Abstract: This paper introduces the structure and characteristics of combined surge arresters suitable for vacuum switchgear, and discusses the selection requirements for their technical parameters and pre-commissioning tests. Keywords: Combined surge arrester, characteristics, structure, parameter selection, commissioning test 1 Introduction Combined overvoltage protectors are a new type of overvoltage protection device, mainly used in 35kV and below power systems. They are used to limit lightning overvoltages, vacuum circuit breaker switching overvoltages, and various transient overvoltages that may occur in the power system. They can effectively protect the insulation of power equipment such as motors, transformers, switches, capacitors, cables, and busbars from damage, and can reliably limit phase-to-phase and phase-to-ground overvoltages. The widespread application of vacuum circuit breaker devices has led to increasing attention to the hazards caused by switching overvoltages. The variety of combined overvoltage protectors provides us with a wide range of choices, but also makes the selection more cautious. This paper aims to discuss the selection of combined overvoltage protectors (combined zinc oxide surge arresters) in vacuum circuit breaker devices. 2. Origin of the Application of Combined Overvoltage Protectors The development of surge arrester products in China has gone through several stages, including ordinary valve-type surge arresters, magnetic blowout surge arresters, and metal oxide surge arresters (MOA). In recent years, the overall manufacturing level and quality of surge arresters have greatly improved. With the widespread application of vacuum circuit breakers, many measures have been taken to limit their operating overvoltage and avoid damage to the insulation of the equipment. Generally, the protection devices for operating overvoltage of vacuum circuit breaker devices are as follows: (1) RC absorption devices; (2) gapless zinc oxide surge arresters; (3) zinc oxide surge arresters with series gaps. The biggest advantage of RC absorption devices is that they can mitigate the steepness of the overvoltage wave intruding into the protected equipment and improve the voltage gradient on the equipment winding. However, they have problems such as large size, no obvious overvoltage limit value, small overvoltage energy absorption capacity, and high-order harmonic pollution. Gapless zinc oxide surge arresters are a relatively advanced overvoltage protection device. Compared with traditional silicon carbide surge arresters, they have superior characteristics in protection features, breaking capacity, and pollution resistance. Their ZnO resistance elements exhibit exceptional nonlinearity, allowing them to approach an insulating state under normal operating conditions. However, they have a relatively high residual voltage, which cannot meet the requirements for frequent operation under switching overvoltages. They also have limitations in power frequency aging and withstand charge rate and thermal balance conditions, which are insufficient for devices protecting the insulation withstand voltage of motors. Zinc oxide surge arresters with series gaps, due to the added series gap, allow for a smaller number of ZnO resistance elements in the MOA (Mean Adjustment Ability). This results in a very low residual voltage. If the discharge voltage of the spark gap is also low, the arrester can achieve a very low protection level without suffering from the degradation and power loss caused by the large resistive component of leakage current. Compared to gapless MOAs, MOAs with series gaps have a higher withstand capability for system transient overvoltages, ensuring their own safety during system ground faults. They also have lower lightning impulse discharge voltages and residual voltage levels, providing good protection for equipment with weak insulation, and are particularly suitable for neutral-point non-effectively grounded systems. In recent years, China has developed various three-phase combined zinc oxide surge arresters with or without series gaps. These arresters have a certain proportion of ZnO resistance elements or spark gaps connected between phases and between phase and ground, making them a composite type of surge arrester. This overvoltage protection device provides good protection against phase-to-phase overvoltages. Because of their composite insulation structure, combined overvoltage protectors are less affected by switchgear dimensions during installation, and are therefore increasingly accepted. 3. Gap Structure and Characteristics of Combined Overvoltage Protectors Combined overvoltage protectors are divided into gapless and gapless types. This article mainly discusses zinc oxide surge arresters with series gaps. The combined zinc oxide surge arrester is composed of a special gap body and zinc oxide varistors (ZnO). The gap structure varies depending on the manufacturer's technical scheme. The gaps are mainly four-gap, three-gap, diamond-shaped gap (single gap), six-gap, etc. There are two types of gaps: those with parallel resistors and those without parallel resistors. The technical characteristics of the gaps are also different. (1) The four-gap star connection combined overvoltage protector is composed of four identical protection units. Each unit is composed of a discharge gap and a ZnO varistor. Its wiring diagram is shown in Figure 1. In this protector, ZnO and the discharge gap are combined to protect each other. The discharge gap makes the charge rate of ZnO zero. The excellent