1. CIGRE Working Group C4.307:
„Resonance and ferroresonance in power networks and transformer energisation studies”
Chairman: Lubomir Kocis EGU HV Laboratory, Czech Rep.
Co-chairman: Manuel Martinez EdF, France
Except of WG for ferroresonance, creation of WG Transformer energisation studies was proposed in 2008
It was decided to join both topics, that are closely related, to one WG.

2. WG members: Name: Company: Herivelto Bronzeado CHESF
Name: Company:
Herivelto Bronzeado CHESF
Orla Burke ESB International
Bruno Caillault EDF DTG
Zia Emin National Grid
William Phang ESB International
David Jacobson Manitoba Hydro
Nicola Chiesa SINTEF
Terrence G. Martinich BC Hydro
J. A. Martinez Velasco UPC
Stephan Pack TU Graz
Eung-Bo Shim KEPRI
François Zgainski EDF DTG
In total: 14 members

3. The first meeting: Prague, 26 - 27 May 2010, EGU HV Laboratory
Attended by (9 members ):
Orla Burke, Bruno Caillault, Zia Emin, William Phang, Manuel Martinez,
Nicola Chiesa, Lubomir Kocis, Terrence G. Martinich, François Zgainski
Introductory contributions were presented dealing mainly with
transformer energisation during „black starts“
Proposed structure of the future document (brochure)
was discussed in the meeting
It will consist of two parts:

4. I. Resonance and ferroresonance in power systems
Proposed by L. Kocis, March 2010
a) Resonance
 1. Resonant circuits and their characteristics (F)
2. Resonances in networks- recorded cases (T,F, O)
3. Analyse of possible resonant circuits
- series compensation, shunt compenation, FACTS (F)
- double cicuit lines, very long lines, capacitor banks, cables
- application of long HVAC cables
- non-standard circuits configured during restoration of system operation (start from  dark) - network islands, acceptable scenarios (T)
4. Resonant overvoltages in hv and uhv networks - summary of risks of their appearance
  5. Mittigation techniques (O,..)

5. b) Ferroresonance
 1. Oscillating circuit with core saturation
- free oscillations, lossless, damped
2. Key parameters
- magnetizing curves
- stray capacitance
- series capacitance
 3. Feroresonance - steady state
- with small losses, large losses
- stability limits
- ferroresonance 50 Hz and subharmonic ferroresonance
4. Transition between low mode and high mode
- criteria for transition (50 Hz, subharmonic)
 5. Ferroresonance of VTs in effectively earthed systems (Z, O)
- influence of circuit capacitances (grading capacitors of CB)
- role of VT magnetizing curve
- criteria for VT ferroresonance
- starting manipulations (switchings)

6. 6. Feroresonances (FR) of VTs in non-effectively earthed systems (T)
- FR in tertiary circuits of large power transformers
- FR in power generator circuits
7. FR of power transformers connected to double circuit line ( Z, T,)
8. Consequencies of FR
- overvoltage, overcurrent, degradation, failures
- 50 Hz vs subharmonic
- requirements for degree of suppression
9. Measures for suppression of ferroresonance (O,T)
- additional losses
- devices for FR suppression
- VT design and dimensioning
- circuit configurations

7. II. Transformer Energization Studies
 Proposed by Manuel Martinez, June 2010
1. Overvoltages and undervoltages due to transformer energization
a. Overvoltage generation due to LF harmonic resonance in weak networks  (transformer energization through long lines or cables) [L, T, F, M]
· System restoration : Transmission and Distribution Transformers & Auxiliary
Transformers of Nuclear Power Plants
· Offshore wind farm transformers linked to the shore network by long AC cables.
· Examples: field recorded cases
 b. Voltage drop in distribution networks at the energization of transformers of distributed generation units1 [M, T, H]
· Examples: Field recorded cases

8. 2. Computing the overvoltages or undervoltages by simulation
 a. Component modeling
· Transformers (single phase/three-phase core, saturation, residual flux, etc.) [N,M, H]
· Overhead-lines and cables [M]
· Network equivalents
· Generators (X’’d simple models versus complete Park’s models, voltage regulation…)[M]
· Loads
b. Random initial conditions: range & discretization
· Circuit-breaker closing times (dispersion between CB phases, discretization) [F, L]
· Residual fluxes (values, phase distribution patterns, decay with time,
discretization) [F, N]
c. Assessing the sensitivity of the computed overvoltages to the   modeling of the upstream network (because overvoltages are due to resonance,  they can be very sensitive to small differences in the modeling of the upstream network (location of impedance poles, etc.))

