Réforme pour économie d'énergie avec variateur de fréquence dans pompe de condenseur
Réforme pour économie d'énergie avec variateur de fréquence dans pompe de condenseur
2010-1-4 13:38:00
Résumé : avec le développement économique, l’économie d’énergie et la réduction des émissions sont des points importants pour l’avenir. La technologie du variateur de fréquence moyenne tension VFD est de plus en plus promue. Cette page démontre le design du variateur de fréquence VFD, les connexions électriques, la configuration logique du contrôle thermique, la mise en service et les bénéfices dans le projet du générateur de 300MW.
Mots clés : pompe de condenseur, fréquence variable, économie d’énergie, réforme.
1. System introduction
1.1 Equipment parameters:
Capacity of phase I project of Shanxi Zhaoguang Power Generating Co., Ltd is 2×300MW. Steam turbine is produced by Shanghai Steam Turbine Plant. Condensing water system is middle pressure system, equipped with condenser pump produced by KSB plant. Parameters are following:
Motor parameters | ||||||||
Model | YLKK500--4 | Rated voltage（U0） | 6kV | Cooling mode G | Q161 | |||
Rated power（Pdn） | 1120kW | Speed （n0） | 1491r/min | Insulation class | F | |||
Rated current（I0） | 124.7A | Power factor | 0.901 | IP class | IP54 | |||
Condenser pump parameters | ||||||||
Rated flow | 912.98m3/h | Rated speed | 1480r/min | Rated lift | 258.4m | |||
Power | 1120kW | Model | NLT350—400*6 | |||||
1.2 Operation state before reform
Condesing pump runs in constant speed. After being boosted pressure by pump, condensing water passes through axes and governing valve (inlet gate of deaerator, serial No C-1), then enters into deaerator. Condensing flow is controlled by governing valve opening to maintain stability of water level of deaerator. In addition, condensing water is also supplied to other auxiliary equipments. In order to avoid condensing system superpressure and cavitation during operation in low load, we design recycle pipeline. (see following figure).
Condensing water system
Circulating governing valve C-2 cooperates with C-1 to adjust water level of deareator, keeps normal pressure for system.
2. Reform scheme
2.1 Basic scheme
“One drive two” auto bypass scheme.
That is, one VFD and two switches. Connect VFD to condenser pump by switch. Power source of variable frequency system is main power source 6kV. It is controlled by main control system in site. VFD implements speed control for pump according to programe. Its main function:
Medium voltage VFD can drive pump A to run in variable speed, and also for pump B. But VFD can not drive two pumps simultaneously. Two motors of pumps have bypass function, either can run in variable frequency. The other is standby in direct on line. When medium voltage VFD is broken, system start the other in direct on line.
2. Work principle
1# pump and 2# pump share one VFD. 3QF is medium voltage switch. TF is VFD. M is motor. There are electrical interlock and logic interlock between 4QF and 1QF, 5QF and 2QF, 4QF and 5QF to avoid short circuit between VFD output and 6kV power source. 1QF, 2QF and 3QF are existing equipments in site. 3QF, 4QF and 5QF only has manual close function. Medium voltage switch 3QF is installed in “6kV power distribution room” and connects to bus. 2 routes DC control power, 2 routes dynamic DC power source of all switches in “ VFD room” are supplied by DC bus of 2#.
When 1# pump runs in variable frequency, open 1QF, close 3QF and 4QF. 2# pump is in standby status in direct on line. When 1# VFD is broken and trips off, system link starts 2# condenser pump to run in direct on line. When 2# pump runs in variable frequency, open 2QF, close 3QF and 5QF. 1# pump is standby in direct on line. When 2# VFD is broken and trips off, system link starts 1# pump to run in direct on line.
3. Operation mode and logic control
DCS adjusts control object and control method automatically according to operation mode of pumps. There are two main control methods:
First: variable frequency adjustment. Adjust speed of pump to control water level of deareator according to variety of deareator water level in different loads.
Second: valve adjustment. As long as one pump runs in direct on line in condensing water system, system would adopt valve adjustment. At the moment, medium voltage VFD should run in high frequency range（45～50Hz）to avoid no water coming out.
Condenser pump has operation modes of variable frequency and line frequency. In normal state, variable frequency pump works and line frequency pump is standby. VFD has local control and remote control. In local control, speed signal from DCS trails VFD speed feedback. Remote control operation is invalid.
When VFD is controlled by DCS, there are two modes: auto/manual. In manual mode, personnel controls speed by changing in VFD interface. In auto mode, speed is controlled automatically according to constant value of deareator water level. When variable pump and line frequency pump run in parallel, water level adjustment eliminates automatically, governing valve C-1 adjusts water level in manual mode. When variable frequency pump is running, “VFD heavy fault”interlinks with medium voltage switch. Signal of “ medium voltage swith trips off”transfers to DCS and standby pump starts in direct on line.
