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Abstract— This paper presents the impact of waveform

distortion of Thyristor-Controlled Series Capacitor (TCSC) operating as a Fault Current Limiter (FCL) on transmission line distance protection. Although the poor waveform and the high valve stress of TCSC in inductive boost mode is less attractive for steady state operation, it is more desirable for limiting the fault current by increasing the inductance of the line higher than the parallel combination of the capacitor and the reactor. This mode of TCSC operation is accompanied by high current distortion which can seriously affect the commercial distance relay functionality not suitably designed for such circumstances. Simulation results show the extent of waveform distortion. A real relay widely applied in power systems is used for testing. The results indicate poor behavior of the relay when TCSC operates as a FCL. Index Terms-- Distance relay, Flexible Alternating Current Transmission Systems (FACTS), power system protection, protective relaying, Thyristor-Controlled Series Capacitor (TCSC).

I. INTRODUCTION

EW fault current limiter (FCL) technologies has been gaining more interest in recent years as the need for these

devices has noticeably been increased [1]-[2]. Thyristor Controlled Series Capacitors (TCSCs) are being used for control of the equivalent impedances of transmission lines, and therefore of the power flow in a network [3]. Meanwhile, TCSC has a remarkable fault current limiting feature. During normal operation, it generally operates in the capacitive region. During system faults, it operates in the inductive region to lower the fault current contribution.

TCSC, like most of the new FCL technologies investigated today, will cause a distorted (i.e., non-sinusoidal) fault current. Only a few FCL technologies, which temporarily insert passive elements, such as resistive or inductive impedances into the circuit, cause non-distorted fault currents. This current distortion significantly affects the protection system [4]. A few publications discuss various impacts of TCSC [5] on the protection system using conventional techniques regarding the rapid changes introduced by the associated TCSC control actions in primary system

Mojtaba Khederzadeh is with the Department of Electrical Engineering,

Power & Water University of Technology (PWUT), Postal Code: 1658953571 Tehran, IRAN. (E-mail: khederzadeh@pwut.ac.ir).

parameters such as line impedances and load currents. In these investigations, the most important singularity lays in the fact that the positive sequence impedance measured by traditional distance relays is no longer an indicator of the distance to a fault. The apparent reactance and resistance seen by the relay are affected due to the uncertain variation of series compensation voltage during the fault period.

When TCSC operates in FCL mode the fault current amplitude is reduced as desired but also waveform distortion is caused undesirably. It is important to carefully examine these impacts with respect to protection relay functionalities. This is a distinguished issue other than the distance relay overreaching caused by insertion of TCSC in the fault loop. It can be considered as an independent point caused by waveform distortion even if the relay confronts with the overreaching effect by some delicate solutions.

In this paper fault current distortion caused by TCSC in FCL mode is simulated and analyzed. Simulation results show the extent of waveform distortion, and then a real commercial distance relay widely used in power systems is tested by injecting distorted signals based on the results obtained from simulations. The results indicate poor behavior of the relay when TCSC operates as a FCL.

II. TCSC OPERATION IN FCL MODE

A. TCSC Model

The basic conceptual TCSC module comprises a series capacitor, C, in parallel with a thyristor-controlled reactor, LS. However, a practical TCSC module also includes protective equipment normally installed with series capacitors, as shown in Fig. 1. A metal-oxide varistor (MOV), essentially a nonlinear resistor, is connected across the series capacitor to prevent the occurrence of high-capacitor over-voltages. Not only does the MOV limit the voltage across the capacitor, but it allows the capacitor to remain in the circuit even during fault conditions and helps improve the transient stability. A circuit breaker is also installed across the TCSC module to bypass it if a severe fault or equipment malfunction occurs. A current limiting inductor, Ld, is incorporated in the circuit to restrict both the magnitude and the frequency of the capacitor current during the capacitor bypass operation.

B. TCSC Modes of Operation

In normal operating conditions, there are four modes of operation; blocking mode; bypass mode; capacitive boost

Waveform Distortion Impact of TCSC in FCL Mode on Transmission Line Protection

Mojtaba Khederzadeh, Senior Member, IEEE

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978-1-4244-4241-6/09/$25.00 ©2009 IEEE

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mode; and inductive boost mode [3], [5]. When the thyristor valve is not triggered and the thyristors

are kept in non-conducting state, the TCSC is operating in blocking mode. In this mode, the TCSC performs like a fixed series capacitor.

