Feature: Switchgear Arc-fault Protection gets Ultra-fast with UFES
Gerhard Salge, Technology Manager for ABB’s Medium Voltage Power Products Business Unit, explains the advantages of the new UFES technology for active arc-fault protection
There are rare cases where failure inside a medium-voltage switchgear cabinet, due to a defect, exceptional service condition or incorrect operation can cause an internal arc – a short circuit current flowing through the air - which presents a significant hazard. This is due to the instantaneous increase in temperatures at the fault location to around 20,000°C, well above the melting point of steel, copper and insulation materials. Internal components are vaporized and the sudden release of heat and plasma (ionized gas) creates an explosive blast.
Absolute protection of all personnel during an arc-fault is of course the number one priority. However, it is even better to take active measures to prevent such an event from happening in the first place. This not only eliminates risk to people, it also offers protection against damage and even destruction of system components. There is now a solution in the form of the ultra-fast earthing switch (UFES) internal arc protection system that combines ABB’s fast acting vacuum interrupter and the world’s fastest limiting and switching device, the Is-limiter.
The UFES operates on the principle that the uncontrolled release of energy from an internal arc fault is prevented by rapid metallic 3-phase earthing. Characterized by a significantly low impedance, this type of connection causes the short-circuit current of an arc fault to commutate immediately to the fast-acting earthing switch and this extinguishes the arc.
Primary switching units combined with an electronic quick release
The UFES contains three complete U1 primary switching elements (fig 1) and a QRU electronic quick release unit. Each primary switching element is similar in dimensions (height 210 mm, diameter 137 mm), shape and fastening points to a 24 kV pin-type insulator, and consists of a two-part vacuum chamber embedded in epoxy resin to protect it from the environment.
Fig 1 Primary switching elements
From a dielectric point of view, the chamber actually constitutes two vacuum gaps separated by a membrane. One gap contains a contact pin at earth potential while the other accommodates a fixed contact at
high-voltage potential. Each element also features an integrated ultra-fast micro gas generator (SMGG), which is comparable in type and functionality to the gas generators used in automotive airbags. The SMGG ( fig 2) drives a piston and is designed as a single-shot piston actuator. The electronic unit, based on durable and fast analog technology, is phase independent in structure, and ensures current and light detection and reliable tripping within the shortest possible time.
When an internal arc fault occurs, the electronic unit detects the fault current (supplied by a current transformer) and the arc light in the compartment (measured by optical sensors). At almost the same time, the gas generator is activated. More specifically, the gas pressure drives the movable piston. This piston slides into the first part of the vacuum chamber and eventually causes the contact pin to penetrate the membrane and engage with the fixed contact permanently and without bouncing to create a solid metal short-circuit to earth. The arc fault is therefore short-circuited and extinguished in less than 4 ms after it is first detected. The entire sequence, illustrated in Fig 3, leads to the safe connection of the piston to earth potential via a moving contact system.
The electronic unit has three input channels that enable continuous monitoring of the instantaneous current. The response level, the criterion for detection of a fault current, can be adjusted to suit a wide variety of protection conditions by means of simple controls. With a low input burden of less than 1 VA, the current measurement system can simply be looped into the secondary wiring of the existing current protection transformers.
Fig 2 Primary switching element for one phase
In addition to current monitoring, nine optical inputs are available for arc-fault detection. The status of the arc-fault protection system is indicated by LEDs and a 7-segment display on the front panel of the unit.
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March 2012 Page 8
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