Recycle valve C-2 adjusts according to outlet flow and main pipe pressure. When flow is smaller than 300T/H, flow signal adjusts, C-2 opens to maintain the least flow; when flow is more than 300T/H, pressure signal adjusts, maintaining outlet pressure ≤2.6Pa. When pressure is beyond constant value, C-2 opens. Opening angle is determined by pressure difference.
For variable frequency pump, deareator water level is adjusted by changing speed. Pressure of condensing water is low. For line frequency pump, deareator water level is adjusted by valve. Pressure of condensing water is high. During operation, once variable frequency pump trips off, after starting standby pump in direct on line, sudden-increase of water pressure and flow affect deareator water level and deareator safety greatly. We make special control logic for the problem. After starting pump B, start signal of motor B is as touch signal of C-1. Makes governing valve C-1 of deareator close to small quickly. Valve position has function relation with load. Then personnel adjusts opening manually.
Relation of C-1valve position and load
Load | 180MW | 270MW | 300MW |
Valve opening | 55 | 70 | 82 |
During operation, pressure of condensing water lowers with load reducing. Original logic of “link start standby pump in low pressure”may start standby pump, which increases instability of system. So we reduce constant value to 0.18Mpa, keep logic same, preserve “ condensing water pressure alarm”. In order to keep standby pump in a good state, outlet of standby pump is in full open state.
Condensing water low pressure close bypass protection: adopt original circuit and constant value. During early variable frequency operation, close C-1 to some position. Make condensing water with enough pressure to ensure low pressure bypass normal operation. Pump low level protection: adopt original circuit and constant value.
In order to improve adjustment quality of deareator water level and control level, we utilize interim control theory which is the newest research result. Its main idea is to introduce speed signal to control system, balance with water level deviation, ensuring stability of water level. Introduce steam flow signal to heighten adaptive capacity of system and conpensate deviation caused by speed signal. Make system act only when load changes and water level self-disturbance changes. After variable frequency speed signal balance with above disturbance variables, system exists in waiting state to adapt thermal system lag and kinds of uncertainties.
4. Thermal system cooperation during commissioning
First pump start-up can adopt variable frequency mode. Because no water is in system, there is a charging course. Measure signal has lags inertially, so “minium flow protection” acts easily. Therefore, the protection constant value is: increase 50t as redundancy on basis of original measure. Meanwhile, stop pump 9s later.
In order to avoid water hammer, governing valve C-1 and outlet of pump are closed. Recycle valve is full open. Whole system is manual control. Medium voltage VFD accelerates from zero to 900rpm/min (minimum speed) automatically, then adjusts speed according to demand. After charging water, valve C-1 opens big gradually. Recycle valve C-2 cooperates. When C-1 opens more than 95%, system runs in variable frequency auto control.
5. Economic benefit
5.1 Theory analysis
According to knowledge of fluid mechanics, flow Q is in direct rate to speed n. Pressure n is in direct rate to square of speed n. Power is in direct rate to cube of speed n. When flow reduces from rated Q0 to 70%Q0 by variable frequency control, speed will reduces from rated n0 to 70%n0, at the moment, pressure reduces from Po to 49% Po, shaft power reduces from rated Po to 34.3%Po. Power consumption reduces 65.7%Po theoretically. Even speed reducing may cuase efficiency down, energy saving of VFD is still considerable. According to calculation, when flow reduces from Q0 to 70%Q0, power consumption of variable frequency adjustment is 52%Po less than that of valve adjustment.