In bypass mode the thyristor valve is triggered continuously and the valve stays conducting all the time; so the TCSC behaves like a parallel connection of the series capacitor with the inductor, Ls, in the thyristor valve branch. In this mode, the resulting voltage in the steady state across the TCSC is inductive and the valve current is somewhat bigger than the line current due to the current generation in the capacitor bank. For practical TCSCs with XL/XC ratio between 0.1 to 0.3 range, the capacitor voltage at a given line current is much lower in bypass than in blocking mode. Therefore, the bypass mode is utilized as a means to reduce the capacitor stress during faults.

Fig. 1: A practical TCSC module.

In capacitive boost mode a trigger pulse is supplied to the thyristor having forward voltage just before the capacitor voltage crosses the zero line, so a capacitor discharge current pulse will circulate through the parallel inductive branch. The discharge current pulse adds to the line current through the capacitor and causes a capacitor voltage that adds to the voltage caused by the line current. The capacitor peak voltage thus will be increased in proportion to the charge that passes through the thyristor branch. The fundamental voltage also increases almost proportionally to the charge. From the system point of view, this mode inserts capacitors to the line up to nearly three times the fixed capacitor. This is the normal operating mode of TCSC.

In inductive boost mode the circulating current in the TCSC thyristor branch is bigger than the line current. In this mode, large thyristor currents result and further the capacitor voltage waveform is very much distorted from its sinusoidal shape. The peak voltage appears close to the turn on. The poor waveform and the high valve stress make the inductive boost mode less attractive for steady state operation. This mode increases the inductance of the line, so it is in contrast to the advantages associated with the application of TCSC. It is only desirable for limiting the fault current. The maximum inductive operating conditions are limited by high circulating currents and TCSC voltage. If the TCSC is applied to an existing transmission line, one choice for fault control strategy

is to make the total impedance of the path (XEQ) equal to the original equivalent impedance before the TCSC addition; thereby, the fault current contribution remains the same during system faults. This important feature is essential to TCSC application on power systems with fault currents close to or exceeding the circuit breaker interrupting capabilities. It may also be possible to reduce the fault current below original levels by selecting a larger XEQ. At the limit, the thyristors can be fired so that the equivalent reactor reactance is equal to the capacitor reactance, pulling the TCSC into parallel resonance mode where the fault current is completely blocked. However, larger XEQ means a higher voltage rating and therefore a higher price for the TCSC [6].

In [7], TCSC design requirements are tabulated and clearly mentioned that the purchaser should specify the required sequences of faults, dynamic overload, temporary overload, and continuous currents for the TCSC bank, and also the power system fault currents, the type of bypass that can occur during internal and external faults, and the desired post-fault control mode(s), reactance range, and voltage across the TCSC bank. These sequences form the duty cycles that all of the components of the TCSC bank should be designed to withstand. The duty cycle should be consistent with the manner in which the surrounding power system will be operated for both internal and external line faults. The purchaser should define duty cycles for faults of normal and extended duration and for faults of different types (multiple and single phase). It means that there is no unique behavior of the TCSC’s in the compensated lines, and line protective relay may respond completely differently because of the different design criteria. Depending on the pre-fault and fault conditions, TCSC transits from the existing mode to one of the possible modes based on its control system strategy.

III. SIMULATION RESULTS

A. Sample System

The sample system shown in Fig. 2 is used for simulation. A TCSC is placed on a 500kV, long transmission line, to improve power transfer. The TCSC is bypassed during the first 0.5s of the simulation. The nominal compensation is 75%, i.e. assuming only the capacitors (firing angle of 90°). The natural oscillatory frequency of the TCSC is 163Hz, which is 2.7 times the fundamental frequency. The test system is described in [8]. The TCSC can operate in capacitive or inductive mode. Since the resonance for this TCSC is around 58° firing angle, the operation is prohibited in firing angle range 49°-69°. The resonance for the overall system (when the line impedance is included) is around 67°. The capacitive mode is achieved with firing angles 69°-90°. The impedance is lowest at 90°.

The inductive mode corresponds to the firing angles 0°-49°, and the lowest impedance is at 0°. In the inductive operating mode, the range of impedances is 19-60 Ohm. The inductive mode reduces power transfer over the line. The TCSC is simulated as a controllable voltage source in each

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phase. The voltage magnitude is the product of equivalent complex impedance and the line current [3], [8].

