INVT GD5000 MV VFD is Applied in the Power Plant 3# of Mongolia

Abstract
According to the application of high voltage inverter in induced draft fan and forced draught blower, the energy-saving effect after the transformation is analyzed.

Keywords
Induced draft fan, forced draught blower, MV VFD inverter, energy saving, Ulaanbaatar

I.Overview

Induced draft fan and forced draught blower are the main equipment of the fan system of thermal power plant. They are one of the main auxiliary engines of thermal power plant, and their power consumption accounts for about 1.5-2.5 of the generating capacity of the power plant. The safety and economy of their operation is directly related to the safe and economic operation of generator set and even the whole power plant. Therefore, the reliability and economy of the blower operation has always been the issue of great concern to power generation enterprises, and controlling the volume of air intake and stabilizing the negative pressure value of the furnace are of great significance to ensure the boiler safety and economic operation.

Current medium voltage drive is widely applied to the field of power plant fan, MV VFD to introduce power plant Induced draft fan and forced draught blower for transformation will produce a lot of positive effect, it is able to run to a great extent, the fan in the process of realization of the goal of saving energy and reducing consumption and improve the system performance, can directly bring objective economic benefit for power plant.

II.Transformation situation

1.Operating Conditions
The third power plant of Mongolia is one of the three power plants (the power plant 2#, the power plant 3# and the power plant 4#) in Ulaanbaatar, ranking the second among the three power plants with a total installed capacity of 198MW, including 4*25MW generators, 4*12MW generators and 1*50MW generator. There are 13 high forced draught blowers and 2 induced draft fans. The previous way is direct power frequency start, because the power is not large, so there is no soft start device. As the power generation load is adjusted in a wide range, the wind volume of the fan needs to change with the change of the power generation load, and it often operates inefficiently in the power frequency state, resulting in a large amount of energy wasted on the wind duct baffle.

According to the change of unit load, the unit control system transmits the load instruction to the fuel system to realize the automatic balance of the whole system and achieve the purpose of safe and stable operation. Therefore, by adjusting the primary air pressure and air volume, the rapid load response of the fuel system is realized, and the high requirements are put forward for the supply and use of air volume. On the one hand, the system has strict requirements on wind pressure, which too much influences the flame shape of the burner and too low wind leads to insufficient combustion. On the other hand, the control of the system on the air volume is affected by many factors, the change of wind temperature, boiler load change, all directly affect the control index of the air volume. Since the air volume of the induced draft fan and forced draught blower cannot be adjusted at present and cannot well meet the needs of the boiler's combustion capacity and stable operation, the two forced draught blowers and one induced draft fan are reformed by frequency conversion according to the opinions of INVT to the customer. This can not only reduce energy consumption, but also effectively control the real-time variation of the air volume, and achieve performance regulation at the same time to save energy.

Refer to the following picture for the system process flow,

2.Primary power system strategy
In order to fully ensure the reliability of the system, The MV VFD is equipped with a bypass cabinet at the same time. When the MV VFD is fault, the MV VFD stops running and the motor can be directly manually switched to the power frequency run. The power frequency bypass consists of three MV isolation switches QS1, QS2 and QS3 (QF is the original MV switch of Party A). It is required that QS2 cannot be turn on with QS3 at the same time and interlock is realized mechanically. When MV VFD is running, QS1 and QS2 are turn on and QS3 is turn off. QS3 is turn on and QS1 and QS2 are turn off during power frequency operation. The worker can switch power frequency and frequency conversion according to the actual requirements of the site.

In order to realize the protection of the converter fault, the MV VFD interlocks the 6KV ‘Users' switch’. Once the MV VFD fault occurs, the MV VFD will switch off the Users' switch, and party A is required to appropriately transform the closing and opening circuit of Users' switch. In the case of power frequency bypass, the frequency converter shall allow the Users' switch to turn on and cancel the trip signal to the Users' switch, so that the motor can start normally through the Users' switch to turn on the power frequency.

The design scheme is as follows

3.Control mode
Three MV VFD were placed in the same newly built room, about 60 meters away from the control room. We use the switch to realize two HMI to control the same MV VFD (one HMI in the MV VFD room and other one HMI in the control room), and use the network cable to realize high-speed communication. In this way, the customer can fully control the MV VFD in the control room, and can monitor and view all data of the MV VFD in real time, the most effective way to realize the control of the MV VFD.

