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Analysis of the Performance of Induced Draft (ID) Fans at a

600MW Thermal Power Unit before and after Installation of MV

Variable Frequency Drives (VFD)

Hebei Datang International Wangtan Power

Company, China

Authors: Gao Zhenhua; Lu Changhai

Abstract: This article evaluates the results vis-à-vis energy savings, and enhancement of

reliability after the installation of Variable Frequency Drives (VFD) at the Hebei Datang

International Wangtan Power Company, China (WPCL)

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1. Foreword

1.1 Introduction of parameter

WPCL commissioned two thermal power units of 600 MW each (2x600MW). The boilers

were all supplied by Harbin Boiler Company Limited and each boiler comprises of two rotary

air pre-heaters and two AN37e6 (V19+4) type adjustable Axial Flow type Induced Draft (ID)

Fans (IDF) manufactured by Chengdu Electric Power Equipment Co. The motors are made by

Shanghai Electrical machinery plant.

The primary parameters of the induced draft fan and motor are as follows:

ID Fan Parameters Motor Parameters

Particulars Unit Design Parameter Particulars Unit Design parameter

Type Static-Blade Axial-flow

Fan Manufacturer

Shanghai Electrical

Machinery Co.

Specification AN37e6(V19+4°) Specification YKK900-10

Manufacturer Chengdu Electric Power

Equipment Co. Rated speed r/min 597

Control Mode Inlet Flow Adjustable Rated power kW 3,800

Max Air

quantity m

3/s 488.11 Rated voltage kV 6

Air pressure Pa 4142.7 Rated current A 448

Shaft Power kW 2332 Power factor 0.845

Max Permitted

Medium Temp °C 118.5 Efficiency ％ 96.7

Medium

Density kg/m3 0.8912

Bearing

Lubrication

Method

Forced Oil Cycle

Rotation speed r/min 585 Fan Efficiency ％ 84

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1.2 Analysis of energy consumption for induced draft fan

Units #1 and #2 of WPGCL have been put into operation on December 7, 2005 and

December 28, 2005.

It was observed that the capacity of the boiler ID Fan was considerably higher than required.

When the unit operated at full load, the ID Fan ran at 60% to 70% of its rated load. The

actual current of the fan was only 60% of the rated current when the fan was operating at

maximum power output.

Data showed that the type of fan was not suitable and the induced draft fan operated within

a low power output area over a long period of time, thereby increasing power consumption,

raising coal consumption of unit and increasing costs of operating the unit.

In order to improve the operating environment of the ID fan, and reducing the electricity

consumption of the unit the North China Electric Power Academy of Science (NCEPAS)

carried out a series of tests on the induced draft fan of 1# unit. Primary data of spot tests are

as follows:

Test table of the performance of the fan at typical working condition is as follows:

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# Name Unit Operating

condition1

Operating

condition2

Operating

condition 3

Operating

condition 4

Operating

condition 5

1 Unit load MW 600 500 450 400 300

2 Volume flow of #1 fan M3/s 498.4 413.7 388.6 344.0 245.5

3 Mass Flow Rate of #1 fan kg/s 423.5 353.0 333.0 307.1 217.9

4 Pressure of #1fan Pa 3147.9 2545.5 2049.9 1605.8 1301.8

5 Pressure Energy of #1 fan N·M/kg 3765.4 3021.8 2416.8 1813.1 1475.8

6 Interior input power of #1 fan kW 2376 1800 1584 1224 1116

7 Operating efficiency of #1 fan % 69.12 61.34 52.64 47.36 30.11

8 Volume flow of #2 fan M3/s 496.21 495.08 401.87 325.08 330.96

9 Mass Flow Rate kg/s 444.20 441.86 365.18 305.8 293.39

10 Total press of #2fan Pa 3088.2 2600.6 2179.2 1689.5 1676.3

11 Pressure energy of #2 fan N·M/kg 3504.9 2952.2 2424.4 1811.1 1903.3

12 Interior input power of #2 fan kW 2304 1872 1512 1224 1116

13 Operating efficiency of #2 fan % 69.5 71.9 60.7 47.1 52.1

As per the test data, it was observed that the high capacity of the boiler IDF is resulting in

the poor operating efficiency of the fan and substantially higher energy consumption. A

considerable negative impact on energy conservation was noted.

Furthermore, the operational performance of the two units was mismatched; the output of

#1 fan is far higher that the output of the #2 fan. This poses a threat to the stable operation

of the units.

