6 Fire hazards and preventive measures for hydro-generating units Zhang Xiaozhong Since the construction of the first hydropower station in France in 1878, the world's hydropower construction has come a long way in 122 years. The use of hydroelectric power neither loses water nor pollutes the environment, and it is flexible in operation, efficient, and low in production cost. Therefore, many countries in the world have prioritized the development of hydropower resources. As the core equipment of a hydropower station, the hydro-generator unit has historically received attention from domestic and foreign design and manufacturing units and operating units. 1 The composition and working principle of the hydro-generator unit The hydro-generator is mainly composed of two components, one is a rotor capable of rotating at a fast speed, and the other is a stationary stator with a winding placed on the iron core. The rotor is located in the cavity in the center of the stator and has a constant air gap with the stator. The rotor is provided with a magnetic pole that can generate a magnetic field. When the water flow impacts the rotation of the rotor, mechanical energy is generated. The magnetic pole of the rotor rotates with the rotor at a high speed. At this time, the magnetic field generates relative motion with the stator winding. The magnetic field cuts the conductors of the stator windings, generating an induced potential that causes the machine to convert to electrical energy. 2 Fire hazard of the hydro-generator unit Insulation damage of the stator winding causes a short circuit between the windings to the ground or windings, and the short-circuit arc can blow the copper wire and connect the copper wire and the iron core. High humidity can cause the insulation to burn. If the generator cooling device fails or the wire joints are badly welded, the windings of the stator or the rotor will be overheated, the insulation ageing and the joint between the windings will be melted, and it may progress to arc fire. 3 Fire-fighting safety measures for hydro-generator units According to the size and importance of the type capacity of the generator set, relays Zhang Xiaozhong and Yichang Public Security Fire Brigade shall be installed on the possible short-circuit accidents. Protect automatic devices. When the stator windings are shorted to each other, the protection device shall act to disconnect the generator breaker and the automatic fire extinguishing magnetic device, and act at the same time to stop the accident. When the stator winding over-current over-voltage and loss of magnetic protection, the generator breaker and automatic de-excitation device should be disconnected. However, for units that are not equipped with Permanent Magnet machines, they should also act on accident shutdown. When the grounding and overload of the stator winding with a current less than 5 people and the ground point of the excitation circuit are grounded, the protection device can act on the signal with the time limit. For generators above 80, the phase-to-phase short circuit of the stator windings and their lead-out lines shall be protected and longitudinal differential protection shall be installed. Single-phase earth faults of the stator windings of the generator can be signalled by single-phase grounding devices installed on the bus. An overcurrent caused by an external short circuit in the stator windings of the generator can be overcurrent protected by a composite voltage including a negative sequence voltage and a line voltage or a low voltage start. Generator stator windings should be fitted with overvoltage protection. Its setting value can be 1.5 times the rated voltage, action time limit can be 0.58. 4 Fire-fighting measures for genset fires At present, domestic and foreign ministries use water-oxidation carbon to extinguish hydroelectric generating unit fires. Water firefighting is economical, reliable, safe and simple to operate. The disadvantage is that effective baking is required during the resumption of use of the equipment to achieve a qualified insulation standard. Extinguishing fire with carbon oxide requires a good sealing structure of the generator and a large initial investment. Extinguishing carbon oxides after fire extinguishing can be used for maintenance. Only a few hydropower stations are used in China. Therefore, most water turbine generators in China use water fire extinguishing methods, especially the use of water spray fire extinguishing, so that the fire extinguishing effect is better and more reliable. Generator automatic water spray fire extinguishing system consists of water source filter electrical cabinet mechanical cabinet with water pipe network spray head alarm. According to the accident adjustment, analysis found that the burned parts of the unit are usually at the ends of the stator coils. Therefore, a fire extinguishing ring pipe and a spray head are provided at the upper and lower ends of the stator coil. In the wind turbine enclosure with a unit capacity of 125 and above, an automatic fire alarm system shall be installed. The electrical connection shall use shielded cables. The fire detectors shall be of the type of anti-frequency electromagnetic field. When a fire occurs, fire alarm signals are sent to the electrical control cabinet of the automatic water spray fire extinguishing system of the generator and the central control room of the unit operation control unit. The automatic water spray fire extinguishing system has an automatic semi-automatic manual seeding operation mode, and the operating modes can be arbitrarily converted by the operating personnel as required. Generator automatic water spray fire extinguishing equipment is arranged on the turbine of each unit, and the mechanical cabinet is arranged beside the turbine side of the main plant. The amount of water sprayed on the circumference of the coil at the upper and lower ends of the generator stator should not be less than 10! In the hydro-generator's heating air intake and supplementary air-turbine generators operating personnel should strictly follow the requirements of the generator's operating procedures, to strengthen the inspection on duty, strict operating system. Maintenance personnel should improve the technical level and quality of inspection. When the oil leakage of the low-head rotary turbine is overhauled, the oil leakage on the blades must be prevented. Before the inspection, the traces of oil on the components must be removed first. The spent cotton yarn or rag used for inspection and repair shall be put into the designated iron drum and processed in time. In the event of a fire in the generator, the generator and system should be quickly disassembled and the fire extinguishing system should be immediately used to extinguish the fire in order to quickly control the fire.
Alnico is an alloy of Aluminium, Nickel and Cobalt.It also contains Iron , Copper ,Titanium, Different metal
component constitution will form different magnet properties, application will also change accordingly.
Alnico is produced by sintered or casted technique,Its`surfaces may need machining,or to be ground for a smoother surface finish and looks brightly metallic looking. Casting means can producing AlNiCo forvariety sizes and shapes, Sintered Alnico is subsize and have very tiny tolorance but machanical performance is not good.
The most commonly used Alnico is Alnico5. There are 4 sort of cast Alnico5 plus a singlesort of sintered Alnico5 (sintered Alnico and cast Alnico have slightly different magnetic and mechanical performance). Among the permanent magnetic materials, AlNiCo have the best reversible temperature coefficient, the highest working temperature up to 500 ℃,it has the
hardest and toughest strongness.Most ALNICO Magnet grades are anisotropic (axial / vertical direction add magnetization).The others are isotropic(horizontal direction magnetized and usually have many poles on one magnet) they are weaker on magnetism but offer more options on design and properties.
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