SF6 Gas Density Monitoring
The energy sector is and will remain one of the growth sectors of the future. The thirst for energy of our civilisation is constantly growing. This is why the energy sector constantly has to cope with new demands. Alternative energy sources such as photovoltaics, wind energy and combined heat and power plants have to be integrated into the energy grid increasingly in order to supplement and partially replace the traditional energy sources.
The future belongs to “smart grid” – intelligent energy grids with many decentralised sources and at the same time comprehensive powerful availability for the electric mobility. Intelligent grid management, which connects and disconnects sources and consumers in line with the demand, can only obtain the required flexibility with compact circuit-breakers that have the interconnectivity capacity for online monitoring.
SF6 gas is the key to compact systems that, as hermetically capsuled modules, isolate the functional devices of the switchgear from the environment. The excellent insulation and spark extinguishing properties of the gas minimise the internal switching wear of the system. SF6 gas insulated switchgears impress with high availability and decades of maintenance-free operation. This places high demands on gas density monitoring, which must function reliably in the climatic conditions of all installation locations. From –50 °C in Siberia up to +70 °C in control rooms at the equator or in mining.
Selected instruments
for use in SF6 applications
Mechanical
Bourdon Tube Pressure Gauge
Pressure measuring instruments with uncompensated indication
Measurement of the actual SF6 gas pressure at instantaneous gas temperature. With simultaneous knowledge of the gas temperature, the gas density and the gas pressure at reference temperature +20 °C can be inferred by means of tables or special scales.
Gas Density Indicator
Pressure gauges with compensated indication
The indication of the actual SF6 gas pressure is adjusted on the basis of the device temperature, which should correspond to the gas temperature, so that the gas pressure is indicated, which would prevail in the gas-filled compartment at same gas density and reference temperature +20 °C. The bimetal compensation is dimensioned to a reference isochore of the SF6 gas, the so-called calibration pressure pc, which corresponds to the nominal filling pressure pf of the gas-filled compartment.
Electromechanical
Gas Density Monitor
Pressure gauges with compensated indication and additional electrical accessory
A gas density monitor is a density indicator, which is extended by electrical limit switches with magnetic contacts. The bimetal compensation is dimensioned to a reference isochore of the SF6 gas, the so-called calibration pressure pc, which in this application typically corresponds to the first switch point in falling direction. Calibration pressure, switch point adjustment and scale according to customer specification.
Electronic
Gas Pressure and Gas Density Transmitter
All-in-One – from −40 °C to +60 °C
The DIGPTMvSF6 combines a hermetically sealed, welded stainless steel pressure measuring cell, a platinum temperature sensor and a microcontroller with 2 switching outputs, an RS-485 interface and 2-wire functionality 4…20 mA in one device. An adjustable electronic low-pass suppresses switching errors due to mechanical shock, triggered by switching operations of the switchgear. Device address, switching functions and switch points, software low-pass, offset and scaling can be administered by the user via the USSCOM software.
Facts about SF6
Sulphur Hexafluoride
SF6 is a synthetic gas. The inert gas is colourless and odourless, non-toxic and non-combustible. However, according to the Kyoto protocol it is one of the main six greenhouse gases and thus it is to be surveyed.
Its unique electrical properties predestine SF6 gas and its blends with N2 for switchgear construction up to over 1,000,000 volts. Even in low and medium voltage switchgears, SF6 gas insulated systems are becoming increasingly popular due to the advantages offered by gas insulation.
On the one hand, a minimum gas density is required in order to guarantee the safe functioning of a switchgear. On the other hand, it should be insured to avoid emissions into the atmosphere. It is necessary to monitor the gas-insulated chambers of each switchgear, to trigger safety-related alarms or switching processes and to transmit the current status to a data network.
Main fields of application
» Gas-insulated switchgears (GIS)
» High-voltage systems
» Medium-voltage systems
» Gas-insulated transmission lines (GIL)
» Isolators (breakers)
» Transducers
» Transformers
» Circuit breakers
» Load switches
» Ring main units (RMU)
Technologies
Temperature Compensation
The dielectric strength and arc extinguishing capability of gas-insulated systems is determined by the gas density. This must by no means fall below the planned minimum value, as this would result in an explosive destruction of the system. Gas-insulated systems are often installed outdoors and are subject to fluctuating environmental influences. Temperatures between −40 °C and +70 °C are not uncommon. Special solutions can also be realised beyond this range.
If the gas density remains constant in the isochoric system, the system pressure changes with the ambient temperature. This is why classical contact pressure gauges and pressure transmitters are not suitable for gas density monitoring. In order to be able to switch reliably in case of leakage, the devices have to be temperature compensated. The reference temperature for this is usually +20 °C. The measured value is compensated in such a way that the indication always corresponds to the situation of +20 °C at any temperature in the range.