Medium voltage switchgear has been used for a long time, has been the use of extensive production, high product homogeneity, under the double carbon goal needs to use low-carbon degradable materials, material use should be less, material utilization rate should be high, product life cycle resource consumption is less, through the product fine design, not only can realize the low-carbon of the product, but also help to extend the service life of the product, reduce the cost of the product, and obtain more market benefits.
Schneider MVNEX-12 medium voltage authorized cabinet.
0.Preface.
Under the dual carbon goal, medium-voltage switchgear is developing in the direction of environmental friendliness, compact and reliable, and intelligent perception. At present, the medium-voltage switchgear is highly homogeneous, and the circuit breaker pole, contact box, sleeve and other insulating parts are cast with supporting epoxy resin materials on the market; The conductive circuit is made of copper with high conductivity, and the number and length of copper bars determined by the scheme configuration and current level are basically the same; Cabinets and other structural parts are made of steel plates, and the structural form and size of the product determine the amount of materials.
The traditional switchgear cost and performance are similar, and the extensive design and production have not adapted to the requirements of low-carbon goals, and the refined design of the product is reflected in the use of advanced technology and green ecological product design through technological innovation to reduce the carbon emissions of each link in the product life cycle, which can not only make the product not contain environmental pollution and refractory materials, but also reduce the use of non-ferrous metals and other materials, so as to achieve cost optimization and performance improvement, and help enterprises achieve carbon peak and carbon neutrality transformation. Achieve the goal of power distribution equipment to move towards the middle and high-end.
1.Insulation system.
Epoxy resin production efficiency is low, the casting process consumes a lot of energy, and it is difficult to disassemble at the end of the product's life, so it can only be disposed of by landfill and other means, so from the perspective of environmentally friendly products, epoxy resin insulation system is not a good choice. Epoxy resin is used in large quantities in switchgear, and an air insulated withdrawable 12kV 1250A 31The epoxy resin of the 5KA switchgear weighs about 87kg, accounting for 17% of the total weight of the switchgear. Among them, there are mainly contact boxes, insulators, and wall sleeves with a total weight of 2175kg, circuit breaker solid seal pole 1248kg, current transformer 2265kg, voltage transformer 3045kg。
1.1. Circuit breaker pole.
Compared with the assembled or open structure, the main advantages include high mechanical strength, reducing the influence of environment, humidity and mechanical forces, and the small, sturdy, and integral modular structure can easily achieve rapid installation. The shortcomings of the solid sealing pole are also more obvious, the heat dissipation is difficult, because the upper and lower connections of the arc extinguishing chamber are completely sealed in the epoxy resin, especially the lower part of the dynamic connection is very hot, and the thermal conductivity of the epoxy material is relatively poor, so the heat accumulation is not easy to dissipate inside the pole, especially the top of the pole of the high-current circuit breaker adopts a radiator, and the pole of 3150A needs a special design of the heat dissipation air duct to pass the temperature rise test under the condition of natural air cooling, and the failure of the arc extinguishing chamber needs to be replaced as a whole. It is very difficult to disassemble the copper parts, and the whole landfill causes waste.
1) Engineering plastic poles, which can reduce the weight of the poles by about 35%, and greatly improve the dielectric strength (about 50%), mechanical stiffness (about 100%) and mechanical strength (300 400%) by reducing the density of the material (12%), so as to achieve the purpose of mass reduction, and at the same time reduce the volume of the sealed pole. Due to the high production efficiency of thermoplastic materials, the production is fully automated, and the quality has also improved.
2) Open insulated cylindrical pole, made of DMC material casting, the material can be reused, especially for high-current poles, which can effectively reduce the amount of copper used in the upper and lower conductive parts, and has good heat dissipation.
1.2 Current transformers.
The function of a current transformer (CT) is to convert a large current into a small current, or a high voltage current into a low voltage current (1A or 5A) for use in metering, testing instruments and relay protection devices. The working principle of a current transformer is similar to that of a power transformer, with structural elements such as iron cores and primary coils. The ratio of current in the primary and secondary coils is approximately inversely proportional to the number of turns. In use, the primary coil is connected in series in the circuit where the current is being measured, while the secondary coil is connected in series with the current coil of the measuring instrument and relay. The LZZB epoxy pillar type current transformer is commonly used in the medium voltage switchgear, and the weight of the three LZZBJ9-12 current transformers is 63kg, and the weight of the epoxy resin such as conductors, iron cores, and coils is completely removed to reach 2265kg, the weight of the voltage transformer epoxy resin reaches 3045kg。With the application of modern microcomputer protection, current transformers no longer need to be matched with high secondary loads.
1) Core-through-type current transformer.
The low-voltage through-center current transformer is to measure and monitor the current by passing a conductor through the center hole of the transformer, that is, the transformer does not have a primary winding, and the secondary winding is wound on the core to achieve measurement.
