7/29/2019 A NEW APPROACH FOR TRANSFORMER DIFFERENTIAL PROTECTION http://slidepdf.com/reader/full/a-new-approach-for-transformer-differential-protection 1/15 A NEW APPROACH OR TRANSFORMER ROUND IFFERENTIAL ROTECTION Dr. Tevfik Sezi Siemens Power Transmission and Distribution, LLC Distribution Automation Division P.O. Box 29503 Raleigh, NC 27626-0503 USA Abstract-Existing electromechanical ground differential protection relays are impaired in the event of CT aturation. They might trip when the fault is external (or not trip when the fault is internal) unless the configuration and settings are designed very carefully. In addition, inrush effects can also cause wrong protection behavior. Simulations and field observations have revealed that the phase angle difference between the ground current and zero sequence current, in combination with the ratio of their magnitudes, can be used to identify precisely a transformer ground fault. These observations were used for the development of a new numerical transformer differential protective relay. Simulations and test results have shown that the new solution correctly detects a wider range of phenomena that would indicate an internal fault, while remaining able to not trip in the event of an external fault. Ke y Words-Power distribution protection, power system protection, power transformer protection, power transmission protection,protection, protective relaying. I. INTRODUCTION This paper describes a new approach for transformer ground differential protection, also known as restricted ground fault protection. The algorithm described has been implemented in a new numerical transformer differential relay to obtain better protection coverage for transformers and shunt reactors than the classical solutions. Extensive simulations and field tests have proven the reliability of the implemented algorithms. The new solution does not require any external auxiliary CTs, and the settings are very simple. 11. CLASSICALOLUTIONS Phase-current differential protection schemes for transformers are not sensitive enough to detect an internal phase-to-ground fault if the fault is located near the neutral point of the transformer. Also, it is difficult to detect a ground fault if the transformer is resistance- or reactance-grounded, since the ground current will be limited. One classical solution for detecting an internal ground fault is to use a high-impedance differential-current relay (Fig. 1). This solution is also often used as a compromise solution for providing differential protection to a grounded delta-wye transformer bank when no delta-side CT's are available (or A B C Fig. 1. Conventional Ground Differential Prot ection Sc heme Using a High-Impedance Differential Relay. convenient). This is a common situation for distribution and industrial ties with the delta as the high-voltage side and protected by fuses. An alternative classical solution is to use a directional overcurrent relay or a product relay. This is often done if the characteristics or CT ratios of the CTs are not suitable for using a high-impedance differential relay. This solution is particularly applicable when the ground current is limited or when a sensitive ground CT is used. Fig. 2 shows two different operating principles that use a directional overcurrent relay. In one case, an auxiliary current balancing autotransformer is used, in the other case an auxiliary l:N current transformer. 1
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7/29/2019 A NEW APPROACH FOR TRANSFORMER DIFFERENTIAL PROTECTION
A NEWAPPROACHOR TRANSFORMERROUNDIFFERENTIALROTECTION
Dr. Tevfik Sezi
Sieme ns Power Transmission and Distribution, LLCDistribution Automation Division
P.O. Box 29503Raleigh, NC 27626-0503 USA
Abstract-Existing electromechanical grou nd differentialprotection relays are impaired in the event of CT aturation.They might trip when the fault is external (or not trip when thefault is internal) unless the configuration and settings aredesigned very carefully. In addition, inrush effects can alsocause wrong protection behavior. Simulations and fieldobservations have revealed that the phase angle differencebetween the ground current and zero sequence current, incombination with the ratio of their magnitudes, can b e used to
identify precisely a transformer grou nd fault. Theseobservations were used for the development of a new numericaltransformer differential protective relay. Simulations and testresults have shown that the new solution correctly detects awider range of phenomena that would indicate an intern al fault,while remaining able to not tr ip in the event of a n external fault.
Ke y Words-Power distribution pro tect ion, power systemprotection, power transformer protection, power transmissionprotection,protection, protective relaying.
I. INTRODUCTION
This paper describes a new approach for transformer
ground differential protection, also known as restricted
ground fault protection. The algorithm described has been
implemented in a new numerical transformer differentialrelay to obtain better protection coverage for transformers
and shunt reactors than the classical solutions. Extensive
simulations and field tests have prov en th e reliability of the
implemented algorithms. The new solution does not require
any external auxiliary CTs, and the settings are very simple.
11.CLASSICALOLUTIONS
Phase-current differential protection schemes for
transformers are not sensitive enough to detect an internal
phase-to-ground fault if the fault is located near the neutral
point of the transformer. Also, it is difficult to de tect a grou nd
fault if the transformer is resistance- or reactance-grounded,
since the ground current will be limited.One classical solution for detecting an internal ground fault
is to u se a high-impedan ce differential-current relay (Fig. 1).
