Transformer Practical Turn-to-Turn Fault Detection ... - IPAPS

13th International Conference on Protection & Automation

in Power System

January 8 and 9, 2019

Sharif University of Technology, Tehran

Transformer Practical Turn-to-Turn Fault Detection

Performance using Negative Sequence and Space

Vector-Based Methods

Nima Farzin1, Mehdi Vakilian1, Ehsan Hajipour1

1Department of Electrical Engineering

Sharif University of Technology

Tehran, Iran

[email protected], [email protected], [email protected]

Abstract—Transformer turn-to-turn fault (TTF) is one of the

most difficult failures to detect. The negative sequence

percentage differential current (NSPD) and extended park

vectors approach (EPVA) are the two most promising methods

for detection of the TTF inception. In this paper, in order to

evaluate the performance of these methods, various experimental

tests are carried out to detect the turn-to-turn fault initiation on

the winding of the transformer under different operating

conditions such as: no-load operation, when an external fault or

an open conductor fault is also occurred. The results show that

the setting of the proper thresholds plays the main role in the

reliable and secure performance of NSPD and EPVA-based

methods. Moreover, the experimental results verify that the

selection of a fixed threshold for these detection methods can be

very difficult. Even if the adjustment of the fixed threshold is

possible, it may endanger the reliable and secure performance of

the related protection methods.

Keywords-negative sequence protection; reliability; security;

space vector; turn-to-turn fault;

I. Introduction

Transformers are one of the most important elements in electrical power systems that play a key role in transmission, distribution, and utilization of the electrical energy. The transformer winding fault takes a great participation among various reasons of transformer failures [1]. A turn-to-turn fault (TTF) quite often begins with degradation of the winding insulation between two adjacent turns, but it may lead to an instantaneous or progressive large coil deterioration, which will finally cause the transformer costly damages [2].

The percentage differential relay is the most commonly used protection against the internal faults for transformers of 10 MVA and higher ratings [3]. However, the conventional differential relay is not sensitive enough to detect TTFs in its incipient stage [4]. Alternatively, the sudden pressure relay is traditionally used to detect internal faults of transformers. The sudden pressure relay can detect TTFs with a typical delay of 50-100 ms, which leads to more serious damages to the transformer [3]. Therefore, this relay is too slow to detect TTFs [5].

To overcome these drawbacks, in recent years several techniques have been applied to detect the internal faults

inception such as negative sequence-based methods [6-9], zero sequence-based methods [10], differential equation-based methods [11], wavelet-based methods [12], and hardware-based methods [13]. A brief review of the above-mentioned methods including their operation bases, advantages, disadvantages, and ambiguities have been presented in [14].

Among these methods, negative sequence-based methods are prominent and they have remained the subject of research until the present day [6-9]. Even two commercially available transformer protection relays [7-8], utilized from the negative sequence current protection. The relay described in [7], uses the negative-sequence percentage differential (NSPD) current element which is calculated by the vector addition of secondary and primary currents of the transformer. Whereas, negative sequence directional algorithms such as [8], uses the primary and secondary negative sequence currents along with their phase difference. However, since the impact of internal fault on the phase difference between secondary and primary negative current is ignorable compared to the effect of external fault, the directional algorithm is prone to be unable to detect the turn-to-turn faults in the presence of external faults [15].

In addition to the NSPD, [15] introduces a promising algorithm to detect TTFs based on the application of the space-vector theory to the differential current of power transformers. The so-called Extended Park’s Vector Approach (EPVA) offers enhanced sensitivity to detect TTF inception [15].

It seems that both of the negative-sequence percentage differential and extended park’s vector approach methods have promising features to detect low-level turn-to-turn faults. It should be noted that both of these methods must be blocked during inrush currents and current transformer saturation [15].

