Abstract: This paper analyzes the influence of auxiliary power voltage regulation on motor efficiency, and puts forward some new measures for energy saving and consumption reduction in power plant. Keywords: motor loss, working voltage, auxiliary power consumption rate
Introduction to 0
More than 80% of the auxiliary power of power plants is consumed by motors. Improving motor efficiency is of decisive significance for reducing auxiliary power consumption rate. There are many ways to improve the efficiency of the motor and reduce its loss. The following focuses on the influence of auxiliary power voltage regulation on motor efficiency.
1 Adjustment range of working voltage of auxiliary power supply
The allowable deviation value of auxiliary bus voltage at all levels depends on the nature of the load connected to each bus, which is generally 5% of the rated voltage. Motor manufacturers generally guarantee that the rated voltage varies from-10% to+10%, and the rated output of their motors remains unchanged. When the operating regulations require normal operation, the voltage of 6KV is within 5% and the voltage of 380V is within -5% to+10%. Because the auxiliary load is mainly the motor load, the voltage change within the specified voltage range will not cause the change of motor output and the significant change of unit working conditions.
2 motor loss analysis
The comprehensive power loss of motor includes active loss and reactive equivalent loss.
∑? P C =？ P+K Q Q = P FE+P CU 1+P CU 2+P FJ+K Q Q
These include:
△P Active Loss P Iron Stator Iron Loss P CU 1 Stator Copper Loss P CU2 Rotor Copper Loss P FJ Additional Loss K Q Reactive Economic Equivalent Q Reactive Power
Generally, various losses account for 20%-25% of power loss, 35%-40% of stator iron loss, 20%-25% of rotor copper loss and 15%-35% of additional loss.
3. Influence of voltage change on motor operation
The running voltage of the motor is U, and the rated voltage of the motor nameplate is Ue. When u > UE, the magnetic flux φ in the motor will increase. Because the magnetic circuit of the motor is saturated, the excitation current will increase greatly when the magnetic flux does not increase much. On the other hand, the electromagnetic torque of the motor will increase in direct proportion to the square of the voltage U. When the load torque is constant, the slip rate S will decrease and the rotor current will also decrease. Because the slip rate is very small when running in the rated load range, due to the decrease of slip rate,
The decrease of stator current is less than the increase of excitation current. As a result, the stator current increases and the power factor of the motor decreases. Therefore, when u > UE, not only the iron loss increases, but also the copper loss of the stator winding increases, which may make the stator winding heat more than the allowable value.
When u < UE, the excitation current decreases due to the decrease of magnetic flux φ, and the motor excitation current decreases slightly due to the saturation of motor magnetic circuit. On the other hand, the electromagnetic torque of the motor will decrease in direct proportion to the square of the voltage u, and the load torque will remain unchanged. For example, in the case of rated power output, the reduction of electromagnetic torque will increase the slip rate S more, which will definitely lead to a great increase in rotor current until the electromagnetic torque generated by the motor and the load torque reach a balance. If the voltage drops too much, the increase of rotor current and stator current caused by the increase of slip will be greater than the decrease of excitation current. Therefore, the rotor and stator currents of the motor will exceed the allowable values. Although the iron loss will decrease due to the decrease of magnetic flux, the copper loss will increase in direct proportion to the square of current, so the total loss will increase and the motor will overheat.
The influence of voltage adjustment on various losses of the motor is analyzed in detail below. For the convenience of analysis, footprint 1 and 2 are used to represent the operating parameters of the motor when the voltage drops from U 1 to U2.
3. 1 stator core loss
It is generally believed that the stator iron loss P FE is proportional to the square of voltage. When the iron loss at rated voltage is P FEe, the losses at different voltages are as follows:
△P FE =PFEe (U 12-U 22)
However, when the voltage is higher than the rated value, the above formula will no longer be used because of the saturation of the iron core. In the range above the rated voltage, the relationship between iron loss and voltage of various motors is different, including quartic relationship and 2.5 power relationship, and the iron loss increases obviously when the voltage is higher than the rated voltage.
3.2 Copper loss of stator
The copper loss of stator is proportional to the square of stator current.
△P Cu 1 = PCU 1e(I 12-I22)
It is generally believed that with the decrease of voltage, the current of the motor will increase, so the copper loss will also increase. In fact, this relationship only applies to motors with fixed loads. In fact, when the voltage drops, the motor speed drops, the mechanical shaft power drops, and the excitation reactive current drops, so that the stator current does not increase significantly; For some auxiliary motors, when the voltage decreases, the stator current will decrease synchronously.
3.3 Copper loss of rotor
The copper loss of the rotor is directly proportional to the shaft power and slip of the motor.
△P CU2=Pe K (S 1 - S2)
Where Pe is the rated power of the motor, K is the load rate, and S 1 and S2 are the slip rate of the motor. As the voltage decreases, S 1- S2 is negative, that is, the copper loss of the rotor increases when the voltage decreases.
3.4 Additional losses
The additional loss of motor is divided into no-load additional loss and short-circuit additional loss. The short-circuit additional loss is generally 0.5% of the rated power, which is proportional to the square of the stator current when the load deviates from the rated value.
3.5 Reactive Power Equivalent Loss
If the power supply voltage exceeds the specified range, it will also have a great impact on the power factor. When the supply voltage is higher than 10% of the rated value, the reactive power will increase rapidly due to the influence of magnetic circuit saturation. According to statistics, when the power supply voltage is 1 10% of the rated value, the reactive power of general factories will increase by about 35%. When the power supply voltage is lower than the rated value, the reactive power is also reduced accordingly, thus improving their power factor. When the voltage is reduced, the reactive power required by the motor is reduced, and the equivalent loss of reactive power is also reduced, which is equivalent to reducing the energy consumption of the generator. The unit of reactive power economic equivalent KQ is Kw/Kvar, and the high-voltage auxiliary transformer in power plant is generally directly connected to the generator bus, so the KQ is generally 2%-4%.