nonlinearity of ZnO makes the arc extinguished immediately after the discharge gap operates, without current interception or follow current. The discharge gap no longer undertakes the task of arc extinguishing. The impact coefficient can reach 1. The discharge voltage value does not change with the discharge waveform, thus improving the service life. This wiring method can significantly reduce phase-to-phase overvoltage. Compared with conventional MOA, the phase-to-phase overvoltage is reduced by 60% to 70%. It can operate safely for a long time under single-phase grounding, intermittent arcing grounding and resonant overvoltage. Since both phase-to-phase and phase-to-ground are double gaps, each gap bears 1/2 of the power frequency discharge voltage. Under normal circumstances, the potential at the center point is "zero", so the power frequency voltage is borne by the phase gap. At the same time, there is parasitic capacitance to ground. The existence of parasitic capacitance will cause the actual discharge value to be unstable. (2) The three-gap star connection combined overvoltage protector consists of three gaps and four units. Its wiring diagram is shown in Figure 2. Its structure is different from that of the four gaps in that the ground protection unit gap is eliminated. The phase-to-ground protection adopts a single gap. The ground protection unit is composed of pure resistive material. The parasitic capacitance and stray capacitance at the center point are relatively small. When phase-to-phase overvoltage occurs, it is completed by the phase-to-phase protection unit and the ground protection unit. The phase-to-phase overvoltage is also borne by two gaps. By adjusting the grounding protection unit, the phase-to-phase and phase-to-ground power frequency discharge voltages can be made the same. (3) The diamond gap star connection combined overvoltage protector consists of a diamond gap and four units. Its wiring diagram is shown in Figure 3. Its structure is different from the four gap star connection in that it adopts a diamond gap structure, which reduces the number of discharge gaps of the three-phase combined overvoltage protector with series gaps to 1, thereby reducing the influence of distributed capacitance and stray capacitance on the discharge value. The phase-to-phase overvoltage and phase-to-ground overvoltage processes are completed by one gap. Since the gap and ZnO can be installed separately, ZnO can be directly hot-pressed together with the shell material, so that the cavity around the valve plate is almost non-existent, and the sealing moisture and explosion-proof problems of ZnO are solved well. (4) A high-voltage resistor is connected in parallel on the gap. At the power frequency, the capacitive reactance of the gap is much greater than the impedance of the parallel resistor. The voltage across the gap depends on the voltage division value of the resistor. During an impulse, due to the steep wavefront, its equivalent frequency is much higher than the power frequency. At this time, the capacitive reactance of the gap is much smaller than its impedance, and the voltage distribution is determined by the capacitive reactance, thus unaffected by the parallel resistance. 4. Selection of Combined Overvoltage Protectors When selecting a combined overvoltage protector, it is essential to understand the structural characteristics of the selected product, the quality of the ZnO resistance element and the gap, and the overall insulation and sealing performance, as manufacturing quality is crucial. Simultaneously, it is necessary to ensure that all performance indicators comply with the requirements of ZBK49005-90 "AC Metal Oxide Surge Arresters with Series Gap" and meet the requirements of DL/T620-1997 "Overvoltage Protection and Insulation Coordination of AC Electrical Installations". Important technical parameters for selecting an MOA include rated voltage, maximum continuous voltage, nominal current, lightning impulse protection level, and switching impulse protection level. The following section discusses the selection of surge arresters within 6-35kV system switchgear. (1) Selection of surge arrester rated voltage Ur a. Selection based on surge arrester continuous operating voltage UC Since 6-35kV systems are mostly neutral point ungrounded systems, after a single-phase ground fault occurs, the phase-to-ground voltage rises to the line voltage Um (Um is the highest operating voltage of the system), which is a temporary overvoltage. The fault duration is ≥10s. Therefore, the selection of surge arrester continuous operating voltage is as follows: For 6-10kV, UC≥1.1Um, then 6kV surge arrester UC≥1.1x7.2=7.92kV For 10kV surge arrester UC≥1.1x12=13.2kV ​​For 35kV, UC≥1.0Um, then 35kV surge arrester UC≥1.0x40.5=40.5kV b. Selection based on surge arrester temporary overvoltage Ut Temporary overvoltages include two main categories: power frequency and resonance. Only power frequency overvoltages caused by single-phase ground faults are the main basis for determining and selecting surge arrester rated voltage. According to the Ministry of Electric Power's "Notification on the Relevant Circumstances Regarding the Improvement of Rated Voltage and Continuous Operating Voltage of 3-66kV Gapless Metal Oxide Surge Arresters" dated October 30, 1993, Ur≥1.4Um for 3-15.75kV and Ur≥1.3Um for 35-66kV. In actual selection, the value is slightly smaller than the above values: 6-10kV Ur≥1.38Um, then 6kV surge arrester Ur≥1.38x7.2=9.94kV 10kV surge arrester Ur≥1.38x12=16.6kV 35kV Ur≥1.25Um, then 35kV surge