For special applications such as generator outlet protection, a C two-phase installation with 3 secondary windings, and a total of 4 windings with 1 generator excitation grounding protection are added to phase B, and it is difficult to meet the requirements of epoxy pillar CT; For multiple ratios, CTs with different ratios need to be installed on the same phase bus; Px level protection applications are required, PX level is a low leakage magnetic flux type transformer, generally used for 1A or below the transformation ratio, mainly used in the high impedance differential protection mode, due to the specified inflection point voltage and other values, this kind of transformer is generally larger; For switchgear with high dynamic and thermal stability value, the general dynamic and thermal stability value of epoxy pillar CT with small current transformation ratio is low, while the through-core CT is not affected by the dynamic and thermal stability of the main circuit, which can meet the requirements.
Examples of special applications for marine switchgear are as follows: A C phase 3 windings each: 600 1 0Class 5, 600 1 5p20 15VA, 1200 1 px stage, phase B 4 windings: 600 1 0Level 5 1 0Level 5 1 5p20 15VA, 1200 1 px level; The epoxy pillar type secondary winding has a large number and different transformation ratios, and a px-level protection winding is required.
Low-voltage through-center current transformers are generally used in 720V and below switchgear to use current transformers, and their insulation levels are generally 3kV 1min for power frequency withstand voltage and 10kV for lightning impulse. The medium-voltage switchgear is more than 1000V equipment, the general 10kV switchgear withstand voltage level is 42kV 1min, lightning impulse withstand voltage 75kv, through-center transformer for medium-voltage equipment must solve the insulation problem.
There are several main ways to use through-center transformers in medium-voltage switchgear:
a.Insulating sleeve installation, such as ring main unit, inflatable cabinet and other CT installed on the outlet bushing, as well as the American standard switch cabinet CT sleeve in the front of the primary contact box, generally each phase upper and lower contact box 2, a total of 4, there is also the American standard primary contact box installed on the upper part of the board sleeve CT design;
b.Insulated bus installation, CT is installed on an insulated busbar, the bus bar has a grounding or semiconductor metal shield, and there is also an extended insulating rod installed at the rear of the contact box;
c.For cable installation, CT is sleeved on a primary cable, which is suitable for small current occasions, such as the CT sleeve on the outgoing cable of the ring main unit is more common.
2) Electronic current sensor.
The electronic current sensor uses low-power coils, such as Rogowski coils, to directly output small voltage signals, without the danger of secondary open circuits, with high precision, no saturation, wide frequency range, good linearity, and reliable operation of the protection device in the system failure state. The Rogowski coil is small in size as shown in Figure 1, and the current sensor is mounted on the front of the primary contact box for easy installation and maintenance. Without the use of iron cores, a full range of current measurements can be realized, from tens of amperes to thousands of amperes, no need to replace, no magnetic saturation. Compared with the traditional epoxy casting current transformer, the volume is reduced by 90% and the weight is reduced by more than 95%, which greatly saves space, and avoids the use of thermosetting and unusable materials such as epoxy resin, which is low-carbon and environmentally friendly.
The low power current sensor complies with the national standard GB T 208408 Transformers Part 8: Electronic current transformers with an output signal of 225 mV and an accuracy class of 3 and 5pp20 with a rated load of 20 kohms.
There are also zero-sequence current sensors, sensors for ground fault detection, which cooperate with the microcomputer unit to achieve ground fault protection.
1.3 Voltage transformer.
Similarly, for epoxy cast voltage transformers, the amount of epoxy resin material is large and the volume is large. The electronic voltage sensor adopts resistance, resistance and capacitance voltage divider sensor, which has low cost, high precision, small size, light weight, small environmental impact, no secondary short circuit risk, and the voltage sensor and insulator shown in Figure 1 are of the same size, which can be easily installed. The voltage sensor is made in the form of an insulator and realizes the combination of functions. The sensor has a double accuracy class and the output signal can also be used for measurement, protection and ground fault detection. The output leads of the sensor are connected directly to the auxiliary equipment.
Low-power voltage sensors are based on the principle of resistive voltage divider. It consists of two resistive elements that divide the input signal to achieve a standard output value.
The voltage sensor is essentially a resistor divider, providing 325 V 3 rated primary voltage. The output of the sensor is connected directly to the auxiliary equipment.
The low power voltage sensor complies with GB T 208407 transformers Part 7: electronic voltage transformer standard, primary voltage up to 30kV, secondary voltage 325 V 3, accuracy class or 3 P, rated load 200 kOhm 1% accuracy.
The birth of electronic transformer is the inevitable result of the development trend of transformer sensing accuracy, transmission optical fiber and output digitalization. It has the advantages of small size, light weight, wide frequency band response, no saturation, good anti-electromagnetic interference performance, oil-free structure, reliable insulation, and easy to develop to digitalization and microcomputerization. Compared with traditional current transformers, electronic current transformers usually have a higher bandwidth and are suitable for fundamental and harmonic measurements of currents with large harmonic content. In order to accurately measure the power, the combined voltage and current electronic transformer can also be used, because the combined electronic transformer can better control the phase difference of the voltage and current signal and improve the accuracy of power measurement. Electronic sensors can realize the lightweight design of products, reduce the use of materials, and be low-carbon and environmentally friendly.