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Electrical facilities and systems
ЭЛЕКТРОТЕХНИЧЕСКИЕ КОМПЛЕКСЫ И СИСТЕМЫ ELECTRICAL FACILITIES AND SYSTEMS
Долгицер А. Dolgicers А.
Dr. sc. ing.
Asoc. professor, Power Engineering Institute Riga Technical University Riga, Latvia
Козадаев Е. Kozadajevs Je.
mag. sc. ing. Researcher
Power Engineering Institute Riga Technical University Riga, Latvia
УДК 621.313
IMPROVEMENT OF THE SENSITIVITY OF DIFFERENTIAL PROTECTION
OF POWER TRANSFORMERS
This paper presents the development of microprocessor based device for improving the sensitivity of differential protection of power transformers. Power transformers failure may lead to high scale system’s operations disruption and heavy economic losses, that’s why the lever of requirements for power transformer protections is so high. Differential protection is usually used as a main protection against internal faults such as inter-windings faults, inter-coil faults or coil-core faults. For safe protection against incomplete winding faults protection device must be capable to detect differential current from such a low value as 0.01% from nominal. However, transformer for relay protections needs are produced with error classes 5P and 10P meaning error rate 5 and 10% correspondently. The idea for a new device is to use information obtained from measurement current transformers. The microprocessor based device may “teach” itself by forming some error correction table in it’s memory on the basis of core outputs compare during normal operation. In case of fault device may operate with much higher sensitivity thus sensing fault on it’s early stage.
Key words: Differential protection of power transformers, current transformer, sensitivity of relay protection, internal faults.
повышение чувствительности дифференциальной защиты силовых трансформаторов
Силовые трансформаторы являются ответственными и дорогостоящими элементами энергосистемы, их повреждения способны принести значительный экономический ущерб, привести к системным авариям. В качестве основной защиты от внутренних повреждений, как межвитковых и межкатушечных замыканий, замыканий на корпус и сердечник, используется дифференциальная защита. Для надежной защиты
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трансформаторов от неполных замыканий может потребоваться защита с чувствительностью 0,01% от номинального тока. На сегодняшний день трансформаторы тока, используемые для дифференциальной защиты, не могут обеспечить такую точность. Идея предлагаемого устройства заключается в использовании информации, получаемой от измерительных трансформаторов, для коррекции погрешности основных данных от измерительных токовых трансформаторов. Микропроцессорное устройство релейной защиты может быть «обучено», т. е. по результатам наблюдений, выполненных в нормальном режиме, в памяти устройства может быть сформирована таблица поправок с помощью которой в аварийном режиме устройство сможет действовать с более высокой эффективностью.
Ключевые слова. Дифференциальная защита силовых трансформаторов, токовые трансформаторы, внутренние короткие замыкания, чувствительность защиты.
I. Introduction
Power transformers are highly valued power system’s elements, transformers failure may lead to high scale system’s operations disruption and heavy economic losses. Transformer damages are often linked with long term customer disconnections or available power limitations. That’s why the lever of requirements for power transformer protections is so high. Between transformer damages internal and external damages are distinguished. Under the term “internal damage” are understood any kind of damage, that is located inside the transformer’s shell or tank. Internal damage may cause winding destruction, fire and transformer’s blowup, which leads to complete transformer destruction. Protection against internal faults is a matter of high importance. Differential protection is usually used as a main protection against internal faults such as interwindings faults, inter-coil faults or coil-core faults [1].
To understand the basics of such protection let’s discuss the simplified schematics on the fig. 1.
The current transformer TA1 is inserted into the primary windings circuit of the protected power transformer, another current transformer TA2 is used to monitor current in the secondary winding’s circuit. Both current transformers are selected in such way, that in case of nominal load of protected transformer they secondary currents are equal. Figl.a shows circuit operation in cases of normal load or external damage. The current relay KA is fed by difference of currents from TA1 and TA2, in case of external fault or load this currents are equal by amplitude and opposite by direction, so resulting current in the KA coil is equal to zero and no output is produced. Complete opposite situation is observable in case of internal fault. Both currents are directed toward fault and as shown on Fig 1.b.
KA will receive near double fault current and with KA trip the disconnection signal for power switches will be produced. Reader may come to conclusion, that protection device, based on differential principle will be absolutely selective and will benefit an unlimited sensitivity. In the real world protection sensitivity is limited by the number of factors, partially linked with
Fig. 1. Basic principle of differential protection’s operations
protected transformer, such as magnetizing current, (especially rapid changes in it) or change of real transformation ratio occurring when the voltage regulator changes the taps during it’s operation and partially inflicted by the current transformers errors. Let’s designate the current in the KA under normal regime conditions or in case of external fault as unbalance current. In the ideal case the unbalance current Inb=0, the real value of this current is basic factor of determinacy for protection trip current and a basic limiter for protections sensitivity.
