Fault Analysis and Improvement of Thermal Resistance for Shaft Bearing Temperature Measurement in Large Hydropower Plant Turbines

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Units 1-4 of the Liujiaxia Hydropower Plant are suspension-type hydrogenerators, with thrust, upper guide, and water guide bearings supporting the axial and radial forces generated during operation. Bearing temperature is an important indicator for monitoring the operating condition of these units; the temperature of the bearing bushes is monitored as part of the unit’s protection system, and if the temperature becomes too high, an alarm is triggered or the unit shuts down. The importance of this is self-evident. However, during the operation of the unit, especially when large generators operate for extended periods during the flood season, the Pt100 thermistors used for measuring the temperature of each bearing experience various issues such as false readings, sudden changes, and no readings at all. It is difficult to determine whether these problems stem from the unit itself or from the thermistors used for temperature measurement, which poses a risk to the safe operation of the unit. Thermal resistors: yunrun.com.cn/product/list_80.html 1. Introduction to the temperature measurement system for hydro-turbine units. In industrial applications, there are two principles for temperature measurement: thermocouples and thermal resistors. Thermistors measure temperature based on the thermal effect of resistance, that is, the property whereby the resistance value of a resistive element changes with temperature. Therefore, by measuring the change in resistance of the temperature-sensitive platinum resistor, it is possible to determine the temperature. The most widely used metal thermoresistive materials at present are platinum and copper, with the commonly used calibration codes being Pt100 and Cu50. The bearing temperature measurement in the units of the Liujiaxia Hydropower Plant is carried out using Pt100 thermistors. The Pt100 thermistor is installed at the production site, at a certain distance from the control panel cabinet; the resistance signal is transmitted to the computer acquisition module via wires. Common lead configurations for thermal resistors include two-wire, three-wire, and four-wire systems. Since the units at the Liujiaxia Hydropower Plant trigger signaling and tripping mechanisms as soon as the temperature of their bearing shells exceeds a set value, high accuracy in temperature measurement is required. To minimize the impact of cable resistance imbalances on the measurement results, three-wire Pt100 thermistors are used for measuring the bearing shell temperatures. 2. Existing problems: Statistical data (see Table 1) show that the main issues encountered during the operation of Pt100 thermoresistors are frequent fluctuations in the measurement values ； The temperature sensing element is short-circuited, with a resistance of 0Ω ； The temperature sensing element is open-circuited, with a resistance of∞ ； Problems such as large deviations between the displayed values and the actual measured values. Table 1: Statistics on Defects in the Temperature Measurement System from April to November http://yunrun.com.cn/upload/201908/31/201908312113198518.png 3. Analysis of the Causes of Failures ① Long operating time and difficulty in maintenance: The temperature measurement resistors for the thrust and various bearing shells are installed in spaces that are tight and difficult to access for maintenance or replacement. During operation, these sensors and some wires remain submerged in high-temperature turbine oil, and they are constantly subjected to the impact of oil flow as well as the vibrations of the machine, which leads to severe mechanical damage. Since the temperature sensing elements at the bearing locations generally only have the opportunity to be inspected and maintained during Class A maintenance when the unit is disassembled. In normal circumstances, if it is necessary to deal with a faulty component, an unplanned shutdown of the unit must be requested, and tasks such as draining oil, removing the spring oil tank, and removing the bearing shells must be carried out; this results in a long shutdown time and high difficulty. Therefore, when phenomena such as jumps or breaks occur in the temperature-sensing resistor during normal operation, it is often not possible to address them promptly, and the faulty component must be taken out of service temporarily. Bearing temperature is an important parameter in the operation of the unit; a prolonged lack of temperature data from the bearing sensors prevents operators from accurately assessing the unit’s operating conditions in a timely manner, thereby posing a threat to the safe and stable operation of the equipment. ②Problems with the temperature-sensing resistor itself: a. Lack of springs at the fixing location – Since the temperature-sensing element installed in the bearing area is subject to continuous vibration during operation, the absence of springs at the fixing location prevents any cushioning against the