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Prototype of a Negative-Sequence Turn-to-Turn Fault Detection Scheme for Transformers
Mr.Umakant Kothoke P G Student
Electrical Engineering Dept.
SSGBCOET, Bhusawal [email protected]
Prof.Ajit P. Chaudhari Associate Professor Electrical Engineering Dept.
SSGBCOET, Bhusawal [email protected]
Prof.Girish K Mahajan Associate Professor Electrical Engineering Dept.
SSGBCOET, Bhusawal [email protected]
ABSTRACT
In this paper, Negative Sequence Component scheme for analysis of inter-turn winding fault detection at initial level has been discussed. Inter-turn faults are the most difficult types of faults to detect within the power transformer. When small percent of turns are shorted, the sensitivity of current differential protection is limited so it does not cause false tripping due to normal imbalances in current. Digital relays are capable of computing the negative sequence current on both primary and secondary sides of the transformer along with the phase difference between these two negative sequence currents. Algorithm discussed is faster and more sensitive than conventional differential protection and capable of detecting inter-turn faults occurring during transformer energization.
The mathematical analysis is illustrated and compared with simulation studies performed with MATLAB/ SIMULINK..
Index Term
Digital protection, transformer protection.
1. INTRODUCTION
The electric power transformer is a vital component of the modern AC power grid. Invented at the end of the nineteenth century, transformers have allowed for the more efficient production of electrical power. Today‟s large scale power systems are very complex networks, formed by large power generation & transmission-distribution systems to supply generated energy to end users. The protection system should be planned to manage with any adverse expected conditions like faults, dielectric stress and, over load condition in order to withstand the accessibility of electrical energy supply.
As a technology that has been refined for more than a hundred years, large power transformers are rarely the cause of a power outage. But when a failure does occur, the results can be disastrous. An arc burning inside a large, oil filled transformer produces hydrogen gas. The time between the striking of an arc and catastrophic failure depends on the type of fault and the size of the fault involved [1]. One of the leading causes of an arc is an inter-turn fault. Such a fault can develop when the insulation between turns breaks down. Insulation can deteriorate due to excessive heat, oxidation, the acidity of the insulating oil, and the presence of moisture in the insulating oil. Electrical faults, occurring external to the transformer, also contribute to the aging of the transformer's insulation. The mechanical stresses accumulated during years of external faults can weaken the transformer's insulation to the point where the insulation dielectric-withstand strength fails and a turn-to-turn fault occurs. [2].
Fig.1 Inter-turn fault at primary side of Transformer Turn-to-turn faults are particularly insidious in that they can avoid the transformer's protection system until much damage has been done. There can be a large current flowing between small groups of shorted windings while there is little change in the magnitude of the current at the transformer's terminals.
Currently, the best defense against such faults is the sudden pressure relay, overcurrent protection, differential relay. This relay will signal a trip before the arcing, inside the transformer's tank, releases enough gas to cause an explosion.
There are various methods to spot a TTF in a transformer on the basis of terminal voltage and current. Such as V-I locus diagram, Dissolve Gas Analysis (DGA), Doernenburg Ratio Method (DRM), Frequency response Analysis (FRA), Reverse Voltage Measurement Technique (RVMT),on-load exciting current extended Park‟s Vector, Leakage flux, Zero Sequence Component and Negative Sequence Component etc [2]. The Negative Sequence Component is more sensitive to sense minor ITF and its action is based on evaluating the magnitude and phase of Negative Sequence Component like current, voltage and phase angle [3].
Different method of negative sequence based relay operated area against CT saturation, magnetizing inrush etc. is implemented and all this is probably without an addition of any cost as it is based on CT signals which are freely existing [10].
2. LITERETURE SURVEY.
2.1. Transformer Theory
In the power system, a Transformer is a stationary machine with the pair or more windings which are extensively used.
Also, a transformer is crucial and the most valuable part of a power system. The transformer work as a channel among the generation and the transmission system. Protection of transformer thus become the complication for the engineers.
2.1.1 Working Principle of a Transformer
The relationship between the induced voltage and the flux is given by reference to Faraday‟s law which states that its magnitude is proportional to the rate of change of flux linkage, and Lenz‟s law which states that its polarity is such as to
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oppose that flux linkage change if the current were allowed to flow. If the circuit of the later winding is closed, there must be a current flowing through it. Whenever alternating current is applied to an electric coil, there will be an alternating flux surrounding that coil. Now if another coil is kept near the first one, there will be an alternating flux linkage with that second coil. As the flux is alternating, there will be obviously a rate of change in flux linkage with respect to time in the second coil [6].
Fig. 2. Model diagram of single phase transformer
2.2 Different Failures in Transformer
Any number of conditions has been the reason for an electrical transformer failure. Statistics show that winding failures most frequently cause transformer faults. Different faults can occur inside the transformer and at the electrical system where the transformer is connected. The transformer protection is an essential part of overall system protection strategy.
