Problem Analysis and Improvement Measures of Relay Pump in Power Plant
**1. Overview**
I installed three relay pumps for the steam turbine's shared equipment, model J122–2, which are driven by squirrel-cage motors. Each motor has a rated power of 185 kW, voltage of 380 V, current of 331 A, and speed of 2960 rpm. These pumps are responsible for delivering chemically treated demineralized water to the condenser, ensuring that the condenser’s water level remains within acceptable limits. From September to May of the following year, especially during winter when heating demands are high, the pumps must be started frequently—sometimes as often as once every half hour. On some days, they may need to start up to 24 times.
Frequent starting can lead to overheating and damage in the area where the rotor cage connects to the end ring, causing rotor failure. It also poses a serious risk to the stator winding insulation. According to rough estimates, these three pumps have been damaged nine times in total. During inspections, three motors were found with varying degrees of main stator winding insulation damage and inter-turn short circuits. Each low-voltage motor costs around 40,000 yuan, and the total economic loss from this issue reached 360,000 yuan.
With the ongoing reforms in the power system, power plants are facing increased competition. Reducing failure rates, lowering operational costs, and ensuring safe and efficient unit performance have become key concerns for us.
**2. Problems in Relay Pump Operation**
The relay pump uses full-voltage starting, which is standard for squirrel-cage motors. However, this method results in a starting torque of 1–2 times the rated torque, with a starting current that can be 5–7 times the rated current. Frequent starts create significant stress on both the motor and the switchgear.
**2.1 Hazards of Frequent Starts on the Motor**
2.1.1 **Stator Winding Damage**
Frequent starts cause the stator windings to heat up due to high currents, leading to insulation aging. This makes the insulation more vulnerable to breakdown under even minor overvoltages, resulting in short circuits and failures. Additionally, the mechanical stress from repeated starts can loosen the windings, causing bar displacement and fatigue wear.
2.1.2 **Rotor Cage Fracture**
During startup, the rotor bars experience intense thermal stress and centrifugal force. This is particularly problematic for high-speed two-pole motors (close to 3000 rpm). At start-up, the temperature of the rotor cage can reach 300°C quickly, reducing its mechanical strength. The skin effect causes uneven current distribution, creating large temperature differences and thermal stress. Magnetic leakage also generates electromagnetic forces, pulling the cage against the tank and causing vibrations that eventually lead to fractures.
**2.2 Hazards of Frequent Starts on the Switch**
The relay pump uses a DW15-C630 switch with a rated current of 630A and an instantaneous current of 6300A. The switch has poor arc performance, and each start or stop produces high-temperature arcs between contacts. Over time, this leads to oxidation, increased contact resistance, and eventual burning or explosion of the switch. Statistics show that eight switches have been damaged due to frequent starts.
**2.3 Economic Losses from Frequent Starts**
Repeated starts increase power consumption and reduce efficiency. When the pump flow is too high, adjusting the valve opening causes throttling losses, further reducing economic efficiency.
**3. Countermeasures**
**3.1 Choosing Wound Rotor Motors**
Wound rotor motors have a three-phase winding on the rotor, connected through slip rings. During startup, resistance is added to the rotor circuit to limit current and improve starting torque. While this improves performance, it also increases cost and maintenance requirements. It doesn’t fully solve the problem of frequent starts but offers better control than squirrel-cage motors.
**3.2 Selecting Frequency-Controlled Asynchronous Motors**
Frequency control adjusts the motor speed by changing the supply frequency. The relationship between motor speed and frequency is given by:
n = 60f(1−s)/p
Where n is motor speed, f is frequency, s is slip, and p is the number of pole pairs. By adjusting the frequency, we can control the pump’s output without frequent starts.
Advantages include:
- Reduced motor starts, lowering failure rates.
- Lower starting current, extending motor life.
- Reduced power consumption and plant power rate.
- Elimination of throttling losses.
- No harmonic interference or special motor requirements.
- Improved reliability and energy efficiency.
**4. Economic Analysis**
Changing the pump speed via frequency control significantly reduces shaft power. For example, at constant speed, the pump operates at point A with power P1. Adjusting the valve only slightly reduces power (P2), but using variable speed control lowers the power to P3, showing a clear energy-saving benefit. This approach not only reduces motor wear and switch damage but also improves overall efficiency and cost savings.
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