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A liquid resistor is an electrical
A common use in the electrical power generating and distribution industry is as a fault current limiter in the common neutral leg of large three-phase transformers and generators. In the UK they are known as Liquid Neutral Earthing Resistors (LNERs).
A rating of 0.5 megawatt for 30 seconds would not be unusual to protect the winding of a 660 MW generator. In this application where a neutral leg current flows due to a current imbalance between the generator or transformer windings due to a fault, it is limited in magnitude by the resistor.
A permanently installed current transformer (CT) fixed around the neutral leg feeder to the LNER senses the current. The CT sends a signal current to an external sensing circuit (the protection system) which then sends a signal to the relevant circuit breaker to open and disconnect the supply or isolate the generator (or transformer) from the fault. The ohmic value of the resistor is calculated as a function of the permissible fault current and the system voltage to earth.A.C. Franklin, D.P.Franklin et al (1983) The J&P Transformer Book 11th ed.P484: Butterworth & Co Ltd.
resistor
A resistor is a passive two-terminal electronic component that implements electrical resistance as a circuit element. In electronic circuits, resistors are used to reduce current flow, adjust signal levels, to divide voltages, bias active e ...
in which the resistive element is a solution. Fixed-value liquid resistors are typically used where very high power dissipation is required. They are used in the rotor circuits of large slip ring induction motors to control starting current, torque and to limit large electrical fault currents (while other protection systems operate to clear or isolate the fault). They typically have electrode
An electrode is an electrical conductor used to make contact with a nonmetallic part of a circuit (e.g. a semiconductor, an electrolyte, a vacuum or a gas). In electrochemical cells, electrodes are essential parts that can consist of a varie ...
s made of welded steel plate (galvanised
Galvanization ( also spelled galvanisation) is the process of applying a protective zinc coating to steel or iron, to prevent rusting. The most common method is hot-dip galvanizing, in which the parts are coated by submerging them in a bath o ...
to reduce corrosion), suspended by insulated connections in a conductive chemical solution held in a tank - which may be open or enclosed. The tank body is normally solidly grounded or earthed. A typical unit can be rated for continuous use, or for short periods when used for current limitation in protection systems.
Liquid neutral earthing resistor
A common use in the electrical power generating and distribution industry is as a fault current limiter in the common neutral leg of large three-phase transformers and generators. In the UK they are known as Liquid Neutral Earthing Resistors (LNERs).
A rating of 0.5 megawatt for 30 seconds would not be unusual to protect the winding of a 660 MW generator. In this application where a neutral leg current flows due to a current imbalance between the generator or transformer windings due to a fault, it is limited in magnitude by the resistor.
A permanently installed current transformer (CT) fixed around the neutral leg feeder to the LNER senses the current. The CT sends a signal current to an external sensing circuit (the protection system) which then sends a signal to the relevant circuit breaker to open and disconnect the supply or isolate the generator (or transformer) from the fault. The ohmic value of the resistor is calculated as a function of the permissible fault current and the system voltage to earth.A.C. Franklin, D.P.Franklin et al (1983) The J&P Transformer Book 11th ed.P484: Butterworth & Co Ltd.
Electrolyte in power industry LNERs
Theelectrolyte
An electrolyte is a substance that conducts electricity through the movement of ions, but not through the movement of electrons. This includes most soluble Salt (chemistry), salts, acids, and Base (chemistry), bases, dissolved in a polar solven ...
is normally sodium carbonate
Sodium carbonate (also known as washing soda, soda ash, sal soda, and soda crystals) is the inorganic compound with the formula and its various hydrates. All forms are white, odourless, water-soluble salts that yield alkaline solutions in water ...
(soda ash) and/or sodium bicarbonate
Sodium bicarbonate ( IUPAC name: sodium hydrogencarbonate), commonly known as baking soda or bicarbonate of soda (or simply “bicarb” especially in the UK) is a chemical compound with the formula NaHCO3. It is a salt composed of a sodium cat ...
(baking soda). This salt solution is alkaline
In chemistry, an alkali (; from the Arabic word , ) is a basic salt of an alkali metal or an alkaline earth metal. An alkali can also be defined as a base that dissolves in water. A solution of a soluble base has a pH greater than 7.0. The ...
and electrode corrosion effects on galvanised steel tanks is manageable if re-galvanisation is possible.
The resistance of the bulk electrolyte is a function of temperature and the concentration of the saltG.R. Jones, M.A. Laughton, M.G. Say (1993) Electrical Engineer's Reference Book / edited P2/7: Reed Educational and Professional Publishing. . governed by the formula:
:''R''''θ'' = {{sfrac, ''R''0, 1 + 0.03''θ''
where ''θ'' is the temperature (in Celsius) and ''R''0 is the standardised or datum calibration temperature.
A specialised technique is required to accurately calibrate an LNER electrolyte bulk resistance value and it is normally carried out by experienced power engineers and technicians. This task must be done off-load at routine intervals (roughly 2–4 years depending on results from routine monitoring) to maintain the resistance value within tolerance and facilitate inspection of the LNER.
See also
* Liquid rheostatReferences