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TOMORROW'S TECHNOLOGY TODAY

DC GROUND FAULT ANALYZER AND LOCATOR (GFAL A&B)

GFAL A&B 

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Ground faults on battery operate DC Multi feeder Distribution System are very difficult to find without proper instrumentation and training.

 

Samco Engineering has design a new microprocessor control instrument with a single purpose “to analyze and locate even the most difficult DC ground faults on "DC Multi feeder Distribution System”, for the safeguard of the DC system integrity.

 

The DC Ground Fault Analyzer and Locator instrument (GFAL-A/B) make use of a very small interrupted and synchronize DC signal to analyze and locate DC ground faults on multi feeder distribution system on power plants, refineries and switchyards.

Remark from one of our nuclear customer,

 "We have a split 250VDC system with high stray capacitance. We have had no luck previously locating grounds on the common bus. We have tried the Bender, Biddle, DC Scout and Ground Hog equipment with no success. We were able to locate a positive ground on Unit One,and an intermittent common ground on Unit Two using the GFAL-A/B equipment.Prior to this, we were living with the grounds."

Abstract

 

The GFAL-A/B instrument has been used for many years in stations,substations and nuclear power plants to troubleshoot their DC systems ground faults and has been effective in locating the faults without ever causing a system trip, or taking a unit off-line. 

THE GFAL EXPLAINED

The GFAL-A (interrupter) and GFAL-B (receiver) are battery operated portable instruments that utilizes a DC tracking signal create by the interrupter and the fault resistance.

A Hall Effect magnetic Sensor Assembly is attach to the GFAL-B that wrap around the conductors to track the tracking signal.

There are three mayor components that make the GFAL-A/B instrument.

 

Item 1) The interrupter unit GFAL-A creates a DC interrupted tracking signal utilizing the fault resistance.

 

Item 2) The hand held unit GFAL-B house the microprocessor and the analog

electronics and track the tracking signal.

 

Item 3) The magnetic sensor assembly (MSA) detect the magnetic field produce by

the tracking signal.

This MSA wrap around the conductors under test and is connected to the

GFAL-B.

Equipment and Method highlights:

 

* Create a tracing signal to locate the DC ground faults up to the point of failure

 

* Functionally test the DC alarm unit operation

 

* Test the DC Distribution System for spike faults and AC contamination

 

* Help to find the value of the fault resistance from each polarity with respect to site ground

 

* Set up the basis for a ground fault predictive maintenance program

 

* Will trace and locate the DC ground faults on DC multi-feeder distribution system, up to the point of failure without service interruption

 

* Detect and process the signal produce by the interrupter with a magnetic sensor (unique design) that wrap around the conductors

 

* Never have to open a circuit or a breaker to troubleshoot a DC distribution system

GFAL-A, Analyzer unit

Background

 

Multi-feeder DC distribution systems utilize ungrounded 12 VDC to 250 VDC battery assemblies for control, instrumentation and plant equipment operations.

 

DC ground faults in one polarity of a multi-feeder system will diminish the reliability of the system if they remain uncorrected.

 

Multiple DC ground faults in opposite polarities can cause an electrical short and system failure.

 

If a DC alarm unit fails to operate and indicate the presence of a ground fault [that would have tripped the alarm] the reliability of the DC system is in jeopardy.

 

The presence of AC power on a DC multi-feeder distribution system can produce none, partial or total system failure and the possibility of electrocution to any person who comes in direct contact with the DC system.

Present methodology to indicate DC ground faults

 

The existence of a DC ground fault in a multi-feeder DC distribution system, is commonly indicated by voltmeters that test for voltage imbalance.

 

The voltmeters are attached to the DC distribution system and will indicate ground faults; however, if a second ground fault with a equivalent resistance is present on the opposite polarity the voltage imbalance with respect to ground is restore and the voltmeters WILL NOT indicate the ground faults.

 

There are many other variations of this voltage imbalance measurements test, a few examples are: the replacement of voltmeters with light bulbs, ground fault detector/alarm relays, or a current meter connected at the center point of a voltage divider.

 

The major problem with all the cited DC ground fault indication methods is that they use the voltage imbalance technology for indication.

 

Consequently, as mentioned earlier, these methods and instruments will have erroneous indication on multiple ground faults, if those faults are located on opposite polarities.

GFAL-A Operation and Applications

 

DC distribution system ground fault test

 

For this application, a selected resistance is applied to the polarity with the bigger voltage indication.

 

A set of voltage data and appropriate calculation will find the value of the total fault resistance from each polarity with respect to site ground.

 

There is another method design by Samco Engineering that will find the fault resistance on each polarity for 250 VDC dual battery distribution system.

 

Site DC Alarm

 

This test is performed to verify and adjust the operation and response of the DC Alarm System.

 

A variable selected resistance is connected to one polarity at a tine to control the amount of fault resistance being introduced to the DC system with respect to site ground.

 

If the site DC Alarm it is properly working should trip when the introduced resistance is below the set point needed to trip the alarm.

 

Disconnected, out of commission and out of specifications site DC alarms are quickly exposed by the GFAL-A instrument.

AC Power and spikes on DC systems

 

The more common AC contamination on DC systems with respect to ground is either wiring connections errors or paths of leakage resistance between the DC system and the AC power system.

