Medium voltage auto-reclosers are increasingly being installed on electrical distribution networks to improve reliability levels and performance. Installing these devices on existing networks often requires making live-line connections in order to minimise disruption to customers. Although this is common practice, what are the safety issues, and how can switchgear manufacturers design products which increase safety for live-line workers?
According to NOJA Power 's Managing Director, Mr Neil O'Sullivan, the answer is to use reclosers that are arc fault contained and vented in accordance with the requirements of IEC60298 Annex AA.
"Not all reclosers on the market today are arc-fault contained and this should concern those responsible for the safety of live-line workers", he said.
"Although some SF6 gas insulated products are arc-vented, these are increasingly avoided due to the associated health and greenhouse gas issues."
"NOJA Power has the only arc-vented, solid dielectric, SF6 free, product on the market today"
"In situations where SF6 gas is not desirable, and installation is to be live-line, our reclosers are the only safe choice", he said.
Pole mounted switchgear have evolved dramatically over the years. Early switchgear used oil for interruption and insulation. The next generation of products used vacuum interruption with oil insulation. Sulphur hexafluoride (SF6) gas was then discovered and used for interruption and insulation. It was then found that SF6 gas is a greenhouse gas and the by-products created when using SF6 gas for interrupting arcs is carcinogenic and can cause cancer.
The challenge for pole mounted switchgear manufacturers has been to find safe, viable, long life alternatives to using oil and SF6 gas in their products.
This has seen the introduction of a range of vacuum interrupted solid dielectric products into the market. The majority of these products use vacuum interrupters embedded inside cyclo-aliphatic epoxy resin poles which are exposed to the environment. Magnetic actuator type mechanisms are used on each pole to actuate the device. The actuators are normally encased in a mild steel or stainless steel housing which also provides the base plate for mounting the cyclo-aliphatic resin pole.
This type of switchgear design is available from several manufacturers and while it has achieved the goal of eliminating the use of harmful insulants such as SF6 and oil, is this really a safer, longer life product? The main problem with this design is that while it does address the environmental issues it does not address safety and long life issues in an acceptable manner.
These types of products do not provide arc fault containment and venting in accordance with the requirements of IEC60298 Annex AA. It can be said that with three separate poles it is not possible to achieve a phase to phase arc fault and therefore arc fault containment is not required. However, it is still possible to achieve a phase to ground fault and this can propagate. If this occurs, arc fault containment is essential for the safety of anyone in the immediate vicinity. Unfortunately, the design described above lacks this essential feature.
This particular concern must be taken into consideration with the ever increasing live line installation practices used by electricity utilities around the world when installing new medium voltage plant and equipment.
A combined insulation system with arc fault containment is the answer.
Electrical breakdown in air occurs when an electrical field exceeds 2.4kV per mm. In practical applications, in order to achieve a minimum 125kV BIL (required for 27kV switchgear), a 200-300mm clearance is required due to the non-even distribution of electrical fields.
However if this air gap has a barrier placed between the two electrodes this greatly reduces the clearances required.
The dielectric strength of a combined insulation system is higher than that of the insulating material itself. This effect is generated by the barrier hindering the chain reaction of molecule ionisation required for electrical breakdown across air.
The combined insulation system must also provide the mechanical strength to withstand the forces associated with arc fault containment and fault level interruption. The insulation system must also be IP66 water proof and shaped to ensure electrical fields are minimised to withstand the required BIL/flashover levels.
Various insulation materials are used within this design and are chosen for their different properties: Silicon for joins and flexible areas, polymers for rigid structures that provide strength. High pressure processes are used for each material to eliminate voids that can lead to partial discharge problems.
NOJA Power's patented Arc fault containment and venting is achieved by manufacturing the tank from 304 grade stainless steel and providing an arc vent in the pole side of the tank.
Pole mounted medium voltage auto reclosers will continue to play a critical role in the future of electricity distribution overhead line protection. Pole mounted auto reclosers offering a longer life and solid dielectric insulation systems with arc fault containment and venting should be specified by all electricity utilities when considering future recloser purchase specifications.