Regarding DC transmission

Compared with AC transmission, DC transmission has the following advantages: ① When transmitting the same power, DC lines have lower construction costs; the tower structures for overhead lines are simpler, the required corridor width is smaller, and cables with the same insulation level can operate at higher voltages； ②DC transmission has low power and energy losses ； ③Minimal communication interference ； ④During steady-state operation of the line, there is no capacitive current nor reactive voltage drop; the voltage distribution along the line is relatively uniform, and no reactive power compensation is required for the line itself ； ⑤The AC systems connected by a DC transmission line do not need to operate in sync; therefore, it enables asynchronous connection between AC systems with different frequencies or those with the same frequency ； ⑥DC transmission lines do not suffer from the stability issues inherent in AC transmission, and their transmission distance and capacity are not constrained by the stability requirements of synchronous operation in power systems ； ⑦The short-circuit capacity of each AC system connected to one another via DC transmission lines does not increase significantly as a result of this interconnection ； ⑧The regulation and control of power and current in DC transmission lines are relatively easy and rapid, allowing for various types of regulation and control. If AC and DC systems operate in parallel, it helps improve the stability of the AC system and enhance the operational performance of the entire system. Generation of harmonics: The principle of inversion relies on the alternating switching of thyristors in the inversion module, which converts direct current into a standard sinusoidal alternating current wave. Due to the switching operations of semiconductor thyristors (the inverter-type IGBTs in frequency converters; I have not worked with high-power inverters), as well as the nonlinear characteristics of diodes and semiconductor thyristors, certain devices in power systems, such as power converters, deviate significantly from a sine wave shape, resulting in many small peak interference waves in the waveform. Harm of harmonics: 1. Large amounts of harmonic voltage and current circulate in the power grid and accumulate, leading to increased line losses and overheating of electrical equipment. This in turn raises the costs of electricity operation and increases electricity bills. 2. Harmonic voltages can make sine waves sharper in many cases, not only increasing the hysteresis and eddy current losses of electrical devices such as transformers and capacitors, but also raising the electrical stress on insulating materials. Harmonic currents can increase the copper loss in transformers; as a result, under severe harmonic load conditions, the transformers experience local overheating and increased noise, which accelerates the aging of the insulation. This **shortens the service life of transformers and motors, reduces the reliability of power supply, and can lead to serious consequences such as power outages during production processes. 3. The power grid contains a large number of harmonic sources (frequency conversion or rectification equipment), as well as loads such as power capacitors, transformers, cables, and motors. These electrical devices are in constant change, which easily creates conditions for series or parallel resonance. When the grid parameters are not properly matched, harmonic oscillations occur at certain frequencies, resulting in overvoltage or overcurrent that threatens the safe operation of the power system. If left unaddressed, this can easily lead to transmission and distribution accidents. 4. Grid harmonics can cause errors in measuring instruments and metering devices, preventing them from providing accurate readings and measurements (the errors in these instruments are most evident in electricity meters). When a circuit breaker breaks current with a high harmonic content, its breaking capacity **decreases**, which leads to arc re-ignition, short circuits, and even explosion of the circuit breaker. 5. When power transmission lines run parallel to or close to communication lines, electrostatic and electromagnetic induction occur between them, resulting in electric field coupling and magnetic field coupling. Harmonic components generate audio interference within the communication system, thereby reducing the quality of signal transmission and disrupting its normal flow. This not only affects the clarity of calls but, in severe cases, can also pose a threat to the safety of communication equipment and people. 　　Harmonics can interfere with adjacent communication systems; in mild cases, this results in noise that reduces the quality of communication ； In severe cases, it leads to the loss of the residence, preventing the communication system from functioning properly.