Energy Storage & Stationary Battery CommitteeWinter 2020 Meeting

Orlando, FLTechnical Symposium 1

Ground Fault Problems & Locating

Energy Storage & Stationary Battery CommitteeWinter 2020 Meeting

Orlando, FLTechnical Symposium 1

Ground Fault Problems & LocatingKurt Uhlir - Standby Power System Consultants, Inc.Haissam Nasrat – Primax TechnologiesSal Salgia – Exelon NuclearAndrew Sagl– MeggerGeorge Pedersen – Carran Associates

Stationary Battery and DC Power System Applications

• Switchgear and control• Telecom and Communications• UPS

Switchgear & Control DC System

Ungrounded DC Systems

Ground Detection Often Used

Figure 1 – Single Battery Ground (Normal Operation)

_ Coil

+ 50ohms

Relay

Figure 2 - Two Battery Grounds (Misoperation)

_

+ 50ohms 50

ohmsCoilRelay

• At a minimum, a floating battery system requires at least two battery grounds before misoperation can occur.

Telecom DC System

Positive Grounded System

Environmentally Friendly andClean Power Rooms.

Open Rack Cabling

Ground Detection Not Used

UPS

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Isolated AC Input

Un-Isolated AC Input

Ground Detection Generally Used

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Un-Isolated UPS with Battery Ground Fault

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Isolated Input UPS – 12 Pulse RectifierBalanced Voltage Divider Ground Detection Circuit

Sources of Battery Grounds• Commercial ground detectors• Water in conduit• Water entering junction boxes or switch/sensor terminations• Wire splices soaking in water• Wildlife• Sharp objects piercing wire insulation• Constant abrasion wearing away wire insulation• Wires that have pulled out of their terminations and touch ground or water• Failed capacitor or semiconductor surge suppressors• Wire insulation cracking

Source of Battery Grounds

• Grounds can occur on the battery itself

• Electrolyte in a cell is conductive• Racks are grounded

Sources of Battery Grounds

• Water is the Number 1 causes of most grounds because it often forms the bond between the exposed conductor and earth ground

Why do we search for grounds?

• Combined battery ground resistance can become so low that high voltage circuit breaker control schemes are unable to open or close breakers when required.

• Circuit breakers can inadvertently open shutting off vital equipment.

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What I know about unintentional ground faultsin floating dc systems

Safety and operational risksHaïssam Nasrat E.E.

Primax Technologies Inc.

• Ground fault• Earth leakage• Leakage current detection• Residual current detection

Common equivalent terms16

Essential standards17

- NEC 250 PART VIII: DIRECT CURRENT SYSTEMS (Safety)A 2-wire, dc system supplying premises wiring and operating at greater than 50 volts but not greater than 300 volts shall be grounded.Exception No. 1: A system equipped with a ground detectorand supplying only industrial equipment in limited areas shall not be required to be grounded.- OSHA (safety)- IEEE946 (safety and operational)

Floating systems vs. non-floating18

• Floating dc systems:- no direct connection to ground such in 125Vdc – 48V Switchgear

applications-High impedance connection to ground is generally accepted

• Non-floating:-One solid connection to ground.Ex. -48Vdc in telecom: the positive is solidly connected to groundEx. 24Vdc or 12Vdc genset starting: the negative polarity is bonded

to ground.

Floating battery with no connection to ground19

V=?

125Vdc Positive post

DC Leakage types20

• Hard grounds: damage cables or conduits: near 0-3V from polarity to gnd

• Soft grounds: caused by battery acid leakage, moisture, solenoid contacts. Usually 3-40V from polarity to gnd.

Simple ground detection circuit21

Electronic type detection circuit22

R1

Ground fault circuit

R2

Positive

Negative

ReferenceK

Ground faults issues23

SAFETY ISSUES

OPETATIONAL ISSUES

Safety24

Source: OSHA

AC vs. DC shock25

DC(mA)

AC at 60Hz(mA)

Effects

<4 <1.5 Perception

<15 <3 Reaction and surprise

<88 <22 Reflex action: let go

<160 <40 Muscular Inhibition

<300 <100 Respiratory block

>300 >100 Fatal

Source: NETA fall 2007

Safety: Ohm’s Law26

Charger 1 Other relays

+125VDCPositive bus

0V (-125VDC)Negative bus

R2=150k

R1=150k

Voltmeter

High impedance

High impedance

Leakeage

Safety: Ohm’s Law27

Charger 1 Other relays

+125VDCPositive bus

0V (-125VDC)Negative bus

R2=150k

R1=150k

Voltmeter

High impedance

High impedance

Wet body resistance

Rb=1k

Leakeage

Other cases:

250Vdc or 480Vdc bus operating at 265Vdc or 560Vdc

Operational issues in power stations28

• The first short between earth and (+) or (-) is not a major operational issue.

