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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.

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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, Franois 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:

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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,..)

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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)

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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

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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

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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 (Xd simple models versus complete Parks 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.))

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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

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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

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Conceptual approach

Type A

- Measuring of magnetizing curves of all used types of VTs byimpulse 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 oflarge power transformers and for generator circuits

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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)

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

magnetizing curve

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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

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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)

Frequ

ency

(Hz)

VT3

VT2

VT1

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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)

Coron

alosses(kW/km)

DCAC(amp)

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Figure Sensitivity of FR to paralel capacity

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TABLE Combination of CB and VTs - risk of ferroresonance 50 Hz

In very short bays equipped by CBs with grading capacitors more than 400pF, we install VTs with very high immunity to FR (high knee ofmagnetizing curve, air gap in core)

Cs = 800 pF(HPL420, VSV420.1)

Cs = 500 pF Cs = 200pF (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 - - -

SVS420/1G**

650kV

550kV

- - - - - - - -

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

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Type B Ferroresonance in MV circuits of large power

transformers or circuits of generators

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Measures for suppression of FR in systems with isolated neutral

Historically - damped resistorin 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.

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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.

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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% Inbut it can be set as needed).

However there is nothing that prevents their triggering fromsubstation 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.

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Fault recorders provide basically two types of information:

current and voltage curves (single phases and zero) withsampling 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.

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Data transfer to central network server

The records from fault recorders are transferred to terminals (one terminalin every substation) and then once a day records are sent from

substation terminals via WAN (intranet) to central CEPS server inPrague .

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 QMPRUcan recognize a restrike during circuit breaker opening

Module PREPcan recognize and calculate levels and durations oftemporary overvoltages in substation bays

.

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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.

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