Video: AGRU-4

Video transcript

What the AGRU-4 does with the gas of a breaker, how its circuit works, and how to recover, clean, evacuate and fill with it. Opening a breaker for maintenance means taking its SF₆ out first. The gas cannot be vented to the atmosphere, and it is too valuable to waste. So it is recovered, cleaned, stored, and put back when the work is done. It is sized for breakers and switchgear up to 69 kilovolts. It moves up to two cubic metres of gas an hour, compresses to 34 bar with an oil-less compressor, and stores up to 22.7 kilograms as liquid, on board.

On the cart: Three gauges, for suction, discharge and the equipment. Two alarm lights, for high pressure and a full tank. The mode valves. The equipment and auxiliary connections. The vacuum pump. The storage tank underneath. And two large tyres, so one person can roll it over a kerb. Everything happens in one circuit. The compressor in the middle, a condenser after it, and then the treatment line: drier, purifier, particle filter and a moisture indicator.

The storage tank on the right, an auxiliary cylinder beside it, and, below, the vacuum line, which works on its own. Recovery. The compressor draws the gas from the equipment through the inlet regulator and compresses it, and the condenser turns it into liquid. On its way to the tank it crosses the drier, the purifier and the filter, so the gas is already clean when it is stored. In practice: the hose from the equipment connection to the breaker. On the tank, the liquid valve open and the vapour valve closed.

The mode valves to recover, and start. Open the valve on the equipment and watch the suction gauge: when it drops into vacuum, the breaker is nearly empty. When the tank is not enough, the same recovery goes to an external cylinder through the auxiliary connection, so storage has no limit. It also consolidates partly used cylinders into one. External vessels must be rated for at least 35 bar. The gas can also be cleaned inside the unit, as long as needed.

With both mode valves on purify and the tank valves open, the compressor circulates the gas from the tank through the filters and back. A hygrometer or an SF₆ analyser on the auxiliary connection shows when it is done. Before filling, the compartment is emptied of air and moisture with the vacuum pump, below one millibar. The hose goes to the vacuum connection, and if the power fails, the anti-suckback valve closes, so no pump oil can reach the equipment. Two details from the manual. A vacuum gauge on the equipment gives the true reading.

And to check for leaks, close the pump isolation valve while it runs: if the vacuum then rises, there is a leak, or moisture still coming out. Filling runs the circuit the other way. Vapour leaves the tank, the compressor pushes it through the same filters, and the charging regulator sets the pressure, read on the equipment gauge. So the gas is cleaned a second time on its way back. Step by step: the charging regulator backed out to zero, the hose connected, and both mode valves to charge.

Open the vapour valve of the tank and close the liquid one. Set the regulator, start, open the equipment valve, and let the gauge on the equipment decide when to stop. Maintenance follows the compressor hour counter. The drier and the filter every 50 hours, a minor compressor kit every 200, a major one every 400, and the pump oil once a year. The filters change one at a time, without removing any tubing. And the safety rules. The unit makes very high pressures,

and liquid SF₆ becomes cold enough to burn the skin: gloves and eye protection when handling hoses. Hoses are connected and disconnected below 7 bar. The tank is for temporary storage while servicing, and it travels empty, upright, with every valve closed. And the specification, in one table. SF₆ gas handling, from Amperis.