II. Estimation of required protections sensitivity
Let’s find an estimation of required protections sensitivity. The hardest task for the differential protection is opening of so called incomplete winding faults, which may occur in the multi-wire transformer windings, when a coil is formed from multiple parallel wires (such design is preferable for high current transformer coils for technology reasons) [1]. Let’s discuss transformer’s winding made of two parallel wires (Fig 2.a) with common terminals a and b. Any insulation damage will cause short circuiting between windings of different parallel branches (points c and d) and, in common case, between different winding (at last
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one winding shift will occur). Faults current is limited by resistance of two contour’s cad and cbd (see Fig 2.b) formed by parallel winded wires and may be estimated
Fig. 2. Transformer’s winding formed by two parallel wires
by the method proposed by L. Leites considering the short-circuited part of common winding as a separate winding [3]. Damaged leg of transformer may be represented by schematic shown on Fig. 3. To simplify equations let’s consider, that before the fault transformer didn’t carry any load. Two opposite EDF EK and EK' are inserted between points of fault c and d both equal to multiplication of single wind voltage Uw by number of faulted winds Wk, power network is represented by the EDF En behind resistance ZN. Let’s use superposition method [4] representing real regime as combination of two partial regimes, in our case prefault and fault. Schematic, representing the fault regime is shown on the Fig. 4.
Fig. 3. Equivalent schematic of one transformer’s leg
Fig. 5. Simplified schematics of fault regime
Should be noted, that winding’s parts вс and вс', as well as ad and ad' are bifilar and may be represented by only active resistances and it is possible to simplify Fig. 4 schematics coming to Fig.5. Where
R’ = 2Rad+Rk (D
and
R’’=2Rd+R, (2)
By separation of active resistance Rk of short-circuited winds Wk and considering voltage Ue equal to U Fig. 5 a may be further simplified to Fig. 5b, where Ze marks equivalent resistance of network and transformer relative to Wk. Let’s introduce parameter
1
Wad + TWk
« =-----—--------, (3)
Wad + Wk + Wcb
with meaning of relative distance from the start point of winding to the point of failure. Thus
R'=2a • Rv (4)
and
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R" = 2 (l-a) Rv, (5)
where Rv denotes active resistance of single windings wire. For winding made of m parallel wires
R = R, • m, R' = 2a • m • R, (6)
and
R" = 2 (l-a) • m • Rab. (7)
Current in the place of fault may be found as
Z +-----
R'+R" (8)
Uk
Ze+a(\-a)2m-Rab
The topmost value of I corresponds to
2(Wad+Wk+Wcb)
1
(9)
and (l-a ), smallest value is cc^ =— i. e. fault is
v max'’ 111Ш O’
in the middle of the winding. The fault component IN, which determines operation of differential protection may be found as
/ ■W
z„=^. (I0)
According to the works of Alexander Zasypkin [5] calculus of IN currents for transformers produced by ZTZ factory with rated power from 25 to 1000 МВА yielded IN in range from 6% to 30% of transformer’s nominal current. Meanwhile the fault current is not less than (a+(1-a)) • Inom and poses a real danger for transformer. Thus for safe protection against incomplete winding faults protection device must be capable to detect differential current from such a low value as 0.01% from nominal. Must be mentioned, that failing to detect incomplete winding fault timely will lead to intercoil insulation burn-off and transition from incomplete winding fault to the full coil fault between coil levels or coil - coil fault with typical IN varying from 10% to 150% of nominal current (see Fig. 6).
Fig. 6. Transition from incomplete winding fault to the complete winding fault (a) and coil-coil fault (b)
III. Current transformer’s errors and unbalance current
Let’s discuss a steel core based current transformer loaded with load resistance Zs (see Fig. 7). In case of
ideal transformer secondary current is proportional to
,, Г-W
the primary as 7 = —— but in case of real transformer w
non-linearity of transformer steel should be taken into consideration.
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Fig. 8. Magnetizing curve for steel 3411
Chart on Fig. 8 shows typical magnetizing curve (flux density B as function of magnetizing force H) for type 3411 transformer steel. The dependency curve is highly non-linear at beginning (H from 0 to 40-45 A/M corresponding to flux densities below 0.15 Tesla), then curve shows relatively low disturbance until saturation at levels above 1 Tesla. Error characteristics of current transformer with core made from such steel will be close to the ABB produced current transformer’s characteristics (see Fig. 9) [6].
