Medium voltage switching during faulty operation Fault localization

1
Medium voltage switching during faulty
operation Fault localization

• Behaviour
• Fault localization for phase-to-earth faults in
  resonant earthed or insulated systems
• Fault localization for three-phase short circuit

2
Behaviour during faults
A fault ist an unintentional change of normal
operating conditions, which are characterized by

• Sufficent voltage

• Intended switching state

• Insulation in good order

• Equipment in good order

• 1. Stay cool - constant communication with
  power system control center
• 2. Get an overview (first note, than quit)
  - collect information (time, area, related
  equipment, readings)
• - no overhasty activity and conclusion
• 3. Look for saftey at fault location
• 4. Avoid enlargement of fault
• 5. Restore power(list of priority)- do
  essential switchings at once without breaks
• Initiate identification of damage and fault
  clearing
• - use adequate equipment

3
Faults requiring power down

• Unskilled persons nearby live parts
• Partial discharges
• Discoloring of cable boxes or insulating parts
• Arcing traces
• Outages on auxiliary devices (batteries,
  protection system)
• Breakage of insulating supports at switchgears
  or busbars
• Foreign particles in high-voltage switchgears
• Leakages of air-pressure systems
• Oil-or gasleakages of equipment
• Insulation-fault on cables
• Faults on monitoring systems
• Hints from fire-brigade, police department or
  people
• Alarms (missing or abnormal readings)

4
Temporal limitation of fault clearing
Absolute times depend on voltage level, toplogy
of network, power control system, Traffic
situation, season etc.
5
Fault localization at phase-to-earth fault in a
resonant earthed or insulated system

• The phase-to-earth fault in a resonant earthed
  or insulated system is not disconnected by the
  protection relays. Therefore the staff in the
  power system control center need some tools
• a) to recognize, that the fault has happend and
• b) to localize it.
• Usefull devices are
• - Reading /notification of neutral displacement
  voltage.
• - Reading of phase-to-earth voltages.
• - Earth fault detection by wattmetrical
  protection relays, transient earth-fault relay
  or high-order harmonics relay.

6
Fault localization at phase-to-earth fault in a
resonant earthed or insulated system

• In case of an earth fault in a MV sub-network
  without direction earth-fault relay the staff
  gets merely the information that an earth-fault
  has happened but no information about a certain
  cable or switchgear.
• If there are no external hints for the failure
  location the location must be determined by
  power cycle the cables in the substations-
• By reading the neutral displacement voltage the
  faulty cable can be identified

7
Earth-fault location in resonant earthed systems
Carryover of faults in healthy systems by
switching Square danger increase depending on
system area Bear in mind the quality of
switchgears
8

• Standard practice should be to choose a certain
  sequence of power-cyling so that the cable with
  a minor effect ist disconnected first.
• Dont connect a proper system with a earth-fault
  tained system by coupling busbars. The risk of
  double earth-fault is very high. The overvoltage
  may produce a cycle earth fault eben in the
  proper system.

9
Fault localization a short circuit

• Normally a short circuit ist disconnected by the
  protection relay and the circuit breaker.
• The staff in the power system controll center
  need some tools to localize the certain cable
  within the cable run with a lot of substations
  and sections.

• Short circuit indicator
• buildt in ring cable feeders
• They will show a colour change in case of a
  certain, adjustable over-current.
• The electronic types can give a signal to the
  system power control center so that it can be
  displayed on the MMI (time saving)
• Fault locator
• They ar a part of the distance protection
  relay, using current and voltage for calculating
  the fault-distance (normaly in m oder km).
• Due to the fault arc und the resistance of
  failure there is a blur in the accuracy of
  distance.

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Short circuit
Relais- anregung
Relais- auslösung
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Short circuit indicator
12
Reset of short circuit indicator
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Reset of short circuit indicator
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Short circuit localization
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Short circuit localization by protection relay
Inspection of overhead transmission line
Measuring tolerance in localization area
Protection relay
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Short circuit localization by indicator
K
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Short circuit localization by indicator
K
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Short circuit localization by indicator
K
Mind the traffic!
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Short circuit localization by indicator
Mind the traffic!
20
System of fault localization

• Measurement of insulating resistance
• All cores against earth
• All cores jointly

L1 EL2 E L3 E
L1 L2L2 L3 L3 L1
If the insulating resistance of all cores is
lower than 200 W it is to be checked that all
service fuses are removed !
21
System of fault localization
2. Pre-locating by reflectometry

1. Low resistance faults (lt 50 W)or
   breaksimpule-echo mode(time domain
   reflectometer TDR)
2. High resistance faultsarcing mode

