A generator is a mechanical device that converts other forms of energy into electrical energy. It is driven by a water turbine, steam turbine, diesel engine or other power machinery, which converts the energy generated by water flow, air flow, fuel combustion or nuclear fission into mechanical energy and then transmits it to the generator, which then converts it into electrical energy.
Generators have wide applications in industrial and agricultural production, national defense, science and technology, and daily life. While there are many types of generators, their working principles are all based on the laws of electromagnetic induction and electromagnetic force. Therefore, the general principle of their construction is to use appropriate magnetic and electrical materials to create mutually electromagnetically inductive magnetic and electrical circuits to generate electromagnetic power and achieve energy conversion.
diesel generator
The diesel engine drives the generator to convert the energy of diesel fuel into electrical energy.
dynamo
Inside the diesel engine cylinder, clean air filtered by the air filter mixes thoroughly with high-pressure atomized diesel fuel injected by the fuel injector. Under the compression of the upward-moving piston, the mixture shrinks in volume and rapidly heats up, reaching the diesel fuel's ignition point. The diesel fuel ignites, the mixture burns violently, and its volume expands rapidly, pushing the piston downwards—this is called 'work'.
Gasoline generator
A gasoline engine drives a generator to convert the energy of gasoline into electrical energy.
Inside the cylinder of a gasoline engine, the air-fuel mixture burns violently, its volume expands rapidly, and it pushes the piston downward to do work.
Whether it's a diesel generator or a gasoline generator, each cylinder performs work in a specific sequence. The thrust acting on the piston is converted into a force that drives the crankshaft to rotate via the connecting rod. By mounting a brushless synchronous AC generator coaxially with the crankshaft of the power unit, the rotation of the power unit can drive the generator's rotor. Utilizing the principle of 'electromagnetic induction,' the generator outputs an induced electromotive force, which generates current through a closed load circuit.
Principles of generator underexcitation limitation and underexcitation protection
What is underexcitation limitation?
A: During underexcitation operation of a generator, the magnetic field connection between its stator and rotor weakens, making the generator prone to losing static stability. To ensure a certain static stability margin, the excitation control system (AVR) is designed with an underexcitation limiting circuit. That is, when the generator outputs a certain amount of active power, it is limited by the heating of the stator end core and the limitation that the power angle cannot exceed the stability limit. To ensure the safety of the generator equipment, it is necessary to ensure that the generator operates within the power limiting circle and the thermal stability limiting line. The specific settings are based on the test and the leading phase capability limit data provided by the unit, and must be coordinated with the generator loss-of-excitation protection.
What is underexcitation protection?
A: The underexcitation protection of the excitation regulator switches the regulator to a backup channel to maintain operation when the underexcitation limit fails. This backup channel can be another automatic channel or a manual channel. The stability of manual operation is lower than that of automatic operation. To ensure stable operation after switching to manual mode underexcitation protection, a minimum excitation limit for manual mode must be set, and its value is related to the generator operating point.
What are the principles for setting under-excitation limits and under-excitation protection?
A: The underexcitation limiting and protection functions of Cummins generator sets must be matched with the generator loss of excitation protection. Any operation or power system disturbance should ensure that the underexcitation limiting operates before the underexcitation protection, and the underexcitation protection operates before the generator loss of excitation protection, with a certain margin between them.
What is a forced excitation limit?
A: To prevent the rotor windings from overheating and being damaged, this limit is activated when the current of the Cummins generator exceeds a certain value, and an adjustment signal to reduce excitation is output through the AVR integrated amplifier circuit.
What are the symptoms of generator overexcitation?
A: Overexcitation of a generator is reflected in changes in various electrical parameters, mainly manifested as high excitation current, no-power overload, and stator overcurrent.
What are the different maximum excitation limits for generators?
A: The maximum excitation current limit for Cummins diesel generators is divided into two types: no-load maximum excitation current limit to prevent generator overvoltage and load maximum excitation current limit to prevent overcurrent on the excitation winding side.
What is the purpose of limiting the maximum excitation current of the load?
A: The purpose of the maximum excitation current limit is to prevent strong excitation current from occurring during generator rotor-side faults. During generator operation, when a fault occurs on the DC side of the excitation power cabinet or an inter-turn short circuit occurs in the field winding, the generator excitation current rises rapidly. If this current is not effectively suppressed, the fault point will expand further and develop into an arc short circuit, which will cause greater damage to the generator. At the same time, the fault current will also damage the equipment of the excitation system.
