Variateur de fréquence moyenne tension HARSVERT—A dans pompe de condenseur ultra-supercritique de 600MW
2009-12-28 10:41:32
Résumé : l’unité de génération au charbon ultra-supercritique est dotée des avantages comme sa basse consommation de charbon, sa haute technologie, sa meilleure performance environnementale et sa caractéristique d’économie en ressources. Cet article se focalise sur la stimulation de l’efficience énergétique de la pompe de condenseur ultra-supercritique 600MW de la centrale de Huaneng Yingkou par le variateur de fréquence moyenne tension multi niveau VFD HARSVERT-A. Les résultats montrent qu’en utilisant le variateur de fréquence moyenne tension multi niveau VFD HARSVERT-A dans la pompe de condenseur, on peut la rendre économe en énergie et jouir d’avantages comme son bas investissement, ses résultats rapides, etc.
Mots clés : ultra-supercritique, eau condensée, variateur de fréquence VFD, boucle fermée.

1.      Introduction

Huaneng Yingkou Power Plant is located at the Liaoning Yingkou Economic-technology Development Zone. There is sea very close to the north of the power plant, and the southwest part is separated from Yingkou port only by a wall, to the north of the sea side making the typical port power plant. Power plant second phase was two sets of 600MW ultra-supercritical coal-burning generator units which put into production in August 31, 2007 and October 14, 2008 respectively. Since the operation, the units have been showing good operating results.

3 # 600MW steam turbine is configured with 2 vertical s with 100% capacity, driven by the motor. Operation method is one unit is for operation and the other unit is for backup. Using the method of driving the condensate pumps at constant speed, the system shows the following problems：

1. Great damper loss of valve adjustment, high pressure on exit, serious damage in pipe, low system efficiency and waste of energy.

2. When the flow is reduced, the valve opening is reduced , the pressure difference before and after the adjustment of the valve increases. The work safety also decreases, great loss in pressure, and more energy is consumed.

3.Opening low valve 40 ~ 70% for long accelerates wear and tear in the valve body, resulting in deterioration of the valve control characteristics.

4. The high pressure of pipe network threats the sealing properties of the system device, leading to severe valve leakage.

5. Equipment has short life-time, needs more daily maintenance, high maintenance costs, resulting in a great waste of resource.

One important means to solve the above problems is using frequency converter technology for adjustment, using medium voltage variable frequency drive for the operation of condensate pump, achieving variable flow load regulation of deaerator. In this way, it not only solves the control of linear difference of valve regulating, pure lag, delay and difficult in control, but also improves the reliability of the system; more importantly, it reduces flow loss while adjusting the valve, reduces wear and tear of the valve, reduces the damage and sealing performance of the system pipeline, increases the device lifetime, reduces maintenance cost and improves system efficiency and saves the energy, provides a good way to reduce power consumption rate of the plant.

Table 1:Turbine specification

Table 2:Condensate pump specification

2.      Power system plan

After the plant research, combining with the actual situation in our plant, our plant ultimately decide using frequency converter with manual bypass for condensation water system and we choose the HARVEST-A model medium voltage VFD manufactured by Beijing Leader & Harvest Electrical Technologies Co., Ltd. Equip a medium voltage inverter, by switching medium voltage isolation switch with medium voltage VFD to operate the condensate water pump. Medium Voltage VFD can drive A condensate pump motor to achieve the variable frequency operation, by switching can also drive B condensate pump motor to achieve variable frequency operation. Condensate pump motors on both sides have the frequency bypass function. Any motor can run in variable frequency state and the other motor run in normal frequency. When the medium voltage VFD has fault, the system can inter-connect another motor to normal frequency working state. Principle of the System is shown in Figure 1:

Figure 1: Diagram showing the conversion of condensate water system with VFD

 The basic principle of the system: It consists of six medium-voltage isolating switches QS1 ~ QS6 (see Figure 1). Among them QS2 and QS3, QS5 and QS6 are electrical interlocked; QS1 and QS5, QS4 and QS6 are mechanically interlocked; QS2 and QS5, QS3 and QS6 are electrical interlocked. If there is power supply from both way at the same time, A condensate pump works in variable frequency, B condensate pump works in normal frequency, QS1 and QS3, QS6 open, QS2, QS4 and QS5 close; when B condensate pump works in variable frequency, A condensate pump works in normal frequency, QS2 and QS4, QS5 open, QS1, QS3 and QS6 close; if VFD has to be repaired, QS1 and QS4 can be closed and the other isolating switches are in open state, two loads can operate direct on line at the same time; when the power supply in one way has to be repaired, you can operate any motor in variable frequency state by using division isolation switch.

When A VFD condensate pump trips off, the system connects and drives condensation pump B, QF2 operates in direct on line. When B VFD condensate pump trips off, the system connects and drives condensation pump A, QF1 operates in direct on line.

3. System control method:

(1) Reform principle

Condensate Pump with VFD reform has to ensure that the quality of deaerator water level regulation is the same, and work under some special working conditions such as pump trip-off, low water pressure, etc. By using existing equipments and systems, the original two water level regulating gates are full open to reduce the throttle loss. After VFD trips off, the backup condensate pump starts immediately in direct on line. Condensate water reaches the exit of main pipe to ensure the stability of the water in the deaerator during trip-off of the VFD. The degree of the opening of two regulating valve doors is calculated by actual load. Close up to the indicated position within 10 seconds quickly to reduce the system disturbance to minimum and maintain deaerator water level within normal range and make sure units normal operation.

