Problem Analysis and Improvement Measures of Relay Pump in Power Plant

**1. Overview** I installed three relay pumps for the steam turbine's public equipment, model J122-2, driven by a squirrel-cage motor with a rated power of 185 kW, voltage of 380 V, current of 331 A, and speed of 2960 rpm. The main function of these pumps is to deliver chemically treated demineralized water to the condenser, ensuring that the water level remains within acceptable limits. From September each year until May or June of the following year, especially during winter when heating demands are high, the relay pumps must be started frequently. In some cases, they are started once every half hour, which can result in up to 24 starts per day. Frequent starting leads to overheating and potential damage in the area where the rotor cage connects to the end ring, causing rotor failure. It also poses a serious threat to the insulation of the stator windings. According to incomplete records, three of the relay pumps have been damaged nine times in total. During maintenance, it was found that the main stator winding insulation had suffered various levels of damage, including inter-turn short circuits. Each low-voltage motor costs approximately 40,000 yuan, and the total economic loss from this single issue reached 360,000 yuan. With the ongoing reform of the power system, power plants are facing increasing competition. Reducing failure rates, lowering operational costs, and ensuring safe and efficient operation have become key priorities. --- **2. Problems in Relay Pump Operation** The relay pump uses full-voltage starting, which causes the squirrel-cage motor to draw 5–7 times the rated current and produce 1–2 times the rated torque during startup. This method places significant stress on both the motor and the switchgear. **2.1 Hazards of Frequent Motor Starts** Frequent starting leads to several issues: - **Insulation Damage:** The stator windings experience heat and electric field stress, leading to gradual aging of the insulation. This makes them more susceptible to breakdown under low overvoltage conditions, resulting in short circuits and failures. - **Mechanical Wear:** The repeated starting process causes the end windings to loosen and the rotor bars to shift, leading to fatigue and wear. - **Rotor Cage Fracture:** During start-up, the rotor bars are subjected to high thermal stress and centrifugal force, making them prone to cracking, especially in high-speed motors (close to 3000 rpm). The temperature in the cage can rise to 300°C during startup, reducing mechanical strength and causing uneven current distribution due to skin effect. This results in thermal stress and potential rupture. **2.2 Hazards to the Switch** The relay pump uses a DW15-C630 type switch with a rated current of 630A and an instantaneous current of 6300A. The arc tube is made of copper-plated steel, which has poor arc performance. Each start and stop generates high-temperature arcs between the contacts, accelerating oxidation and increasing contact resistance. Over time, this can lead to burning, damage, or even explosion of the switch. According to records, eight switches have been burned out due to frequent starts. **2.3 Economic Impact of Frequent Starts** Frequent starting increases power consumption and reduces efficiency. When the pump flow is too high, valve adjustment is needed, causing throttling losses and further reducing economic efficiency. --- **3. Countermeasures** **3.1 Choosing Wound Rotor Motors** Wound rotor motors have a three-phase winding on the rotor, which is shorted through slip rings during normal operation. During startup, resistors are added to the rotor circuit to reduce starting current and increase starting torque. While this improves starting performance, it does not eliminate the need for frequent starts and requires significant investment in new motors, limiting energy savings. **3.2 Selecting Frequency-Controlled Induction Motors** Using frequency control allows the motor speed to be adjusted by changing the power frequency. The formula for asynchronous motor speed is: n = ns(1-s) = 60f(1-s)/p By adjusting the frequency f, the motor speed n changes proportionally. This enables better control over pump output, reducing the number of starts and minimizing stress on the motor and switchgear. Key benefits include: - **Reduced Starting Current:** Lower impact on the motor, extending its lifespan. - **Energy Savings:** Pump power is proportional to the cube of the speed, so reducing speed significantly lowers energy consumption. - **Avoid Throttling Losses:** Adjusting speed instead of using valves improves efficiency. - **No Harmonic Interference:** Inverter-based control avoids grid disturbances and doesn’t require special motor modifications. - **Improved Reliability:** Smooth speed control and stable performance enhance overall system reliability. --- **4. Economic Analysis** Changing the relay pump’s speed through frequency control can significantly reduce shaft power and improve energy efficiency. As shown in the diagram, when using speed control instead of valve adjustment, the power required drops dramatically. For example, at point C, the shaft power P3 is much lower than P2 or P1, demonstrating a substantial energy-saving effect. In addition to saving energy, frequency control reduces the number of starts, minimizes motor and switch damage, and lowers maintenance costs. This approach not only enhances operational safety but also supports long-term cost savings and improved plant performance.

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