In this paper, various experimental tests carried out to evaluate the performance of the NSPD and EPVA-based methods. The purpose of this paper is to show the performance of both methods under different conditions such as in the presence of the external fault, turn-to-turn fault, and open conductor faults. Moreover, the experimental results represent that the selection of a fixed threshold can be very difficult. Even if the adjustment of a fixed threshold is possible, it may endanger the reliable and secure performance of these protection methods.

mailto:[email protected]

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2RESI

Internal Fault

2OPI

2

Min

OPI

tanSLP

2

Min

RESI

II. NSPD and EPVA Protection Algorithms

Since the turn-to-turn fault occurs typically only in one of the transformer windings, TTF is considered as an asymmetrical fault. Both of the negative-sequence percentage differential and extended park’s vector approaches introduce some criterions derived from the unbalanced three-phase currents caused by TTF inception. Their protection algorithms are described in the following sections.

A. Negative Sequence Percentage Differential Protection

NSPD algorithm is proposed in detail in [9]. Fig. 1 represents the single-slope characteristic curve of the negative-sequence current differential protection. In this method, similar to the conventional percentage differential protection, restraining current ( 2RESI ) and operating current ( 2OPI ) are

calculated by using the negative-sequence currents from

primary ( 2pI ) and secondary ( 2sI ) sides of the transformer

as follows:

2 2 2

2 2 2

OP p s

RES p s

I I I

I I I

where subscripts p and s refer to the values of primary and secondary sides of transformer and subscript 2 indicates the negative sequence component of the currents. Moreover, subscripts OP and RES denote the operating current and restraining current of the algorithm.

Figure 1. The operating characteristic of the negative-sequence percentage

differential.

A fault is detected as an internal fault if the negative-sequence operating current is greater than the negative-sequence restraining current multiplied by the slope of the single-slope characteristic curve ( 2 2OP RESI I SLP ).

Moreover, the operating current must be greater than the

minimum pickup current ( 2MinOPI ). This constraint is essential to

prevent false tripping due to minor imbalances.

The edge of this operating region is shown in Fig. 1 by a

horizontal line defining 2MinOPI and a straight line with slope

SLP which passes through the origin. Therefore, minimum

restraining current ( 2MinRESI ) can be defined as the ratio of

2MinOPI to the SLP .

B. Extended Park’s Vector Approach Protection

EPVA-based protection is fully described in [16]. In this method Park’s transformation is applied to the transformer

three-phase differential current ( , ,dA dB dCi i i ) and Park’s vector

component ( ,D Qi i ) obtained as:

2 1 1

3 6 6

1 10

2 2

dAD

dBQ

dC

ii

ii

i

In addition, the Park’s vector module ( PVI ) defined as the

magnitude of the differential space vector as follows:

2 2

PV D QI i i

In the normal operating condition of the transformer, PVI

just has a DC component ( ( )PV DCI ), whereas, in the case of

the turn-to-turn fault initiation, an AC component at twice the

supply frequency ( (2 )PV fI ) also appears. The existence of

(2 )PV fI is directly related to the asymmetries in the

transformer [16] and can be used as the operating signal if the percentage differential principle is applied. By applying the above Park’s transformation to the classic restraint differential

current ( res p sI I I ) [9], (2 )RPV fI obtained and can be

used as the restraining signal.

III. Experimental Performance

Evaluation

In order to investigate the performance of NSPD and EPVA-based methods, some experimental tests are carried out on a three-phase transformer (2kVA, 400/400 V, 50 Hz, three-leg core), which is a specially designed transformer to perform turn-to-turn fault tests. This custom-built transformer has been equipped with various externally accessible taps on both HV and LV sides at desired intervals (based on the percentage of turns). By using these taps, turn-to-turn faults with different percentages (1 to 25 % of the whole number of turns) are applied to the windings of the transformer through a combination of a switch and a variable fault resistor. Fig. 2 illustrates the differential currents when turn-to-turn faults with different percentage and different fault resistor occur. It can be seen that the severe fault inception in which more turns with smaller fault resistor is involved, leads to a higher increase in the magnitude of differential current. Therefore, detection of fault initiation is more difficult under occurrence of minor TTFs. This is the main reason why the conventional differential relay cannot protect the transformer against low-level internal faults (under 6% internal faults).