Fig. 2: Sample network used for simulation

B. Waveform Distortion in FCL Mode

A single-phase fault is simulated at the 75% of the line AB. Fig. 3 shows the current waveform measured at the relaying point R1 near bus A in sample system. Fig. 4 shows the amplitude of 3rd harmonic measured at relaying point R1. The simulation time is 3s, the fault is applied at 1.0s and is cleared at 1.2s.

0 0.5 1 1.5 2 2.5 3-6

-4

-2

0

2

4

6

Time [s]

Fau

lt C

urre

nt [

kA]

Fault Current Measured by Relay R1 [kV]

Fig. 3: Fault current measured at relay point R1 due to a single-phase fault at 75% of line AB with TCSC in FCL mode.

0 0.5 1 1.5 2 2.5 30

10

20

30

40

50

60

70

80

90

100

Time (s)

Har

mon

ic A

mpl

itude

(%

)

3rd Harmonic Amplitude of Fault Current at Relay R1 [%]

Fig. 4: 3rd harmonic amplitude of fault current measured at relay point R1 in % due to a single-phase fault at 75% of line AB with TCSC in FCL mode.

It is worth noting that operation of MOV is observed during the fault period to alleviate the voltage stress on TCSC module. TCSC is bypassed during the first 0.5s of the simulation. Although Total Harmonic Distortion (THD) is a criterion to show the extent of waveform distortion, it is a cumulative index and does not clearly show the impact of each individual harmonic on the protective relay functionality, so it is decided to extract each harmonic separately and analyze the relay behavior for that harmonic. This method could more useful since the impact of harmonics on the relay performance, even if not generated by TCSC, would be clarified.

Fig. 5 shows the magnitude of the 3rd harmonic of voltage measured with the same procedure. It is deduced from Figs. 4 and 5 that the 3rd harmonic amplitude is higher for current than the voltage.

Fig. 6 shows the THD of fault current measured at the relaying point R1. During the insertion of TCSC and also during the fault period it is remarkable.

0 0.5 1 1.5 2 2.5 30

5

10

15

20

25

Time (s)

Har

mon

ic A

mpl

itude

(%

)

3rd Harmonic Amplitude of Fault Voltage at Relay R1 [%]

Fig. 5: 3rd harmonic amplitude of fault voltage measured at relay point R1 in % due to a single-phase fault at 75% of line AB with TCSC in FCL mode.

0 0.5 1 1.5 2 2.5 30

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Time [s]

TH

D o

f F

ault

Cur

rent

Total Harmonic Distortion (THD) of Fault Current at Relay R1

Fig. 6: Total Harmonic Distortion (THD) of fault current measured at relay point R1 due to a single-phase fault at 75% of line AB with TCSC in FCL mode.

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0 0.5 1 1.5 2 2.5 30

0.05

0.1

0.15

0.2

0.25

0.3

Time [s]

TH

D o

f F

ault

Vol

tage

Total Harmonic Distortion (THD) of Fault Voltage at Relay R1

Fig. 7: Total Harmonic Distortion (THD) of fault voltage measured at relay point R1 due to a single-phase fault at 75% of line AB with TCSC in FCL mode.

0 0.5 1 1.5 2 2.5 30

10

20

30

40

50

60

70

80

90

100

Time [s]

Har

mon

ic A

mpl

itude

[%

]

5th Harmonic Amplitude of Fault Current at Relay R1 [%]

Fig. 8: 5th harmonic amplitude of fault current measured at relay point R1 due to a single-phase fault at 75% of line AB with TCSC in FCL mode.

0 0.5 1 1.5 2 2.5 30

1

2

3

4

5

6

Time [s]

Har

mon

ic A

mpl

itude

[%

]

5th Harmonic Amplitude of Fault Voltage at Relay R1 [%]

Fig. 9: 5th harmonic amplitude of fault voltage measured at relay point R1 due to a single-phase fault at 75% of line AB with TCSC in FCL mode.

Fig. 7 shows the THD for voltage. It is less than the THD of current, but still significant in fault period. Figs. 8 and 9 show the amplitude of 5th harmonic for current and voltage, respectively. They show less severity for 5th harmonic than 3rd harmonic. This is true for both current and voltage. The amplitude of 7th harmonic of current and voltage waveforms during the same fault are shown in Figs. 10 and 11, respectively.