The following is the communication diagram

III.VFD principle

INVT GD5000 series VFD is a high-tech energy-saving product, which adopts advanced IGBT power unit multi-level technology, digital control technology and SPWM pulse width modulation technology. It has the advantages of high power factor, high efficiency, energy saving, high stability, etc, and has the rated power energy feedback ability.

Each phase of the VFD is composed of six power units in series, and the three-phase Y is connected to the output. The output adopts the SPWM wave superimposed by six power units of each phase to form high-quality output waveform, which is supplied to the motor to drive the load.

Optical fiber communication is adopted between control cabinet, power unit cabinet, transformer cabinet and bypass cabinet to prevent electromagnetic interference. Achieve complete isolation of high and low voltage, improve safety.

The input side of the power unit is composed of fuse, rectifier and filter parts, and the rectifier part is composed of six diodes. The inverter consists of four IGBT H bridge inverter outputs, and the motor is driven by the unit series.

Power unit topology

Topology of VFD system

IV.Application advantages

1.Energy saving space is large
Before the transformation of the VFD, the induced draft fan is operated at a power frequency of 50HZ with no speed regulating device, the damper plate is adjusted to meet the demand of air volume. The actual operating voltage is 6kV, the current is 74A and the motor power factor is 0.78.

After the transformation of the VFD, the actual operating frequency is 44HZ and the actual input current is 52A.Baffle changed to maximum to reduce ventilation resistance. The power factor is 0.96 and the input voltage is 6kV.

Before the transformation of the VFD, the induced draft fan is operated at a power frequency of 50HZ with no speed regulating device, the damper plate is adjusted to meet the demand of air volume. The actual operating voltage is 6kV, the current is 39A and the motor power factor is 0.78. After the transformation of the VFD Drive, the actual operating frequency is 20HZ and the actual input current is 17A.Baffle changed to maximum to reduce ventilation resistance. The power factor is 0.96 and the input voltage is 6kV.

2.Reduce the current shock when the motor is started
When the motor starts directly, the maximum starting current is 7 times of the rated current. Motor soft starter is also 2.5 times. Double inspection of actual operation of the INVT MV VFD, it can be found that the start is basically no impact. The current from zero, only with the increase of speed and rise, basically will not exceed the rated current. Therefore, frequency conversion operation solves the problem of large current impact when the motor is started, eliminates the impact response force of large starting current on the motor, the transmission system and the host, and greatly reduces the daily maintenance cost.

3.Extend equipment life
The use of MV VFD can make the motor speed change along the acceleration and deceleration curve of the fan, without the effect of stress load on the bearing, prolong the life of the bearing. At the same time, the relevant data show that the mechanical life is proportional to the reciprocal of the speed, reducing the speed of the fan can double the life of the fan, the fan's use cost will naturally reduce.

4.Noise reduction
Fan after using MV VFD, reduce the speed of the fan running at the same time, the noise greatly reduced, when the speed reduced by 50%, the noise can be reduced by more than ten decibels. At the same time, it eliminates the skid and sharp contrast during shutdown and startup, and overcomes the defects of strong resonance caused by hammer strike and resonance of pipes due to poor linearity and poor quality of adjustment. After the fan frequency conversion operation, the noise and resonance are greatly reduced and the changes are considerable.

Photos of the converter in operation

V.Conclusion

After being used to adjust the blower and induced draft fan in the thermal power plant, the high voltage frequency converter can prolong the service life of the motor and blower, improve the production efficiency and remember the automation level, improve the reliability of the operation and power supply of the thermal power plant, save the energy and the cost of inspection, and bring great emergency benefit and social benefit to the thermal power plant. Especially the thermal power unit takes up the proportion of the total installed capacity is very high, the high-power auxiliary machines have traditional baffle valve control for inverter adjusting condition a lot, after transforming energy saving potential is very large, especially for coal-fired power plants in saving energy and reducing consumption, improve the economic benefit, improve the competitiveness of the feed-in tariff plays a huge role.

Reference
[1] Goodrive 5000 Series Medium Voltage Variable Frequency Speed Control System