As is well known, a power system consists of a small number of power sources, a large number of users, and a power grid. The power grid is divided into three parts: substation, transmission, and distribution. The electricity generated by the generators in the power plant is stepped up by power transformers and then transmitted over long distances before being delivered to households through the distribution network.
When transmission lines operate under no-load or light-load conditions, an abnormal phenomenon may occur where the voltage on the distribution/consumption side is higher than that on the transmission side. The higher the voltage of the transmission line, the larger the equivalent capacitance to ground; the longer the transmission distance, the larger the equivalent series inductance. The equivalent inductance, equivalent resistance, and terminal capacitance to ground of a transmission line form an RLC series circuit structure. Because the line inductance is much larger than the resistance, according to the vector relationship of voltage, the voltage amplitude at the end of the transmission line will be larger than that at the beginning. As the active load transmitted by the transmission line gradually increases, the total current I through the transmission line increases due to the increased load current. Consequently, the equivalent resistance, inductance, and voltage drop of the line also gradually increase, ultimately resulting in the voltage at the end of the transmission line being lower than that at the beginning.
When a synchronous generator is generating power, the generator rotor, connected to the prime mover, forms a rotating magnetic field, while the three-phase current output from the generator stator forms another rotating magnetic field. The stator's rotating magnetic field affects the rotor's rotating magnetic field, exerting a reaction force on the rotor and preventing it from accelerating. When these forces are balanced, the generator maintains a constant rotational speed. When the load suddenly decreases, the generator's output current decreases, and the stator force preventing rotor rotation decreases synchronously, causing the rotor to accelerate. The accelerated rotation of multiple synchronous generators leads to an increase in the system frequency. Since the output power of the load motor is directly proportional to its frequency (cubic power), the system frequency causes the load to increase synchronously, thus achieving a dynamic balance between frequency and load.
Unlike traditional synchronous generators, the amplitude of the rotating magnetic field of the internal rotor is adjustable at any time (equivalent to the dynamic adjustment of the excitation current of a synchronous generator), and the rotation speed of the internal rotor is synchronized with the external power grid. When the load suddenly decreases, the output current of the new energy generator decreases synchronously. Since there is no prime mover and a phase-locked loop control strategy is used, the generator "rotor" will not accelerate. Using a constant output power control strategy, when the converter detects a decrease in output current, it considers the "voltage difference" across the AC filter inductor to be insufficient, thus adjusting the PWM modulation to increase the equivalent AC output voltage of the DC capacitor (equivalent to increasing the excitation current). This results in a more severe high-voltage phenomenon at the generator end .
Meanwhile, many power generation centers, mainly based on new energy wind turbine generators, are located far from the developed power-consuming areas of the energy industry, and the large number of existing and planned ultra-high voltage direct current transmission lines have exacerbated the high voltage problem.
In power systems with ultra-high voltage direct current (UHVDC) transmission, various types of DC faults (single-pole and bi-pole blocking, DC line fault restart, commutation failure, etc.) can affect AC-side voltage . Among the various types of DC faults, continuous commutation failures at the DC receiving end often result in the longest duration and most complex phenomena of overvoltage in the sending-end system. Continuous commutation failures at the DC receiving end will lead to periodic power interruption and recovery, resulting in 3 to 4 periodic periods of low and high voltage at the sending end.