0 0.5 1 1.5 2 2.5 30

10

20

30

40

50

60

70

80

90

100

Time [s]

Har

mon

ic A

mpl

itude

[%

]

7th Harmonic Amplitude of Fault Current at Relay R1 [%]

Fig.10: 7th harmonic amplitude of fault current measured at relay point R1 due to a single-phase fault at 75% of line AB with TCSC in FCL mode.

0 0.5 1 1.5 2 2.5 30

0.5

1

1.5

2

2.5

3

3.5

Time [s]

Har

mon

ic A

mpl

itude

7th Harmonic Amplitude of Fault Voltage at Relay R1 [%]

Fig.11: 7th harmonic amplitude of fault voltage measured at relay point R1 due to a single-phase fault at 75% of line AB with TCSC in FCL mode.

IV. TEST RESULTS

In order to evaluate the behavior of the transmission line distance relay under waveform distortion caused by TCSC operation in FCL mode, a commercial relay is used. This relay is widely used in different power systems. Fig. 12 shows the front of the relay. This is a computer-based relay with the capability of connection to external PC.

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Fig. 12: Front side of the distance relay used for testing

As can be deduced from Figs. 4 and 5, the amplitude of 3rd

harmonic is remarkable, hence, in order to check an extensive range of voltage and current distortion, a test system is set up according to Fig. 13. The relay test set is Freja 300 model from Programma Company.

This relay test set has 3 current output channels with the accuracy of ±1mA and three voltage channels with the accuracy of ±10mV. Current and voltage channels are used as inputs for testing the sample distance relay.

Initially, the relay settings are selected as KZPH=1, KZ1=1, KZ2=2, KZ3=10, KZ0=15, KZn=1, KZ'3=0.1 KZ'6=1.5, θn=70° and θph=75°. For the 1st and 2nd zones, mho characteristics are used which have been set at 5Ω and 10Ω with θline=75°, respectively. 3rd zone is offset-mho with 50Ω forward and 5Ω reverse settings. Operating times are 300ms and 1000ms for 2nd and 3rd zones, respectively. For this testthe injected current is 1∠282.5° and the voltage is 9.49∠0°, i.e., a point in the 1st zone.

Table 1 shows the result for voltage and current waveform distortion. |V| and |I| stands for the amplitude of different voltage and current harmonics added to the fundamental component with relative zero degrees. The results are for a single phase fault, which the relay operates after 22 ms when pure sine waves are injected.

As can be deduced from this table the operating time has gradually increased from 22 ms (1st zone) to 341 (more than the operating time of 2nd zone). For the current distortion alone, first column is used. It shows the prolongation of the operating time with respect to increasing the harmonic.

Fig. 13: Test system set-up using a relay test set capable of generating different harmonics.

Table 1: Distance Relay Operating Times for Different 3rd Harmonic Contents (Angle between Signals=0°)

3rd Harmonic Evaluation Distance Relay Operating Time in milliseconds

|V| |I|

0% 1% 2% 3% 4% 5% 10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

0% 22 53 52 52 52 52 52 53 53 52 52 52 52 52 53 52 1% 52 52 53 52 54 62 52 52 53 52 53 52 53 53 54 53 2% 54 53 52 53 52 52 52 53 52 53 52 54 53 53 52 53 3% 52 52 54 52 52 53 58 63 47 52 48 48 53 47 52 48 4% 52 62 53 63 54 58 57 57 58 57 57 57 52 52 53 59 5% 54 63 47 47 47 59 58 58 58 58 87 58 58 68 87 57

10% 77 340 341 341 340 341 340 340 340 340 339 339 341 341 341 341 20% 87 287 340 340 341 340 341 341 339 341 339 340 340 341 340 330 30% 147 339 341 341 340 340 340 340 340 341 340 340 340 340 340 340 40% 227 329 338 339 339 340 339 340 341 340 340 341 341 341 338 341 50% 148 168 339 339 341 340 339 339 341 340 340 341 341 340 339 340 60% 117 341 339 339 340 341 340 338 341 338 340 341 341 341 339 320 70% 228 247 339 340 340 341 339 339 341 339 341 339 342 341 341 341 80% 88 340 341 341 339 340 340 339 340 340 340 341 341 341 341 341 90% 147 340 341 340 341 339 340 340 341 339 341 341 340 340 341 341

100% 138 340 340 341 340 340 340 341 340 340 340 340 340 340 340 341

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Table 1 clearly shows that during a fault period, when TCSC transfers to FCL mode, the relay may operate falsely in 2nd zone instead of 1st zone due to 3rd harmonic component in current and voltage signals.

