The Xinjiang Tianhong Paper Industry 35 kV workshop substation has seven 35 kV high-voltage switchgear cabinets, consisting of six 35/0.4 kV transformers with a capacity of 6×1600 kVA. The 35 kV incoming line is supplied by the 35 kV outgoing line of the Tianfucheng West 110 kV substation. In 2001, due to the intrusion of system harmonics, the 35 kV voltage transformer TV cabinet burned out. Since the 35 kV system uses oil circuit breakers, this caused the entire 35 kV high-voltage switchgear cabinet to burn out and explode, resulting in the termination of power supply to the workshop and severely impacting production. [b]1 Generation of Harmonics[/b] Power grid harmonics originate from three aspects: First, poor power quality generates harmonics. Generators, due to the difficulty in achieving absolute symmetry in the manufacturing of three-phase windings and the difficulty in achieving absolute uniformity in the iron core, generate harmonics. Second, harmonics generated during the operation of the power transmission and distribution system; and third, harmonics generated by electrical equipment. Among these, electronic electrical equipment generates the most harmonics. In power transmission and distribution systems, harmonics are primarily generated by power transformers. Due to the saturation of the transformer core, the nonlinearity of the magnetization curve, and economic considerations in transformer design (where the operating magnetic flux density is chosen near the saturation segment of the magnetization curve), the magnetizing current exhibits a peaked waveform, thus containing odd harmonics. The magnitude of these harmonics depends on the structure of the magnetic circuit and the degree of core saturation. The higher the core saturation, the further the transformer's operating point deviates from linearity, and the larger the harmonic current. The third harmonic current can reach 0.5% of the rated current. Harmonics are generated in electrical equipment such as thyristor rectifiers, frequency converters, electric arc furnaces, calcium carbide furnaces, gas discharge light sources, and household appliances. At Tianye Chemical Plant's calcium carbide plant, during calcium carbide production, the electric arc furnace and calcium carbide furnace often experience unstable combustion because the three-phase electrodes cannot simultaneously contact the uneven furnace charge. This leads to an imbalance in the three-phase load, generating harmonic currents that are injected into the power grid through the delta-connected windings of the transformer. The main harmonics are the 2nd to 7th harmonics, averaging 8% to 20% of the fundamental frequency, with a maximum of 45%. [b]2 Harmonic Restriction Measures[/b] One 35 kV outgoing line of the Tianfucheng West 110 kV substation supplies power to the Tianhong Paper Industry 35 kV workshop substation, and another line supplies power to the calcium carbide plant of Tianye Chemical Plant. Because Tianye Chemical Plant generates harmonics during calcium carbide production, primarily the 2nd to 7th harmonics, this large harmonic source flows into the Tianhong Paper Industry 35 kV workshop substation via the transmission lines. The voltage transformer cabinet of the Tianhong Paper Industry 35 kV workshop substation lacks harmonic suppression and arc suppression devices. When system harmonics intrude, it will inevitably cause a burn-out accident at the Tianhong Paper Industry 35 kV workshop substation. To address the impact of system harmonics, the 35 kV cabinet circuit breaker was redesigned to be a vacuum circuit breaker, and the original voltage transformer cabinet was replaced with an XHG arc suppression, harmonic suppression, line selection, and overvoltage protection device cabinet. 2.1 The XHG cabinet is suitable for 3-35 kV medium-voltage power systems. The XHG cabinet consists of the following components: A three-phase combined overvoltage protector (TBP). The TBP is a special metal oxide overvoltage protector that limits system overvoltages. Its impulse coefficient is 1, and the discharge voltage is equal under various voltage waveforms, so its protection performance is unaffected by the overvoltage waveform. A high-voltage vacuum fast contactor (JZ) with phase-by-phase control. The JZ's function is to quickly transform the unstable arc grounding into a stable metallic direct grounding when arc grounding occurs in the system, thus protecting the equipment from damage. A controller (ZK). Based on the voltage signal provided by the voltage transformer, it determines the fault phase and fault type, i.e., voltage transformer open circuit, metallic grounding, and arc grounding, and controls the high-voltage vacuum contactor JZ according to a preset method. A high-voltage current-limiting fuse assembly (FUR). The FUR prevents phase-to-phase short circuits caused by miswiring or device malfunctions. A voltage transformer with auxiliary secondary windings. When a single-phase ground fault occurs in the protected system, the voltage across the open delta of the voltage transformer immediately changes from low to high, activating the controller. 2.2 Basic Functions of the XHG Cabinet: When a single-phase ground fault occurs in the system, the faulty phase can be directly grounded within approximately 30 ms. If it is an arc ground fault, the arc is immediately extinguished, stabilizing the arc ground overvoltage at the line voltage level, effectively preventing phase-to-phase short circuits caused by single-phase ground faults and surge arrester explosions caused by arc ground overvoltages. If it is a metallic ground fault, the contact voltage and step voltage are significantly reduced, which is beneficial for personal safety. If used in a power grid primarily composed of overhead lines, the vacuum contactor trips 5 seconds after the device operates. If it is a transient fault, the system returns to normal; if it is a permanent fault, the device will not trip again after reoperation. When a voltage transformer open-circuit fault occurs, the device outputs a signal so that the user can block relay protection that may malfunction due to a voltage transformer open-circuit fault. After the device operates, the panel displays the fault type, such as identification of voltage transformer disconnection, metallic grounding, or arcing grounding, and simultaneously outputs passive switch contact signals. The device can limit various overvoltages occurring between phases and between phase and ground to approximately 3.5 times. [b]3 Conclusion[/b] At the end of 2001, the Tianhong Paper Industry 35 kV substation was re-commissioned after design and renovation. Based on the operation over the past 5 years, the Tianhong Paper Industry 35 kV substation has performed well when harmonic intrusion occurs in the system.