SF6 - MV circuit breakers
motor insulation according to IEC and practice
The technical comittee of rotating
machinery (working for IEC) and the
different authors of the technical
litterature agree on the rated insulation
levels for rotative machines.
The future IEC is today the 2
(secretariat) 688 document, stipulating
the following levels.
interturn insulation
The rated lighting impulse withstand
voltage is:
4 UN + 5 UN = rated voltage, for the
type test.
In terms of p. u (1 p. u = U N 2 / 3)
this level is:
4.9 p. u + 5 kV (peak) = 31 kV
for UN = 6.6 kV.
This voltage is to apply between the
terminals. A model of the line end coil is
tested at 50 % of this value, which is a
compromise due to the non linear
distribution of the voltage along the
winding. The front time should not be
shorted than 0.5 microsecond.
ground insulation
Power frequency voltage test
The r.m.s. voltage (2 UN + 1) kV shall
be applied for 1 min between coil
terminals and earth, shall then be
increased at the rate of 1 kV/s up to
2 (2 UN + 1) kV and shall then
immedialety be reduced at a rate of at
least 1 kV/s to zero, without failure.
The rated impulse level 4 UN + 5 is
lower than the peak value:
2 2 (2 U N + 1) kV
derived from this test because the
impulse level of a machine is
determined by the interturn voltage
due to longitudinal voltage distribution.
The purpose of the higher a.c. test
level is to produce a voltage gradient
at the slot end more nearly equivalent
to that obtained by the impulse test. In
terms of p. u, this peak value is:
2 2 (2 U N + 1) = 6.9 p. u + 2.8.
Impulse test
The test voltage is the same than for
the interturn insulation
4.9 p. u + 5 kV (peak)
with a front time of 1.2 miscrosecond.
overvoltages and steep fronted voltage waves
The overvoltage problem accompanying
small inductive currents
breaking is one which has received a
large amount of publicity and
undergone much research. To
understand the problem it is necessary
to examine the phenomena which give
rise to different overvoltages. The three
main phenomena associated with small
inductive current switching overvoltages
are prestrikes, restrikes and
current chopping.
Thus all three phenomena have a
statistical basis.
Successive prestrikes and restrikes,
during closing and opening operations,
are due to multiple reignitions of
the CB.
The prestriking phenomenon occurs
during every closing operation but its
severety depends upon the point on
wave on which the process is initiated
(among other things).
Restriking and current chopping which
can occur when opening also depend
on the point of wave of switching for
their initiation.
the steep fronted voltage
waves
They are created by the reignitions:
n when opening (successive restrikes)
and
n when closing (successive pretrikes)
with certain types of CB or contactors.
A reignition can occur when the
contacts separate juste before the
current zero: the CB interrupts a first
time the current at the 50 Hz zero, the
voltage raises between the contacts
which are too close, leading to a
reignition. The inrush current is a high
frequency current which the CB is
capable to interrupt a second time, if its
recovery strength velocity is very high.
Then, the same phenomenon starts
again: the voltage raises and a lot of
reignitions can happen, more than 50
or 100 in the worst case.
These reignitions create travelling
waves, moving dowstream. The
amplitude of these waves depend on
the voltage difference across the CB
contacts, just before the break down.
When the motor is connected by a
cable, the amplitude of the incoming
wave can almost double at its
terminals. To summarize this
phenomenon, the CB can cause
repetitive HF transients if the CB is
capable to interrupt the HF transients
currents, corresponding to the multiple
reignitions. The vacuum CB are
capable to interrupt HF currents, due to
the very high recovery strenght velocity:
only one microsecond after the current
zero, the gap between contacts can
reach 75 % of the full dielectric
insulation. This behaviour depends on
the type of contact alloy, but it remains
still different of the SF6 CB behaviour,
which need roughly ten microsecond to
reach 75 % of the full dielectric
insulation.
Reignitions according to IEC
In I.E.C. document, multiple reignitions
created by the switchgear is qualified of
"abnormal events", in such case, the
windings (of the motor) should:
n either be designed to withstand other
impulse levels
n or be protected in an appropriate
way.
That means that the users have two
solutions to avoid failures of MV
motors:
n either using motors with a very good
interturn insulation in compliance with
the characteristics (front, amplitude) of
the travelling waves,
n or using voltage surge absorbers (for
voltage amplitude limiting) and R-C
devices (for transient currents
shunting).
The IEC document illustrates these
phenomena with figure 10.
Reignitions are defined not only by Ulf
but also by the peak-to-peak value Us.
The number of reignitions is a basic
data, because the repetition of
excessive interturn voltages can lead to
the definitive damage of the interturn
insulation.
With the SF6 breaking technique, a
single reignition is possible and
frequent when contact separate just
before the current zero (50 Hz),
exceptionnaly a few ones and never a
high number.
Restrike transients
Following interruption of current,
whether it be at a normal current zero
or whether it be chopped, the voltage
across the load oscillates between the
load side inductance and capacitance
causing an overvoltage as previously
described. If the voltage rise
momentarily exceeds the dielectric
voltage withstand capability of the
separating contacts, a reignition occurs
across the contacts. This reignition
results in the flow of an oscillating
current through the circuit-breaker.
This phenomena is described in the
following paragraphs with reference to
the circuit of figure 11.
This circuit is identical to the single
phase circuit in which however an
internal loop around the circuit-breaker
consisting of Cp1 and Lp1, has been
included. This internal circuit includes
the inherent parallel capacitance of the
circuit-breaker and connecting leads
plus the capacitance to earth. It also
includes the equivalent inductance in
the circuit of "first parallel oscillation".
On interruption, the source and load
side parallel LC circuits oscillate
practically independently at
frequencies fs and fL as given
previously. The very small parasitic
values of Cp1 and Lp1 result in a first
parallel oscillation of very high
frequency, 1 to 10 MHz, during which
Cp1 is discharged through the circuitbreaker.
f p1 = 1
2 p L p1 C p1
The next phenomena, termed the
second parallel oscillation involves the
next circuit loop as illustrated in figure
n° 10.
During this period of time, energy
transfer occurs between the source
and load capacitances through stray
inductance L0. Thus the oscillation