precision approx. 3 ... 5
In former times high resistance faults were burnt
down to low resistance faultsby arc-ignition
(danger of continuing damages)
22
System of fault localization
Basics of reflectometry
L
R
G
C
Equivalent circuit of an electrical cable
R longitudal resistance G transversal
conductance L longitudal inductance C
transversal capacity
R and G determine the ohmic resistance L and C
determine the characteristic wave impedance
Wave impedance of cable
Z ? high thin wire, great distance high L, low
CZ? low thick wire, small distance low L,
high C
v Paper insulated cable 80 ... 82 m / µs XLPE
cable 84 ... 86 m / µs
Propagation speed of electrical waves
erel relative permittivityair 1, plastics 2
... 4 , water 81
23
System of fault localization
reflectionfactor r
US
UR
Z150W
Z225W
Ideal failure sketches
Cable without fault
Change of diameter
50W
25W
Short circuit
Change of diameter
Transversal resistance
25W
50W
Connection joint
Interruption
Longitudal resistance
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System of fault localization
Reflexionsbilder an einer Meßstrecke
L1, L2 phases interrupted L3 short circuit
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System of fault localization
Reflection pictures of ideal failures
Short circuit
Interruption of two phases
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System of fault localization
cable
7
Cable route location
6
Searching coil
5
4
Display crossways to cable route
3
ca. 3 m
2
Determination of depth of laying
1
Sequence of searching
a
a
a
a
depth
27
System of fault localization
Ground microphone
4. Pin-pointing
a. Acoustic pin-pointing by surge voltage
generator
C
Automatic discharge device
Flashover of capacitor C creates anacoustic
noise at the fault position
An additional solenoid displays the magnetical
field of the surge impuls.The time difference
between undelayed magnatical wave and acoustic
wave is a rate of distance to fault position
(the deciding factor ist the minimum time)
Be carefull at sheath tubes shielding,
megaphone effectNot usable when solid metallic
contacts at fault-position are given Diffficult
to locate in pedestrian areas and frequently used
roads (background noises) Be carefull when
natural gas pipelines are around! (diagnose
leakages!)
28
System of fault localization
b. audio-frequency twist method
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System of fault localization
5. Cable identifying
Direction of arrow on tongs has to agree
with current direction of emitter !
Flag elected cable at once !
30
Diagnostics on PE and XLPE cables after laying
Disconnect voltage transfomers and transformers
first!
High voltage tests (inulating) Test alternating
voltage 45 bis 65 Hz Level (eff) 2 x U0 Test
time 60 min
Test alternating voltage 0,1
Hz (VLF-test) Level (eff) 3 x U0 Test time 60
min
No dielelctric breakdown
No dielelctric breakdown
Voltage testing on sheath PE-sheath Directvolta
ge 5 kV PVC-Mantel Directvoltage 3 kV
Test time 10 min No dielelctric breakdown
Practical expirience values for leakage
currents PVC 0,8 mA every 1000 m cable( ?
2,5 MW) PE 0,02 mA every 1000
m cable
31
Diagnostics on paper-insulated cables after laying
High voltage tests (inulating)
Test directvoltage U0/U 6/10 kV 34 ... 48
kV 12/20 kV 67 ... 96 kV
18/30 kV 76 ... 108 kV Test time
jeweils 15 ... 30 mi No dielelctric breakdown
Test alternatingvoltage 45 bis 65 Hz Level
(eff) 2 x U0 Test time 30 min
Test
alternatingvoltage 0,1 Hz (VLF-test) Level(eff)
3 x U0 Test time 30 min
No dielelctric breakdown
No dielelctric breakdown
No tests with direct voltage (DC) on mixed
XLPE/paper insulated cables ( beware of space
charge)
Warning disconnect transformers and voltage
transformers Reduce test level on aged cables !
32
Cable testing
By testing online without disconnecting the cable
from the switchgear the isolating distance to the
busbar is charged with the summation of normal
supply voltage and the test voltage. Sometimes
the stress-voltage incloses a DC-part.
No benchmark on technical rules! The asset
manager is responsible ! If necessary consult
manufacturer of switchgear
33
Cable test van
Standard-equipment

• Fault location
• Emergency power generator 230 V, 10 kVA
• Time domain reflectometer (TDR)
• Burn-down transformer
• Surge voltage generator
• Audio frequency receiver
• Transmitter for cable depth evaluation
• fault location bridge (telecomunication cables)
• Echometer
• Walkie-talkies
• Quality diagnostics
• VLF-device
• Partial discharge device

34
In-situ diagnostics of transformers

• Outer failure
• damgages on vessel
• broken bushings
• leakages

2. Inner failuere winding breaks insulation
breaks winding fault between HV and LV short
circuit between HV and LV short circuit between
HV and core short circuit between LV and core
Tests Insulation metering HV against earth
5 kV, 1min HV against LV 5 kV, 1min LV
against earth 2,5 kV, 1 min Insulation
resistance min. 10 MW / kV ( 10 kV ? 10 x
10 MW 100 MW) Voltage-proportion-test
(Identification of interturn faults)
Resistance test The resinstances are very low and
not stable. Therefore you need a special testing
bridge Testing by ohmmeter only for breaks or
continuity.
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Sequence of fault
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Sequence of fault
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Sequence of fault
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Illustrationin Power-Quality-System
39
Special vehicles
emergency power supply 500 kVA, 12 Zyl.
Cherry picker
Container for transfomers
40
Special vehicles
cable test van
trouble shooting van
41
Emergency power generator

• Check fuel (high consumption at full load !)
• Check motor oil
• Check cooling water
• Check battery
• periodic chech of main cables
• periodic check of auxilary units
• periodic tets runs
• before departure
• - brakes o.k. ?
• - tire air-pressure o. k. ?
• accessories on board ?
• - two-phases voltage tester
• - revolving field tester
• - special tools for live working
• - earthing rod
• - cable roller

Emergency power generator 500 kVA, 12 Zyl.