Furthermore, transformers have a wide variety of features, including tap changer, phase shifters and multiple windings that require special consideration in the protective system design. The combination of electrical and non-electrical protection system is installed to protect the transformer due to those all possible faults. To reduce the effects of thermal stress and electrodynamics forces, it is advisable to ensure that the protection package used minimizes the time for disconnection in the event of a fault occurring within the transformer.
There are two ways in which transformer faults are classified:
External faults- developed at exterior of transformer:
Faults due to overvoltage
Faults due to under frequency
External system short circuits.
Internal faults- which are develop at inner of transformer, such as:
Incipient fault: Incipient faults grow gradually into sever faults which its origin remains undetected and corrected. (overheating, over fluxing, overpressure)
Active faults: In situation where there is current flow of current from one phase to other then there is occurrence of active fault like turn-to-ground, TTF, tank fault, and core fault.
Insulation deterioration, often the result of moisture, overheating, vibration, voltage surges, and mechanical stress created during transformer through faults is the major reason for winding failure. The detection of internal ITF has become difficult in a case where transformer protection is concerned [7].
2.3 Existing Techniques for Protection of a Transformer
Survey and study indicate that since the time when first available and commonly used method is differential relay is the only method which provides better protection against internal fault but for application of this type of protection scheme transformer rating must be approximately above 10 MVA below this rating. Even though the differential relay is the universally used and very typical method used for safeguarding of transformer, although it is inadequate for recognizing less number of inter-turn winding snags in power transformers. It is challenging to identify the obscure ITF of winding with the help of differential relay as a result of the change in transformer terminals current will be considered poor due to transformation ratio between healthy and faulty turns as shown in Figure 2.3 (Inter-turn fault) is entirely poor.
A digital relaying algorithm was used to detect transformer winding faults in single and three phase transformer. To restrain the relay during magnetizing inrush algorithm works without using the presence of harmonic currents [10].
Gas Accumulation Relays
Sudden Pressure Relays
Buchholz Devices
Oil analysis
Differential Relays
Table 1 Various Abnormal Conditions and their protection system
Abnormal condition Protective relying scheme Incipient faults: TTF and
phase-earth faults lower oil level resulting in corrosion of oil.
1. Buchholz relay sounds alarm
2. Sudden pressure relay 3. Pressure relief valve Through faults (faults beyond
protective zone)
1. HRC fuse
2.Time graded over current relay
High voltage surges due to lightening switches
1. Horn gaps 2. RC surge arrestor 3. Surge arrestors
Overloads 1.Thermal overload relays
2. Temperature relays sound alarm
2.4 Negative Sequence component
For the analysis of the “normal” differential currents the calculation of the purely negative sequence differential currents is used as same coefficient matrices [10]. For an internal fault (at the point of fault with the imaginary source in negative sequence) the negative sequence currents leave the unhealthy transformer on both sides as shown in Fig. 3 The currents on the particular (Y/Y) sides of the power transformer have the similar path as shown in Fig. 3. Three-phase faults do not contain NSC, so they require a different 20 approach.
Negative sequence network are basically equal to positive sequence network except for the absence of source and different phase shifts in some connection of the power transformer [17] The Negative Sequence Component is more sensitive to sense minor ITF and its action is based on evaluating the magnitude and phase of Negative Sequence Component like current, voltage and phase angle. In detection of ITF in transformer with help of NSC of Current, the calculation of system has been prepared for different operating
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conditions, different number of shorted turns and different connection of power transformer. It observed that even when 2% of turns are shorted, the arrangement will work properly [17].
Fig.3 Improved protection method using (NSC) for Inter- turn fault
There is no existence of Negative Sequence current in case of symmetrical 3 phase fault. But when unsymmetrical faults are concerned they exist for time for relay to come to a proper conclusion. The major advantage of NS current to Zero Sequence current is its ability to provide coverage for phase- to-phase failures as well as TTF along with ground failures.
The method can be successful only when NSC that are on the sides of the transformer are considerable. This method works subject to the relative position of the two phasors. Moreover in simpler terms we can state that this techniques compares the phase shift between the Negative Sequence Current on both the primary as well as secondary side of transformers.
Table 2. Comparative analysis of conventional differential protection and Negative sequence component
Criteria Conventional Differential protection
Negative Sequence Component Relibility for
uneuqal load, no fault,
unequal source
Stable Stable
Turn to-Turn fault dignosis for 1 p.u.fault current
10-15% faulty turns
3-5 % faulty turns
Turn to Turn fault dignosis of dead short circuit
At 10 no. of turns At 2 no. of turns
Precision and swiftness
As compare with negative sequence component fails to detect fault at initial level
At initial level fault detection is accurate and fast
3. SYSTEM DEVELOPMENT
The major advantage of NS current to Zero Sequence current is its ability to provide coverage for phase-to-phase failures as well as TTF along with ground failures.
