Hebei Datang International Wangtan Power Company, China

Transcription

1 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) 1

2 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 Shanghai Electrical Manufacturer Fan Machinery Co. Specification AN37e6(V19+4 ) Specification YKK 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 Rated voltage kv 6 Air pressure Pa Rated current A 448 Shaft Power kw 2332 Power factor Max Permitted Medium Temp C Efficiency % 96.7 Bearing Medium kg/m Lubrication Density Method Forced Oil Cycle Rotation speed r/min 585 Fan Efficiency % 84 2

3 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, 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: 3

4 # Name Unit Operating condition1 Operating condition2 Operating condition 3 Operating Operating condition 4 condition 5 1 Unit load MW Volume flow of #1 fan M 3 /s Mass Flow Rate of #1 fan kg/s Pressure of #1fan Pa Pressure Energy of #1 fan N M/kg Interior input power of #1 fan kw Operating efficiency of #1 fan % Volume flow of #2 fan M 3 /s Mass Flow Rate kg/s Total press of #2fan Pa Pressure energy of #2 fan N M/kg Interior input power of #2 fan kw Operating efficiency of #2 fan % 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 = 2986KW 4

5 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 = 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 5

6 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. 6.0kV Main Breaker QF VFD Isolator QS1 Contactor KM1 Contactor KM2 Isolator QS2 M 6.0kV MOTOR Bypass Breaker QF2 6

7 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 Start Main breaker connected VFD fault Yes VFD running No Request runnig Reset Adjust speed DCS sends running instruction Continue Yes No Pre-charging Stop Finish pre-charging Yes Turn on main breaker No Main breaker is unconnected End 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 7

8 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 8

9 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 FAN Fan 2 Figure 3: Relationship Curve between VFD Frequency (Hz) and Static Blade Opening Degree 9

10 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. 10

11 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 VF mode Operating parameter on 450MW load 11

12 Parameter of #1 fan Parameter of #2 fan Current A Power consumption kw h Current A Power consumption kw h SF mode VF mode 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 VF mode t/h 800t/h 1000t/h power consumption on FF mode of #1 power consumption on VF mode of # t/h 800t/h 1000t/h power consumption on FF mode of #2 power consumption on VF mode of #2 Figure 4: Power consumption comparing of the fan on the same load 12

13 At a loading rate of 75% in a year, the electricity each boiler conserves every year after installation of the VFD is: ΔP= [( 3.I 1. V.cosø - 3.I 2. V.cosø) + ( 3.I 3. V.cosø - 3.I 4. V.cosø)] = 19,403,400 kw h In the equation above, I 1: Current of FAN-1 when unit is operating at 75% of rated load at Standard Frequency I 2 : Current of FAN-1 when unit is operating at 75% of rated load at Variable Frequency I 3: Current of FAN-2 when unit is operating at 75% of rated load at Standard Frequency I 4 : Current of FAN-2 when unit is operating at 75% of rated load at Variable Frequency V: Voltage; Cos : Power factor,

14 Coal consumption reduction was as follows: B g = (P 1 -P 2 ) f = 0.782g/ kw h In the equation above: P 1: Average power consumption before installing the VFD (at 75% loading rate) P 2 : 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 Δb g = 3, MT In the equation above, F: Generating capacity of the total year when (loading rate at 75%) Δb g : 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 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 14

15 Water level Coal Firing Automatic 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 15

16 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

17 For more details please contact: Epsilon Asia Group, X-29 Hauz Khas, New Delhi , India T: ; F: Epsilon Engineering (Thailand) Co. Ltd. Level 29, The Offices at Centralworld, 999/9 Rama-I Road Bangkok 10330, Thailand T: ; F: Neelesh@epsilonasia.com 17