Yangzhou Second Power Generation Co.Ltd. is the biggest thermal power plant in Jiangsu province. It has 4 sets of 600MW generator unit. 1#, 2# unit are subcritical coal-burning generators. 3#, 4# unit are supercritical coal-burning generators. With increasing market competition and grid peak-valley difference, large turbine generator with base load is required to join peak adjustment. Generator unit joins peak adjustment mainly in low load. It is a conventional mode. In order to increase force, generator unit should run in load as low as possible. But auxiliary equipments are designed as biggest load. It controls flow just by adjusting valve and damper opening in low load, which causes lots of energy wasted. How to optimize operation mode and reduce energy wasted is a question that power plant has to face to.

In order to solve the question, we orgnize an expert board to research. Finally, we find variable frequency technology is the best way to save energy. Then we purchase VFDs produced by Leader Harvest through inviting bidding. It saves energy a lot and runs stably.

I. Reform scheme

1. Speed adjusting principal

Work characteristics of pump is shown in Fig 1:

Fig1. speed adjusting principal

     When flow is from Q₁ to Q₂, if pump runs in constant speed, work point is from A to B. Pressure increases, threating network safety. Work point will be from A to C in speed adjusting mode. It provides flow demand and keeps pressure same. Difference of output power between B and C: P_B －P_C＝（H₃－H₂）×Q₂

      On point A, C, although speed is different, the outlet pressure and network pressure still keep balanced because of different flows in two situations. For need of pressure balanced, when pump runs in parallel, pump speed can not be less than N₃，otherwise there is no water output.

    If adjusting speed in system of no changeable characteristic, without requirement for water pressure, this mode is better than constant pressure in energy saving.

2. Principal of speed adjusting

HARSVERT series VFD adopts multi-level series structure. Drawing is shown in Fig2:

Fig 2  structure of Harsvert VFD

    There are 24 power cells in 10kV series. Each phase is composed of 8 power cells. Each cell has the same structure and they can be replaced by each other. Circuit is AC-DC-AC single phase inverter. Rectifier is diode three phase full bridge. It realizes PWM control by IGBT.

3. Bypass control

Fig 3. Bypass control drawing

Principal: There is a breaker QF2 equipped on VFD as output switch. Keep original 2 breakers. These 3 breakers switch between variable frequency/direct on line. QF1 interlocks with input/output breaker to ensure VFD circuit and direct online circuit not to close simultaneously. In variable frequency state, VFD input/output switch close, QF1 opens. In direct on line state, open VFD output and input switch first, then close QF1.

In addition, keep motor differential protection. Change motor neutral point CT again, connect its control signal to QF2 switch cabinet, make it join differential protection to improve motor protection.

II. Energy saving analyse

1. Parameters

2. Calculation

III.Cooling mode

Although VFD efficiency is more than 96%, there is still 4% loss in full load. Transformer loss accounts for 50% and inverter circuit loss accounts for 50%. For 2000kW VFD, max loss is 80kW in full load. All these is converted to heat. So, the room termperature increases quickly after VFD operation. VFD can not work stably. In order to enhance VFD stability and solve heat problem, we use air conditioner for power cell cabinet and wind duct for transformer.

Fig 4: power cell cabinet cooling

Cooling device is on top of power cell cabinet. Two compressors cool for power cell cabinet. The connection between cooling device and site is simple. It only needs two 380V AC. In normal operation, each power source supplies power to each compressor. When a power source faults, the other supplies power to two compressors. When two compressors stop, power cell cabinet can cool itself by air channel. The integrated cooling method solves the heat problem and reduces system fault.

Fig 5: VFD is running in site

IV. Conclusion

Leader Harvest VFD meets user requirement as its advanced technology and stable performance. Cooling method reduces heat and cost greatly.

System realizes soft start through VFD. Start current is less than rated current and start-up time is prolonged. No big impact to power grid and machinery damage is reduced. It also reduces noise and vibration, prolongs equipment lifetime, saves much maintenance cost.

VFD application in our plant indicates the feasibility and good economy effect of VFD. We are going to extend variable frequency technology in reconstruction in future and make progress for energy saving.

Reference:

1. HARSVERT-VA series VFD technology manual

2. VFD reconstruction material collection of Yangzhou Second Power Generation Co., ltd.

3. Leader Harvest power material collection