ancp0801 mutual coupling measurements

AN CP0801Application Note

MUTUAL COUPLING MEASUREMENTS

2AN CP0801 Application Note

Manual Version: ANCP0801.AE.1

OMICRON electronics 2008. All rights reserved.

This Application Note is a publication of OMICRON electronics GmbH.

All rights including translation reserved. Reproduction of any kind, e.g., photocopying, microfilming orstorage in electronic data processing systems, requires the explicit consent of OMICRON electronics.Reprinting, wholly or in part, is not permitted.

This Application Note represents the technical status at the time of printing. The product information,specifications, and all technical data contained within this Application Note are not contractuallybinding. OMICRON electronics reserves the right to make changes at any time to the technology and/orconfiguration without announcement. OMICRON electronics is not to be held liable for statements anddeclarations given in this Application Note. The user is responsible for every application described inthis Application Note and its results. OMICRON electronics explicitly exonerates itself from all liabilityfor mistakes in this document.

OMICRON electronics translates this manual from its source language English into a number of otherlanguages. Any translation of this manual is done for local requirements, and in the event of a disputebetween the English and any non-English versions, the English version of this manual shall govern.

3Contents

Contents1 Using This Document . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4

1.1 Operator Qualifications and Safety Standards . . . . . . . . . . . . . . . . . . . . .41.2 Safety Measures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .51.3 Conventions and Symbols Used . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .51.4 Related Documents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5

2 Safety Instructions for Connecting CP CU1 to Power Lines . . . . . . . . . . . .62.1 Before Starting. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .62.2 Recommended Current Range Settings . . . . . . . . . . . . . . . . . . . . . . . . . .62.3 Estimating the Open-Line Voltage. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .72.4 Connecting the Measurement Setup to Power Lines . . . . . . . . . . . . . . . .8

3 k Factor and Mutual Coupling Factor Measurement . . . . . . . . . . . . . . . . .103.1 Why k Factor Measurement? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .103.2 Mutual Coupling. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .123.3 Performing Measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .133.4 Interpretation of Measurement Results . . . . . . . . . . . . . . . . . . . . . . . . . .19

3.4.1 Excel CPC 100 File Loader . . . . . . . . . . . . . . . . . . . . . . . . . . . .20

AN CP0801 Application Note

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1 Using This DocumentReading the AN CP0801 Application Note alone does not release you from the duty of complying with all national and international safety regulations relevant to working with the CPC 100 and the CP CU1. The regulation EN 50191 "The Erection and Operation of Electrical Test Equipment" as well as all the applicable regulations for accident prevention in the country and at the site of operation has to be fulfilled.

1.1 Operator Qualifications and Safety StandardsWorking on overhead lines is extremely dangerous. The mutual coupling measurements described in this Application Note must be carried out only by qualified, skilled and authorized personnel. Before starting to work, clearly establish the responsibilities. Personnel receiving training, instructions, directions, or education on the measurement setup must be under constant supervision of an experienced operator while working with the equipment.

The measurements must comply with the relevant national and international safety standards listed below:

EN 50191 (VDE 0104) "Erection and Operation of Electrical Equipment"

EN 50110-1 (VDE 0105 Part 100) "Operation of Electrical Installations"

IEEE 510 "IEEE Recommended Practices for Safety in High-Voltage and High-Power Testing"

LAPG 1710.6 NASA "Electrical Safety"

Moreover, additional relevant laws and internal safety standards have to be followed.

5Using This Document

1.2 Safety MeasuresBefore starting a measurement, read the safety rules in the CPC 100 User and Reference Manual and CP CU1 Reference Manual carefully and observe the application specific safety instructions in this Application Note when performing measurements to protect yourself from high-voltage hazards.

1.3 Conventions and Symbols UsedIn this document, the following symbols indicate paragraphs with special safety relevant meaning.

1.4 Related DocumentsThe following documents complete the information covered in this Application Note:

Symbol DescriptionEquipment damage or loss of data possible.

Personal injury or severe damage to objects possible.

