Variateur de fréquence moyenne tension dans turbine de générateur 300MW
2010-1-4 10:59:56
L’entraînement par variateur de fréquence peut permettre une meilleure opération des équipements et une augmentation de l’efficience, tout en économisant de l’énergie. La technologie de variation de fréquence est grandement utilisée dans les pompes et ventilateur. A présent, le variateur de fréquence moyenne tension MV VFD est également en développement. Nous avons participé à la reconstruction de générateur 2*300MW avec variateur de fréquence moyenne tension MV VFD et avons eu de considérables bénéfices économiques ainsi que beaucoup d’expérience.

Reconstruction scheme

Condenser pump is one of main auxiliary equipments in turbine system. It brings condensing water to deaerator through heating. Because of constant speed operation, flow only can be controlled by valve. Great throttle loss, high outlet pressure, damage on pipe and low efficiency lead to frequent leakages and energy waste. Furthermore, this kind of mechanic adjustment structure has bad quality, linearity and low switch rate. Frequent adjustments cause lots of faults, which increases maintenance cost and wastes resources.

2 100% capacity condenser pumps are equipped with 300MW generator. Model is 9LDTNA4. Rated flow is 841m³/h. Water lift is 280m. Speed is 1480rpm. Asynchronous motor power is 1000KW. During reconstruction, we add MV VFD to implement stepless adjustment. Power varies with load variety. It heightens efficiency and optimizes operation. Reconstruction complies tenet of “ small change, more reliability, best economic benefit”.

Advantages: make full use of existing equipment; low cost; keep original control mode same and ensure motor to run in direct on line when VFD is broken; easy maintenance.

Disvantages: complicated connection; need to add MV breaker; need to add close circuit.

Normal operation: take an example of 51# pump in variable frequency state. Power goes to VFD through 50# switch, then output to 51# motor through 510#. At this time, direct on line switch of 52# is spare. Logics of each switch is following:

Conditions of switch on in direct on line:

Conditions of switch on of VFD

Operation process is following when a pump is broken.

When the pump controlled by VFD is broken, the other pump switches to direct on line automatically (same as original switch mode). When the broken pump is well repaired, it switches to variable frequency state. The spare pump runs only in direct on line, can not run in variable frequency state.

Switch sequence:

     a）close 52# direct on line switch; start 52# pump in direct on line;

     b）open 510 switch; close 51 switch; 51# pump run in direct on line;

     c）open 52 switch; close 520# switch and 52# pump connect to VFD; start 52# pump in variable frequency state.

     d）after 52# pump receives total load, stop 51# pump as spare, switch finishes.

Choose VFD

After collecting relevant material and research, we finally decide to use Leader Harvest VFD. Parameters are in table:

VFD characteristics:

（1）Medium-medium voltage source VFD; no need to add output transformer; VFD is connected between 6kV and motor.

（2）Single phase multi-level inverter circuit is good for both power source and motor. For power source side, it is perfect without harmonic; for motor side, its output is perfect sine waveform. VFD connects to motor directly, no need to add filter.

（3）High efficiency and high power factor. Components are reduced greatly because of diode rectifier and IGBT. So it is more reliable and less loss. VFD reaches up to 97% because of distinct control technology. Power factor reaches ip to 0.95 because of phase-shifting.

（4）controller is composed of SCM, HMI and PLC. It is PWM control and V/f control with high precision and quick response.

Economic benefit

1. Obvious energy saving effect

Yearly power generation is 1.5 billion kWh, power saved is 3300000 kWh, cost saved is 1 million yuan(USD 147058).

2. Reduce current impact

In the case of direct start-up, current is 7 times of rated current; in the case of star-delta start -up, current is 4.5 times of rated current; in the case of soft start, current is 2.5 times of rated current. Observing curve of VFD start-up, we find there is little impact. Current climbs up from zero, increases with speed rising and would not beyond rated current. Thus VFD eliminates impact on motor, reducing maintenance cost.

3. Prolong equipment lifetime

  VFD adjusts motor speed according to accelerating/decelerating curve of condenser pump. Due to no force on bearing, its lifetime is prolonged. Some data show that lifetime is directly proportional to reciprocal of speed. Reducing condenser pump speed can prolong its lifetime greatly.

4. Reduce noise

VFD not only reduces pump speed, but also lowers noise. When speed is 50% down, noise reduces 10 dB. Meanwhile, skid and scream during stop/start are eliminated. VFD overcomes faults of pipe impact, resonance and strong vibration resulted from bad linearity and quality.

In conclusion, VFD reduces power generation cost a lot, heightens competitive power.