In order for the successful operation of the NSC method of Inter-turn fault detection, the development of Inter-turn faults on the low-voltage side of the transformer will be examined. In a situation when there is a short circuit of turns in one phase the impedance of this phase is difficult to compare with the other phase. There is no existence of Negative Sequence current in case of symmetrical 3 phase fault. But when unsymmetrical faults are concerned they exist for time for relay to come to a proper conclusion.
The method can be successful only when NSC that are on the sides of the transformer are considerable. This method works subject to the relative position of the two phasors. Moreover in simpler terms we can state that this techniques compares the phase shift between the Negative Sequence Current on both the primary as well as secondary side of transformers. When NSC exists beyond the normal unbalances it proves that there is a fault in transformer winding.
3.1. Block Diagram
A load is connected to the transformer to be protected so as to achieve bidirectional current flow. The NS fault detection component is included in the method that has been implemented here. The extraction of magnitude is done using FFT block. The FFT helps in evaluating the magnitude of harmonics as well as input signal phase in terms of time. If the NSC on both sides is more compared to the previously set level then comparison based on the direction is done and the phase angle between the two NSC is found.
The following Fig. 4 shows block diagram using NSC for inter-turn Fault protection. It consist of input load which is measure by V-I Measurement block.Two transformer are connected in system in which one is faulty and other is healthy transformer. The fault created in faulty transformer at primary side of winding. For detection of this fault using logic circuit the NSC relay logic is used which find the magnitude of current, voltage, and phase angle between them. The phase and magnitude signal block gives waveform for magnitude of primary as well as secondary NS Current and phase angle between NS Current at primary and secondary side.
Figure 4 Block diagram for Inter-turn Fault Protection using Negative Sequence Component.
431 3.2 Introduction to the NSC Method for
Protection of Transformer
There is no existence of Negative Sequence current in case of symmetrical 3 phase fault. But when unsymmetrical faults are concerned they exist for time for relay to come to a proper conclusion. The major advantage of NS current to Zero Sequence current is its ability to provide coverage for phase- to-phase failures as well as TTF along with ground failures.
The method can be successful only when NSC that are on the sides of the transformer are considerable. This method works subject to the relative position of the two phasors.
Moreover in simpler terms we can state that this techniques compares the phase shift between the Negative Sequence Current on both the primary as well as secondary side of transformers.
The theory of symmetrical components is used to derive algorithm for the new TTF detection based on NS currents.
From the theory of symmetrical components we can deduce that:
Distribution of Negative sequence currents can be done through the negative sequence network.
At the point where fault occurs the NSC source is E2= -I2 Z2
The Kirchhoff‟s first law is followed by NSC.
3.3. Simulation Analysis
Fig. 4 .Simulation model for protection of ITF using NSC Fig.4 shows the MATLAB model of improved protective relying scheme using Negative Sequence Component. The
MATLAB model is simulated with two transformer for three phase 50 MVA, 132/66kV, 50 Hz, and Y/Y connected transformer. One transformer is healthy and other is faulty transformer. To the primary side of transformer three phase AC supply takes 132 kV. The nominal voltage to secondary side of transformer gives 66 kV which is connected to the three phase load.
The model is simulated for 1) Unprotected System and 2) For protection System using NSC for different percentages of ITF as 2 %, 3%, 5% and 10%.The fault is created at phase „A‟ with Inter-turn short circuit winding for different percentages.
4. PERFORMANCE ANALYSIS 4.1 Simulation Results
In this paper, the protective relaying scheme for ITF in winding of transformer using negative sequence component system is simulated in MATLAB/SIMULINK. Results are given for unprotected system and for protection using negative sequence component system due to occurrence of 2 %, 3%, 5%, and 10% of inter-turn fault respectively.
The system operates under steady state condition prior to the occurrence of fault.The new method being focused on NSC, there is a need to observe NS current for different percentages of ITF
.
4.2 Unprotected system
As inrush current is not a faulty state, the protection of transformer is unchanged in inrush condition. Fig. 5 shows the inrush currents in unprotected system. When primary side CB1 is closed at 0.5 sec, an inrush current flows in primary winding of transformer and no current flows through secondary side of transformer and gives a trip signal as shown in Fig.6 which received at 0.5 sec but it is not analyzed by the fault.
Fig. 5 Waveforms of effect of ITF on inrush current with unprotected system for (a) phase A , (b) phase B, and (c) phase respectively
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Fig. 6 Trip signal with unprotected system
4.3 Protection Using Negative Sequence Component
4.3.1 For 2 % IT
Fault is created and security of algorithm is tested. In Fig.7 (a) significant change in primary negative sequence current takes place at 0.15 sec due 2 % of ITF inside the protected zone and Fig. 7 (b) shows secondary Negative sequence current.