Table 2 shows the relay operating time in the presence of 5th harmonic. Again, |V| and |I| stands for the amplitude of different voltage and current harmonics added to the fundamental with relative zero degrees. The results are for a combination of voltage and current harmonics simulating a single phase fault. According to the column with 0% voltage distortion, the operating time has gradually increased from 22 ms (1st zone) to 341 ms (more than the operating time of 2nd zone) and finally to no operation for harmonics more than 20% of the fundamental. It shows the impact of 5th harmonic on relay malfunction. According to Fig. 8, 5th harmonic of current with amplitudes higher than 20% is probable for the

sample system when TCSC operates in FCL mode. The relay would not trip correctly in these conditions. As Table 2 shows, the relay does not operate when current has more than 30% 5th harmonic and for any % of voltage harmonic. Needless to say, the relay should operate in nearly 22 ms at 1st zone.

An interesting point in Table 2 is the malfunction of the relay for even 1% of 5th harmonic in current when the voltage contains more than 1% 5th harmonic. This part of the test has been repeated 5 times in order to reduce the testing error, but the result was the same in all cases.

Another remarkable point in Table 2 is the variance of relay operating times for 20% harmonic content and different % of voltage harmonics. These variations in relay operating times indicate that the relay operation in these areas is not reliable.

Table 2: Distance Relay Operating Times for Different 5th Harmonic Contents (Angle between Signals=0°)

Note: * NO stands for No Operation of the relay.

As can be deduced from Table 2, for 10% 5th harmonic

in current, the relay operated in 2nd zone up to 20% 5th harmonic in voltage, but the operating time decreased significantly for higher % of harmonic in voltage. It can be interpreted as counter effects of voltage and currents harmonics in some instances. Comparison of Tables 1 and 2 reveals that the impact of 5th harmonic on relay functionality is more severe than 3rd harmonic. This depends on the signal conditioning methods used in the relay, which is not clear for the user.

Table 3 shows the results obtained for the 7th and 9th harmonics. According to Figs. 10 and 11, the amount of 7th harmonic is not so high for the sample system, so the harmonics are injected up to 10% in contrast to 3rd and 5th harmonics which are up to 100%. The results are better than the 3rd and 5th harmonics, but there are some points

that need more attention. For 1% 7th harmonic in current and for any % of harmonic in voltage, the relay does not operate. This can only be justified that the relay is very sensitive for this amount of 7th harmonic in current waveform. The other operating times in Table 3 are nearly 2-3 times the 1st zone. This can be interpreted as delayed operation of the first zone.

The test has been repeated for odd harmonics up to 19, the results are not presented for the sake of brevity. As can be seen from the above results, the harmonic contents in voltage and current has forced the relay to operate in a longer time. In order to see if the harmonics can force the relay to operate faster than settings, a few operating points has been selected in the 2nd and 3rd zones and the relay operating times were measured. The results indicate the relay delayed operation than the normal case.

5th Harmonic Evaluation Distance Relay Operating Time in milliseconds

|V| |I|

0% 1% 2% 3% 4% 5% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100%

0% 23 57 52 52 52 52 52 63 58 58 59 57 59 59 58 58 1% 53 NO NO NO NO NO NO NO NO NO NO NO NO NO NO NO 2% 52 355 52 53 52 52 43 52 62 61 61 59 58 58 59 58 3% 60 60 61 60 61 61 51 52 61 60 62 60 61 60 60 61 4% 60 60 60 60 61 60 61 62 61 60 60 61 61 60 61 61 5% 59 51 59 60 60 61 60 61 61 62 50 61 61 52 51 52

10% 342 43 339 342 338 342 327 328 34 33 91 31 85 33 49 44 20% 341 44 341 342 339 326 335 327 84 63 34 34 114 33 34 43 30% NO NO NO NO NO NO NO NO NO NO NO NO NO NO NO NO* 40% NO NO NO NO NO NO NO NO NO NO NO NO NO NO NO NO 50% NO NO NO NO NO NO NO NO NO NO NO NO NO NO NO NO 60% NO NO NO NO NO NO NO NO NO NO NO NO NO NO NO NO 70% NO NO NO NO NO NO NO NO NO NO NO NO NO NO NO NO 80% NO NO NO NO NO NO NO NO NO NO NO NO NO NO NO NO 90% NO NO NO NO NO NO NO NO NO NO NO NO NO NO NO NO

