At what ratings (I n and U n ) are fuses used in power systems? What are the main advantages and disadvantages by using fuses instead of breakers to interrupt current?
Sketch how a typical high voltage fuse is designed, indicate what materials are used and briefly explain the functions of the main parts.
The circuit diagram of Fig. 4.39
will be used to analyse the interruption of a short circuit current with a fuse in a 50 Hz single-phase network. The short circuit is modelled by a switch in parallel to the load that is closed at the moment of the short circuit.
For the entire exercise assume that the short circuit happens at the source voltage zero crossing (t = 0). The short circuit current is much larger than the load current so sufficient accuracy in the calculations is achieved by setting i(0) = 0.
Find an expression for the prospective fault current (the current that goes through the fuse assuming it is being replaces with a lossless conductor).
How large becomes the prospective current’s maximum value when U = 12 kV and L = 6.37 mH?
Assume that the fuse element warms up adiabatically from an initial temperature T 0. Assume that the heat capacitance c and the resistivity ρ of the fuse element are constant (temperature independent) during the heating process. Derive an expression for the fuse melting time t s as a function of network parameters, fuse element cross-section A, as well as its temperature and material proprieties.
Hint: Sufficient accuracy is achieved by assuming that cos x = 1 − x 2 /2 for small values of x.
Assume an initial temperature T 0
= 60 °C, and insert the given values of the network parameters U
, and ω
, together with the following material parameters:
ρ = 3 × 10−8 Ω m
η = 10,500 kg/m3
- heat capacitance
c = 235 J/kg K
- melting heat
c m = 105,000 J/kg
- melting point
T m = 960 °C
and show that with A = 0.7 mm2 the fuse element melts 5 ms after the short circuit occurred.
When the fuse element melts after 5 ms a number of arcs are ignited in the fuse. The arcs cause a voltage drop across the fuse, u b . Find an expression for the current through the fuse while the arcs are burning (assume u b constant).
How large must u b be to reduce the interruption time to 1.5 ms after the melting of the fuse element?
Hint: Enough accuracy is achieved by assuming sin x = x for small x.
Sketch the current waveform from the instant of the short circuit to the instant of the current interruption.
Often the melting element has distributed narrow sections. What is the function of these narrowings?
It is important to adjust the number of narrowings to the rated voltage of the fuse. Explain briefly what problems can arise if the number of narrowings is too high at a given rated voltage.
It is also usual to attach a small piece of tin to the melting element (so called “M-spot”). Explain briefly their function.
A popular breaker solution in 12 kV grids is a load break switch in series with a fuse equipped with a striker. Explain how the breaker and fuse work together during interruption of:
Moderate over currents (like 2I n where I n is maximum load current in the circuit),
In which of these cases will it be necessary to replace the fuse afterwards?
Another popular switchgear technology for 12 kV grids is the vacuum interrupter. Explain briefly and by use of a figure how a vacuum interrupter is constructed.
The electric arc in a vacuum interrupter can assume two different modes or states. What are these called, and which parameter is decisive for what mode the arc takes. What characterises the appearance of the arc and the voltage drop in the two cases?
SF6 circuit breakers are commonly sorted into three generations. Describe briefly the characteristics of each generation with regard to gas blow/arc extinction principle, and do also outline the major advantages and disadvantages associated with each generation.
This breaker employs, like many breaker designs do, two sets/pairs of contacts. What is the reason for using two contact sets, and what are they usually called?
What is the main technical requirement for each of the two contact sets? Give examples of materials or material combinations that are being used in the two cases.
In what sequence do the two contact sets mate during closing and separate during opening operations?
Oil is a common arc quenching medium. Describe briefly—for example by including a schematic drawing showing the various substances in and nearby the arc—the conditions observed after a pair of current carrying breaker contacts immersed in oil is separated.
Why is it so important to keep the arc immersed in oil and thereby avoid contact with air?
During arcing in oil soot or carbon particles are produced. Why is this a clear disadvantage with this quenching medium?
Describe briefly and by means of a drawing how the arcing chamber of a vacuum interrupter (the “vacuum bottle”) is built up.
What are the main areas of application (current and voltage ratings, types of power networks) of vacuum interrupters? What are their main advantages compared to competing technologies?
Substations in the Norwegian 145, 300 and 420 kV systems are a mixture of gas insulated substations (GIS) and air insulted substations (AIS). What is the single most important reason for choosing GIS instead of AIS technology, and at what kind of locations are these GISs typically installed?
What are the most important drawbacks with the GIS compared to the AIS technology.
The solid insulation inside GIS is provided by epoxy supports. Why are these normally bell-shaped? Besides providing dielectric insulation they serve a second purpose. Which purpose?