Figure 8 (a) shows magnitude of comparison of primary as well as secondary NS current and Fig 8 (b) shows phase angle for difference between primary as well as secondary NS current which is less than 1800. For 2% of IFT Fig.9 shows that a trip signal is received at 0.15 sec and the fault is analyzed up to 0.2 sec.
Fig .7 Simulation result for 2 % Inter-turn fault, shows (a) Negative Sequence current of primary winding (b) Negative Sequence current of secondary winding.
Fig 8 Simulation results of 2% Inter-turn fault for changes between primary and secondary NS current of (a) Magnitude (b) phase angle
Fig. 9 Trip signal for 2 % of ITF
4.3.2 For 3 % ITF
Figure 10 (a) and (b) gives simulation result for negative sequence current of primary and secondary winding for inter- turn fault of 3% at 0.145 sec. Fig 11 (a) shows magnitude and phase angle between primary and secondary negative sequence current. For 3% of IFT, fault is analyzed from 0.145 sec to 0.156 sec in Fig.12.
Fig. 10 For 3 % Inter-turn fault, simulated result shows (a) Negative Sequence current of primary winding (b) Negative Sequence current of secondary winding
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Fig.11 Simulation results of 3% Inter-turn fault for changes between primary and secondary NS current of (a) magnitude (b) phase angle
Fig. 12 Trip signal for 3 % of ITF
4.3.3 For 5 % ITF
Fig. 12 (a) and (b) gives simulation result for NS current of primary and secondary winding respectively at 0.125 sec. for 5
% of IFT Fig 13 (a) shows magnitude and phase angle changes between primary and secondary NS current. Fig. 14(c) a trip signal received at 0.125 sec and fault is analyzed up to 0.130 sec.
Fig.12 Simulation results for 5 % Inter-turn fault, simulated result shows (a) Negative Sequence current of primary winding (b) Negative Sequence current of secondary winding
Fig. 13 Simulation results of 5% Inter-turn fault for changes between primary and secondary NS current of (a) Magnitude (b) phase angle
Fig. 14 Trip signal for 5 % of ITF
4.3.4 For 10 % ITF
Fig. 15 (a) and (b) gives simulation result for negative sequence current of primary and secondary winding respectively, for 10 % of IFT at 0.10 sec. Fig 16 (a) shows magnitude and phase angle between primary and secondary negative sequence current. For 10% of IFT fault is sensed at 0.1 sec and analyzed up to 0.132 sec as shown in Fig. 17 (d).
Fig. 15 Simulation results of 10 % Inter-turn fault for (a) Negative Sequence current of primary winding (b) Negative Sequence current of secondary winding.
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Fig. 16 Simulation results of 10% Inter-turn fault for changes between primary and secondary NS current of (a) Magnitude (b) phase angle
Fig.17 Trip signal for 10 % of ITF
The Table 3 shows the results obtained due to mathematical analysis as well as simulation in MATLAB. Also the difference in these two measurements is given. From the table a minimum difference is observed and thus we can say that the results that are obtained are according to our expectations.
Table.3 Comparison of Mathematical Analysis and Simulation Results
From table 4 it seen that for 2% of IFT a trip signal is received
at 0.15 sec, for 3% of IFT fault is analysed from 0.145 sec, a trip signal received at 0.125 sec for 5% of ITF, and . For 10%
of IFT fault is sensed at 0.10 sec
Table 4 Trip Signal Time for Various Percentages of Faulted Turns
% Turns Faulted Trip Signal Time (s)
2% 0.15
3% 0.145
5% 0.125
10% 0.10
CONCLUSION
The comparison of NSC in a differential relay accumulates sensitiveness to identify ground faults with large resistance and ITFs. At the time of an internal fault with large resistance, there is not significant change in phase current. If we apply NSC, it improves the sensitivity of detecting faults for different percentage of fault. It is beneficial to various applications in power system as the negative-sequence currents are almost faultlessly in phase for internal faults and out of phase for outside faults. Likewise, it gives more scope to high resistance ITF.
Simulation results for different percentages of Inter-turn Fault are discussed in this paper. ITF fault for various percentages as 2 % ITF, 3 % ITF, 5 % ITF, and 10 % ITF are represented and discussed in detail. The results are satisfactory for all cases.
Using Negative Sequence Component, the value of current and phase angle is obtained to identify ITF on related winding. The algorithm is given in which comparison of primary and secondary current with NSC trip signal is carried out. This relaying scheme i.e. NSC is stable and very sensitive to offer better fault detection capability and that too without any additional sensor requirement.
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