Tehnîk Bir Analiz, Daxili Hêzda Tir Êdilî & SF6 Li Girtina Pêşniyarên Bistûr Kirin a Tir Êdilî (RMUs)

Yekamên RMU (Gas-Insulated Ring Main Units) da ku ji SF6 têne çêtirin, yekamên ku amadeyên switch-yên barka ve û yekamên kombinasyonî yên AC barka mehenda (kombinasyonî yên barka-fuse). Dibeke li ser bîcirên bikarhêner, wan dikarin bi rêzeyên common-tank an unitized pêşkêş kirin.

Li sererastina xebitandinên mühendisî, qateryanên elektrik hatine di navbera busbarên solid-insulated yên ser demê an busbarên plug-in yên rast demê hilîn. Li navbera parametreyên teknîkî yên din, girîngîna transfer current yên yekamên kombinasyonî û closing capability yên yekamên switch barka reprezentivên pirrûbarên sereke yên çavderek. Di demê de, li ser piştguhên parastina higer, arc faults ên derveyî yên naverokî hate wekî pirrûbarên herî girîng yên bikarhêner dide.

1. Analîza Pirrûbarên Teknîkî

Di dema çavderek û parastina RMUs de, an jî ber hemî şertên jêrîn heye ke dikarin bi tevahiyê bînin:

1.1 Transfer Current

Transfer current ya yekamên kombinasyonî ya ku wergera interrupting function ji fuse derketiwe bike load switch. Ji bo currents ku ji vê girîngîya zêdetir in, interrupting dikeji têne ji fuses ve. Di navbera girîngîya kêm a fault current de, time of melting fuses ên triphasîk hatine girîngîya taybetand. Fuse ku ji navbera yekemîn melting time dike, dikeji û striker-a we tiştekî bike trip mechanism ku load switch vekirin.

Interrupting di navbera du faseyên din dike bi verbiguhera di navbera actual time-current characteristics fuses û opening time load switch ku ji striker-a fuse yekemîn dake. Ji bo currents ku ji navbera yekemîn phase dike, interrupting dikeji têne ji load switch ve. Buna, interrupting di navbera girîngîya kêm a fault current de têne ji navbera fuse û load switch.

Transfer current ya yekamên kombinasyonî têne li ser du faktorên girîng: tripping time load switch ku ji striker-a fuse dest pêk biki û actual time-current characteristics fuse. Rated transfer current ya ku têne girîngîya teknîkî yên girîngî, wê reprezentivên girîngîya maximum load switch ku têne safe interrupt. Ji bo select kurter fuses, time-current characteristics fuse-an dikarin bine evaluate ku bide bînin ke transfer current ku ji navbera rated transfer current yekamên kombinasyonî be. Ev bide bînin ke coordination reliable û safe be di navbera load switch û fuse, bixebitîne protection effective transformers.

1.2 Closing Capability

Li ser testîn load switch, closing operations unsuccessful hatine bibin, genînîn li ser du categories: failure to meet the required number of closing operations or inability to close at rated short-circuit currents. Analysis of test results indicates that such failures are predominantly caused by excessive erosion of the main contacts, which compromises their ability to carry the rated short-circuit current.

Therefore, minimizing or preventing main contact erosion is crucial to achieving successful test outcomes. Research and extensive testing have demonstrated that adding auxiliary contacts made of high-melting-point copper-chromium alloy to the original main contacts can indirectly protect the lower-melting-point copper main contacts. The specific design approach can be flexibly adapted based on the contact structure used—whether linear motion or rotary blade type.

2. Withstanding Internal Arc Faults

An electric arc reacts violently with surrounding air, causing rapid increases in temperature and pressure. If not properly contained, it can pose severe risks to personnel and equipment. Internal arc fault tests should be conducted separately for the gas compartment (switch compartment) and cable compartment of the RMU. To pass the test, the following criteria must be met:

• Panels and doors of the switchgear must remain closed; limited deformation is acceptable.

• The enclosure must not rupture, and no fragments heavier than 60 g may be ejected.

• No holes may form on accessible surfaces of the switchgear up to a height of 2 m.

• Horizontal and vertical indicators used during the test must not be ignited by hot gases.

• The enclosure must remain connected to the grounding point throughout the test.

2.1 Rated Short-Circuit Breaking Current

The rated short-circuit breaking current of the combination appliance is determined by the selected fuse. The following considerations apply:

• The fuse’s rated short-circuit breaking current must be greater than or equal to the maximum prospective fault current at the installation point in the distribution system.

• The fuse’s rated short-circuit breaking current must be reasonably matched with the load switch’s rated short-time withstand current within the combination appliance.

• Three fuses of the same model and specification must be installed; otherwise, interruption performance may be adversely affected.

• Fuses must be correctly and fully installed to ensure the striker activates at the appropriate time and reliably triggers the load switch trip mechanism.

• After one or two fuses operate, all three should be replaced unless it is certain that the unfused fuses did not carry current.

2.2 High-Altitude Operation

The design of sealed gas compartments in RMUs is typically based on operation at altitudes below 1,000 m. At higher altitudes, air becomes thinner and atmospheric pressure decreases. Since the internal gas density remains constant, the relative pressure within the sealed compartment increases. This can lead to increased mechanical stress on the enclosure, resulting in deformation and a higher risk of gas leakage. In such cases, the enclosure strength should be appropriately reinforced and validated through testing. Reducing the gas filling pressure (or density) is not a scientifically sound or recommended solution.

2.3 Moisture Content Control

Clause 6.5.1 of DL/T 791-2001, Guidelines for Selection of Indoor AC Gas-Insulated Switchgear, specifies moisture content in gas compartments: “When the rated filling pressure is no more than 0.05 MPa, moisture content shall not exceed 2,000 μL/L (by volume).” Other standards do not provide specific guidance. In RMU production, controlling moisture content at 1,000 μL/L (at 20°C) is considered reasonable, based on the following:

• The load switch interrupts relatively small currents (630 A), with a maximum of transfer current (approximately 1,500–2,200 A).

• The filling pressure is low (rated at 0.03–0.05 MPa), significantly lower than that of high-voltage GIS (around 0.5 MPa).

• Sealing performance is excellent, resulting in very slow moisture ingress from the external environment.

• Test results show minimal SF6 decomposition products after interruption.

• During testing, samples were not deliberately moisture-controlled, yet no failures due to excessive moisture were observed.

Therefore, completely neglecting moisture control during production is unjustifiable, as is strictly adhering to insulation-based limits without considering arc-quenching requirements. Based on years of practical production and operational experience, maintaining moisture content at 1,000 μL/L (at 20°C) during manufacturing is both technically sound and reasonable.

3. Conclusion

RMUs have been manufactured and operated in China for many years, demonstrating mature technology, stable performance, and strong market acceptance. It is hoped that more manufacturers will enter this field and continue to explore, discuss, and share insights on technical challenges encountered in research, manufacturing, and operation, thereby collectively advancing RMU technology and promoting its continuous improvement.