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Digital EnergyMultilin
Problems and Solutions of Line Differential
Application in Cable Transformer Protection
Jorge Cardenas. GE Digital Energy Mahesh Kumar.
GE Digital EnergyJesus Romero. RasGas

• CIGRE 7-10 September 2009, Moscow

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Application problem in differential
CableTransformer
Transformer Inrush phenomena affects the
operation of the differential protection in the
same way as in case of a Power Transformer. In
the classical approach, an inhibition or blocking
action based on the harmonic (usually 2nd
harmonic) content on the differential algorithm,
is normally used. Because differential
protection is practically disabled during some
time after the energization, a complementary
protection is needed to disconnect the circuit if
a fault is produced during that time Another
problem is the Inrush phenomena after the voltage
recovery during external three-phase or
phase-to-phase faults..
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On-line diagram and Line Differential Protection
scheme
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Initial Protection Scheme
As Current Differential Relays installed in the
plant have not specific functions to prevent
Transformer Inrush phenomena, initially a logic
based on setting group change was implemented to
avoid a non-desirable trip when the Line is
energized (there are no CBs on the 33 kV
receiving end of the individual Power
Transformers) and interposing CTs are used at LV
side to correct the vectors similar to
conventional transformer differential protection.
At time zero, the relays work with a high pickup
differential settings (typically 2.0 p.u.) and
with a time delay (800 ms) after the CB close,
the pickup is changed to a lower value (typically
0.3 p.u.).
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Initial Logic to prevent false trip during
energization
Relays (UR from GE) installed allows on-line
logic change of the differential pickup level.
Setting Group is changed in less than 2.5 ms.
This is the maximum additional delay expected to
trip on internal faults.
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Problems with the Initial Protection Scheme

• Definition was correct, but incomplete. Logic
  only contemplates
• Energization, but no Inrush after voltage
  recovery.
• Pickup setting were too low for Energization,
  particularly on 6.6
• kV. Side
• No complementary protection was active to
  prevent faults during
• the energization, only TOC as backup.

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Critical Irest vs Idiff in 6.6 kV with Actual
Setting
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Changes Recommended
1.   Switch on to fault and distance protections
implemented in Setting Group 2 to provide backup
to current differential protection during
transformer energizing or when operation is
blocked due to loss in communications. This new
setting effectively covered all 3 phase and phase
to phase faults up to 120 of the line. Ground
TOCs in 33KV and 6.6KV provide the protection for
phase to ground fault as per existing settings.
The current differential pickup is adjusted to
avoid unwanted trip due to inrush
currents.   2.   Setting Group 3 is introduced in
the scheme to provide the correct setting for
inrush current produce during voltage recovery
after the isolation of an external fault. The new
setting used distance protection to cover 3 phase
and phase to phase internal faults. Ground TOCs
in 33KV and 6.6KV cover phase to ground faults as
the actual coverage is done with Setting Group
1.    3.  Distance protection is implemented in
Setting Group 1 to cover all faults up to
transformer HV windings, this also provided
backup to current differential protection when
the operation is blocked due to loss of
communications.    
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Changes Recommended (cont.)

• 4.  With the existing or proposed current
  differential settings in Group 1, it was found
  out that for loads in 33KV of more than 200A
  approximately, differential protection does not
  operate on phase to ground faults. This is due to
  the increased in the restraint current magnitudes
  with increased load currents. Enabling of
  distance Gnd Z2 proved to address this problem
  plus the existing 33KV directional Ground TOC.
•  
• It has also been noted that only trip on current
  differential is programmed for circuit breaker
  inter trip. A new inter trip logic based also in
  the other protection functions (Distance, SOFT
  and Ground TOC) has been tested and it is
  recommended for implementation.
• The Solutions Proposed were validated initially
  with test on RTDS ans later with tests in Field.

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Validation of the model for Inrush tests
a) Phase A b)
Phase B c) Phase C
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Breaker Trip time estimation from a Real Event
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New Setting Group Proposed for Inrush recovery
after external faults
Modifications in Group 2 were as follows   Relay
at 33 kV Change the slope 2 to 70 and the
Breakpoint to 2 Relays at 6.6 kV Raise the
pickup to 4.0, slope 2 to 70 and Breakpoint to 2
Critical Irest vs Idiff in 6.6 kV modified
Setting of Group 2
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New Setting Group Proposed for Inrush recovery
after external faults
It was decided an intermediate setting, higher
than the maximum peak estimated according to
table 1 (3,2 p.u.). Settings on relays at 33 kV
and 6.6 kV were as follows  
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Logic Implemented to complement the Line
Differential Algorithm
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Energization tests in the worst close angle
condition after the new logic implementation
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Inrush voltage recovery tests after the new logic
implementation
Inrush after voltage recovery.
Operation considering load
condition,
that is trip on external fault caused by an
instantaneous protective relay.
3PH fault on 1
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Cable and Transformers Fault tests
PH_PH fault on pos 5.
PH-PH in Pos 7.
3PH in pos 7.
External 3PH fault on 6.
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Cable and Transformers Fault tests
Direct trip by Ground TOC and Distance during
internal PH_G fault on 4.
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Conclusions

• Proposed solution makes the energization of
  transformers without any
• nuisance trips and proved several time for
  last 2 years. As these feeders
• were crucial in LNG production of Rasgas, this
  solution helped them to
• save time and money with an improved
  reliability.
• The solution implemented has been be extended to
  any other similar
• substations w/o additional hardware, nor
  modifications on it.
• The numeric technology in the multifunction
  relays give us many tools
• to implement new solutions to problems that in
  the past were only
• possible to solve using another techniques as
  the second harmonic
• restrain to prevent a false trip during
  transformer energization. We
• have demonstrated that other possible schemes
  are also possible
• maintaining a similar level of reliability and
  security as the traditional
• ones.
• The use of the RTDS has allowed as a very good
  approach with the
• problems encountered in field, helping in test
  new solution for Differential
• Application on schemes composed by cables
  transformers.