Vt Guard-ironing Out Resonance
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42 ABB Review 4/2005 Grid reliability Every engineer knows the phenomenon of resonance. Without it, there would be no musical instruments, no radio-based communication and many natural phenomena, ranging from the astronomical to the sub-atomic would not occur. The occurrence is not always benign: Oscillations can reach amplitudes for which the system was not designed, leading to damage and failure. In 1940 the Tacoma Narrows Bridge in the USA collapsed spectacularly as the result of undamped resonance. Resonance is also a frequent cause of malfunction in electronic systems. Fortunately, the frequencies at which resonance occurs can be influenced by design. The calculation of resonance gets trickier for non-linear effects: When the magnetic flux in a core (of for example, a transformer) exceeds a certain value, resonance becomes much more difficult to predict. Such occurrences are frequent in the voltage transformers (VT) that transform high and medium voltage levels into low voltage for instrumentation or protection purposes. Ironing out resonance Ferroresonance prevention in MV voltage transformers Wojciech Piasecki, Marek Florkowski, Marek Fulczyk, Pentti Mahonen, Mariusz Luto, Wieslaw Nowak, Otto Preiss
Transcript of Vt Guard-ironing Out Resonance
42 ABB Review 4/2005
Grid reliability
Every engineer knows the phenomenon of resonance. Without it, there wouldbe no musical instruments, no radio-based communication and many naturalphenomena, ranging from the astronomical to the sub-atomic would not occur.The occurrence is not always benign: Oscillations can reach amplitudes forwhich the system was not designed, leading to damage and failure. In 1940 theTacoma Narrows Bridge in the USA collapsed spectacularly as the result ofundamped resonance. Resonance is also a frequent cause of malfunction inelectronic systems.
Fortunately, the frequencies at which resonance occurs can be influenced bydesign. The calculation of resonance gets trickier for non-linear effects: Whenthe magnetic flux in a core (of for example, a transformer) exceeds a certainvalue, resonance becomes much more difficult to predict. Such occurrencesare frequent in the voltage transformers (VT) that transform high and mediumvoltage levels into low voltage for instrumentation or protection purposes.
Ironing out resonanceFerroresonance prevention in MV voltage transformersWojciech Piasecki, Marek Florkowski, Marek Fulczyk, Pentti Mahonen, Mariusz Luto, Wieslaw Nowak, Otto Preiss
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Ironig out resonace
Grid reliability
The rated power of voltage trans-formers (VTs) is usually very low
due to their metrological, rather thanpower supply function. Nominalprimary currents in the transformerwinding are typically of the order ofsingle milliamps (at several up to tensof kilovolts).
The so-called ferroresonance phenom-enon can occur when VTs are con-nected phase to ground in an un-grounded network. Currents can occurthat exceed nominal values by ordersof magnitude, risking damage to theVTs.
The ferroresonance phenomenonThe resonance of a circuit containingan inductance and a capacitance is awell known physical phenomenon.The simplest forms of this are the par-allel and the series resonant circuits .
In the series circuit, the equivalent im-pedance is the sum of the impedancesof the individual components:
In the parallel resonant circuit theequivalent admittance is the sum ofthe admittances of the individual com-ponents:
The resonance pulsation is
in both cases. At and near to this fre-quency in the series circuit, voltagesacross the capacitor and the induc-tance can reach values that exceedthe source voltage significantly. In theparallel circuit, it is the currentsthrough these components that aresimilarly amplified. Such extreme val-ues can damage the equipment if noremedial action is taken.
For known values of L and C, theresonant frequency can be predicted.Resonance-related hazards can beavoided by maintaining an appropri-ate safety margin from the power-supply frequency.
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ωCΖ(ω) = jωL – j 1__ + Rs
ωLY(ω) = jωC – j 1__ + 1__
RP
In the so-called ferroresonance phe-nomenon, however, resonance fre-quencies are more difficult to predict.The phenomenon occurs when themagnetic core of an inductive deviceis saturated, making its current-fluxcharacteristic non-linear . Becauseof this non-linearity, resonance canoccur at various frequencies.
In practice, ferroresonant oscillationsare initiated by momentary saturationof the core of the inductive elementas a a result of switching operations,for example. The effects of such reso-nance are further aggravated if damp-ing is insufficient.
Currents can occur thatexceed nominal values by orders of magnitude,risking damage to theVTs.
In many cases of non-transient reso-nance involving a saturated inductiveelement, some nodes in the equiva-lent network circuit are electricallyfloating (or connected to the fixedpotential points through very highimpedances). Such a situation is illus-trated in the network of .