9. 3. Evaluating the effects of the overvoltages : quantification of the stress
· Transformer withstand capability to TOV with harmonics [M, L]
· Surge arresters withstand capability to TOV with harmonics [M, L]
4. Mitigation techniques
· Controlled switching, shunt reactances, closing resistors, surge arresters, network
modification, local magnetization… [F, M, T, L]
· Domain of application, effectiveness…
5. Some case studies: Simulation results vs. field measurements [F, M, N]
1 For instance, in France DU/U must be less than 5 %.
Next meeting of WG C4.307:
 CIGRE 2010 General Session, Paris,
 Thursday 26 August 2010 from 14 to 18 h
 Room 332 M Level 3.5

10. Ferroresonance in Czech Transmission Network and Power Generation
Two types of ferroresonance time to time occured:
Type A (400 kV)- Ferroresonance of CB grading capacitors with VTs
Type B (MV) - Ferroresonance of VTs in tertiary circuits of large power transformers and in generator circuits

11. Conceptual approach
Type A
·        - Measuring of magnetizing curves of all used types of VTs by impulse method
·       - Simulation methods
·        - Finding of circuits sensitive to FR - field measurements
·        - Combining of VTs and CBs immune to FR
Type B
·        - Design of VTs with low magnetisation for tertiary circuits of large power transformers and for generator circuits

12. Measuring of magnetizing curve by free oscillations
Figure Circuit for measurment and evaluation of VT magnetizing curve
  AI(t) = uVT(t) – Rw . i(t) (1)   
(t) = ui(t).dt + (0) (2)

13. Figure Reconstruction of magnetizing curve from measuring of VT free oscillations, (record of voltage and current, integrated flux and resulting magnetizing curve

14. Comparison of magnetizing curves of three different types of VT used in the network 400 kV

15. Figure Frequency of free oscillations as a function of initial voltage Uco for three types of VTs and capacities 1 nF

16. Figure Corona losses in kW/km vs voltage

17. Figure Sensitivity of FR to paralel capacity

18. TABLE Combination of CB and VTs - risk of ferroresonance 50 Hz
In very short bays equipped by CBs with grading capacitors more than 400 pF, we install VTs with very high immunity to FR (high knee of magnetizing curve, air gap in core)

19. Type B Ferroresonance in MV circuits of large power transformers or circuits of generators

20. Measures for suppression of FR in systems with isolated neutral
Historically - damped resistor in opened delta secondary of VTs
Experience showed, that damped resistor is effectively able to suppressed steady-state FR but, in some cases not its initial transition part.
Modern automatic systems of remote control of substatios ( e.g.action as a response to earth fault) have very fast times of reactions with adjusted time of insucceptibility of about 150 miliseconds.
That is reason, why FR in tertiary must be suppressed from the beginning including its transition part lasting several period.

21. Advanced designs of digital relays with very low burden led to construction
of VTs with very low power of order of units of VA.
It enables much more effective manner how to suppress FR in MV circuits.
Classic VTs have steady state magnetisation about 0,8 T.
Todays designs of VTs can be constructed (keeping required accuracy 0,2%) with magnetisation 0,4 - 0,6 T.
It was proved, that after replacing clasic VTs in circuit with ferroresonance by VTs with low level of magnetization 0,4 T, FR was suppressed at all.

22. ACM - Automatic central monitoring - on line diagnostic system
Fault recorders are sources of a huge amount of data that are not always effectively used for equipment service condition assessment.
They are usually triggered only from relay system operation or by exceeding a threshold value of phase current or voltage (usually 120% Unr.m.s. and % In but it can be set as needed).
However there is nothing that prevents their triggering from substation control systems to record normal service switching operations too. Doing that the fault recorders become unique sources of data describing some transient events in substations.

23. Fault recorders provide basically two types of information:
·        current and voltage curves (single phases and zero) with sampling frequency 1 kHz for 0.2 to 0.3 seconds before and 3 to 5 seconds after the fault recorder function was triggered
·        timing characteristics (the beginning and the end) of different signals, e.g. start and end of protection relays, start and end of O or C impulse, transfer of the impulse to CB coils (O,C), start of pole discrepancy, start of CB interlocking (for auto-reclosing and for opening operations).
But with general triggering condition (fault, overvoltage, overcurrent + before every switching), fault recorders can record many other transients than only short circuit faults.

24. Data transfer to central network server
The records from fault recorders are transferred to terminals (one terminal in every substation) and then once a day records are sent from substation terminals via WAN (intranet) to central CEPS server in Prague .
AROPO
AROPO is a modular system consisiting of independent software modules - that have differrent functions - recognition of specific event or evaluation of the records.
There are already running 12 AROPO expert modules called e.g.
Module QMPRU – can recognize a restrike during circuit breaker opening
Module PREP – can recognize and calculate levels and durations of temporary overvoltages in substation bays ….

25. Module FERO: The module is able automatically to recognize transient or stable ferroresonance 50 Hz or subharmonic in any record of failure recorders collecting from the all substations of the CEPS network.
Module FERO is in operation 6 months. It found two cases of steady-state subharmonic ferroresonance of VTs 16,6 Hz in bus breaker circuits. This subharmonic ferroresonance is not dangerous for VTs. The circuits were subjected to analysis of sensitivity to possible appearance of dangerous ferroresonance 50 Hz.