Title DescriptionCPC 100 User Manual Provides basic information on the CPC 100

test system and relevant safety instructions.CPC 100 Reference Manual Provides detailed hardware and software

information on the CPC 100 test system including relevant safety instructions.

CP CU1 Reference Manual Provides information on the CP CU1 coupling unit and the CP GB1 grounding box including typical application examples.


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2 Safety Instructions for Connecting CP CU1 to Power Lines

2.1 Before StartingCaution: A lightning discharge to the line under test can cause injury or possibly death of the operating staff. Do not connect the measurement setup to overhead lines if there is a possibility of a thunderstorm over any part of the lines to be measured.

Caution: Connecting the measurement setup to overhead lines with a life parallel system brings about high-voltage hazards. It is strongly recommended to take all parallel lines out of service before proceeding.

Before connecting CP CU1 to overhead lines or power cables (further on referred to as power lines), you must estimate the open-line voltage as described in 2.3 "Estimating the Open-Line Voltage". Follow the instructions below exactly and sequentially to protect yourself from high-voltage hazards. In addition to the following safety instructions, observe "Safety Rules" on page 7 of the CP CU1 Reference Manual V1.4.

2.2 Recommended Current Range SettingsThe highest current range allowed by the open-line voltage (see 2.4 "Connecting the Measurement Setup to Power Lines" on page 8) provides the best measurement accuracy. However, depending on the length of the power line under test, this setting may result in CPC 100 overload due to low driving voltage. As a rule of thumb, the current range required for the power line length is given in Table 2-1: "Recommended Current Range Settings" below. Set the current range switch of CP CU1 to the value according to the table.

Table 2-1:Recommended Current Range Settings Line Impedance Line Length Current

RangeCompliance Voltage

01.6 02 km/01.5 miles 100 A 50 V0.88 110 km/0.55 miles 50 A 100 V4.040 550 km/330 miles 20 A 250 V> 16 > 20 km/15 miles 10 A 500 V

7Safety Instructions for Connecting CP CU1 to Power Lines

2.3 Estimating the Open-Line VoltageTo estimate the open-line voltage:

1. Switch off, short-circuit and ground the power line on both sides using an installed grounding switch or, if no grounding switch is available on site, using grounding cables (further on, the grounding switch or these extra grounding cables are referred to as grounding switch).

2. Make sure that the connection to ground at the far end of the power line is not removed during the complete test procedure.

3. In addition to the grounding switch, ground the line at the near end using a grounding set consisting of three cables rated for the maximum short-circuit current possible on the line (further on, this connection is called working ground).

4. Open the grounding switch at the near end of the power line and measure the current through the working ground using a clamp-on ammeter on all three phases.

5. Close the grounding switch.

6. Calculate the estimated open-line voltage after removal of the grounding cables as follows:

Vest [V] = meas [A] 0.4 [/km] 2 lline [km] (Eq. 2-1)or

Vest [V] = meas [A] 0.64 [/mile] 2 lline [miles] (Eq. 2-2)where Vest [V] is the estimated open-loop voltage in volts,meas is the highest measured current in ampers,0.4 [/km] = 0.64 [/mile] is the constant of a typical overhead line per wireand lline [km] and lline [miles] is the length of the line in km and miles respectively.

Caution: If the estimated open-line voltage is

> 500 V, stop. The measurement is not possible due to high-voltage hazard. Try to take parallel lines out of service.

250500 V, the measurement is possible only in the 10 A range. 100250 V, the measurement is possible in the 10 A or 20 A range. 50100 V, the measurement is possible in the 10 A, 20 A or 50 A range. < 50 V, the measurement is possible in all current ranges.7. If the current range allowed by the estimated open-line-voltage is lower as

the current range set according to Table 2-1: "Recommended Current Range Settings" on page 6, set the current range switch of CP CU1 to the value allowed by the open-line voltage.


