cabling of mv distribution networks

Cabling of MV Distribution Networks

– a challenge for the distribution sector

Per Norberg

Adjunct professor Chalmers

September 2014

Confidentiality class: None (C1)

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


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.


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


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


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.


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.


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


“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


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


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


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


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



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



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.


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

cabling of mv distribution networks

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