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Application of VFD In Power Plant
Abstract: The paper introduced the application of MV VFD on fan and pump in the thermal power plant.
Keywords: VFD energy saving

Preface
Frequency control is widely used in several fields for its superiority in energy-saving, adjustment precision, adjustment range and easy to communicate with automatic control system. This text is aiming to illustrate its broad prospects by introducing and concluding several success cases.

1. Frequency control of boiler's fan
1.1 Application of the boiler's SA fan
The boiler is circulation boiler which is equipped with ID fan, PA fan, SA fan. The parameters as below:
ID fan: Y400-6, 400kW, 6kV, 47.3A
PA fan: Y355-4, 250kW, 6kV, 29.3A
SA fan: Y315L1-4, 160kW, 380V, 289A

The statistic of fans before reform in 2000year is below:

PA fan
Average current Average opening Maximum opening Maximum current Minimum current
21(A) 50% 55% 23(A) 20(A)
ID fan
25(A) 48% 50% 28(A) 24(A)
SA fan
135(A) 45% 50% 140(A) 130(A)

As is known in boiler's regular service, there are considerable variety in fuel composition, heat load, power load, and seasons. Thus, the air for the boiler varies greatly. However, the fan of boiler is designed according to the maximum air flow in full load. In the mean time, there is certain redundancy for air flow in case of boiler's accident. Generally the power of fan is large. From the statistic, average opening of the wind shield in normal condition, 48% in ID fan, 45% in SA fan. Adjusting shield causes a large quantity of energy waste on overcoming the shield resistance. Plant efficiency is low.

At the end of 2000, we planned to choose SA fan of #1 to reform. Considering the fan redundancy, we purposely chose VFD of 132kW to control the fan of 160kw. VFD model is FR-F540L-132K, 380V. VFD started to run in January 2001. By a long term running, the current of the fan reduces from 135A to 75A, which reaches obvious energy saving and meets the requirement of boiler operation. According to the ammeter, energy saving rate is 45% and the cost is recovered in one year. Furthermore, the motor is improved in therms of start-up, adjustment, controlling and operation. For the perfect benefit, we reform the fans in the other unit later. In design of bypass circuit, we choose double-throw switch in manual as its low-cost, good performance, easy to connect and install and high reliability. The circuit is below:

Figure 1 –bypass circuit of SA fan

2. Application of VFD on ID fan

After the success VFD application of SA fans, we see enormous potential benefit. In 2002, we chose ID fan of #3 to reform. By comparing and studying, we consider HARSVERT-A06/050 as the first choice as its high performance, reasonable price, widely using and good after-sale service. HARSVERT-A06/050 is Multi-level voltage source VFD with the technology of cascade-connected power cells. The rectifier is 30-pulses, diode three-phase full bridge, IGBT inverter bridge series output. In order to fully ensure the system reliability, a bypass device is configured with MV VFD. When VFD is broken, motor can be manually switched to direct on line. The bypass circuit is composed of 3 MV disconnecting switches QS1, QS2 and QS3. When the motor is driven by VFD, QS1, QS2 are closed and QS3 is open; when the motor is in direct on line, QS3 is closed, QS1,QS2 are open.
VFD is remote controlled through the start/stop buttons, switches for speed adjusting on the operation board of boiler, and it is able to communicate with DCS. To make operation convenience, there are many functions, such as alarm instruction, fault instruction, ready state, bypass, running, MV allowable, emergency stop, speed set, actual speed etc..
Period of commissioning, installation and tests was 4 days. We also made some dynamic tests such as fully-loaded, system threshold vibration test, bus voltage-hunting test, interlock test between MV switch and VFD and continuous 72 hours preparation.
According to the data, the current of the fan reduces from 25A to 15A, which means around 130 kW/h is saved. The energy-saving ratio is more than 50% and the benefit is obvious. Due to perfect economic benefit, we would reform other devices.

3.Application of VFD on pump
Plant consumes daily water about 6000~7000t which mainly is used as water-refining, industrial water, water for life. There are three pumps in the pump room. The parameters are below:

Serial number power Rated current Flow
1#pump Y225S-4 37 kW 70.4 A 400t
2#pump Y225S-4 37 kW 70.4 A 400t
3#pump Y180M-4 18.5 kW 36.5 A 200t

On the one hand, 3# pump force is too small to meet the demand of the consumption. Other 2 pumps force is too large to control, and this leads to motor frequent stop/start. On the other hand, the ideal method for purifying is water-supply continuously which is high efficiency and good effect. If the pump frequently stops/starts, it may lead to water break. When water conditioner stops, the plastic pipe is easy to be aged for exposing in the sunshine long term. If the water conditioner stops too long , it should be rewashed so that water is wasted considerably. Furthermore, it is 600 meters long between the pump room and the water conditioner, and the pipe is laid too shallow to be easy burst in winter. All above are caused by long time water-stop, seeper in pipe, water freezing in low temperature. Thus we plan to reform for 1#, 2# pump with VFD.

Figure 3---connecting circuit,dashed is reformed

4. Problems in technology reform
4.1 The boilers' security is basic guarantee for the plant. Although the VFD is reliable, once it faults, system must make sure boiler work well. Thus, bypass circuit is required. Generally, if manual bypass meets demand, it is better not to choose automatic bypass cabinet. For normal low-power motors, manually/automatic bypass by double throw switches is ideal.
4.2 For large inertia-load motors (boilers' ID fans), there might be flexural resonance. Once it happens during operation, fans and motor might be damaged seriously. Thus the critical speed must be calculated and some measure mut be done, such as set frequency leap point, soft connection, add rubber.
4.3 After reform with VFD, if VFD works below 25Hz long time, with speed reducing, motor heat reduces. Thus, temperature of motor declines and it works normally without overheating.
4.4 VFD can not transfer power back to grid, thus it requires that QS2 and QS3 can not switch off at the same time for ID fan.
4.5 Low-voltage VFD which is smaller, easy to choose the installation site in technology reform. But for medium-voltage VFD which is much larger, generally composed of 4-5 cabinets, needs a room. Thus the site can not be too far from the motor. It requires a better circumstances for VFD stable and reliable operation. Temperature must be between 0 and 40℃. Air conditioning should be equipped if temperature is too high. At the same time, room inside must be clean, without dust, corrosive and explosive gas.
4.6 Make sure reliable earth. To avoid signal interfere, user should make independent earth system. Earth resistor is less than 4. signal wire to VFD must be screen cable. One terminal should be well earthed.
4.7 Considering practical operation situation, user can choose lower power VFD. For example, we use 132kW VFD to control 160kW motor in reform of SA fan.

5 Concluding remarks
With the development of power electronics, the performance of VFD are improving. In thermal power plant, there are a large number of pumps and fans, and VFD is widely used in technology reform for energy saving.

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