100% NO NO NO NO NO NO NO NO NO NO NO NO NO NO NO NO

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Table 3: Distance Relay Operating Times for Different 7rd & 9th Harmonic Contents (Angle between Signals=0°)

7th Harmonic Evaluation Distance Relay Operating Time in milliseconds

|V| |I|

0% 1% 2% 3% 4% 5% 6% 7% 8% 9% 10%

0% 22 59 52 51 52 51 52 51 51 51 52 1% NO NO NO NO NO NO NO NO NO NO NO 2% 61 58 61 62 61 62 61 60 60 61 60 3% 61 59 61 62 61 61 61 61 62 61 60 4% 61 37 62 62 61 61 61 61 61 61 61 5% 62 48 62 62 61 62 61 61 61 61 61 6% 61 61 62 61 62 61 62 57 60 61 59 7% 61 60 61 61 61 62 62 61 61 60 61 8% 61 58 58 59 61 61 61 61 61 59 59 9% 61 61 60 58 61 60 60 61 60 60 61 10% 61 61 61 61 60 59 58 59 59 60 59

9th Harmonic Evaluation Distance Relay Operating Time in milliseconds

|V| |I|

0% 1% 2% 3% 4% 5% 6% 7% 8% 9% 10%

0% 23 53 51 52 51 51 52 50 51 53 52 1% 52 52 50 51 53 51 51 50 52 53 51 2% 61 60 61 61 61 60 62 62 60 61 61 3% 60 59 59 60 61 60 62 60 60 61 61 4% 62 62 60 60 61 61 61 61 59 60 60 5% 62 62 60 60 60 60 61 61 61 59 61 6% 61 61 61 60 60 60 60 59 61 61 61 7% 59 60 60 60 61 62 62 63 61 60 61 8% 60 60 60 60 59 59 62 61 60 60 60 9% 59 62 62 61 61 61 61 61 62 62 59 10% 61 60 60 61 61 60 59 61 59 59 62

Table 4 shows the relay operating times for 100% 3rd

harmonic but for different angles between the harmonic and fundamental signal. It clearly shows the angle has a significant impact. For example when the angle of current harmonic is 90°, the relay does not operate even for pure sinusoidal voltage waveform. The same results are for the angles higher than 90°.

Table 4: Relay Operating Times for 100% 3rd harmonic

and different angles between signals. 3rd Harmonic Evaluation

Distance Relay Operating Time in milliseconds 270° 225° 180° 135° 90° 45° Sine ∠V

∠I

20 28 75 84 34 22 21 Sine 69 28 74 75 45 22 22 45° NO NO NO NO NO NO NO 90° NO NO NO NO NO NO NO 135° NO NO NO NO NO NO NO 180° NO NO NO NO NO NO NO 225° NO NO NO NO NO NO NO 270°

According to Table 4, for a pure sinusoidal current waveform, the angle of voltage waveform has increased the relay operating times for 90°, 135° and 180°. Table 5 shows the same results for 5th harmonic. Here, the relay does not operate for angles higher than 45°. It indicates the impact of 5th harmonic is more severe than 3rd harmonic. This has been considered previously by comparison of Tables 1 and 2.

Table 5: Relay Operating Times for 100% 5th harmonic and different angles between signals.

5th Harmonic Evaluation Distance Relay Operating Time in milliseconds

270° 225° 180° 135° 90° 45° Sine ∠V ∠I

58 44 44 40 40 51 22 Sine 59 43 44 39 40 52 52 45° NO NO NO NO NO NO NO 90° NO NO NO NO NO NO NO 135° NO NO NO NO NO NO NO 180° NO NO NO NO NO NO NO 225° NO NO NO NO NO NO NO 270°

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Fig. 14 shows a sine wave contaminated with 100% 3rd harmonic and 135° phase difference between harmonic and fundamental signal. This figure is for visualization.

Figure 14: A sine wave mixed with 100% 3rd harmonics and 135° phase difference between signals.

Table 6 shows the impact of 7th harmonic for different

phase angles between the harmonic and main signal. Here, the relay does not operate for phase angles 135° and 180°. It operates in 2nd zone instead of 1st zone for 225° and 270°.