5.2 Data analysis
Operation data
Load | Main steam pressure | Main steam flow | Deareator pressure | Pump current A | Outlet pressure Mpa | Pump soeed rpm/min | Condensing water flow t/h | |||||
MW | Mpa | t/h | Mpa | Before reform | After reform | difference | Before reform | After reform | Before reform | After reform | Before reform | After reform |
304 | 16.6 | 1040 | 0.83 | 95.5 | 88.4 | 7.1 | 2.5 | 1.64 | 1490 | 1361 | 930 | 926.4 |
292 | 16.1 | 975.1 | 0.79 | 93.6 | 85.15 | 7.45 | 2.6 | 1.609 | 1333 | 878 | 906.9 | |
281 | 16.3 | 959 | 0.78 | 92.9 | 73.23 | 18.67 | 2.6 | 1.301 | 1234 | 865 | 837.2 | |
269 | 16.2 | 908.2 | 0.71 | 91.5 | 72.41 | 19.09 | 2.6 | 1.426 | 1217 | 846 | 822.9 | |
259 | 16 | 878.3 | 0.73 | 91.4 | 67.84 | 23.56 | 2.7 | 1.34 | 1174 | 823 | 788.1 | |
252 | 15.6 | 883.4 | 0.72 | 90.4 | 65.52 | 24.88 | 2.7 | 1.25 | 1151 | 804 | 791.5 | |
242 | 15 | 860.6 | 0.7 | 91.3 | 55.93 | 35.77 | 2.7 | 1.429 | 1143 | 825 | 652.4 | |
231 | 14.7 | 825.5 | 0.67 | 89.9 | 55.96 | 33.84 | 2.7 | 0.942 | 1049 | 797 | 624 | |
222 | 14.1 | 746.5 | 0.62 | 91 | 52.89 | 38.11 | 2.6 | 0.99 | 1012 | 838 | 673.4 | |
211 | 13.3 | 708.3 | 0.59 | 91.2 | 50.92 | 40.28 | 2.7 | 1.17 | 1006 | 821 | 649.4 | |
201 | 12 | 670 | 0.57 | 91.9 | 43.61 | 48.29 | 2.6 | 0.88 | 920.9 | 837 | 553.3 | |
190 | 11.8 | 674.3 | 0.57 | 90.4 | 44.8 | 45.6 | 2.7 | 0.84 | 920.8 | 814 | 595.3 | |
183 | 11.6 | 609.7 | 0.51 | 91.1 | 47.13 | 43.97 | 2.7 | 0.87 | 942.1 | 819 | 618.9 | |
Estimate as per power generating in September, double generator work time is 1440h. Power generating is 3565.2 million kWh. Average load is 24.76MW. Load rate is 82.53%. From above table, 35.77A is saved per hour, corresponding to power:
1.732*UICOS¢ = 1.732×6×35.77×0.901 = 334.92kW·h
Power price: 0.2754 yuan / kW·h, 92.237 yuan is saved per hour, yearly cost saved is 796900yuan (USD 117191). The reform cost is 3000000yuan which can be recovered in 2 years. With change of relation of demand and supply, work time and load rate of machine are lowering. From above data, less load rate is, more energy is saved.
5.3 Safety analysis
Variable frequency adjustment solves problem of impact on motor caused by big start current, prolonging motor lifetime. For asynchronous motor, its biggest start current is about 7 times of rated current and 4~5 times of rated current in star-delta mode. In variable frequency start, there is no impact current. Current increases from zero with speed increasing. Biggest current is not beyond rated current, which eliminates impact force, prolongs motor lifetime.
In variable frequency control, if medium voltage VFD runs under 50% line frequency for long term, motor heat dissipation capacity reduces with speed down. Motor heating also reduces. So motor temperature is down.
VFD heightens reliability of condenser pump, prolongs pump lifetime. In variable frequency control, in low load, pump runs in low speed. Required NPSH lowers. Probability of cavitation lowers. Pump lifetime is prolonged.
In variable frequency adjustment, outlet and governing valve are full open, which avoids impact on pipe during valve adjustment, lowers probability of pipe leakage. Soft start eliminates water hammer and cavitation effectively, reduces impact on pipe and pump, prolongs pump lifetime and reduces loss and waste of pipe and accessories, thereby reducing maintenance labour, heightening safety and reliability.
In variable frequency control, pressure of condensing water is lower than constant pressure, which eliminates and releases big vibration of condensing water pipe, leakage of minimum flow adjustment gate, big noise and vibration of cooling pipe, easy damage of adjustment gate caused by high pressure of condensing water during constant pressure operation.
Maintenance is reduced. In variable frequency control, vibration caused by pump apart from rated work area during valve adjustment is eliminated. In general, purpose of VFD application is to reduce pump speed. Slow start-up and speed down prolong lifetime of many components, especially for seal and bearing.
In variable frequency control, speed is low in small flow, which lowers noise of pump and system, improves environment.
In variable frequency control, adjustment mode is simple and reliable, which heightens utilization factor of automatic device.
Leader&Harvest VFD is voltage source structure. Power factor can reach up to 0.95.
6. Work to do after reform
During start/stop, control speed of variable frequency pump at certain value, adopt inlet adjustment, which keeps deareator water level stable and ensures enough cooling water for other auxiliary equipments. But we should grope critical speed range of condenser pump. Variable frequency pump is forbidden to run in this range during start, stop and operation.
Reliable earth between VFD and ground should be ensured. In order to avoid signal disturbance, it is better to lay isolated earth system for control system. Earth resistor is required not more than 4Ω. Signal wire to VFD should be screen cable. Its terminal earths reliably.
In this project, medium voltage source of VFD is from section I of 6kV bus, same route with condenser pump. So the best operation mode is: 1# pump runs in variable frequency and 2# pump standby in direct on line. If 2# pump runs in variable frequency and 1# pump standby in direct on line, once section I of 6kV bus powers off, 2# pump can not start and stop due to long time of power source switching.
7. Concluding remarks
Leader&Harvest VFD has been running stably since it put into service. After reform with VFD, energy saving effect is very good. It solves problems of big vibration, big throttle loss, control lag, serious intercoupling, uncertainty of control object. Interim adjustment scheme controls variable frequency pump and controls deareator water level. The investment cost can be recovered within 2 years.
