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

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

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.

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.

Background Normal Excitation

Background Under Ferroresonance

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.

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.

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

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

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

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

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).

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

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

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

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

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

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

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

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.

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.

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. 255-265, 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. http://www.ece.mtu.edu/faculty/bamork/FR_WG/ [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. 256-262, Jan. 2007.