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Caution: While the grounding switch at the near end of the power line is open, the area around CP GB1 in the range of 5 m/15 ft and around CP CU1 in the range of 2 m/5 ft is a dangerous zone due to high-voltage and mechanical hazards. Do not enter the dangerous zone. Keep the grounding switch open for a time as short as possible.

Caution: If you see or hear anything uncommon in the test equipment, for example noise of electrical discharge or lightening of surge arrestors, close the grounding switch before touching the measurement setup.

2.4 Connecting the Measurement Setup to Power LinesIf the estimated open-line voltage (see "Estimating the Open-Line Voltage" on page 7) allows measurement in the current range you want to use, connect the measurement setup to the power line as follows:

1. Make sure that the grounding switch is closed.

2. Connect CP GB1 to ground using the delivered cable near the place where the connection to the line is made. Make sure that the grounding stud is in good condition, clean and free of oxidation.

Caution: Depending on the type of grounding points in the substation, the appropriate connection set and socket clamp have to be used. Connecting socket clamps of one type to a grounding point of another system is highly dangerous on both the connection of the grounding set to CP GB1 and the connection of CP GB1 to the grounding point in the substation. The 16 to 20 mm socket clamps are designed and tested for fault currents up to 26.5 kA, the 25 mm (1 inch) socket clamp for fault currents up to 30 kA, both for a maximum duration of 100 ms. On locations where higher fault currents are possible, CP CU1 and CP GB1 must not be used.

3. Disconnect the grounding cables from the ground (the grounding switch is closed!) and connect them to the CP GB1s line studs.

4. Position CP CU1 at a minimum distance of 5 m/15 ft from CP GB1.

5. Position CPC 100 at a minimum distance of 5 m/15 ft from CP CU1 and 10 m/30 ft from CP GB1.

6. Ground CP CU1 using a cable of at least 6 mm2 cross-section close to CPC 100 and the position of the operator.

7. Connect CP CU1 with CP GB1 as shown in Figure 2-1: "Wiring the Measurement Setup" on page 9.

9Safety Instructions for Connecting CP CU1 to Power Lines

Figure 2-1:Wiring the Measurement Setup

8. Ground CPC 100 using a cable of at least 6 mm2 cross-section close to the position of the operator.

9. Connect CP CU1 with CPC 100 as shown in Figure 2-1: "Wiring the Measurement Setup" above.

10.Mark the area around CP GB1 in the range of at least 5 m/15 ft and around CP CU1 in the range of at least 2 m/5 ft as dangerous zone.

11.Open the grounding switch and read the voltmeter on the CP CU1s front panel from outside of the dangerous zone.

Caution: If the voltmeters reading is

> 500 V, stop. The measurement is not possible due to high-voltage hazard.

250500 V, the measurement is possible only in the 10 A range. 100250 V, the measurement is possible in the 10 A or 20 A range. 50100 V, the measurement is possible in the 10 A, 20 A or 50 A range. < 50 V, the measurement is possible in all current ranges.If the open-line voltage allows measurement, proceed as described in "Performing Measurements" on page 13.

Make sure that the grounding switch is always closed when no measurement is performed and especially when the wiring is modified or the current range switch of CP CU1 is set.

L3/C L2/B L1/A

Connection using grounding sets on site


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3 k Factor and Mutual Coupling Factor Measurement

3.1 Why k Factor Measurement?On most modern secondary distance protection relays, the value of the positive-sequence (line) and zero-sequence (line-to-ground) impedance or the ground impedance matching factor (k factor) is required to make the relay settings. The line impedance can be readily calculated but the chosen values for the ground impedance often do not match the actual conditions. This is because that nearby parallel systems have an influence on the measurement and thus discrepancies between the calculated and actual values are generated. Therefore, the mutual coupling factor between two systems has to be determined to consider these influences for the evaluation of the measurement results. The accuracy of these settings is crucial to the operation of the relay because they directly affect the reach of the different protection zones, for example in case of a line-to-ground fault. Measurements show that in a significant number of cases the k factor of the measured lines is set more than 20% from its actual value. This can result in zone under- or overreach and consequently, the selectivity is lost. This situation is particularly relevant to underground power cables.