Although ferroresonance can also beinitiated in other situations (eg, capac-itive coupling between parallel lines,ferroresonance between the VT andthe power transformer’s internal ca-pacitance or single-phase disconnec-tion in grounded networks) the con-figuration shown in reflects a typi-cal unearthed distribution system withsingle-pole VTs.
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3
2
ωr = 1√LC
Series and parallel linear resonant circuits.1
Series resonant circuit Parellel resonant circuit
Z (ω) U
RS L C
UL UC
IY (ω)
I
U
IL IC
L C
RP
Non-linear characteristic of an inductivecomponent with a saturable magnetic core.
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flux ψ
current i
norm
al o
pera
ting
cond
ition
s
saturationLn
Ls
The result of the ferroresonance in unprotected VT.
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Ungrounded MV network with three single-phase voltage transformers connected phase-to-ground.
3
VT1 VT2 VT3
C C CUN
R
S
T
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this day. No universally applicablemitigation methods exist other than adamping resistor connected to theopen-delta auxiliary windings of thethree individual VTs.
Risk to equipmentUnder normal operating conditions,the primary currents in the MV volt-age transformers are typically well be-low 10mA. In ferroresonance, the coreof the VT operates in the deep satura-tion region and primary currents canreach amp values. The two order ofmagnitude difference between normaland ferroresonant conditions leads to
thermal damage to the primary wind-ing if not appropriately damped .
The compact design of modern volt-age transformers and the high qualityof the magnetic material (low losses)makes damping difficult. The resistiveload must have a low value to dissi-pate sufficient oscillation energy. Toosmall a value, however, draws toomuch power from the VT when a sus-tained zero-sequence voltage occurs(eg, due to a ground fault that is notcleared), and so overloads the VTthermally. Selecting the right resist-ance is therefore crucial.
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Grid reliability
Despite first publications on the fer-roresonance phenomenon appearingat the beginning of the 20th century,
no reliable criteria on the risk of fer-roresonance have been formulated to
Simulated and experimentally obtained primary VT current and neutral voltage for capacitance C within the hazardous range.5
System voltage 20 kV/ 3, C=70nF/phase
Simulation Measurement
2.0
0
-2.0
prim
ary
curr
ent
(A)
0 0.2 0.4 0.6 0.8 1.0
time ( seconds)
20
0
-20
neut
ral v
olta
ge (k
V)
0 0.2 0.4 0.6 0.8 1.0
time ( seconds)
2.0
0
-2.0
prim
ary
curr
ent
(A)
0 0.2 0.4 0.6 0.8 1.0
time ( seconds)
20
0
-20
neut
ral v
olta
ge (k
V)
0 0.2 0.4 0.6 0.8 1.0
time ( seconds)
Simulated and experimentally obtained primary VT current and neutral voltage for capacitance C above the hazardous range.6
System voltage 20 kV/ 3, C=240nF/phase
Simulation Measurement
500
0
-500
prim
ary
curr
ent
(mA
)
0 0.2 0.4 0.6 0.8 1.0
time ( seconds)
2.0
0
-2.0
neut
ral v
olta
ge (k
V)
0 0.2 0.4 0.6 0.8 1.0
time ( seconds)
500
0
-500
prim
ary
curr
ent
(mA
)
0 0.2 0.4 0.6 0.8 1.0
time ( seconds)
2.0
0
-2.0
neut
ral v
olta
ge (k
V)
0 0.2 0.4 0.6 0.8 1.0
time ( seconds)
√
√
The compact design ofmodern voltage trans-formers and the highquality of the magneticmaterial (low losses)makes damping difficult.
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Analysis of potential ferroresonanceTo analyze the network conditions forwhich ferroresonance is initiated inparticular VT types, simulations wereperformed using dedicated transient
simulation software (PSpice,ATP/EMTP). Simulating transient re-sponses to switching events involvedmaking models of various VT types,based on magnetic parameters. These
models were inserted into an equiva-lent network model with ungroundedvoltage sources and line-to-groundcapacitances. A worst-case analysis ofthe unloaded network was performedto determine the capacitance range inwhich hazardous ferroresonance oc-curs. Including the auxiliary windingsand damping resistors in the modelshelped identify the optimal resistorvalues.
Simulation results were verified exper-imentally. shows a switching re-sponse with ferroresonant oscillationand shows a response without.
