79 Key Professional Knowledge Areas in Electrical Engineering （21-30）

By clyde March 5th, 2025 19 views

21.Why is a DC insulation monitoring device installed?

It is not permissible for one pole of the DC system in a substation to remain grounded for an extended period. If another ground fault occurs at another location on the same pole, it may cause incorrect operation of signaling devices, relay protection, and control circuits. Additionally, if one pole is already grounded and another ground fault occurs on the opposite pole, it will result in a DC short circuit.

22.Explain what float charging is.

Float charging involves the installation of two charging units: one as the main charging unit and the other as the float charging unit. Float charging compensates for the self-discharge losses of the battery, ensuring that the battery bank remains fully charged at all times.

23.Why do outdoor busbar joints tend to overheat?

Outdoor busbars are frequently exposed to wind, rain, snow, sunlight, and freezing conditions. These environmental factors can accelerate oxidation and corrosion of the busbar joints, increasing contact resistance and causing temperature rise.

24.What are the phenomena during system oscillations?

The phenomena during system oscillations include:
(1) Current, voltage, and power meters in substations exhibit periodic fluctuations. If there is a tie line, the meter fluctuations are most noticeable.
(2) The closer to the system oscillation center, the greater the voltage fluctuation. Incandescent lamps flicker noticeably, making the phenomenon very visible.

25.What is the function of the "unreset signal" indicator?

The unreset signal light is used to ensure that operators do not miss recording the actions of protective devices, which could lead to misjudgment. It is important to promptly reset the signal to avoid repeated actions and confusion between consecutive events.

26.Can AC and DC low-voltage circuits share the same cable? Why or why not?

No, they cannot. Because:
(1) Sharing the same cable can reduce the insulation level of the DC system.
(2) If the DC insulation is compromised, DC mixing could cause short circuits or unintended activation of relays.

27.What size of megohmmeter should be used to measure the insulation of secondary circuits? What are the insulation standards in megohms?

It is best to use a 1000V megohmmeter to measure the insulation resistance of secondary circuits. If unavailable, a 500V megohmmeter can also be used.
Insulation standards:
In operation: no less than 1 megohm.
For new installations: indoor circuits should have no less than 20 megohms, and outdoor circuits should have no less than 10 megohms.

28.What are the functions of auxiliary contacts in oil circuit breakers?

The normally open and normally closed auxiliary contacts of an oil circuit breaker change their positions to connect the closing and tripping circuits of the circuit breaker mechanism, as well as the audio signal circuits. This achieves the purpose of opening or closing the circuit and correctly issuing audio signals. They also activate automatic devices and protective interlocking circuits. When auxiliary contacts are used in closing and tripping circuits, a delay is typically included.

29.What are the chemical properties of SF6 gas?

SF6 gas is insoluble in water and transformer oil. At high temperatures, it does not react with oxygen, argon, aluminum, or many other substances. However, under the influence of electric arcs and corona effects, SF6 gas decomposes into low-fluorine compounds, which can damage insulating materials and are highly toxic. The decomposition of SF6 gas is closely related to moisture content, so dehumidification measures are necessary.

30.What is the operating principle of overcurrent protection?

During phase-to-phase short-circuit faults in the power grid, current suddenly increases while voltage drops. Overcurrent protection is set based on the selectivity requirements of the line, adjusting the operating current of the current relay accordingly. When the fault current reaches the operating value of the current relay, the relay operates, selectively disconnecting the faulty line according to the selectivity requirements of the protection device.

79 Key Professional Knowledge Areas in Electrical Engineering （11-20）

79 Key Professional Knowledge Areas in Electrical Engineering （31-40）