effects of this vibration on the element. b. The temperature sensing wire has no protection on its outside; it has poor resistance to oil, corrosion, and heat ； Prolonged exposure to an oil-immersed environment can lead to hardening, brittleness, and cracking of the outer layer, thereby reducing its service life ； Prolonged vibration can cause the wire to break at the base of the sensor (see Figures 1 and 2). On-site inspections revealed that failures due to broken wires at the roots accounted for nearly 50% of all failures in the temperature-sensing resistors. Figure 1: Break in the wire at the base of the sensor. http://yunrun.com.cn/upload/201908/31/201908312155342368.png Figure 2: Break in the wire at the base of the sensor. ③ Issues with the installation of the temperature-sensing resistor: a) Problems with wire connections. The short length of the wires leading from the temperature-sensing resistor results in many connection points within the oil tank; each of these connection points needs to be welded. Due to the limited space in the oil tank, welding is difficult, and problems such as weak welds or loose connections can occur. This can lead to the breakdown of the welds under the effects of vibrations and fluid flow during operation. For example, taking Unit 4 of the Liujiaxia Hydropower Plant as an example, screw connectors are used for the connection of the thrust components and the temperature-sensing resistor wires in the upper oil guide groove. The leads of the temperature-sensing elements and the wires of the temperature-sensing cables are fixed to the epoxy plate using nuts in order to achieve the purpose of connection. However, during the maintenance and disassembly of the unit, it was found that the wiring terminals of the transfer epoxy board tend to loosen and oxidize, resulting in poor contact and causing fluctuations in temperature readings. b、The wiring in the oil tank is not properly arranged; plastic ties are used for fixing and binding the wires, which can easily damage the outer insulation of the wires. The fixing clamps in the oil tank are made of thin iron sheet, with sharp edges; if the wire’s insulation is not protected, it can easily be damaged by these sharp edges, creating potential hazards. In some cases, the wiring is not securely fixed in the oil tank; as the unit operates, the flow of oil and surges can cause the temperature sensing wires to break at the joints, as shown in Figure 3. http://yunrun.com.cn/upload/201908/31/201908312159048363.png Figure 3: The temperature sensing leads are worn out. c) The temperature sensing resistor and its wires lack effective shielding. Due to the thinness of the leads used for the temperature sensing resistor, the shielding layer is weak; as a result, during connection, the outer shielding of the temperature sensing resistor does not connect properly with the outer shielding of the temperature sensing cable. The strong electric fields generated by the generator during operation, as well as the strong magnetic fields resulting from magnetic leakage, cause significant interference in the temperature sensing circuit, leading to fluctuations in temperature. 4. Improvement measures: ① Improving the temperature-sensing resistors. Changhui Instrument Manufacturing Co., Ltd. has adopted imported foreign technologies in the production of platinum resistors for steam turbines. During the process of improving these temperature-sensing platinum resistors, it received advice and technical support from Changhui Instrument. Special springs are installed at the fixing points of the newly replaced Pt100 thermoresistors, as shown in Figure 4, thereby reducing the impact of unit vibrations on the temperature-sensing elements ； Select an outer insulating material with excellent oil resistance, corrosion resistance, and heat resistance for the temperature sensing lead in order to extend the service life of the wire ； Use temperature measurement wires with a dense mesh shield to reduce the interference of strong magnetic fields on the temperature sensing element. http://yunrun.com.cn/upload/201908/31/201908312204195095.png Figure 4: Installation of special springs at fixed locations. At the junction between the wire and the temperature-sensing resistor, bellows and armored wires are used for additional protection, as shown in Figure 5. It is particularly important to emphasize here the method of using armored wires for protection; the armored wires inside the sensor are extended outward, so that the entire part of the wire that is exposed to the impact of oil flow is made up of these armored wires. In fact, armored wires are stainless steel wire coatings that can be bent freely, and they offer excellent resistance to corrosion, impact, and vibration. This approach allows for a complete solution to the problems of wire corrosion and damage caused by oil. Longer service life and more reliable performance. http://yunrun.com.cn/upload/201908/31/201908312208026363.png Figure 5: Extended protection ② Standardized installation and wiring of temperature sensing resistors. a) Within the thrust, upper guide, and