(2) The implementation of the actual reform

Turn on/off the VFD by connecting and disconnecting the 6kV switch of the condensate pump, in other words, by pressing “ON” and “OFF” button in the operator panel of the condensate pump, the operator can connect/disconnect the 6kV switch as well as on/off the VFD. As one VFD controls two condensate pumps so at a time only one pump can work with VFD and the other pump is direct on line. Logically pump has VFD mode and direct on line mode. At the same time we add a set of safeguard and a set of interconnection to the original system, namely when the VFD is in serious fault, the condensate pump trips with protection and the backup condensate pump starts.

During normal operation, one condensate pump works in variable frequency state, while the other condensation pump works in direct on line. When VFD runs and enters into automatic mode, the regulator door of the deaerator is forced to open up to 95% according to a certain rate (to reduce disturbance). By means of the variable output frequency the VFD adjusts the speed of the condensate pump, and by controlling the water amount from condensate pump to the deaerator it ensures the stability of deaerator water level within expected range. When the water level fluctuation occurs, cascade loop composed of main water flow, deaerator level and condensate water flow in DCS outputs speed command to VFD, adjusts water amount in the condensate pump to keep stability of deaerator water level.

When the local equipments are broken, such as the “serious fault alarm” in the VFD or a sudden trip in condensation pump; etc, the MV switch of condensate pump gets open, and MV switch of the other spare pump gets closed to start the spare pump in direct on line. VFD automatically switches to "Manual" mode and the two adjusting doors automatically switch to "auto" mode. When the direct on line pump starts, the opening degree of deaerator door is still 95%, which causes condensation water sudden-increase. To prevent water level in deaerator from exceeding the set value, two adjustment doors must close to the appropriate position in the shortest time. So it is logically designed that during the “automatic” mode of the VFD, the opening of the adjustment door changes according to the actual load variety. Once the VFD changes into “manual” mode from “auto” mode, the adjustment doors are forced to close up to as required by the load and then enters into “auto” mode. This opening degree is also the ideal degree in direct on line state. When the adjustment door closes up to the value as calculated from the load and keeps stable, the entire VFD fault switch completes.

4.       Cooling system method

Because VFD has heat loss during operation, in order to ensure that VFD has a good environment for operation, an independent cooling system is required to equipped with VFD. According to the actual condition of the field, investment and operating costs of the integrated cooling system, equipment maintenance, trouble-free run-time, actual installation location, total heat, total operating costs, construction costs and other factors, the Forced closed loop cooling method is chosen for this system.

In order to protect the safe operation of VFD, to avoid the negative effect of environmental temperature and dust in the equipment, the forced loop cooling is independently installed in the side of power-cell cabinet of VFD. This system acts as the supplementary installation to the VFD power cell cabinet, keeps power cell cabinet operating at range of 25 ~ 35 ℃. It significantly extends the replacement cycle and reduces the maintenance cost. No need to re-construct a room for VFD. Transformer cabinet adopts open type cooling mode. Forced cooling device and VFD power cell cabinet are integrated in design, attached to the top power cell cabinet. Refrigeration compressor is installed near to VFD cabinet. Forced closed loop cooling system is shown in Figure 2:

                                 Figure 2: Forced closed loop cooling system

During actual operation, the forced closed-loop cooling system can fulfill the radiating requirement of the medium voltage VFD. Equipment installation is simple, fast and high efficient in heat exchange。

4.       Analysis of energy-saving effect

In different conditions, the parameters before and after the reform are shown in Table 3:

Table 3:Acutal operating parameters of the condensation pump and motor

After calculation, energy saving rates for different loads are shown in the diagram 3:


                             Figure 3: Energy saving rates at different loads

Calculating as #3 Units average power generation in 2008 July, operating hour 744 hours, power generation capacity 0.32809 billion kW • h, average load 440.9MW, load rate 73.5%. From Table 3 it can be seen that current saved per hour is 63.3A. Power saving rate is : P=1.732×6×63.3×0.93=611.77kW·h.

Calculating as 0.393 yuan/kw.h, the electric energy saved per hour is equal to 240.43 yuan RMB, taking 8348 hours as operating time in one year, the energy saved by each unit in one year is equal to 2,007,300 yuan RMB (USD 295191). With the change in the supply and demand of electricity market, using hour of the unit and load rate decline. According to data in Table 3 and Figure 3 it can be seen that the lower load factor, the energy-saving effect is more remarkable.

4.       Conclusion

In reform of 600MW ultra-supercritical generating units condensate system with medium voltage VFD, the installation location of VFD is near to the condensation pumps and thus it saves the cost of medium voltage cable and construction. Cooling system adopts closed loop cooling structural design, using wind recycling loop, low dust, stable environment, low external environmental influence, greatly reduces maintenance, work strength of the maintenance personnel.

After operating Condensation water system it shows good performance of the tests. Noise and vibration during the closing of the two adjustment doors are significantly reduced. Energy saving rate of the condensation pump motor reaches up to 50%. Average energy saving rate is 34.6%. The water pressure in deaerator drops from 3.7MPa to 1.2MPa, especially at low load, the electric current in condensate pump reduces from the original 200A to as low as about 60A. Energy-saving effect is very significant and reform with VFD is very successful.

About the author: Zhu Dongsheng, male, senior engineer, Huaneng Yingkou Power Plant Production Department, is mainly engaged in electric professional and technical management.