Download - Cabling of MV Distribution Networks
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Cabling of MV Distribution Networks
– a challenge for the distribution sector
Per Norberg
Adjunct professor Chalmers
September 2014
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Content
- Background – the hurricane Gudrun – demand for weather safe
distribution systems
- L-G fault in high impedance earthed networks
- The difference between 30 bays * 10 km – urban systems and
10 bays * 30 km – rural systems
- Distributed grounding reactors
- Small asynchronous generators
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Gudrun
First week of jan 2005 Gudrun hit the southern parts of Sweden. We have
had harder storms if you look at wind speed but in history most hurricanes
lost power after hitting the coast line. Gudrun did not…
Roughly we can say from a line Gothenburg and east several hundred
thousand people was out of service for weeks up to months. In the
central part of southern Sweden also parts of the regional networks was
out of service for weeks.
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SAIDI for swedish LV customers 1998-2011
381 1620 529
For the authorities including the regulator this was enough. They took
command and the order was that the system should be “weather-safe”
and in the law text it was printed that disruptances over 24 h was
illegal. SAIDI = System Average Interruption Duration Index
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Cabling of OHL
The existing rural MV networks was built as overhead lines, OHL
“Weather safe” meant that you either replace the OHL with cables or (my
idea) give rural customers a reserve generator, a users guide and some
petrol.
The general solution was cabling. And many
managers (today few of them are electrical
engineers) saw no problems since a city MV
network could consist of many 100 km of cables.
But what they did not realize is that it is a great difference between
30 bays * 10 km and 3 bays * 100 km
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Basic legal demands
There are two basic legal demands for MV networks coming from national
safety demands:
- Faults, including phase to ground, should be detected and disconnected
within 5 sec
- The “touch” voltage during earth faults should be limited to 100 V
meaning Ifault-to-ground * Rearth < 100 V
The easiest way to handle the voltage demand has been to tune the so
called Peterson coil to a value that gives a resulting current of some
amps.
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Data Overhead line - Cable
In pu 22 kV, 10 MVA
****POSITIVE SEQUENCE**** ******ZERO SEQUENCE******
Type Length R X B Ro Xo Bo
157ACSR 5.0 0.0217 0.0341 0.00084 0.06915 0.15653 0.0003
3*95+16 Cable 5.0 0.033 0.012 0.01672 0.13 0.165 0.0167
The series impedances are in the same decade for OHL and cables but the
capacitive charging increase 20 times in positive sequence and more than
50 times in zero sequence.
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Jordfel i kabelnät – resulterande impedans
In length units with length=S R’ = Rres = rlo*S + (S/rlo )*(xlo - xbo/S2)2
”Norbergs formula”
-jXc +jXr R’
-jX’
+jXr -jXc
Ro + jXo
-jXc
Xr can compensate
any X’//Xc but not R’
Zero sequence equivalent – one branch
Xr = XNP+Xt0
XNP = Petersen
coil
Xt0 = transformer
zero seq
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“Active” current 22 kV radial cable 3*95+16
3*Io resistive part
-5
0
5
10
15
20
25
30
35
40
45
50
0 10 20 30 40 50 60 70 80 90 100
Length km
Am
pere
3*Io
Io
resistive part
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Typical rural network
If we cable the above network the different bays will have
more or less ”active” current
10.7 A
j162 A
1ph LG
fault
0.3 A
j19 A
9 .0 A
j180 A
6.0 A
j157 A
Total fault current
26.1 + j518 A
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Tuning of Petersen coils
The reactance in the neutral is usually
called a Petersen-coil after Waldemar
Petersen 1880-1946, former chief of
AEG.
The common practice today is to use
3-limb YN/yn transformers without a
∆ winding.
This can give problems if the neutral
reactor gets to big compared with the
attached transformer.
The reason is that the transformer zero
no load admittance is non-linear and
part of the resonance circuit.
∆Xt0
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Distributed compensation
The simple solution to handle the situation with a lot of cables is to install
local shunt compensation. Either as a part of the MV/LV transformer (only
in zero seq) or as a combined 3 phase/1 phase reactor.
Since the reactive power must be compensated somewhere the 3 ph/1 ph
is a better solution from a system point but more expensive.
Our philosophy is to set a limit for
central compensation. The
distributed reactors should be
placed in the outer parts of the
network.
22/0.4 kV distribution transformer 200 kVA 22 kV reactor
100 kVA, 10-20 A compensation 15 A compensation
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Problems with small generators in uncompensated
networks
In the technical swedish guidelines regarding small generation (non-
synchronous) we have the demand that they are not allowed to consume
reactive power. So in history the standard solution has been to install
capacitors up to say 0.5*Pgen.
But what happens then the OHL system is replaced by cables ?
Unfortunately we learned it the hard way since no one thought it could be
a problem
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Problems with small generators in uncompensated
networks
What happened was that after correct handling of line faults we got a lot
of claims from customers regarding destroyed LV equipment like electric
lamps, electronics and typical television units. And fault recorders showed
high voltages. We decided to run a Master Thesis on the subject.
Simulation without
hydro plant.
Simulation with
hydro plant. Breaker opens at
1.6 s. Generator
trips at 1.9 s
Max overvoltage ≈ 1.7 pu
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Problems with small generators in uncompensated
networks
It was failures that was the reason for the breaker to open but you will get
the same result without any fault if you opens the breaker.
One way to solve the problem is to compensate 3 phase and 1 phase
until you have a lack of reactive power seen from the generator.
Another is to have faster relay protections schemes for disconnecting the
generator.
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Conclusions
We can handle the situation by using distributed reactors. Both positive and
zero sequence must be compensated.
It would be interesting to investigate if
other methods to handle the grounding
of MV systems could be more effective.
For example if fault time is < 0.2 sec
(as in transmissions networks) we may
be able to accept 600 V as “touch”
voltage instead of 100 V.
See picture
Acceptable touch voltage versus time for human body
Source: IEC