Diesel generator frequency is too high, what is the impact?

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The impact of high frequency on a generator: The frequency should not exceed 52.5 Hz, which is 5% above the rated value. High frequency is mainly limited by the mechanical strength of the rotating parts. When the frequency increases, the rotational speed of the motor rises, increasing the centrifugal force on the rotor, which can cause damage to components such as the rotor itself.

Low frequency has several negative effects on a generator. First, it reduces the rotational speed of the rotor, decreasing the air flow from the cooling fans at both ends, leading to worse cooling and higher temperatures in various parts. Second, low frequency can cause the temperature of the rotor coil to rise, potentially reducing output. Third, it may lead to blade failure in turbines. Fourth, when frequency drops, the terminal voltage must be kept constant, requiring an increase in magnetic flux, which can cause saturation in the stator core, leading to local overheating and structural damage. Fifth, lower frequency reduces the motor speed in factories, lowering production output and affecting user safety, product quality, and efficiency. Lastly, low frequency results in lower voltage because the induced electromotive force depends on the rotational speed, and the generator's exciter output also decreases, impacting reactive power generation.

Q16: What should the operator pay attention to when the generator enters phase?

A: In theory, a generator can enter phase operation, which means the power factor is leading, and the current leads the terminal voltage. At this point, the generator still delivers active power but absorbs reactive power. The excitation current is low, and the generator operates under low excitation conditions. Operators must monitor two key issues: (1) reduced static stability, and (2) increased temperature at the stator end due to leakage, which can raise the temperature of the stator’s end sections.

Q17: Explain the principle, advantages, and disadvantages of a 600MW brushless excitation system.

A: The brushless excitation system uses a high-frequency 400Hz power supply generated by a permanent magnet machine's stator to supply the main exciter's field winding through two sets of fully controlled rectifier bridges. The intermediate frequency 200Hz power from the main exciter's armature is supplied to the rotary rectifier, and the DC output from the rectifier provides the generator's excitation power. This power is delivered to the generator's field winding through the central bore of the rotor.

Advantages of the brushless excitation system include eliminating large current collector rings and carbon brushes, thus preventing sparks that are common in conventional systems. It also offers a compact structure and reduces maintenance workload.

Disadvantages include the time lag in the excitation control system. To improve response time, hard and negative feedback from the generator’s rotor voltage is added to the excitation regulator circuit, which reduces the time constant but increases the exciter's capacity and voltage requirements.