Table 6: Relay Operating Times for 100% 7th harmonic and different angles between signals.

7th Harmonic Evaluation Distance Relay Operating Time in milliseconds

270° 225° 180° 135° 90° 45° Sine ∠V ∠I

52 52 52 52 51 52 22 Sine 52 51 52 51 52 52 52 45° 51 52 52 52 52 52 51 90° NO NO NO NO NO NO NO 135°

NO NO NO NO NO NO NO 180° 341 341 342 343 341 341 339 225° 341 341 342 343 341 342 341 270°

Table 7: Relay Operating Times for 100% 9th harmonic and different angles between signals.

9th Harmonic Evaluation Distance Relay Operating Time in milliseconds

270° 225° 180° 135° 90° 45° Sine ∠V ∠I

52 55 55 51 51 52 22 Sine 52 53 54 53 51 53 53 45° 52 55 55 52 52 52 53 90° 52 53 55 55 52 52 52 135° 53 52 51 51 52 52 52 180° 52 52 52 52 51 53 52 225° 52 52 53 52 52 53 53 270° Table 7 shows the same results for 9th harmonic. This

Table shows the delayed operation of the relay in the 1st zone

for all the phase differences. The results indicate the less severity of higher harmonics on relay operation.

V. CONCLUSION

This paper analyzes the impact of waveform distortion on transmission line distance protection, when the line is compensated by TCSC, and TCSC operates as a Fault Current Limiter (FCL). The simulation results show the extent of voltage and current distortion measured at the relaying point. Simulation results indicated the significant harmonic content of voltage and current signals. In order to show the impact of such waveform distortion on commonly used protective relays, a commercial distance relay widely used in power systems is selected and distorted waveforms is injected by a sophisticated test set to the relay. The test results are encouraging and show the severe impact of waveform distortion on relay functionality. It can be concluded that for such applications a sophisticated protective relay prone to voltage and current waveform distortion should be designed and selected.

VI. REFERENCES [1] Survey of Fault Current Limiter (FCL) Technologies EPRI. Palo Alto, CA,

2005, 1010760. [2] CIGRE WG A3.10, “Fault current limiters in electrical medium and high

voltage systems,” CIGRE Tech. Brochure, No. 239. [3] L. Gyugyi, “Unified Power-Flow Control Concept for Flexible AC

Transmission Systems,” IEE Proceedings-C, vol. 139, no. 4, pp. 323–331, July 1992.

[4] Y. Pan, M. Steurer, T. L. Baldwin, and P. G. McLaren, "Impact of Waveform Distorting Fault Current Limiters on Previously Installed Overcurrent Relays," IEEE Trans. on Power Delivery, vol 23, Issue 3, July 2008, pp. 1310-1318.

[5] M. Khederzadeh, and T. S. Sidhu, "Impact of TCSC on the Protection of Transmission Lines," IEEE Trans. on Power Delivery, vol 21, Issue 1, Jan. 2006, pp. 80-87.

[6] T.F. Godart, A.F. Imece, J.C. McIver, and E.A. Chebli, "Feasibilty of TCSC for Distribution Substation Enhancements," IEEE Trans. on Power Delivery, vol 10, Issue 1, Jan. 1995, pp. 203-209.

[7] IEEE Recommended Practice for Specifying Thyristor-Controlled Series Capacitor (TCSC), 2002. IEEE Std. 1534.

[8] D. Jovcic, G. N. Pillai, "Analytical Modelling of TCSC Dynamics," IEEE Trans. on Power Delivery, vol 20, Issue 2, April 2005, pp. 1097-1104.

VII. BIOGRAPHIES

Mojtaba Khederzadeh (SM’06) received the B.Sc. degree in electrical engineering from Sharif University of Technology, Tehran, Iran, in 1980, the M.Sc. degree in electrical engineering from Tehran University, Tehran, in 1990, and the Ph.D. degree in electrical engineering from Sharif University of Technology in 1996. He worked as post doctorate fellow at the University of Western Ontario (UWO) during 2004-2005.

Currently, he is an Associate Professor and Director of Power System Protection and Control Research Activities in the Department of Electrical Engineering, Power and Water University of Technology, Tehran, Iran. His research interests include power system protection, control and monitoring, application of FACTS devices in power systems and power system dynamics.

Dr. Khederzadeh is a Senior Member of the Institution of Electrical and Electronics Engineers (IEEE), a member of IET and a member of Cigre.