At the unit load 600MW:

The current of #2 fan is 313A~317A

The max actual current of each unit is not more than 340A

The Max Actual Power of the fan is: 340A × 6kV × √3 × 0.845 = 2986KW

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Redundant capability is: 3800kW – 2986kW = 814kW

At a Unit Load of between 300MW to 350MW:

The current of #2 fan is 205A~225A

The max actual current of each unit is not more than 225A

The Max Actual Power of the fan is: 225A × 6kV × √3 × 0.845 = 1975kW

Redundant capability is: 3800kW – 1975kW = 1825kW

With modification using a VFD it would be possible to:

Fix the opening of the stable blade at total opening degree (100%)

Minimize resistance to flue gas flow

Decrease system throttle losses

Raise the efficiency of the fan

Upon changing throttling adjustment to shifting adjustment there could be:

An effective avoidance of flue gas surges

Increased stability of the fan

Increased reliability of the fan

As per the analysis above pertaining to the stability, reliability and operating costs of the fan,

it was concluded that the fan would be suitable to be driven by a VFD

1.3 Operating stability analysis of the fan

From the operating data of the fan it was observed that with a unit load 70% of the rated

load, the fan usually operates in an unstable manner. When the unit operated at a variable

load condition, the fan often stalled or surged - this situation was detrimental to the

operational safety and life of the fan.

1.4 Selecting the appropriate VFD

Considering the relationship between the stability of the fans and the economical operation

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of the unit, earlier generation VFDs converted “standard frequency” (SF) to “variable

frequency” (VF), or vice versa. In order to do this, one of the ID Fans needed to be stopped

and therefore there was a 50% decrease in the unit’s output. This situation negatively

impacted the safety and economical operation of the unit.

In view of this, when a VFD is selected, an important feature to consider is the ability of the

VFD to switch operating conditions while the equipment is operational. In order to satisfy

this condition we selected the DHVECTOL VFD provided by DHC.

2. Proposal to use the appropriate DHC VFD

2.1 The unit must operate stably after installation of VFD

One VFD was installed per ID fan with control unit housed in a temperature-controlled

environment located at a permitted distance of within 1,000 meters of the two fans.

Entries and configurations for the operation of VFDs were made in the DCS system being

used in the plant. Operating personnel can adjust the speed of the ID fan or put the VFD into

operation by means of the DCS system. It can also send automatic controlled messages for

the fan in case of indicated negative pressure in the boiler. This will result in controlling the

speed of the fans and adjust boiler pressure.

VFD M6.0kV

6.0kV

MOTOR

Main Breaker QF

Bypass Breaker QF2

Isolator QS1 Contactor KM1 Isolator QS2Contactor KM2

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Figure 1: Primary circuit figure of the fan VFD

In order to avoid operational errors, interlocking is provided between the VFD contactor

(KM2) and the bypass breaker (QF2) of the VFD, so that only one switch can be activated at a

time.

2.2 VFD Logic

Request runnig

VFD fault

Start

DCS sends running

instruction

Pre-charging

Finish pre-charging

Turn on main

breaker

Main breaker

connected

VFD running

Adjust speed

Continue

Stop

Main breaker is

unconnected

End

Reset

Yes

No

Yes

No

Yes

No

Figure 2: the operating procedure to start and stop the VFD

2.3 Mode of fan operation after installation of VFD

Existing interlocks and protection logic is maintained even after installation of the VFDs. For

better control, a switch on the control panel in the control room is provided to change over

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between standard frequency and variable frequency. The switch can be operated as per

requirement even when the VFD is running.

In order to ensure smooth operation, only one of the two permissible modes to control the

ID fans – this is, through VFD or through static blade – is possible. This is done by providing

appropriate interlocks.

Operation to start the fan VFD: Before starting the VFD, QF2 is to be put into “open position”

(off) and that QS1, QS2 are in “closed position” (on).