This solution is also often used as a compromise solution for
providing differential protection to a grounded delta-wye
transformer bank when no delta-side CT's are available (or
A B C
Fig. 1. Conventional Ground Differential Protection Scheme Using a
High-Impedance Differential Relay.
convenient). This is a common situation for distribution and
industrial ties with the delta as the high-voltage side and
protected by fuses.
An alternative classical solution is to use a directional
overcurrent relay or a p roduct relay. This is often done if the
characteristics or CT ratios of the CTs are not suitable for
using a high-impedance differential relay. This solution is
particularly applicable when the ground current is limited orwhen a sensitive ground CT is used. Fig. 2 shows two
different operating principles that use a directional
overcurrent relay. In one case, an aux iliary curren t balancing
autotransformer is used, in the other case an aux iliary l:N
current transformer.
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7/29/2019 A NEW APPROACH FOR TRANSFORMER DIFFERENTIAL PROTECTION
Directional Overcu rrent Relay withAuxilialy Current Transformer
,I
,8
Fig. 2.Ground Differential Protection Scheme Using a Directional Overcurrent Relay with Either anAuxiliary CT (left dashed-line box) or an A utotransformer (right dashed-line box).
If the directional overcurrent relay solution is used, therelay has a directional unit that operates as a product unit.
The overcurrent unit itself is non-directional and operates
only in response to the amplitude of the current. In Fig. 2, it isshown as the coil without an indicated polarity. This non-
directional unit has an inverse time characteristic, but
operates only if the directional unit operates.
In an y classical solution, the relay operates if the product
of the amplitude of the ground current, the amplitude of the
zero sequence current, and the cosine of the phase angle
between the two currents exceeds a certain limit. For any
particular current amplitudes, the m aximum operating torque
occurs if the phase angle between the two currents is O",
while the maximum restraining torque occurs if the phase
angle is 180". Zero torque occurs at k90". With the classical
protection scheme, detailed consideration must be given to
ensuring that the relay w ill operate correctly even if no zerosequence current is present [11.
111. THENEW LGORITHM
The new, low-impedance ground differential protectionalgorithm is based on Kirchoff's law. The inform ation
provided to the algorithm is sampled values of the phasecurrents and the ground current.
Using the known phase and ground CT ratio information
(specified as relay settings), the sampled current values are
normalized relative to the nominal current of the protected
transformer winding, In. This simply means that the unit of
measure for all currents is In, not amperes. Then, the
quantities used by the algo rithm are calculated:
A . Calculated Quantities
The restraining current, ZR , is the scalar sum of the separate
amplitudes of the measured phase and grou nd currents. It is a
measure of the total amount of current flowing through the
transformer, regardless of whether the currents are balanced.
It is calculated according to equations (1) an d (2):
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7/29/2019 A NEW APPROACH FOR TRANSFORMER DIFFERENTIAL PROTECTION
C.H. Einvall and J.R. Linders, “A Three-phase DifferentialRelay for Transformer Protection,” IEEE Transactions onPAS, Vol. PAS-94, No. 6, Nov/Dec 1975.
W.A. Elmore, editor, Protective Relaying Theory andApplications. New York: Marcel Dekker, 1994.
L.F. Kennedy and C.D. Hayward, “Harmonic-Current-Restrained Relays for Differential Protection,” AlEETransactions, Vol. 57, pp. 262-266, 1938.
O.P. Mal& P.K. Dash, and G.S. ope, “Digital Protection ofPower Transformer,” Paper No. A76 191-7 IEEE PES 19 76Winter Power Meeting , New York.
C.A. Mathews, “An Improved Transformer DifferentialRelay,” AlEE Transactions, Vol. 73, Part 111, pp. 645-650,1954.
J.A. Sykes, “A New Technique for High-speed TransformerFault Protection Suitable for Digital ComputerImplem entation,” IEEE paper No. C72 429-9, Summer PowerMeeting of PES, 1972.
J.A. Sykes and I.F. Morrison, “A Proposed Method forHarmonic-Restraint Differential Protection for PowerTransformers,” IEEE Transactions on PAS, Vol. PAS-9 1, No .3, pp. 1260-1272,1972.
VII. BIOGRAPHY
Dr. Tevfik Sezi (M’ 997) was born
in 1953 in Adana, Turkey. He studiedpower electronics at the Technical
University of Berlin (Germany),
obtaining his Ph.D . (Dr.-Ing.) in 1 985
after being an assistant professor there
from 198 0 to 1985. His research
areas have included frequency
variable drives, protection algorithms,
and optimized software structures for
protective relays. He has been with
Siemens since 1985, working as a development engineer for
protective relays from 1985 to 1996, and was responsible for
the relay development department between 1993 and 1996.
He holds several patents on protection algorithms. Since
August 1996 he has been in the United States as ProductManage r for Protective Relays.
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7/29/2019 A NEW APPROACH FOR TRANSFORMER DIFFERENTIAL PROTECTION