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,

FranceExcept 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.

WG members: Name: Company:Herivelto Bronzeado CHESF Orla Burke ESB International Bruno Caillault EDF DTGZia Emin National GridWilliam Phang ESB InternationalDavid Jacobson Manitoba Hydro Nicola Chiesa SINTEF Terrence G. Martinich BC HydroJ. A. Martinez Velasco UPCStephan Pack TU Graz Eung-Bo Shim KEPRIFrançois Zgainski EDF DTG In total: 14 members

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 meetingIt will consist of two parts:

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,..)

b) Ferroresonance 1. Oscillating circuit with core saturation

- free oscillations, lossless, damped2. 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. 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

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 & AuxiliaryTransformers 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

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]

· Loadsb. 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.))

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, networkmodification, 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

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

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 

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)

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

magnetizing curve

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

VT1

0

500

1000

1500

2000

2500

0 0,2 0,4 0,6 0,8 1

VT3

0

500

1000

1500

2000

2500

3000

3500

4000

4500

0 0,2 0,4 0,6 0,8 1

VT2

0

500

1000

1500

2000

2500

3000

0 0,2 0,4 0,6 0,8 1

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

C=1 nF

0

20

40

60

80

100

120

140

160

180

0 200 400 600 800 1000 1200 1400 1600

Voltage (kV)

Freq

uenc

y (H

z)VT3VT2VT1

Figure Corona losses in kW/km vs voltage

0

200

400

600

800

1000

1200

1400

1600

0 200 400 600 800 1000 1200 1400

Voltage (kV)

Cor

ona

loss

es (k

W/k

m)

DC

AC(amp)

Figure Sensitivity of FR to paralel capacity

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)

Cs = 800 pF (HPL420, VSV420.1)

Cs = 500 pF Cs = 200 pF (VVR,

3AQ2) Cp (pF) = 500 1000 1500 2000 500 1000 1500 2000 500 1000 OTEF420 yes yes - - yes - - - - - NKF400-65 yes yes yes yes * yes yes yes - yes - VT1 420 yes - - - yes - - - - - UTF 420 yes - - - - - - - - - VEOS 420 yes yes - - yes yes yes - - - SVS 420/1G**

650 kV

550 kV

- - - - - - - -

SVAS420/1G - - - - - - - - - -

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

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.

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.  

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 150-200% 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.

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.

Data transfer to central network serverThe 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 .

 AROPOAROPO 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 ….

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.