In the allowable voltage range, the fixed loss can be reduced by reducing the voltage level of the motor, and finally the auxiliary power consumption rate can be reduced. Theoretical research shows that when the motor load rate is higher than 75%, the efficiency of the motor increases with the increase of voltage, and when the motor load rate is lower than 75%, the efficiency of the motor increases with the decrease of voltage, the power factor also increases, and the reactive power loss also decreases. Therefore, the 6KV power plant should adopt reverse voltage regulation operation, that is, the 6KV voltage increases when the load is high and decreases when the load is low. When the unit is running at low load, the load rate of the six fans, which account for a large proportion of auxiliary power, is very low. When the voltage drops, the slip of the motor increases, the speed of the fan decreases, and the efficiency of the fan is improved to a certain extent, so the voltage drop has obvious influence on their power consumption. However, excessively lowering the voltage will cause the rotor current to increase, the copper consumption of stator and rotor to increase, and the auxiliary power rate will increase instead, and the low voltage will also cause the unstable operation of some auxiliary machines. Therefore, whether the voltage adjustment in the operation of auxiliary power is reasonable plays an important role in ensuring the safe and economic operation of the unit.
4 Field test and analysis
The field test of a 600MW unit was carried out. The voltage of 6KV high-voltage auxiliary bus can be changed by adjusting the terminal voltage of on-load tap changer or generator of high-voltage auxiliary transformer. The field test is divided into two parts: the active power change of a single motor and the active power change of a high-voltage auxiliary transformer. The following data comes from field measurement.
4. 1 single motor voltage change test
The primary fan motor is YKK630-4 1800KW, and the active power of the unit is 550MW. It can be seen that the active power loss of the primary fan increases gradually with the increase of voltage. The active power loss at 6.2KV is 38.4KW higher than that at 5.97KV. See Figure 1 for details.
Figure 1 500 MW Relationship between Primary Fan Power Consumption and Voltage
Boiler water pump motor 400KW. When the unit is loaded with 400MW, the power consumption of the boiler water pump gradually decreases with the voltage drop, and the active loss at 6.29KV is 3 1.5KW higher than that at 5.88KV, as shown in Figure 2.
Fig. 2 Relationship between power consumption and voltage of water pump in 400 MW boiler
When the unit is loaded with 300MW, the power consumption of the primary fan gradually decreases with the voltage drop, and the active loss at 6.35KV is 72KW higher than that at 6. 17KV. See Figure 3 for details.
Fig. 3 Relationship between primary fan power consumption and voltage at 300 MW
4.2 Voltage change test of high-voltage auxiliary transformer
Under three different loads, the unit adjusts the 6KV bus voltage by changing the generator terminal voltage, so the resulting change of active power of high auxiliary transformer is shown in Table 1:
Table 1 change of active power in high voltage substation
It can be seen from the table 1 that when the unit load is 450MW and the high-voltage auxiliary power voltage drops 199V, the auxiliary power decreases 180V, and when the load is 520MW and the high-voltage auxiliary power voltage drops 6560 MW, the auxiliary power decreases 630KW. That is, 6KV voltage drops by 2.5%, which can save auxiliary power 1.8%, reduce auxiliary power rate by about 0.08%, and save 0.27 g standard coal per kWh.
5 Reasonable range of working voltage
It is unscientific to think that the higher the voltage, the lower the power consumption, and it is extremely uneconomical to run the voltage higher than the rated voltage of the motor for a long time.
Practice shows that with the decrease of voltage, the power consumption of main auxiliary machines decreases synchronously, and so does the power consumption of auxiliary machines. The lower the voltage, the smaller the power consumption. In order to ensure that the terminal voltage of 400V will not drop too low, the voltage of 6KV bus section can be adjusted to about 5.9KV, so that the auxiliary power consumption rate can be reduced to 0.08%-0. 16% without increasing investment, and the power saving effect is obvious.
Some people think that lowering the voltage of 6KV will reduce the operation reliability of the auxiliary motor, which is also an unnecessary worry. After calculation, the voltage of 6KV is -5%(5.7KV), and the auxiliary voltage is still about 80%Ue when the electric pump is started under the worst working conditions, which meets the requirements, because the design of auxiliary power has fully considered the margin. Therefore, from the point of view of safety and economy, it is more energy-saving to control the operation between 5.9 and 6.0 kV when the unit voltage is 6KV, compared with the usual control between 6. 1-6.3KV.
With the increasing demand for automatic voltage regulation in power grid system, super-large power plants gradually require the use of VC automatic voltage regulation system. With the fluctuation of system voltage, the voltage of auxiliary bus will also fluctuate. It is suggested that the on-load switch of high-voltage auxiliary transformer should choose products with high reliability to meet the requirements of power grid for system voltage. At the same time, the influence on the auxiliary system should be considered comprehensively when setting the AVC automatic adjustment conditions. Power plants should also adjust the auxiliary bus voltage in time from the perspective of economic operation, and try to run the high-voltage auxiliary bus voltage to the lower limit. It is suggested that it is reasonable around 5.9KV (for 6KV auxiliary voltage), and the position of the tap of the low-voltage auxiliary transformer at the end of the power supply should be adjusted during maintenance.