1. Introduction Auxiliary equipment in power plants is generally driven by electric motors. For a 200MW unit, auxiliary power consumption accounts for approximately 9% to 10% of total power generation. Reducing the power consumption of these auxiliary equipment is a crucial task for every manager, engineer, and employee. Currently, the operating voltage of our company's 6kV auxiliary bus is 1.67% to 3.3% higher than the rated voltage of the motors under full load conditions. The tap position of the high-voltage transformer corresponding to this voltage was determined during the early stages of the Qinling Power Plant's construction, when the voltage in the eastern Shaanxi power grid was low. Since the plant's completion, the low-voltage situation in the region has improved. In recent years, a large number of auxiliary motors have been operating at higher voltages, which not only endangers motor insulation but also significantly increases motor losses (copper losses are proportional to the square of the operating voltage, and iron losses also increase sharply when the voltage is higher than the rated voltage). Excessively high operating voltages lead to unnecessary energy waste. Therefore, it is necessary to select a reasonable voltage range for the auxiliary equipment to further explore its energy-saving potential and reduce the power consumption rate to a normal level. The following are some steps that can be taken to select the economical voltage for the plant auxiliary busbar. 2. Industry Standards Regarding Operating Voltage of Plant Auxiliary Busbars: The "200MW Electrical Operation Regulations" (Plant Auxiliary Motor Regulations) stipulates: ① Plant auxiliary motors are allowed to operate within a voltage variation range of +10% to 5% of the rated voltage, with their rated output remaining unchanged. ② When the motor's supply voltage exceeds the range of 95% to 110%, the shift leader and electrical foreman should be notified immediately. ③ When the operating voltage of the motor under rated cooling conditions is higher (lower) than the rated voltage by +10% (-5%), the current should be correspondingly allowed to be lower (higher) than the rated current by +5% (-10%). 3. Characteristics of Losses in Plant Auxiliary Motors and Ways to Reduce These Losses: The losses in plant auxiliary motors include iron losses and copper losses, and their variations are related to changes in the applied voltage. To fully utilize the core material and reduce costs, manufacturers operate the motor under rated conditions with the core in a near-saturated state, as shown at point A in the attached diagram. When the voltage changes, the saturation level of the motor's iron core changes, potentially causing a sharp change in the excitation current. In the case of iron core saturation, the change in excitation current is much larger than the voltage change. Simultaneously, the leakage reactance of the motor also changes with the change in iron core saturation. Therefore, the change in the stator terminal voltage U of the motor has a significant impact on the motor's operating performance. When U > U, the magnetic flux in the motor will increase. Due to the saturation of the motor's magnetic circuit, the excitation current I will increase significantly even with a small increase in magnetic flux. On the other hand, the electromagnetic torque M of the motor will increase proportionally to the square of the voltage U. With a constant load torque, the slip s will decrease, and the rotor current I will also decrease accordingly. Since the slip is very small when operating within the rated load range, the decrease in rotor current caused by the decrease in slip is less than the increase in excitation current. As a result, the stator current increases, and the power factor of the motor deteriorates. Therefore, when U > U, not only do the iron losses increase, but the copper losses in the stator windings also increase, which may cause the stator windings to overheat beyond permissible limits. When U > U, the decrease in magnetic flux leads to a decrease in excitation current I, resulting in unsaturated magnetic circuitry in the motor. Therefore, the change in excitation current is not significant. On the other hand, the electromagnetic torque M of the motor will decrease proportionally to the square of the voltage U. Given a relatively heavy load torque, if the motor outputs rated power, the decrease in electromagnetic torque M will significantly increase slip S, worsening the power factor (COS) of the rotor circuit. Due to the decrease in magnetic flux and COS, the rotor current I will inevitably increase significantly until the electromagnetic torque generated by the motor balances the load torque. If the voltage drop is excessive, the increase in rotor current caused by the increased slip will exceed the decrease in excitation current. This results in a worse power factor, causing the rotor and stator currents to exceed permissible values. Although the decrease in magnetic flux may reduce iron losses somewhat, copper losses increase proportionally to the square of the current, so the total losses still increase, leading to motor overheating. Therefore, industry standards and enterprise standards stipulate that the applied voltage of a motor must not exceed 110% of the rated voltage and must not be lower than 95% of the rated voltage. Even within the 95%–110% rated voltage range, the total losses of the motor are different. The purpose of analyzing the applied voltage is to find the most economical operating voltage. Currently, the taps of our plant's No. 3–6 high-voltage transformers are all operating at tap VI. This tap was set at the initial stage of the plant's construction, when the voltage level of the eastern Shaanxi power grid was low, to meet the operating requirements of the plant's auxiliary equipment. After more than 20 years of rapid development and continuous expansion, low-voltage operation is no longer an issue in the local power grid. At our plant, the phenomenon of unadjustable high voltage at the control point during off-peak hours occurs frequently, and the high voltage is even more severe when the units are running under no-load and low load. Since the motor's own impedance remains constant, the actual power consumption is generally closely related to the operating current and the supply voltage. Ferromagnetic losses increase sharply with the increase of operating voltage above the rated voltage. Copper losses are directly proportional to the square of the operating current. The operating current of a motor with a fixed load is also proportional to the voltage. For each motor, the question is whether the current supply voltage is the economical voltage that minimizes the total iron and copper losses. Therefore, it is necessary to find the most economical operating voltage within the 95%–110% rated voltage range while ensuring equipment safety. If YXV=d(U) is known, the minimum value can be found by taking the derivative. This is feasible for a single motor, but finding the function expression that minimizes the losses of all auxiliary motors in the entire plant is extremely cumbersome. Therefore, for the numerous auxiliary motors in the plant, the extreme value of the function can be found through experimentation (voltage adjustment) to determine the corresponding 6kV bus voltage U, i.e., the economical voltage. Our company's allowable voltage range for the 6kV bus is 5.7–6.6kV. Currently, when the unit is at full load, all auxiliary motors are operating at maximum capacity. Observations show that the voltage of each plant bus is between 6.1 and 6.2kV, or even higher, which is +1.67% to +3.3% of the motor's voltage U. The question is whether this voltage represents the economical operating voltage for all auxiliary equipment. A significant deviation from the economical motor voltage inevitably increases losses. Our company's main power-consuming equipment, the auxiliary motors, has sufficient design capacity margin. Currently, most equipment output adjustment relies on baffles, valves, or recirculation installed at the mechanical end, such as boiler suction and blower baffles operating in a throttling state (approximately 50% opening). This throttling loss is severe. Appropriately lowering the voltage will not cause the operating current to exceed the rated value. Therefore, under full unit load, without changing the operating mode of the auxiliary equipment in the plant system, and under different voltage conditions, we will measure the actual load of the plant power transformer to determine the most economical voltage for the plant bus voltage, close to the motor's rated value. This will minimize the losses (iron and copper losses) and even the input power of the auxiliary motors, achieving energy savings. Based on the current operating voltage, appropriately lowering the plant bus voltage should be feasible. 4. Analysis and Calculation of Current Plant Auxiliary Power Operation We consulted the parameters of the plant's high-voltage working transformers to calculate the 6 kV bus voltage when the generator voltage varies by ±10%U and the load on the high-voltage working transformers varies from 50% to 100% of their rated capacity. Table 1 shows the relevant parameters for transformers #3 to #6. Observation shows that under maximum operating conditions, the load rate of the high-voltage transformers is close to 70% of their capacity. Table 2 shows the calculation of the plant's auxiliary bus voltage variation at each transformer tap when the generator terminal voltage varies by ±10%U under these operating conditions.