When the self-diagnostic result of the VFD is indicated as ‘normal’, the local panel

indicates that the VFD is ready for start

Upon issuing the “start” command at the DCS the main breaker QF will close (on)

When the VFD detects the switch has been closed, it sends a “high-voltage ready”

message and “request run” message to DCS

The VFD starts up with 10Hz (preset as the minimum start frequency) after 5 seconds

If the fan is required to startup with standard frequency, the procedure is to:

Switch-off the 6kV power source switch

Put the switch to test position

Switch-off the breakers QS1 and QS2 (disconnected position)

Set the bypass switch QF2 to closed (on) position

Close the Main breaker QF (On position)

For the Operation to stop the ID Fan VFD normally:

Using the DCS:

o Gradually decrease the frequency of the VFD to 10Hz before stopping the VFD

o Gradually close the stable blade open degree to 0°

o Issue “stop” command at the DCS to stop VFD operation

In case emergency shutdown of the VFD is required, the DCS will issue “emergency

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stop” command

The VFD controls the VF-SF/SF-VF switchovers during normal operation

It is essential to test the ID Fans to know corresponding values between VFD

Frequency (in Hz) and Static Blade opening degree (in °)

In the event of VFD failure or bypass, the DCS uses the predetermined relationship

curve between VFD Frequency and Static Blade opening degree to ensure boiler

pressure remains stable by closing the Static Blade to a degree that corresponds to

the frequency of the VFD at the point of failure

Figure 3: Relationship Curve between VFD Frequency (Hz) and Static Blade Opening Degree

0

20

40

60

80

100

20 25 30 35 38 43 45 50

FAN 1

0

20

40

60

80

100

20 25 30 35 40 45 47 50

Fan 2

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3. Comparison of Parameters after installation of VFD

3.1 Safety and stability analysis of the fan after installation of VFD

With direct-drive startup at standard frequency, the starting current exceeded 2600A and

start time exceeded 8 seconds

With startup through VFD, the electric motor startup was considerably smoother due to the

gradual increase in current. There was also no surge on account of inrush current leading to

overall system stability, reduced electro-mechanical stresses on the equipment and

therefore an extended working life of the motor.

After installation of the VFD:

The control mode of the ID fan changed from ‘throttling’ to ‘frequency converting’

A decrease of loss of throttle was observed

An Improvement in the performance curve of the fan due to variable speed

adjustment was noted

A head difference caused by the fixed pressure adjustment during working condition

was removed

The Pressure head of the fan and the pressure head of the system matched perfectly

There was elimination of fan instability when the system resistance line was located

above the stall line of the performance curve

There was a decrease in the chances of blade-fatigue cracks developing during

unstable fan operation

The phenomenon of stall speed of the two fans was avoided which greatly enhanced

the operating safety of the boiler

In order to ensure the smooth operation of VFDs for use with ID Fans, the VF-SF switchover

function was thoroughly examined and tested.

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The momentary loss of power to the electric motor during switchover impacted the rotating

speed of the fan. Thus, the negative pressure in the boiler did fluctuate, but well within

permissible limits.

The test results at 450MW load showed that a certain degree of positive pressure in the

boiler during the SF-VF switchover process was found, but again within permissible levels. In

the event that no human intervention occurs, the automatic system was able to control

operations during the SF-VF switchover. The initial rotating speed was high therefore the

impact on the negative pressure in the boiler was low.

In order to test the disturbance to the boiler when the VFD switched automatically, we

simulated a catastrophic fault and checked the operation of the fans and boiler. All

operations continued in a stable manner and with minimal impact to the boiler.

3.2 Operating Cost Analysis

The energy conservation effect is extremely notable through the modification of the fan VFD

in our company, now we compare the test data which partial get before the modification

and the other get after the modify of the fan VFD:

1. Operating Parameters at 600MW (100%) Load

Parameter of #1 fan Parameter of #2 fan

Current A

Power consumption

kW·h Current A

Power consumption

kW·h

SF mode 325.11 2792.93 335.69 2883.82

VF mode 252.05 2165.29 238.74 2050.95

2. Operating parameter on 450MW load

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Parameter of #1 fan Parameter of #2 fan

Current A

Power consumption

kW·h

Current

A Power consumption kW·h

SF mode 257.59 2212.88 256.26 2201.46

VF mode 122.21 1049.87 125.21 1075.64

3. Operating parameter on 300MW load

Parameter of #1 fan Parameter of #2 fan

Current

A

Power consumption

kW·h

Current

A Power consumption kW·h

IF mode 238.06 2045.11 216.37 1858.77

VF mode 53.15 456.60 60.85 522.75

Figure 4: Power consumption comparing of the fan on the same load

0

500

1000

1500

2000

2500

3000

550t/h 800t/h 1000t/h

1717.28

2157.3

2985.1

455.22

1524.08

2052.15

power consumption on FF mode of #1 power consumption on VF mode of #1

0

1000

2000

3000

4000

550t/h 800t/h 1000t/h

1801.47 2129.55

3011.73

480.65

1211.72

2248.02

power consumption on FF mode of #2 power consumption on VF mode of #2

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At a loading rate of 75% in a year, the electricity each boiler conserves every year after

installation of the VFD is:

ΔP= [(√3.I1.V.cosø - √3.I2.V.cosø) + (√3.I3.V.cosø - √3.I4.V.cosø)] ×365×24 = 19,403,400 kW·h

In the equation above,

I1: Current of FAN-1 when unit is operating at 75% of rated load at Standard Frequency

I2: Current of FAN-1 when unit is operating at 75% of rated load at Variable Frequency

I3: Current of FAN-2 when unit is operating at 75% of rated load at Standard Frequency

I4: Current of FAN-2 when unit is operating at 75% of rated load at Variable Frequency

V: Voltage;

Cos : Power factor, 0.8

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Coal consumption reduction was as follows:

Bg = (P1－P2) × f = 0.782g/ kW·h

In the equation above:

P1: Average power consumption before installing the VFD (at 75% loading rate)

P2: Average power consumption after installing the VFD (at 75% loading rate)

f: Coal consumption rate taken as 3.4g/kWh

Each boiler conserves coal every year:

B= F·Δbg= 3,082.64 MT

In the equation above,

F: Generating capacity of the total year when (loading rate at 75%)

Δbg: coal consumption difference between the VFD used before and after;

4. Related Testing Data after the fan installed the VFD

4.1 IF-VF switchover test of #1 fan VFD of #1 boiler(on July 18,2008)

Operating parameter before Test

Unit load: 500MW

Parameter Name Value

Boiler Pressure: -133Pa

Fan-1 Current 276.7 A

FAN-2 Current of #2 fan 281.7A

Boiler Oxygen Content 3.8%, 3.2%

FAN-1 Opening degree of Static Blade 59.95%

FAN-1 Opening degree of Static Blade 60.6%

Steam temperature Automatic

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Water level Automatic

Coal Firing Automatic

Test Procedure:

1. The output command of FAN-1 VFD was set at 50Hz

2. DCS “SF-VF switchover” command was given in the control room

3. After a programmed time-delay of 5 seconds, the VFD switched to VF mode

4. During the switchover, FAN-1 current decreased from 276.7A to 131A and then came

back to 206A and operated stably at this level

5. Negative pressure in the boiler increased from -139Pa to 74Pa then returned to normal

6. No obvious impact to the other primary operating parameters was noted

Curve of “IF-VF” switchover Test on July 18, 2008

Power Output of the Unit (in MW)

IDF-1 Motor Current

IDF-2 Motor Current

Boiler Pressure

IDF-1 Static Blade Position

IDF-2 Static Blade Position

VFD-1 Output Frequency

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VFD-2 Output Frequency

Tests proved that the VFD was able to meet operating demand the switchover from “SF-VF”

or “VF-SF” was made with the fan running.

5. Summary

The VFDs at our Wangtan Power Generation Co., Ltd operated stably.

1. The VFD system was delivered, installed and commissioned without posing any

challenges or problems

2. The installations not only avoided the phenomenon of stall speed of the two fans,

but also greatly enhanced the operating safety of the boiler

3. No change to the infrastructure or setup was required except the construction of an

enclosure to house the VFDs at remote location within the site premises

4. Intensive training for maintenance staff for VFD operations was not required. The

learning curve for operation and maintenance was not difficult

5. The ability of the Dongfang-Hitachi DHVECTOL VFD to switch from Standard

Frequency to Variable Frequency (and vice versa) while the equipment is operational

(as compared to other VFDs on the market) was very useful for increasing the Load

Factor of the unit and also avoided tripping of the fan

The results of installing VFDs at our plant were found to be excellent. Along with the

benefit of extending the life of our equipment and lowering maintenance costs, we have

found substantial saving in energy consumption and have been recognized for our efforts

in reducing overall plant Auxiliary Power Consumption and meeting our commitments to

the environment by reducing emissions.

Address:

Gao Zhenhua, Electricity Generation Department,

Wangtan Power Generation Co. Ltd,

Donghai Road, Harbor Development Area,

Tangshan, Hebei province 063611

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