water-guided oil channels, the oil rotates at high speeds, generating significant impact forces on the connection points between the temperature sensing resistor probes and the wires. During installation, it is advisable to route the wires in a direction that is parallel to the flow of the oil, in order to reduce the impact of the oil flow on the wires at the base of the probes. b. The wires of the Pt100 thermistor used for temperature measurement are routed within the oil tank; they are tied together every 500 mm using white cloth straps, and then the prepared insulating paint is applied to those tie points so that they merge together. The wires are grouped into bundles in this way to reduce the impact of oil flow. c. Gather the temperature sensing resistor leads together along the bearing bracket, and use wire clamps to secure the wire bundle to the oil sump wall or bracket. Wire clips covered with oil-resistant rubber are used to prevent the insulation of the wires from being worn out, thus avoiding mechanical damage to the wires inside the oil tank. When the wire clip is large, first wrap the wire harness with a white cloth strip, and then secure it with the wire clip. ③Improved oil tank outlet device: A specially customized oil tank outlet device is used, as shown in Figure 6, to address issues such as oil leakage when wires exit the oil tank and difficulties in installation. The base plate and oil-sealing latch of the oil tank outlet device are both made of 304 stainless steel, ensuring no deformation during the disassembly and maintenance of the unit. The steel plate is equipped with several transparent wire-passing holes; after the wires pass through, the oil-proof latch is tightened, and the sealing strip inside the latch fits tightly around the outer layer of the wires, preventing pressure oil from leaking out through these holes during the operation of the unit. With this device, the wires do not need to be welded individually or connected using screws; instead, they pass directly from the oil tank to the terminal box on the outer wall of the oil pan. It reduces intermediate transfer steps, offering advantages such as simple processing and a high protection level. At present, all the oil tank outlet devices for units 1-4 at the Liujiaxia Hydropower Plant have been replaced, and no oil leakage has been observed during operation. http://yunrun.com.cn/upload/201908/31/201908312210257899.png Figure 6: Oil tank outlet device. ④ Redundant configurations are used for critical monitoring points. Since it is extremely difficult to replace a damaged temperature sensing element during the normal operation of the unit, in order to improve operational reliability, dual-type temperature resistance elements should be used in important areas such as the thrust bearings and guide bearings. That is, two thermal resistors with the same division number and equal precision are placed within the same armored outer sheath to measure the temperature at the same point. The lead wires of the temperature sensing element should use a six-wire system to eliminate the impact of uneven lead resistance on the measurement results. http://yunrun.com.cn/upload/201908/31/201908312213255292.png Figure 7 Schematic diagram of the dual-arm temperature sensing resistor structure. By using a dual-arm temperature sensing resistor, two measurement points can be obtained at the same location; the wires from these two points lead to the terminal box located on the outer wall of the oil tank. Under normal circumstances, one is in operation while the other serves as a backup. When a component fails, it is sufficient to connect an external cable to the spare component at the terminal box, which reduces the risk associated with equipment failures resulting from the inability to replace a damaged resistor. This paper analyzes the causes of problems such as wire breaks and instantaneous maximum measurement values that occur during the operation of temperature measurement resistors in hydropower plants, and proposes the following improvement measures: ① Improve the temperature measurement resistors. ②Standardize the installation and wiring of temperature sensing resistors. ③Improve the outlet device for the oil tank. ④Redundant configurations are used for key monitoring points. Following the above method, the temperature-sensing resistors of the bearings in units 1–4 of the Liujiaxia Hydropower Plant were modified step by step. Operational results show that the improved Pt100 temperature-sensing resistors can operate safely, reliably, and stably during the annual Class A maintenance periods over a period of five years, thereby avoiding shutdowns for repairs caused by false readings or damage to these components. Practice has proven that this method is highly practical and suitable for implementation on-site; it can serve as a reference for modifying temperature-sensing resistors in other operating hydropower stations, as well as for selecting and installing such resistors in newly built hydropower stations. Authors: Wang Ruyu, Gou Xiaojun, Luo Hongping, Liujiaxia Hydropower Plant, State Grid Gansu