arrester Ur≥1.25x40.5=50.6kV (2) Selection of nominal discharge current The nominal discharge current In of the surge arrester is an important parameter used to classify its level with a waveform of 8/20μs. There are five levels: 1.5, 2.5, 5, 10, and 20kA. The first three levels correspond to the neutral point, motor surge arrester, and capacitor surge arrester, respectively. The power station surge arrester is divided into the latter three types. Generally, 5kA is selected for 6-35kV systems. (3) The residual voltage value under the nominal discharge current (8/20μs) of the surge arrester is the lightning impulse protection level of the surge arrester. The ratio of the residual voltage value under the nominal discharge current (1/5μs) of the steep wave to the residual voltage value under the nominal discharge current shall not be greater than 1.15. The lightning impulse protection level of the surge arrester shall meet the requirements of insulation coordination for the protection of power equipment, that is, the ratio of the full-wave impulse insulation level of the electrical equipment to the lightning impulse protection level shall not be less than 1.4. According to GB11032-2000 "AC Gapless Metal Oxide Surge Arresters" based on the continuous operating voltage: For 6kV surge arresters with UC≥7.92kV, the residual voltage of a power station type MOA is 27kA, and that of a distribution type MOA is 30kV; For 10kV surge arresters with UC≥13.2kV, the residual voltage of a power station type MOA is 45kA, and that of a distribution type MOA is 50kV; For 35kV surge arresters with UC≥40.5kV, the residual voltage of a power station type MOA is 134kA. (4) The maximum residual voltage of the switching impulse current (within 30～100μs) of the surge arrester with switching impulse protection level. The switching impulse insulation coordination coefficient should meet the requirement that the ratio of the switching insulation level of the electrical equipment to the switching impulse protection level shall not be less than 1.15. According to GB11032-2000 "AC Gapless Metal Oxide Surge Arresters" standard, when the continuous operating voltage is UC≥7.92kV, the residual voltage of a substation type MOA is 23kA, and that of a distribution type MOA is 25.6kV; when the UC of a 10kV surge arrester is UC≥13.2kV, the residual voltage of a substation type MOA is 38.3kA, and that of a distribution type MOA is 42.5kV; when the UC of a 35kV surge arrester is UC≥40.5kV, the residual voltage of a substation type MOA is 114kA. Additionally, creepage distance and other factors must be considered to ensure compliance with acceptance test requirements. 5. Pre-commissioning Inspection To prevent damage to the product from unexpected factors, tests and periodic inspections should be conducted before the surge arrester is put into operation. Tests should be performed between "phase-to-phase" and between "ground-to-phase," with three measurements taken and the average value calculated. The time interval between each two tests should be no less than 10 seconds, and the power frequency power supply should be cut off within 0.2 seconds after discharge. During the test, an ammeter of 10A or higher can be connected in series next to the test transformer to observe the current value. When the current suddenly changes, it indicates that the test object has discharged, and the voltage value at this moment is the power frequency discharge voltage value. If the site conditions permit, the pulse voltage value can be directly read through a high-voltage tester. Routine power frequency discharge tests should be performed every 3-4 years. The preventive test regulations for power equipment stipulate that the insulation resistance of MOA surge arresters of 35kV and below should be measured with a 2500V megohmmeter and should not be less than 1000MΩ. For gapless MOA surge arresters, the critical operating voltage U1mA at 1mA (DC) and the leakage current at 75%U1mA DC should also be measured. The measured U1mA is mainly to check whether the valve plate is damp and to determine whether its operating performance meets the requirements. The measured value of U1mA should not differ from the initial or manufacturing value by more than 5%. Excessive U1mA reduces the insulation margin of the protected electrical equipment, while excessively low U1mA can cause the MOA surge arrester to explode under transient overvoltages during various operations and faults. Measuring the DC leakage current at 75% U1mA primarily detects changes in the long-term allowable operating current. The regulations stipulate that the leakage current at 75% U1mA should not exceed 50μA. 6. Summary Combined overvoltage protectors, due to their composite insulation structure, have a lower phase-to-phase residual voltage level and provide good overvoltage protection for both phase-to-phase and phase-to-ground connections. Especially in switchgear installations where size limitations limit performance, their more compact structure compared to single-unit surge arresters makes them highly suitable for high-voltage switchgear. Currently, the reliability of combined zinc oxide surge arresters is still being improved, and manufacturing processes vary among manufacturers; therefore, product quality should be the primary consideration when selecting them. The selection of technical parameters for combined overvoltage protectors is also very important, as different parameters result in different levels of protection. Therefore, the parameters should be selected appropriately for different applications.