The k factors are line parameters independent of the fault location describing the ratio of the line and ground impedances. The following k factor definitions are commonly used:

The complex ratio of the ground impedance and the line impedance

, (Eq. 3-1)

Note:

the complex ratio of the zero-sequence impedance and the positive-

sequence impedance (see Figure 3-1: "Zero-Sequence Impedance Definition" below)

(Eq. 3-2)

and a couple of real values

ZE ZL

kLZEZL------

Z0Z1----- 1

3--------------------==

Z1 Z= L

Z0Z1

k0Z0Z1-----=

11

k Factor and Mutual Coupling Factor Measurement

(Eq. 3-3)

(Eq. 3-4)

where and are the real and imaginary parts respectively of the ground impedance and and are the real and imaginary parts respectively of the line impedance.

Figure 3-1:Zero-Sequence Impedance Definition

The single-phase zero-sequence impedance corresponds to a serial connection from the line impedance and the triple ground impedance . The k factor is an important setting of distance protection relays. The precision of this setting affects the accuracy of distance protection relays dramatically. The k factor can be calculated, but the calculation results give only a rough estimate of the actual value. As a wrong k factor setting can cause worse power quality, higher risk to lose the system stability and loss of power supply, k factor measurements are essential for fast, selective and reliable distance protection.

Because there are usually strong disturbances by other lines in service, measurement at the mains frequency is not feasible. All measurements running according to the templates are done below and above the mains frequency and the results are interpolated. From these results the positive- and zero-sequence impedances as well as the k factor in various formats are calculated.

RERL------

XEXL------

RE XERL XL

Z1 ZE


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3.2 Mutual CouplingBasically, mutual coupling is nothing but a voltage induced in the parallel system II, which is caused by a current in system I. Due to the voltage induced in system II, a current also flows in the parallel system, which in turn induces a voltage in system I. This relation is shown in the following figure.

Figure 3-2:Mutual Coupling Between Two Wires

For measuring the coupling impedance , the template requires two measurements. The advantage of this measurement is that no measurement is required on system II. Rather, all measurements are conducted on system I. In the first measurement, system II is separated from the ground on at least one end. Consequently, no current can flow through system II. The result is the zero-sequence impedance for the case that no current can flow in system II. For the second measurement, both ends of the line have to be grounded to ensure a flow of current. However, the voltage in system II becomes zero. The result of this measurement is the zero-sequence impedance . The coupling

impedance is now calculated from and :

(Eq. 3-5)

The coupling factor can be presented in two versions. The following equation shows the complex coupling factor kM.

(Eq. 3-6)

In the two equations below, the real and imaginary components are split.

and (Eq. 3-7)

ZM

Z01

Z02ZM Z01 Z02

Z 13--- Z01 Z02( ) Z01=

kMZMZ1-------=

RMRL-------

XMXL-------

13


3.3 Performing MeasurementsConnect the measurement setup to the overhead lines or power cables under test following 2 "Safety Instructions for Connecting CP CU1 to Power Lines" on page 6.

Note: For line length below 5 km/3 miles it is recommended to connect the V SENSE input of CP CU1 as close as possible to the VT of the line to reduce the additional impedance of the current feed in the path. For longer lines, you can connect the V SENSE input with the Kelvin clamps directly on CP GB1.


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In the course of the k factor test procedure, the following measurements are performed:

Line-to-line impedance measurements: L1-L2, L1-L3, L2-L3 (Figure 3-3: "Line-to-Line Impedance Measurements" below shows the L1-L2 measurement as example.)

Figure 3-3:Line-to-Line Impedance Measurements

CPC 100 CP CU1CP GB1

Near end

Far end

Overhead line

V1 AC V1 ACI ACI AC I OUT

V SENSEBOOSTEREXT. BOOSTER

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Line-to-ground impedance measurements: L1-E, L2-E, L3-E (Figure 3-5: "Zero-Sequence Impedance Measurement - System II is in operation" below shows the L1-E measurement as example.)