Good current transformer design exploits linear areas of core magnetizing curve in the most optimal way. Nowadays two main types of current transformers are produced - for purposes of relay protection and for measurements which are mostly tailored to the needs of commercial metering. Measurement transformers are available with precision class 0.2 and 0.5 and maximal measurable current up to double nominal. Transformer for relay protections needs are produced with error classes 5P and 10P meaning error rate 5
Fig. 9. Error boundaries for current transformers
and 10 % correspondently. Measurable current for such type of transformers is up to 40 times of nominal. As a test object transformer from widely used series TPL-10 with nominal current 75A was selected. Design of this transformer incorporates two separate cores - one for protection and another for measurement purposes, molded in hard epoxy enclosure with common primary winding. Factory characteristics of TPL-10 are shown on Fig. 10, curve 1 [9]. As long as difference between measuring and protective core measures in parts of percent direct measuring of currents will not produce trustable results, since error of best ammeters (0.2% from 5A) is compatible with measurable difference. To solve this problem differential schematic was selected (See Fig. 11). Both cores are loaded with equal resistance of 4.5 Ohm which corresponds to allowable by precision current up to 150 A. Sensitive ammeter measures differential current, assuming measurement core as reference we can think this current as some
Secondary load, Ohm
Fig. 10. Factory characteristics of TPL-10 type current transformers
form of protective core errors estimation. Chart on the Fig. 12 shows this differences current in dependence from primary current. As reader can see, protective
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core has quite moderate error in the area of currents under nominal, but even a 2-3% of error seriously limits protection ability to discover incomplete winding fault timely until it transits to the more severe damage. Modern protective devices do incorporate some technics to adjust to limitations caused unbalance current, one of such technics is to use functional restrain characteristics like one shown on Fig. 13, where necessary to trip differential current is changed accordingly to absolute value of load current, but such technics can’t solve problem of unsatisfactory protection sensitivity limited by current transformers error.
Fig. 11. Schematic, used to measure difference between current transformer’s cores
Fig. 12. Difference between secondary currents of measurement and protection cores in dependency of primary current
Significant improvement in sensitivity may be high current and will saturate putting into jeopardy the achieved by usage of measuring cores, but such usage whole operation of protective device. is not allowable because such cores are not designed for
Fig. 13. Restrained operational characteristics of differential protection
Significant improvement in sensitivity may be IV. Usage of measuring current transformers achieved by usage of measuring cores, but such usage for error correction
is not allowable because such cores are not designed for As mentioned, simple usage of metering cores for high current and will saturate putting into jeopardy the protection purposes is not allowable due to unavoidable whole operation of protective device. saturation, such saturation often considered as
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necessary to protect measuring equipment from damage by overcurrent. But information obtained from measurement current transformers may be used (of coarse when such information is relevant) to correct main current transformer’s errors. The microprocessor based device may “teach” itself by forming some error correction table in it’s memory on the basis of core outputs compare during normal operation. In case of fault device may operate with much higher sensitivity thus sensing fault on it’s early stage.
Let’s discuss proposed technics on example of developed by RTU transformers protection device [10].
Simplified structure of device is presented on the Fig. 14. Currents from primary and secondary side of protected power transformer are accepted by the set of intermediate current transformers 1а-1с for primary side and 1d-1e for secondary, after that current is transformed into proportional voltage and filtered by two sets of analogue filters 2а-2с and 2d-2f Voltages from
A phase from both sides of protected transformer are received thru intermediate transformers 1g and 1h and are filtered by analogue filters 2g and 2h. All analogue signals are converted to digital form by ADC module 3, later digital filtration is incorporated. Numerical filters 4a and 4c are used to separate orthogonal components of fundamental harmonics from primary and secondary side currents, filters 4в and 4d are separating second harmonics components from currents. Maxi-selector 6 is used to determine larger amplitude and if it is above threshold level m2max comparator 7 produces binary signal m2. Threshold constant is never less, than 0.17 from absolute value of primary current value li'l, produced by module 10. Module 5 in connection with orthogonal components filter 4f produces real value of power transformer transformation ratio, allowing to compensate transformation ratio changes occurring when taps are changed by voltage regulator.