 

A spike is produce when a leakage fault resistance is intermittent.

 

The GFAL-A has an AC voltage detector circuit that will reveal spikes and AC power that is superimposed on the DC distribution system respect to site ground

Predictive Maintenance Programs

 

A periodic testing on-line on DC multi-feeder distribution systems can produce a database file to support a predictive maintenance program.

 

Important data to save are: Voltage with respect to ground, DC ground fault magnitude from each polarity, DC alarm response and AC contamination.

 

All the above data can be obtain with the help of the GFAL-A Unit.

Typical users for the GFAL-A

 

The DC ground fault analyzer GFAL-A is utilized in situations where a high degree of service reliability and safety from the multi-feeder DC distribution system is required and desirable.

        *       All type of Generating Power Plants

        *       Distribution Substations

        *       Refineries

        *       Cogeneration Facilities

        *       All locations with DC ungrounded systems

References for DC ground faults

ANSI/IEEE:

Standard 450-1987, 484-1987 and Std. 946-1992 “Recommended Practiced for Maintenance, Testing and Replacement of Large Lead Storage Batteries for Generation Stations and Substations”.

NRC: (Nuclear Regulatory Commission)

Information Notice 88-86. “Operating with Multiple Grounds”

EPRI : ( Electric Power Research Institute”

 “DC Distribution Systems” Book. 

Specifications for the Model GFAL-A  

        Operating DC range                               12 to 240 VDC    

        Impedance range                                 5 K to 10 Mohm

        "Switching pseudo grounds”                  5 K to 80 K “Cold opto switching”

        Accuracy                                              2%, full scale

        Environmental operation                       30 to 125 Degrees F

        Power adapter input                              + 12-18 VDC

        Internal battery operation                     24 hr, Rechargeable

        Display                                                Volts and milliamps

        Dimensions                                          12” x 7.5” x 11”

        Weight                                                 6 Lb

        Case (2 colors)                                     Gray and yellow

Model: GFAL-A Highlights

 

*    Test for ground fault resistance in multi-feeder distribution systems

 

*    Help to calculate the fault resistance in each polarity with respect to ground

 

*    Analyze and functional test the DC alarm system operation

 

*    Detect spikes and AC power contamination in battery distribution systems

 

*    Will perform the function of a ground fault indicator unit

 

*    Create the DC tracking signal for troubleshooting with the GFAL-B tracking unit

Performance

    *     Operate on 12 to 250 VDC ungrounded DC systems

    *     Variable load resistance selection for safety operation

    *     Plant operation remains uninterrupted during test

    *     Does not use or inject AC signals to operate 

GFAL-B, Tracking Unit

 Highlights:

* Does not require system interruption

* Locate DC ground faults with this hand held portable instrument

* Protect your sensitive electronic equipment

* Unique design reduces system outages

* No need to be connected to the AC main to operate

* New: Will detect how much fault exist on each branch     

Traditional methods of indicating DC ground faults are ineffective when two or more faults are in opposite polarities of the DC multi-feeder distribution system.

 

Samco engineering has created a new method that quickly will identify these faults on each polarity.

 

However once a fault has been identified, the fault must be located in order to be corrected.

 

Locating a DC ground fault is currently done in two ways. The first method consists of  opening circuit breakers or disconnecting branches and interrupting the system until the fault is cleared, a procedure that is time consuming inaccurate and costly, since it requires shutting down the system..

 

The second method to locate DC ground fault injects an AC signal with respect to ground into the DC system. This process can inadvertently damage sensitive electronic equipment or cause expensive nuisance trips.

Specifications for the Model GFAL-B

Sensitivity                          3mA, will locate up to 40,000 Ohms fault in a 130VDC                                                         distribution system.

Operating DC range            12 to 240 VDC

Impedance range                No load to the DC distribution system

Accuracy                            10 % (full scale)

Environmental                    30 to 125 Degrees F

Power adapter input           + 12-18 VDC

Internal battery operation   More than 24 hr (Rechargeable)

Display                              Alphanumeric 16 characters

Dimensions                        10” x 11” x 4.3”

Weight                               4 Lb.

Case (2 colors)                   Gray and yellow   

PROVEN RESULTS

 

Below is a partial list of users that have benefited from the innovative method implemented by the GFAL-A/B

              Plant Bowen 3,540 MW (GA)

              Plant Scherer 3,564 MW (GA)

              Ray Olinger Plant 420 MW (TX)

              City of Anaheim (250 KV) Substation (CA)

              El Segundo GS 684 MW (CA)

              Exxon Mobil Refinery (CA)

              Callaway Nuclear Plant (MO)

              Comanche Peak Nuclear Plant (TX)

              Wolf Creek Nuclear Plant (KS)

              Brunswick Nuclear Plant (NC)

              Diablo Canyon Nuclear Plant (CA)

              Palo Verde Nuclear Plant (AZ)

              Hatch Nuclear Plant (GA)

              Vogtle Nuclear Plant (GA)

               Three Mile Island Nuclear Plant (PA

               Bonneville Power Administration, Gov (OR)

               Arnold Airforce Base, Gov (TN)

              Paradise Power Plant,TVA Gov (KY)

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