1. Performance of Differential Protection under the Influence of Ferroresonance
WP-1-P5
2018 Georgia Tech Annual
Protective Relaying Conference, Atlanta, GA
May 2-4, 2018
Hemanth Kumar Vemprala and Dr. Bruce A. Mork
Michigan Technological University
Houghton, MI

2. Outline Introduction to Ferroresonance Motivation Background
Case study
Overall Summary and Future works

3. Ferroresonance
Nonlinear resonance, Localized behavior, Excitation currents
Ill-effects are broad & could propagate (ex. misoperation of relay, harmonics distortions, overvoltages, arrester failure)
Low damping coupled with near knee-point operation.

4. Motivation
Ability to study bifurcation using developed 3-phase EMTP non- linear tool.
The behavior is nonlinear and non-deterministic.
The ill-effects of Ferroresonance impacts the Protective relays.
EMTP studies provide a more accurate time-domain observation than analytically simplified methods.

5. Background
Normal Excitation

6. Background
Under Ferroresonance

7. Background Mitigating strategies of Ferroresonance
Avoid risky configuration such as grading capacitors, buswork, network conditions, unbalanced phase operations.
Damping devices or controllers.
Avoid using High Voltage Inductive transformer and opt for CVT, optical VT etc.,
Minimum load maintained on the system.
Surge protection such as arresters in FR prone areas.

8. Background - Common characteristic of nonlinear system
Leg
Yoke
Ipeak [A] 80 70 60 50 40 30 20 10
Fluxlinkage [Wb] 35 34 33 32 31 29 28 27 26 25 24 23 22 21 19 18 17 16 15 14 13 12 11 9 8 7 6 5 4 3 2 1
Background - Common characteristic of nonlinear system
Jump resonance
Subharmonic generation
Multiple equilibrium points
Chaotic behavior
Bifurcation analysis
Identifies any jump resonance and hidden modes.
Time varying parameters (cable length or capacitances).
Need to perform twice in both direction.

9. Background - Transformer Differential Relay
Challenges
Energization inrush
Overexcitation and excitation
CT performance and saturations
Mitigation
Harmonic Restraint
Harmonic Blocking
Wave shaping recognition

10. Case Study: Simulation Test System
Unbalanced phase operation at the terminal
Generic Transformer Differential relay
2nd Harmonic restraint = 20%
Harmonic blocking function modes

11. Test case - Simulation for inrush
Unbalanced phase operation
Generic Transformer Differential relay
2nd Harmonic restraint = 20%
20% setting was sufficient to avoid misoperation

12. Bifurcation diagram – single phase trafo eq
Bifurcation study : Poincare mapping of X1 terminal voltage
Time domain sim results for C= 10uF and 35uF.

13. Bifurcation diagram for Power Transformer
For a three-phase transformer bifurcation study
Core Model is topologically correct.
Hybrid model approach.
Various possible bifurcation behaviors (depends on the ferroresonant risk factors, and parameter configuration).

14. Case Study: Ferroresonance case
Unbalanced phase operation due to CB/Fuse/ open line
2nd Harmonic restraint = 20%
Harmonic blocking function modes are designed

15. Ferroresonance case: (false differential current case)
Primary terminal line voltage
Secondary terminal line voltage

16. Test case - Simulation for Ferroresonance
Unbalanced phase operation.
Periodic ferroresonance is simulated.
Current sufficient for operation.
Ih2 < I(fund)
Ih2 > I(fund)

17. Differential relay security violation:
Line current on primary terminal
Percent second harmonic current

18. Differential relay security violation:
Phase A
Phase B
Phase C
Trip zone
I restraint [A]
2E4
1.9E4
1.8E4
1.7E4
1.6E4
1.5E4
1.4E4
1.3E4
1.2E4
1.1E4
1E4
9E3
8E3
7E3
6E3
5E3
4E3
3E3
2E3
1E3
I difference [A]
700
650
600
550
500
450
400
350
300
250
200
150
100 50
Phase A
Phase B
Phase C
Trip zone
I restraint [A]
340
320
300
280
260
240
220
200
180
160
140
120
100 80 60 40 20
I difference [A]
Trip signal asserted by 87T relay
Inrush security violation with harmonic restraint of 20% or more

19. % second harmonic for ferroresonance
Harmonic restraint + blocking function??

20. Per-phase function for harmonic restraint
87T under different restraint + blocking function
Per-phase function for harmonic restraint
Two out of three
One out of three
Average

21. Overall summary of finding
The ferroresonance simulation results reconfirm that the system is sensitive to the initial conditions.
ATP-EMTP simulation is used for the study.
Case with Bifurcation analysis for two winding Δ-Y transformer
Exciting currents during ferroresonance were simulated and misoperation of 87T due to this false differential current was observed.

22. Overall summary of finding
Different modes of inrush restraint + blocking methods were explored.
Under severe ferroresonance modes, harmonic restraint or blocking modes could be violated.
Future work needs to be carried out to correlate between core saturation extent vs. FR configurations, and develop improved relaying strategies.

23. References
[1.] M. R. Iravani et al., “Modeling and analysis guidelines for slow transients- Part III. The study of ferroresonance,” in IEEE Transactions on Power Delivery, vol. 15, no. 1, pp , Jan 2000.
[2.] CIGRE WG C4.307, “Resonance and Ferroresonance in Power networks”, Feb 2014.
[3.] Mork, B.A., Ferroresonance and Chaos: Observation and Simulation of Ferroresonance in a Five- Legged Core Distribution Transformer, Ph.D. Thesis, North Dakota State University © May 1992.
[4.] A.P. Kunze, B.A. Mork, "Prediction of Ferroresonance Using Moving Window Techniques," European EMTP Users Group Meeting, Leon, Spain, September 24-26, 2007.
[5.] IEEE WG on Practical Aspects of Ferroresonance, Bruce Mork, Chair.
[8.] Mork, B. A., Understanding and Dealing with Ferroresonance, Proceeding of Minnesota Power systems conference, St. Paul, MN, November 7-9, 2006.
[19.] B.A. Mork, F. Gonzalez, D. Ishchenko, D. L. Stuehm, J. Mitra, “Hybrid Transformer Model for Transient Simulation-Part II: Laboratory Measurements and Benchmarking”, IEEE TPWRD, Vol. 22, pp , Jan