps. One pump works and the other is standby.

Adjusting water level of condenser is main work of condensing pump. In normal state, water level of condenser can not be too high or too low. When load rises, condensing flow increases. Water level of condenser rises. When load lowers, water level is down.

Before using VFD, water level was adjusted by valve opening of pump outlet. Linearity was inferior and much energy was wasted on valve. In the mean time, frequent start/stop of pump led to low reliability of valve, affecting stable operation.

After using VFD, no need to adjust valve frequently. Valve opens in large scope. Flow is adjusted by changing output frequency of VFD.

2. Operation control

In normal working condition, DCS selects #11 condensing pump first. DCS implements variable frequency control according to water level of condenser. At the moment, condensing water adjustemnt gate is full open. Once #12 pump powers on, adjustment gate opens at 40% when load is 600MW, at 30% when load is 300MW. Other load points determine gate opening as per above slope. After in place, #11 pump powers off. Adjustment gate plunges automatically. After water levels of condenser and deaerator are balanced, enlarge outlet gate of #12 pump gradually. In order to ensure pressure of main condensing water pipe not less than 0.6Mpa, set minimum frequency of #11 pump as 30Hz (900round/min).

#11 pump has two working modes, line frequency operation and variable frequency operation. Close 6kV switch first, then start VFD. if VFD is broken, VFD is isolated and motor runs in direct on line through bypass cabinet. Water level is controlled by adjustment gate.

3. Safety estimation

Although system adds VFD, there are still protections. Safety factors are considered completely in early of VFD design. When VFD is broken or pump trips off, original protections are still valid. It does not threaten system operation.

Here are some protection measures to ensure stable system operation:

a.       Flow protection: set minimum speed to avoid cavitation. When flow is less than 96kg/s or more than 580kg/s, stop machine 28s later.

b.      Temperature protection: trips off when temperature of pump and motor bearing is over 100 degree.

c.       Set high speed and low speed, with function of quick accelerating and quick decelerating. Heighten response speed when load changes suddenly. Lower variety rate of water level, avoid alarm of high water level and low water level.

d.      When line frequency pump is running and variable frequency pump is recovering, implement pumps parallel, pump exit smoothly, implement smooth transition of VFD upward adjustment.

III. Technology characters of medium voltage VFD

1. Parameters

Quantity of condensing pumps: 2, one works and one is standby.

Motor parameters

Parameters of HARSVERT—A06/300VFD

2. External ports of VFD

3. Special design of VFD on 600MW pump

a. N 1 redundancy design in power cell

b. Quick start in 20s

4. Cooling effect

For big power VFD, air cooling can not satisfy normal operation. Leader&Harvest provides air-water cooling scheme for user. Practise proves the cooling system has advantages of easy installation, long lifetime, low fault rate and free maintaince.

IV. Energy saving

According to formula: P_d =1.732×U×I×cosφ, calculate power consumption of each load.

Following is figure of energy saving rate in each load.

Average load rate is 75% (500MW), yearly work time is 7200h, yearly power saved: 5688800kW.h. Average energy saving rate is 44.58%. The investment can recover within one year.

V. Other advantages of VFD

a. Reduce current impact during starting

start current of motor is 7 times of rated current in direct start-up, 4~5 times in star-delta start-up and 2.5 times by soft starter. There is no current impact in variable frequency start-up.

b. Prolong equipment lifetime

cavitation is avoided in low load in variable frequency adjustment. In the mean time, there is no force on bearing, so bearing lifetime is prolonged.

c. Heighten reliability of pipeline

Compared with valve adjustment, variable frequency adjustment relieves impact on pipeline, which lowers noise and perfect environment.

VI. Concluding

This VFD reconstruction is the first application on generator of 660MW. It provides experience for other generators 600MW upward, also extends wide prospect for equipments of 2500kW upward. HARSVERT VFD is regarded as first choice product of plant as it high cost performance.