The overall experimental set-up circuit is shown in Fig. 3. Six current transformers (CTs) are provided to measure the


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transformer primary and secondary currents during a TTF initiation. These currents are recorded by two digital storage scopes and Discrete Fourier Transformation (DFT) with a sampling frequency of 1.6 kHz (32 samples per cycle in a 50 Hz power system) is employed to extract the currents phasor. Transformer load is a three-phase fully resistive load.

Moreover, after performing various experimental tests, 2MinopI

and (2 )RPV fI are determined to be 0.15 A and 0.17 A (almost

5% of the transformer rated current), respectively. The slopes of EPVA and NSPD characteristic curve are determined to be constant values of 0.12 and 0.14, respectively.

Figure 2. The differential currents during turn-to-turn fault inception with

different percentages and different fault resistors.

Figure 3. The experimental Test Setup

In order to evaluate the performance of NSPD and EPVA-based methods, security and reliability of these methods should be checked. Reliability means that they can detect minor TTFs under different operating conditions and security means that any disturbances other than TTF should not lead to malfunction of these methods. In the following, different scenarios are investigated to evaluate the performance of these TTF detection methods.

A. Turn-to-Turn Fault Inception

In this section, a turn-to-turn fault is initiated on the secondary side winding of the ∆-Yn transformer. Fig. 4 depicts the recorded currents under the fault initiation between two adjacent winding turns (1% of the winding turns). The transformer supplies a purely resistive load equivalent to 50%

of its rated capacity. The current in the shorted turns is limited to 3 times the rated current of the faulted winding.

Fig. 4(a) and Fig. 4(b) respectively represent the recorded primary and secondary currents of the transformer. Internal fault inception leads to a low-level increase in the current of primary lines connected to the faulted phase (phase A and C in Fig. 4(a)). On the contrary, changes on the secondary currents are ignorable (as shown in Fig. 4(b)). Fig. 4(c) illustrates the instantaneous differential currents which significantly increased due to the TTF inception. Fig. 4(d) and Fig. 4(e) respectively present the performance of the EPVA-based protection and NSPD method. In these figures, red squares show the steady-state operating points of the transformer after the fault initiation and dash-line circles represent the estimated margin for the location of the steady-state operating points. As it can be seen, by proper adjustment of the characteristic curves, NSPD and EPVA-based method can clearly identify the fault after 16.2 and 19.4 ms, respectively.

Figure 4. Transformer currents during 1% turn-to-turn fault inception: (a)

primary currents, (b) secondary currents, (c) instantaneous differential

currents, (d) performance of the EPVA-based method, (e) performance of

NSPD.

B. Internal Fault Inception on a No-load Transformer

In this section, a 1% internal fault is applied to the primary winding of the ∆-Yn transformer under a no-load condition. Fig. 5(a) depicts the recorded primary currents of the transformer during the TTF inception. The primary currents are


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equal to the differential currents, due to the fact that the secondary currents are zero in a no-load condition. As it can be seen in Fig. 5(a), TTF initiation leads to an increase in the magnitude of the differential currents. Fig. 5(b) and Fig. 5(c) respectively present the performance of the EPVA-based protection and NSPD method. In this case, since secondary currents are zero, 2 2op resI I according to (1) and

(2 ) (2 )PV f RPV fI I according to the definition provided in [9].

Therefore, as it can be seen in Fig. 5(b) and Fig. 5(c), operating points of the no-load transformer lie on a line with slope 1. Moreover, by proper adjustment of the characteristic curves as shown in Fig. 5(b) and Fig. 5(c), the transformer operating points after the turn-to-turn fault initiation are located inside the internal fault zone and both methods detect the TTF incipient after 18.2 ms.