Figure 3-4:Line-to-Ground Impedance Measurements

Zero-sequence impedance measurements: L1||L2||L3-E (see Figure 3-5: "Zero-Sequence Impedance Measurement - System II is in operation", Figure 3-6: "Zero-Sequence Impedance Measurement - System II is off and disconnected from the ground on at least one end", and Figure 3-7: "Zero-Sequence Impedance Measurement - System II is off and connected to the ground on both ends" below).

CPC 100 CP CU1CP GB1

Near end

Far end

Overhead line




16

Short the three phases with the delivered three-lead cable as shown in "Shorting the Phases" on page 17 of the CP CU1 Reference Manual V1.4.

Figure 3-5:Zero-Sequence Impedance Measurement - System II is in operation

CPC 100 CP CU1

CP GB1

Near end

Far end

Overhead line



17


Figure 3-6:Zero-Sequence Impedance Measurement - System II is off and disconnected from the ground on at least one end

CPC 100 CP CU1

CP GB1

Near end

Far end

Overhead line




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Figure 3-7:Zero-Sequence Impedance Measurement - System II is off and connected to the ground on both ends

The test procedure is controlled by templates available on the CPC Explorer CD-ROM shipped with your CP CU1 or in the customer area of www.omicron.at.

It is recommended to use the same test current for all measurements. To find out the highest test current possible, start the test procedure with the measurement featuring the highest impedance, that is the L1-L3 measurement on power cables and the L1-E measurement on overhead lines.

After wiring the measurement setup to the line under test proceed as follows:

1. Configure CPC 100 as described in 2.3 "Configuring CPC 100" on page 22 of the CP CU1 Reference Manual V1.4 for the CP CU1s current range set by the current range switch.

Caution: The configured current range must not exceed the limit by the open-line voltage.

CPC 100 CP CU1

CP GB1

Near end

Far end

Overhead line



19


2. Choose the XML template for the mains frequency(e.g. "Line Imp CU1 60Hz.xmt" for the 60 Hz mains frequency) and open the template.

Caution: Open the grounding switch at the near end before making the test and keep it open only during the measurement. Close the grounding switch after the test and before reconnecting the measurement setup.

3. Run the test procedure.The following measurements are performed:

Line-to-line measurements: L1-L2, L1-L3, L2-L3For each measurement, connect the I OUT and V SENSE inputs of CP CU1 to the corresponding CP GB1s line studs.

Line-to-ground measurements: L1-E, L2-E, L3-EFor each measurement, connect the I OUT and V SENSE inputs of CP CU1 to the corresponding CP GB1s line studs.

Zero-sequence impedance measurements: L1||L2||L3-E

4. If an overload of CPC 100 occurs, reduce the test current or set a lower current range and run the test procedure once again.Lower test currents at the two highest frequencies are recommended.

5. Save the test procedure as a file on CPC 100.

6. Download the test file from CPC 100 to the PC using CPC Explorer.

Load the test file into the Microsoft Excel template.The measurement results are displayed.

3.4 Interpretation of Measurement ResultsTo interpret the results of line impedance measurements correctly, you have to know details about the overhead line or power cable under test. You will find below some useful notes about how to interpret the measurement results.

Usually, the resistive part of the line impedance is relatively constant over the L1-L2, L1-L3 and L2-L3 as well as L1-E, L2-E and L3-E measurements. If the measurement results differ considerably, typically contact problems are the reason. In some cases, the grounding switches at the far end of the line are not as good as necessary for the measurement. Additional grounding cables could help to avoid the contact problems. For the lines under test shorter than 5 km/3 miles, do not connect the V SENSE input of CP CU1 with the Kelvin clamps, but rather use additional clamps directly on the wires of the power line.

The inductive part of the line impedance increases with the distance between the lines. This is documented by the measurement results stored in an example file delivered with the line impedance templates (see the marked results in


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Figure 3-8: "Measurement Results" on page 21). The measured overhead line with the shortest distance between the lines L1 and L3 is shown in Figure 3-9: "Measured Overhead Line" on page 22.

