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This paper introduces the feasibility of medium voltage inverter technology application in power plants. Some applications are introduced and the prospect of this technology is shown.Feasibility of inverter technology in thermal power plant
Power generation plant is manufacturer of power as well as consumer of power. In structures of Chinese power generation plants, thermal power plants account for 70%. For general coal-fired generator units, plant itself consumes 4%~7% of its generating power and auxiliary equipments, such as big capacity fans and pumps, account for 80%. Coal consumption and power consumption rate of plant have been important index for power plant and they have a direct relationship with economic benefit and company competitive power. Energy saving effect resulted from variable frequency adjustment on auxiliary equipments such as fan and pump can reduce power consuming cost and power generating cost for power plant. So it is very urgent to choose appropriate inverter.
With fast development of power grid, increasing of machinery capacity and peak valley of load, requirement for peak adjustment ability is higher and higher. Operation state of units must vary with load changing. Even for unit with base load, capacities of auxiliary equipments and motor are far more than the actual need, which results in "big horse pulls small carriage". So it requires that auxiliary equipments can work in variable frequency state to meet actual need.
Some relevant statistics show that about 15% faults of motor are caused by big starting current and big electromagnetic force on coil. Start directly also affects stator windings terminal weld-off, rotor squirrel cage strip-break and many other faults of auxiliary equipments. Frequency start-up resulted from peak adjustment operation causes more faults of equipments, shorts maintenance period and lifetime of equipments, increases maintenance cost and labour cost, results in great economic loss.
It is obvious that large auxiliary machinery designed as full load is not applicable to new operation mode. Original adjustment method has problems of source serious waste and bad adjustment quality. Frequent start/stop impacts auxiliary equipment, motor and power grid, which threats safety of units. Medium voltage variable frequency adjustment technology can meet requirement of auxiliary equipments through soft start. It provides a safe and feasible operation mode for large auxiliary equipment in power plant.
As a new electrical technology developing quickly in 1990s, medium voltage variable frequency adjustment technology mainly used in MV motor. Variable frequency adjustment technology is better than any other in speed adjustment mode. It is well known as its remarkable energy saving, high efficiency, high precision, wide adjustment range, perfect electric protection and auto communication. It is also safe, reliable, easy installation & maintenance, which makes it first preferred drive for power plant in energy saving field.
Gozuk inverter applicability
- Direct medium voltage input and medium voltage output, without harmonic filter or power factor compensator;
- Inverter can continue running with 30% voltage drop;
- Inverter does not stop but derates to run automatically when medium voltage is low. Inverter recovers original speed when voltage is normal;
- Inverter does not stop within 3s when medium voltage faults, which can meet demand of no stop during switching bus. But for other same kind drive, this value is only 100 ms.
Corporation with units
- Inverter can be connected to DCS of power plant
- Inverter accepts and outputs standard signals of 0~10V/4~20mA
- Inverter realizes data exchange and interlock control. Inverter can provide alarm signal and keep original output frequency same when analogue signal faults or short circuit.
- Inside PLC can change control logic easily, which meets site changeable need; there is also international external port. Inverter can realize open loop control and close loop control.
User special requirement
- Control power of inverter accepts AC 220V and DC 220V. With inside UPS, inverter can continue running and alarm when control power is broken.
- LV power source can be supplied by transformer secondary of inverter, which enhances reliability.
- User can choose manual or auto bypass cabinet. Motor will be switched to direct on line when inverter is broken through bypass cabinet.
Environment protection
During MV inverter operation in power plant, the biggest problem is harmonic pollution. The active way is that adopting multi-level PWM technology and improving circuit topology structure make electric device not bring harmonic and reactive power. Leader Gozuk inverter is designed according to this idea and it has got inspection certificate from authority institute.
Prospective development of MV inverter
Adjusting speed of auxiliary equipments such as fan and pump by variable frequency technology makes each parameter at the best state, which improves operation, enhances efficiency and saves power, water and coal, reduces production cost, brings lots of benefits for power plant. Meanwhile, inverter cooperates with control system of units, which can improve automatization level, save labour, enhance management level of whole system.
There are two aspects for inverter development in power plant. One is inverter itself development; the other is how to cooperate with progress of plant production technology and automatization.
For inverter itself development: development in big capacity and small dimension, modularization, voltage withstand and hardware universal are research goals in short time.
For application in power plant: manufacturer should improve inverter self-detection function; restore inverter fault quickly; solve problem of damper and valve quick response; digital connection to central system and so on. Furthermore, anti-dirt, heat, anti-disturbance and reliability should be researched and explored for a better mode.
Conclusion
MV inverter provides a feasible operation mode for auxiliary equipments in power plant. Its excellent performance and energy saving effect make its widely application in power plant. User should consider reliability, economy factor, applicability and environment protection when choose a inverter. Inverter is an important way to reduce cost and enhance competitive power for plant.