1. CIGRE 7-10 September 2009, Moscow
Digital Energy Multilin
Problems and Solutions of Line Differential Application in Cable Transformer Protection
Jorge Cardenas. GE Digital Energy Mahesh Kumar. GE Digital Energy Jesus Romero. RasGas
CIGRE 7-10 September 2009, Moscow

2. Application problem in differential Cable+Transformer
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..

3. On-line diagram and Line Differential Protection scheme

4. 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 CB´s on the 33 kV receiving end of the individual Power Transformers) and interposing CT’s 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.).

5. 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.

6. 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.

7. Critical Irest vs Idiff in 6.6 kV with Actual Setting

8. 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.
The Merging Unit (Brick) is designed to be located as close to the source of signals and controls as possible. As such, the brick is designed for outdoor mounting and is completely connectorized for quick replacement. Copper cables for interfacing with Currents, Voltages, Digital Inputs, Transducer Inputs, and control outputs are available. The system is designed for complete redundancy such that 2 MUs – containing the exact same data – can be connected into the same relay. The relay always compares the sources of the data and, depending on the security setting, upon detection of an error either shuts downs the entire system or automatically switches to the back MU as its source of data.

9. 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.

12. Validation of the model for Inrush tests
a) Phase A b) Phase B c) Phase C

13. Breaker Trip time estimation from a Real Event

14. 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
A Merging Unit needs to provide data to multiple relays/meters. This is typically accomplished through the installation of an Ethernet switch on the output of the Merging Unit. In this architecture, the switch is combined with the Merging Unit so no external switch is required. As such, the fiber cable from the field into the control house contains 4 fibers – the outputs of the internal switching – as well as a pair of copper wires in order to provide power for the MU. Note that the IEEE Bi-Direction Ethernet standard is used for Ethernet so that a only a single fiber connection is required for full-duplex communication. Fiber cables from the field are terminated in a Patch Panel (lower left pictures). The Patch Panel provides the junction box that enable connection from a source of data in the field to a consumer (relay) in the control house. Connection between the producer (MU) and the consumer (relay) is realized through a single fiber jumper between the fiber cables.
Critical Irest vs Idiff in 6.6 kV modified Setting of Group 2

15. 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:

16. Logic Implemented to complement the Line Differential Algorithm

17. Energization tests in the worst close angle condition after the new logic implementation

18. 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

19. Cable and Transformers Fault tests
PH_PH fault on pos PH-PH in Pos 7.
3PH in pos External 3PH fault on 6.

20. Cable and Transformers Fault tests
The data input card is designed with 8 fiber optic inputs that feed into another internal Ethernet switch. The inputs are organized as 4 from system A and 4 from system B. As mentioned earlier, in a redundant implementation, the receiving card (Process Card) compares all received packets and flags errors and failures.
The switch serves two functions: first – to collect all the data from all devices and second (as will be seen in the Synchronization slide) provides for simultaneous output of GOOSE messages to all connected devices.
Direct trip by Ground TOC and Distance during internal PH_G fault on 4.

21. 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.