Figure 5. Transformer no-load currents in the presence of 1% turn-to-turn

fault: (a) instantaneous differential currents, (b) performance of the EPVA-

based method, (c) performance of NSPD.

C. Turn-to-Turn Fault Due to The External Fault Initiation

In this section, a 1% turn-to-turn fault is applied to the primary winding of the transformer in the presence of a line to line external fault. The recorded results of this experimental test are presented in Fig. 6. Fig. 6(a) and Fig. 6(b) respectively represent the recorded primary and secondary currents of the faulted transformer. As shown on the left side of Fig. 6(b), due to the external fault initiation on the “a” and “c” phases of the secondary windings, their currents increase and gain almost the same large magnitude, however they are in reverse direction. While, the current in the healthy phase (phase “b”) remains unchanged compared to its normal operating current. As it is expected, and is shown in the left side of Fig. 6(a), due to the inception of line to line fault, the magnitude of primary current in one phase equals 2 times of the current in the other two phases. As it can be seen in the right hand side of Fig. 6(a), the changes in the secondary and primary currents due to the internal fault initiation are insignificant if compared to the currents in the presence of external faults. Fig. 6(c) illustrates the instantaneous differential currents. It can be seen that in

contrast to the line currents, TTF initiation leads to an observable change in the differential currents.

The TTF detection operating points under external fault are shown in Fig. 6(d) and Fig. 6(e), for the two methods, respectively. As it can be seen, steady-state operating points of the transformer in case of line to line fault are located inside the external fault zone. Due to the TTF initiation, operating points go to the internal fault zone. Therefore, NSPD and EPVA-based both method detect the TTF inception after 22.2 and 24.4 ms, respectively. However, since the transformer operating points are too close to the edge of the characteristic curve, it may endanger the performance of this algorithm. Therefore it is very important to precisely determine these algorithms thresholds. More details on the thresholds selection are provided in section III.F.

Figure 6. Transformer currents in the presence of 1% turn-to-turn fault

caused by a line to line external fault inception: (a) primary currents, (b)

secondary currents, (c) instantaneous differential currents, (d) performance of

the EPVA-based method, (e) performance of NSPD.

D. TTF Inception in The Presence of Open Conductor Fault

Open conductor fault acts as a high impedance fault and characteristically has a very low current value [17]. Open conductor fault can also be considered as the worst case of an asymmetrical load. In order to evaluate the reliability of NSPD and EPVA-based methods in case of asymmetrical load, a 1% turn-to-turn fault is applied to the primary windings of the ∆-Yn transformer in the presence of open conductor fault. Fig. 7


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presents the results of this case. Fig. 7(a) and Fig. 7(b) respectively depict the transformer primary and secondary currents. As shown on the left side of Fig 7(b), due to the open conductor fault inception, the faulted phase current decreases to zero.

In addition, as it can be seen in the left side of Fig 7(a), since due to the open conductor fault initiation, the magnitude of the current in one phase of the load becomes zero, two of primary currents decrease to the almost 0.57 of their pre-faulted current and other one remains unchanged. As shown in Fig. 7(c), the turn-to-turn fault initiation leads to a slight increase in the magnitude of differential currents. As depicted in Fig. 7(d) and Fig. 7(e), internal fault initiation changes the operating points of the transformer. By proper adjustment of the characteristic curves, both methods clearly identify the internal fault initiation after 13.1 ms.

Figure 7. Transformer currents in the presence of 1% turn-to-turn fault with

open conductor fault: (a) primary currents, (b) secondary currents, (c)

instantaneous differential currents, (d) performance of the EPVA-based

method, (e) performance of NSPD

E. Security against The External Faults

As it can be seen from the Fig. 6 and Fig. 7, the steady-state operating points of the transformer in the presence of an external fault and open conductor fault are in the external zone of NSPD and EPVA-based methods. However, in order to validate the security of these methods, the transient of transformer operating points between normal operations to the

faulted ones should be investigated. Fig. 8(a) and Fig. 8(b) respectively depict the operating points of the transformer for NSPD and EPVA-based method, in case of external line to line fault and open conductor fault inception. It can be seen from Fig. 8(b) that TTF detection operating points enter the internal fault zone and stay only for a very short period of time (3 samples for the line to line fault and 5 samples for open conductor fault). However, this can lead to the false operation of the relay. To overcome this problem, in order to accept a fault as an internal or external fault, this decision must be confirmed several times in succession (“security count”) [6].