Note: For each line of measurement results there is a separate overload indication top right on the CPC 100s screen (or in the report) explained below.

No overload indication means no overload during that step of the measurement sequence.

Dotted overload indication means that there was an overload during that step of the measurement sequence but not all the time.

Solid overload indication means a permanent overload during that step of the measurement sequence.

3.4.1 Excel CPC 100 File LoaderExcel CPC 100 File Loader allows loading XML files generated with CPC 100 into Microsoft Excel templates for post-processing. Excel CPC 100 File Loader is installed with CPC Explorer. After the installation, a shortcut to start Excel CPC 100 File Loader appears on your desktop. Templates are pairs of XML documents and Microsoft Excel templates designed by OMICRON electronics or end users for designated applications. The XML templates are predefined test procedures, often with comments, that run on CPC 100 and guide the user through the test. Once completed, the XML file is saved, downloaded to the PC using CPC Explorer and then loaded with CPC 100 File Loader into the corresponding Microsoft Excel template. There the results are post-processed and a final test report is generated. The template pairs facilitate and speed testing with CPC 100 and the evaluation of results.

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Figure 3-8:Measurement Results

Click the button "Load XML-File" to open the browse menu to load the desired data to the template. Click "Print Report" to print the calculated data. Under "Measurements", the results of the impedances of the nine conducted


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measurements are shown in Cartesian and Eulerian form. The relevant calculated impedances are listed under "Impedance Results". is the

arithmetic mean value of the first three measurements. is the triple value of the measured three-phase zero-sequence impedance and thus refers to one phase (see Figure 3-1: "Zero-Sequence Impedance Definition"). The coupling impedance is calculated according to (Eq. 3-5) on page 12. The coupling

zero-sequence impedance corresponds to the triple value of the coupling

impedance .

The "Residual Compensation Factor" is the k factor calculated from the determined data for setting the relays. Under "Residual Compensation Factor Format", one of the three manufacturer-dependent formats can be selected. Under "Mutual Coupling Factor", the mutual coupling factor is indicated. Like for the k factor, three different display formats are available.

Figure 3-9:Measured Overhead Line

The L2-E measurement features the lowest X component because the line is very close to the ground wire. The X component of the L3-E measurement is decreased by a parallel system taking course close to L3 on the other side of the

Z1Z0

ZM

ZM0ZM

L2

L1 L3

23


tower. Short-circuiting of the parallel system during the measurement would have increased the effect and would have lead to erroneous results because this is not the normal operating condition.

Another interesting effect can be observed when measuring power cables. If the screen or shield is very close to the conductors but the conductors are relatively wide from each other, the inductive part of the line-to-line measurements is higher than the inductive part of the line-to-ground measurements, resulting in a negative X component of the calculated impedance . This seemingly strange result is explained as follows. Recalling (see 3.1 "Why k Factor Measurement?" on page 10) that the zero-sequence impedance is given by

(Eq. 3-8)

and hence

(Eq. 3-9)

where is the positive-sequence impedance and is defined as a difference between the line-to-ground loop measurement and a half of the line-to-line loop measurement, the X component of can become negative.

ZE

Z0 Z1 3ZE+=

ZEZ0 Z1( )3

----------------------=

Z1 ZE

ZE

1 Using This Document1.1 Operator Qualifications and Safety Standards1.2 Safety Measures1.3 Conventions and Symbols Used1.4 Related Documents

2 Safety Instructions for Connecting CP CU1 to Power Lines2.1 Before Starting2.2 Recommended Current Range Settings2.3 Estimating the Open-Line Voltage2.4 Connecting the Measurement Setup to Power Lines

3 k Factor and Mutual Coupling Factor Measurement3.1 Why k Factor Measurement?3.2 Mutual Coupling3.3 Performing Measurements3.4 Interpretation of Measurement Results3.4.1 Excel CPC 100 File Loader