ground grid testing 101 - tsdos thompson_tsdos... · 2018. 9. 20. · construction. • components...

Ground Grid Testing 101Cliff Thompson

North American Substation Services, LLCDirector of Strategic Business Development

• 36+ year career with Dominion Virginia Power• Retired in 10/16 as the T&D Manager for Electric

Transmission

SEPTEMBER 5 - 7, 2018

ems and Equipment SeminarMemphis, TN

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Safety Moment!Look before Touch!

Always check grounds before touching equipment!


Copper Theft

Check grounds before touching equipment3

SEPTEMBER 5 - 7, 20184Check grounds before touching equipment

Molten Steel

SEPTEMBER 5 - 7, 20185Check grounds before touching equipment

Underload and Cooking


• Grounding Systems in Substations

• Safety Hazards During Testing

•Ground Grid Test Methods

•Case Studies

Topics

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PurposeProvides a surface, under and around a substation that is:

at a uniform voltage as close to zero (0) as is economically feasible (0.75 volts or lower)

Definition: Ground Grid or Ground MatGrounded system of wires and rods interconnected beneath substations Designed to Reduce Potential (Voltage) Hazards Step and Touch Potential Provide Equipment Grounding

Grounding Systems in a Substation

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Grounding System Design

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Grounding accomplished by: Formation of a Ground Mat or Ground Grid Includes the Fence Bury 4/0 Cable in Ground Perimeter Conductor 3 ft. outside fence Perimeter Conductor 6 ft. beyond gates

All structures/equipment connected All Neutrals of the Power System are connected

Goal: Provide a surface, under and around the substation, that is at uniform ground potential. (as close to zero as is economically feasible: 0.75 volts or lower)


Components of a Ground Grid

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Ground Rods • Copper weld or equivalent

• 5/8 inch in diameter

• Free from grease and paint

• Minimum of one 6-foot section at each location



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Ground Conductors Copper

Copper Clad Steel (CCS)



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Fences


Grid Connections

Finished connection12

CADWELL Connections


Grounding Substation Equipment

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Goal: Provide a surface, under and around the substation, that is at uniform ground potential. (as close to zero as is economically feasible: 0.75 volts or lower)

All equipment is grounded to the station ground gridHas same potential


Substation Equipment

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Circuit Breakers Transformers

• Grounded with 2 separate connections• Connections to 2 different sections of the ground grid• When transformer neutral to be grounded directly, neutral and at least one case ground should be visibly connected together

• Tanks connected with 2 separate ground connections • Each to a different section of the grid


Substation Equipment

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Capacitor Banks Voltage Regulators

Tank or cases grounded with 2 separate ground connections, each from a different section of the grid:

• One connects to stand and then to tank• One connects to tank and to SL bushing

• Metal supporting structures are grounded regardless of whether capacitor neutral is grounded or ungrounded.

• Supporting structure is grounded on diagonal corners with the ground lead being the same size conductor as the ground grid.


Personnel Potential Hazards

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Step Voltage

http://electrical-engineering-portal.com/6-voltages-a-person-can-be-exposed-to-in-a-substation

http://electrical-engineering-portal.com/6-voltages-a-person-can-be-exposed-to-in-a-substation



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Touch Voltage



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Mesh Voltage = Maximum Touch Voltage


•Case Studies

Topics

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Safety during a Ground Grid Test

Let’s talk about it!20

NASS Safety First We always consider these 4 Human

Performance questions:1. What are the critical tasks in this project? 2. What errors am I most likely to make? 3. What's the most likely worst-case scenario? 4. What defenses will I use to avoid errors?


Ground Grid Testing

The Ground Grid Testing is used to detect faulty ground connections.

Remember, Ground Grid provides:-Personnel Safety-Equipment Safety-Reliable Operation of Electrical Devices

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Types of Safety Hazards

The following Safety Hazards may occur when Ground Grid Testing:

• Touch Potential - if fault occurs while carrying/pulling the test lead

• Fault Potential - ground fault occurring during connection or disconnection of leads- fault current has the capability of flowing through the test leads and set

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• Case Studies

Topics

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Substation Ground Grid Testing


EPRI’s Smart Ground Multimeter (SGM) Presently Available Functions

1. Ground (System) Impedance Meter 2. Touch Voltage Meter 3. Step Voltage Meter 4. Tower Ground Resistance Meter 5. Soil Resistivity Meter 6. Ground Mat Impedance 7. Transfer Voltage Meter 8. Low Impedance/Continuity Meter 9. Fall of Potential Method 10. Oscilloscopic Function 11. Pole Ground

Testing Methods

Smart Ground Multimeter Training Copyright 1994-2017, A. P. Sakis Meliopoulos


• Relatively High Source Current and Voltage Capability Up to 500 Volts and 15 Amperes