Module 8 perform scaling of currents accordingly to
YAT'
I'a
1a
Vtv-1
TA' a, в C 1
YAT''
Q
TA",
TV,
;3
B
■3
C
■3
v
I'a
2a 3
1
■3
■3
C
•3
TV2" —►
B
Гс
Fig. 14. Simplified structural schematic of protective device
''
A
K
I
4f
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the real transformation ratio found by module 5. Scaled thru current transformer is above 120% - 150% from the orthogonal components of currents are processed by the nominal preventing “poisoning” of correction table by set of three main characteristics modules 9a, 9b and 9c. data from saturated measurement core. After some time in In absence of m2 signal each module 9 realizes restrained operation corrections table containing values of correction trip characteristics combined from three lines. Module of multiplier KC as tabular function from current’s value is vectorial subtraction \i - ktl"\, is used as a tripping value and formed in the non- violent memory of device and device sum of current absolutes \1'\ + \ktl"\ as a restraining. Such may start operate using corrected values of currents I' and design makes possible to easily adopt protective device to I". Should be noted, that correction table contains discrete unbalance current of particular current transformers set. values of KC and device needs an interpolation module Special role is assigned to the binary signal m2 Presence of able to produce value of KC any intermediate value of this signal marks the surge of magnetizing current as shown current. To ensure minimal trip time of protective device by Alexander Drozdov in his work [7] during magnetizing interpolation module must operate in the shortest possible currents surge second harmonics amplitude is no less, than time, that’s why a linear interpolation model in the space 0.17 from amplitude of fundamental harmonics. In case ofl , Re (K), Jm (K) was chosen. of m2 goes high main characteristics module switches to V. Conclusions
the low sensitivity characteristics in form of straight line. Usage of measuring transformers as correction source Trip of any of modules 9 sends signal to the output relays for main current transformers allows to reduce “appearing” module 12 and formation of disconnection impulse for value of unbalance current from 2-3% to 0.2-0.3% from power switches of protected transformer. nominal. As result protective device will become much
Proposed upgrade of this device (see Fig. 13) adds to more sensitive towards inter-winding fault, maintaining the protection’s structure (of Fig. 12) additional current good robustness level in case of other types of fault. channels 1 i, j, k and 1 l, m, n, additional analogue filters
set 2 i, j, k and 2 l, m, n. Also additional channels are added Список литературы
to the ADC module 3 and fundamental harmonicsc filters 1. Федосеев А.М. Релейная защита электро-4 д and 4 h are also added. Such additions will allow to энергетических систем: Учебное пособие для вузов obtain information about measuring transformers currents [Текст] / А.М. Федосеев; 2-е изд., перераб. и доп. -in form of orhogonal components a, b, c and i"a, i"b, i"c. М.: Энергоатомиздат, 1992. - 528 с.
Under the normal condition protective device is 2. Вольдек А.И. Электрические машины: Учеб-“learning”, the comparisement module 13 is forming a ник для студентов высш. техн. учебн. заведений corrections tabular function Kc = f(\i\), this module has two [Текст] / А.И. Вольдек. - 3-е изд., перераб. - Л.: inhibit inputs, one is activated by the m2 signal preventing Энергия, 1978. - 832 с.
“ bad learning” caused by the magnetizing current surge. 3. Лейтес Л.В. Электромагнитные расчеты
The second inhibiting input is activated by the signal трансформаторов и реакторов [Текст] / Л.В. Лейтес. from threshold module 14 and stops “learning” if current - М.: Энергия, 1981 г. - 392 с.
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4. Атабеков Г.И. Основы теории цепей [Текст] / Г.И. Атабеков. - М.: Лань, 7-е изд., 2009. - 592 с.
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6. ABB Instrument transformer application guide. - ABB, 2011.
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1. Fedoseev A.M. Relejnaja zashhita elektrojenergeticheskih sistem: Uchebnoe posobie dlja vuzov [Tekst] / A.M. Fedoseev. - 2-e izd., pererab. i dop. - M.: Energoatomizdat, 1992. - 528 s.
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Исмагилов Ф.Р.
Ismagilov F.R.
доктор технических наук, профессор, заведующий кафедрой «Электромеханика» ФГБОУ ВПО «Уфимский государственный авиационный технический университет», Россия, г. Уфа
Максудов Д.В. Maksudov D. V.
кандидат технических наук, доцент кафедры «Электромеханика» ФГБОУ ВПО «Уфимский государственный авиационный технический университет», Россия, г. Уфа
Гареев А.Ш.
Gareev A.Sh.
аспирант кафедры «Электромеханика» ФГБОУ ВПО «Уфимский государственный авиационный технический университет», Россия, г. Уфа
УДК 621.31.015.2
СПОСОБЫ УМЕНЬШЕНИЯ НЕГАТИВНОГО ВЛИЯНИЯ ПРОВАЛОВ НАПРЯЖЕНИЯ НА ПОТРЕБИТЕЛЕЙ
Обеспечение качества электроэнергии в сетях общего назначения представляет собой важнейшую проблему электроснабжения. Одним из показателей качества электроэнергии является глубина и дли-
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