Although using security count is essential, but it can be effective only if the thresholds of characteristic curves (red line in Fig. 8(a) and Fig. 8(b)) are correctly determined. In other words, thresholds adjustment plays the most critical role in the performance of NSPD and EPVA-based methods.

Figure 8. operating points of the transformer in case of external fault and

open conductor fault inception: (a) performance of NSPD, (b) performance of

the EPVA-based method.

F. Determination of The Thresholds

Generally, there is a tradeoff between reliability and security of the protection methods. Therefore, it can be so difficult to set the proper thresholds which help these methods to demonstrate more reliable and secure performance. In order to select the thresholds of the NSPD and EPVA-based

protection method ( 2MinopI and (2 )RPV fI along with their slope),

various experimental tests are implemented on a ∆-Yn transformer. In these tests, a 1% TTF is applied to the transformer under the following operating conditions: 1) normal operating condition (N), 2) in the presence of an open conductor fault (OC), 3) in the presence of line to line (LL), single line to ground (LG) and three-phase external faults (LLL), 4) power transformer no-load condition (NL).

Fig. 9 presents the results of these tests. Steady-state operating points of the transformer before TTF inception and after that are respectively shown with a dash-line circle and full line circle. The characteristic curve, which is shown with the red line in Fig. 9, should pass between dash-line circles and full line ones to be able to precisely discriminate between the


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faulted and un-faulted conditions. As shown with the red dash.dot lines in Fig. 9(a) and Fig. 9(b), the acceptable range

for 2MinOPI is between 0.10 A and 0.23 A and for Min

PV(2f)I is

between 0.1 A and 0.17 A. Whereas, the acceptable slope values in both methods confide between 0.10 and 0.13. This narrow range denotes that these studies should be performed for each individual transformer.

In addition, it can be so difficult (even impossible) to set a fixed characteristic curve which is enough for the reliable and secure performance of NSPD and EPVA-based protection methods. This is due to the fact that closeness of the pre-fault operating points (dash-line circles in Fig. 9) to the edge of the characteristic curve can endanger the security of these methods. Similarly, the closeness of the post-fault operating points (full line circles in Fig. 9) to the edge of the characteristic curve can threat the reliability of the protection method. Therefore, it seems that using adaptive thresholds for different operating conditions can lead to a better performance for these methods.

Figure 9. Steady-state operating points of the transformer (before turn-to-

turn fault inception and after that) in various operating conditions of the ∆-Yn

transformer: (a) NSPD Method, (b) EPVA-based method.

IV. Summary and Conclusion

. In this paper, in order to evaluate the performance of the negative sequence percentage differential current and extended park vector approach, various experimental tests are implemented on a ∆-Yn transformer. These tests include a 1% TTF on the winding transformer under the different operating conditions such as normal operating condition, in the presence of an open conductor fault, in the presence of phase external faults, and under a no-load condition. The following results can be deducted from the carried out experimental results:

Using security count can prevent the false trip of the NSPD and EPVA-based methods in case of external faults inception.

The security count can be effective only if the thresholds of TTF detection characteristic curves

( 2MinopI and (2 )RPV fI along with their slope) are

determined correctly.

It can be very difficult to set a fixed threshold for these protection methods to precisely discriminate between the faulted and un-faulted conditions.

Even if the selection of a fixed threshold is possible, it may endanger the reliability and security of these protection algorithms.

REFERENCES

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[2] IEEE guide for protecting power transformers, IEEE Std. 37.91, 2008.

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