FerroTool and FerroSim The simulations identified the haz-ardous capacitance range and the
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maximum damping resistance for dif-ferent VT types and voltage levels. To make full use of these results inpractical applications, software toolswere implemented for the fast identifi-cation of potential ferroresonance.FerroTool contains a database map-ping VTs to their parameters, includ-ing hazardous capacitance ranges andsuggested damping resistances. Thetool calculates equivalent capacitancevalues for substations based on theline characteristics to permit fastanalysis of the risk of ferroresonance.
Computer simulations andexperiments show that inmany cases the resistanceneeded to damp ferrores-onant oscillations is verysmall. Such a resistorwould, however, draw toomuch current from the VTin the case of a networkasymmetry.
FerroTool is supported by the FerroSimdedicated software for simulating anetwork response to switching tran-sients . As the topology of the circuitis predefined, the user interface is keptvery simple .
SmartLoad: New ferroresonanceprevention concept Computer simulations and experi-ments show that in many cases theresistance needed to damp ferroreso-nant oscillations is very small (< 20W).
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FerroTool for potential ferroresonance risk identification for particular network VT configuration.7
VT type selection
Substation view
Link to cable manager
Link to VT manager
Predefined circuit model and stimulus.8
eA
t=0
RS LS iA
uA C
ipA
Rp
Rt
k:1
i(ψA)
eC
t=0
RS LS iC
uC C
ipC
Rp k:1
i(ψC)
eB
t=0
RS LS iB
uB C
ipB
Rp k:1
i(ψB) Volta
ge
Time
t0
Overvoltage factorkov=Um/Em
Um
Em
0
phase Aphase B phase C
FerroTool contains a data-base mapping VTs to theirparameters, includinghazardous capacitanceranges and suggesteddamping resistances.
VT Guard: New ferroresonanceprevention concept
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Such a resistor would, however, drawtoo much current from the VT in thecase of a network asymmetry. A newand unique protection approach hasbeen developed by ABB: A two-termi-nal element named SmartLoad takesthe place of the conventional linearresistor.
The SmartLoad protectsagainst thermal overloadand can be used forpractically any typeof IEC-standard in-ductive MV voltagetransformer.
When zero-sequence volt-age occurs due to a naturalsystem unbalance undernormal operating condi-tions, the device acts as avery high resistance and sodoes not drain power fromthe VTs. When the zero-sequence voltage exceedsa predefined threshold lev-el, however, the device’sohmic value drops suffi-ciently to dampen oscilla-tions within a small num-ber of cycles. A device hasbeen created that efficient-ly dampens ferroresonantoscillations without over-loading VTs. The device isvery compact and can bemounted on a standard
both worlds. As it practically elimi-nates the risk of thermally overloadingthe VT in case of non-transient faultsin the system, the SmartLoad can beused for protecting practically anytype of IEC-standard inductive MVvoltage transformer.
W. Piasecki
M. Florkowski
M. Fulczyk
ABB Corporate Research
Krakow, Poland
Mariusz. Luto
ABB Medium Voltage
Przasnysz, Poland
Pentti Mahonen
ABB Medium Voltage
Vaasa, Finland
O. Preiss
ABB Corporate Research
Baden, Switzerland
Wieslaw Nowak
University of Science and Technology
Krakow, Poland
Grid reliability
User interface of the FerroSim and exemplary results showing the primary VT currents and the neutral voltage.9
Pilot installation of the SmartLoad.10
DIN-rail in the secondary equipmentcabinet .
ConclusionsInductive voltage transformers operat-ing in ungrounded networks mustalways be protected against ferroreso-nance. Ferroresonant oscillations ex-ceeding the nominal values by ordersof magnitude pose a serious risk toVTs. Resistive damping is not alwaysviable because the low ohmic valuethis requires overloads the VT. ABB’snovel solution is a self-adjusting load(SmartLoad) that combines the best of
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Inductive voltage transformers operating in ungrounded networks must always be protected against ferroresonance. Ferroresonant oscillations exceeding the nominal values by orders of magnitude pose a serious risk to VTs. Resistive damping is not always viable because the low ohmic value this requires overloads the VT. ABB’s novel solution is a self-adjusting load
which practically eliminates the riskof thermally overloading of VT in case of non-transient faults in the system. The VT Guard can be used for protecting practically any type of IEC-standard inductive MV voltage transformer.
Such a resistor would, however, draw too much current from the VT in the case of a network asymmetry. A new and unique protection approach has been developed by ABB: A two-termi-nal element named VT Guard takes the place of the conventional linear resistor.
The VT Guard protects
Pilot installation of the VT Guard.
deysnl
Rectangle
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