• Random Pulse Width Signal 0-250 hz

• State of the Art Data Acquisition Hardware 24 bit SD A/D Converter 8 Channel Simultaneous Sampling

• Advanced Signal Processing Frequency Domain AveragingCoherence Based Noise Band Rejection

• Advanced Data Analysis State Estimation Based

Smart Ground Multimeter Training Copyright 1994-2017, A. P. Sakis Meliopoulos

SGM Design Parameters


High Current Test MethodPurposeTests integrity of Substation Ground Grids

MethodologyPass high current (300 amperes D.C.) through Ground Grid between Reference Point and Test Ground using:

Sorenson Model DLM8 – 350E

Record Keeping- Record Voltage drop across grid- Path of current flow

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Test Equipment Set-Up

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VARIAC

AMPS VOLTS

P1 C1 C2 P2

Current Up

Current Down


Preparing to Test

1. Position test equipment in central area near planned reference point.

2. Operator should wear Class 2 rubber gloves or be on a portable protective ground mat.

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Preparing to Test

3. Mark up grounding plan with test points and reference point. Also indicate any equipment removed or added to station.

4. Consecutively number all risers to be tested with a paint pencil.

Note: Be sure to include the fence as a test point.

5. Perform the lead to lead. Note: Make sure to record the voltage drop for the lead to lead.

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Total Voltage Drop

Goal of the test is to measure voltage drop of individual test riser only

Reference Riser Voltage Test Riser Voltage

Total VoltageDrop

Voltage Drop is the electrical effort necessary to force the testcurrent through the ground grid and back to the reference.

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Current SplitWhen Testing, the 300 amps applied splits into 2 paths:

current flowing DOWN towards the grid+

UP towards the equipment

Example: Apply 300 amps to the Test Point = Current Up + Current Down

260 amps UP + 40 amps DOWN = 300 amps

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Voltage Drop Results

4/0 Copper Riser

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Total Voltage Drop Interpretation

Less than 0.75 volts Good Grid

0.75 volts to 1.50 volts Safe grid but weakened

1.50 volts + Bad Grid


Voltage Drop Results

Copper Clad Steel (CCS)

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Total Voltage Drop Interpretation Less than 1.40 volts Good Grid

1.41 volts to 2.79 volts Safe grid but weakened

2.80 volts + Bad Grid


Trouble Shooting

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Possible Cause Situation

Current does not flow Possible open test connection

High voltage Possible bad connection or bad specimen

Low voltage(below 0.2 volts)

Possible connection problemPossible equipment failure

Unusual current split Possible break in the ground or loose connection to structure


• Case Studies

Topics

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Case Study:• Substation expansion had recently been completed.• Capacitor bank and additional cable troughs had been added.• Ground grid testing had been performed prior to

construction.• Components had been reported failing during switching of

the new capacitor bank. • Upon further investigation ground grid testing was

recommended.• Testing discovered that during construction digging inside the

station had created an islanding issue with the new capacitor bank.

• Switching created transients that were introduced into the control house via the messenger ground and control cables.


Case Study:• New 1500 MW Generating Station • Collector Station and power equipment had been installed• Control house was complete• Generation plant construction had been completed• Start-up operations had begun.• Unusual line events continued to trip units off line with no

good evidence or explanation.• Discovered that during final construction activities installing

hard surface road the preparation required a final grade.• During the process of final grading sub-surface grounds were

cut between plant and collector station. • No indication or evidence of the damage was detected until

performing the ground grid testing. • Site had been properly tested prior to final grading and start-

up.


Additional thoughts!• If you are under construction for substation expansion or experienced a

failure of substation equipment it is recommended to perform spot checks on the ground grid within the affected area.

• Once new construction is complete a full comprehensive ground grid test is recommended to bench mark and provide assurance for safe operations.

• Performing point to point testing is an effective and convenient method to determine ground grid integrity.

• Performing various soil resistance tests using the SGM on green field sites prior to designing will add value and possibly prevent over or under designing.

• Bench marking ground grid test data is beneficial for engineering when expanding facilities or adding ground grid monitoring.

• Ground Grid monitoring may be considered for early detection of theft or grid deterioration.


Conclusion

• Ground Grids are essential for the safe protection of workers, public, power equipment, electronic components and reliability.

• Establishing and following a ground grid testing program will ensure network reliability and longevity of the installed equipment.

• Copper theft is still among the highest security challenge facing utilities.• Monitoring (Cresatech CuTZ) can provide an early detection of grid integrity

and intrusion. • A healthy and properly engineered ground grid will provide critical equipment

the best operating condition to ride through transient events and power interruptions.

• Ground grid testing can be performed online and not requiring outages. • Ground grid testing provides the assurance that an engineered ground system

is properly protecting your critical high value assets and control components.

SEPTEMBER 5 - 7, 201841

ground grid testing 101 - tsdos thompson_tsdos... · 2018. 9. 20. · construction. • components...

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