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How Does a Diesel Generator Set Form a Magnetic Field

Nov. 21, 2018

The engine of the diesel generator set drives the generator to convert the energy of the diesel into electrical energy. The generator set uses the principle of 'electromagnetic induction', and the generator outputs an induced electromotive force, which can generate current through a closed load circuit. In addition, a range of diesel and generator controls, protection devices and circuits are required to achieve a usable, stable power output. The following describes the working principle of the formation of the internal magnetic field of the diesel generator set.

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Generators are divided into two major categories: DC generators and alternators. The latter can be divided into synchronous generators and asynchronous generators. Synchronous generators are most commonly used in modern power stations. This type of generator is characterized by DC current excitation, which can provide reactive power to meet the needs of various loads. Because there is no independent excitation winding, the asynchronous generator has a simple structure and convenient operation, but can not provide reactive power to the load, and also needs to draw the lag magnetization current from the connected power grid. Therefore, the asynchronous generator must be operated in parallel with other synchronous units or with a considerable number of capacitors. This limits the range of applications of asynchronous generators and can only be applied to small automated hydropower plants. DC power supplies used in urban trams, electrolysis, and electrochemistry industries used DC generators before the 1950s. However, DC generators have commutators, which are complicated in structure, time-consuming to manufacture, expensive, easy to malfunction, difficult to maintain, and inefficient as an alternator. Therefore, since the advent of high-power controllable rectifiers, there has been a trend to use AC power to obtain DC power through semiconductor rectification to replace DC generators.


Synchronous generators are classified into three types: turbo generators, hydroelectric generators and diesel generators according to the different prime movers used. What they have in common is that in addition to the small motor that uses a permanent magnet to generate a magnetic field, the general magnetic field is generated by a direct current excitation coil, and the excitation coil is placed on the rotor, and the armature winding is placed on the stator. Because the voltage of the excitation coil is lower, the power is smaller, and there are only two outlet heads, which are easily led out through the slip ring; the armature winding has a higher voltage and a large power, and the three-phase winding is used, and there are three or four lead-out heads. It is convenient to put on the stator. The armature (stator) core of the generator is laminated with silicon steel sheets to reduce iron loss. The rotor core can be made of a monolithic steel block because the magnetic flux passed through does not change. In large motors, because the rotor is subjected to strong centrifugal force, the material used to make the rotor must be made of high quality steel.



What are the adverse effects of generator loss of magnetism?


When the generator loses magnetism, the rotor loses the excitation current. The generator after demagnetization will generate differential current in the damper winding, rotor surface and rotor winding of the rotor, causing additional temperature rise, which may cause local high temperature of the rotor, which may easily cause some defects. The impact is summarized as follows:


1. After the generator loses excitation, the reactive power is changed to absorb reactive power, and the larger the slip, the smaller the equivalent reactance of the generator, and the greater the reactive power absorbed, causing the stator winding of the loss-generating generator to pass current.


2. After the rotating speed of the rotor and the rotational speed of the rotating magnetic field synthesized by the startor winding are deviated, the rotor surface (including the body, the wedge, the guard ring, etc.) will induce the slip frequency current, causing local overheating of the rotor, which is the most harmful to the generator.


3. During asynchronous operation, the torque changes periodically, so that the stator, rotor and its foundation are constantly subjected to abnormal mechanical moments, and the vibration of the unit is intensified, threatening the safe operation of the generator.


4. When the demagnetization is moderately serious, if the protection is not acted in time, the generator and the turbine rotor will be overspeeded immediately, and the consequences are unimaginable.


5. When a generator is demagnetized, due to the voltage drop, other generators in the power system will increase their reactive output under the action of the automatic adjustment of the excitation device, thereby causing overcurrent of some generators, transformers or lines. The backup protection may be mistaken due to overcurrent, which will widen the scope of the accident.


6. Low-excitation and de-energized generators absorb reactive power from the system, causing the voltage of the power system to decrease. If the reactive power reserve in the power system is insufficient, the voltage in some adjacent points in the power system will be lower than the allowable value. It destroys the stable operation between the load and each power supply, and even collapses the power system voltage.


7. After a generator is demagnetized, due to the swing of the active power of the generator and the drop of the system voltage, it may cause the step-out between the adjacent normal operation generator and the system, or between the parts of the power system, so that the system Oscillation occurred.


8. The larger the rated capacity of the generator, the greater the reactive power shortage caused by low excitation and demagnetization, and the smaller the capacity of the power system, the smaller the ability to compensate for this reactive power shortage. Therefore, the greater the ratio of the unit capacity of the generator to the total capacity of the power system, the more serious the adverse effect on the power system.


It can be seen that the loss of magnetism of the generator is likely to have more serious consequences, which will not only damage the generator itself, but also affect the power system. For safety reasons, the generator management personnel must ensure the inspection of the excitation circuit, find problems in time, and avoid the occurrence of generator loss of magnetic failure.


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