• Unbalanced dc system may operate ok if one leg leakage is unresolved.

• If a ground develops on one leg of the 125Vdc then another one evolves on the other leg so we can have sparks and tripped circuits

• Depending on the leakage(s) location, protection relays may inadvertently trip when their control contact is bypassed by the ground leakage circuit with a low resistance capable to activate the protection relay coil or circuit.

Operational issues29

125VDCPositive bus

Protection relay 2

Protection relay 1

R1

125VDCNegative bus

Protectioncontroller

Protection relay 2 might trip if leakage (R1) value is low enough to match relay 2 energizing level …

Multiple grounds on a dc bus30

Charger 1 Charger 2 Relay 1 Other relays+125VDC

Positive bus

0V (-125VDC)Negative bus

150k

R2=150k 150k

R1=150k

High impedance

High impedance

This might lead to nuisance alarms

Ground faults in IEEE 94631

• Clause 9.2: Ground effect on operation, symatrical and asymmetrical deterioration of insulation possibility and causes….

• Annex E: full annex to explain the effect on substation operation under a single and multiple ground faults in a dc system. Examples are given for guidance.

125Vdc ground repair32

Situation in a 125Vdc system (132Vdc).

• Reading: 81Vdc between positive and gnd + 51Vdc between negative and gnd

• unbalancing is happening when turbine is running• When it shuts down, unbalancing drops to 3V and 129V

IEEE-ESSB Winter 2020 Technical Presentation

Nuclear Power Plant DC System Grounds By: Surendra (Sal) K Salgia Sr. Staff Engineer, Exelon Corporation

Nuclear plants have a commitment to operate plants in the safe & reliable manner

Proactive in preventing inadvertent tripping of plant causing outage

Types of DC System in nuclear plants:

• Ungrounded 125, 250 VDC

Reason for ungrounded system:

• If one of the polarities has ground then system may be degraded based on the severity of ground level but operable

Impact that grounds can have on operability:

In nuclear plant most of the control system is powered by 125, 250 VDC systems for example:

• Low voltage, medium voltage circuit breakers supplying power to pumps, valves (MOVs, SOVs etc.)

• HV switchyard breakers and relay house • Protective relaying for transforms, generators and transmission lines • Engineering safeguard or Reactor protection system (safety injection etc.) • Inverters supplying power to nuclear instrumentation • Fire protection, communication system • Control room alarms, indication & annunciators • Pumps, valves (MOVs, SOVs)

Supplying 250VDC power to maintain critical assets (EBOP & ESOP)

The impact of DC ground could be an inadvertent operation or failing to operate any of the devices noted above by:

• Initiate operation of de-energized dc loads

• Inhibit dropout of energized dc loads

• False operation of a normally de-energized load

• Short-out an actuating coil and/or the dc source

• Fuse or circuit breaker trip

• Preventing operation of de-energized equipment

• Inhibit the trip of breaker for switchgear, generator, or a large motor

It is evident in following figures from IEEE 946:

Figure 1—Ground faults may energize a normally de-energized device or prevent de-energizing a normally energized device

Figure 2—Ground faults may cause contacts in one circuit to actuate devices in a different circuit

Figure 3—Ground faults may cause contacts in one circuit to actuate devices in a different circuit

Typical Causes of DC Grounds:

• Degraded wires & cables • Aging of components (coils, seals, SPRs, solenoids, limit switches, bellows

etc.) • Water intrusion and environment (steam leak, condensation, rain,

corrosion etc.) • Human performance issues (accidently cut insulation, leaky cells or during

battery cleaning etc.) • Faulty annunciators and DC Monitoring equipment

Monitoring of DC Grounds:

• Operator rounds • Alarms from ground detectors or relays • Display of ground readings in control room for the plants which have this

feature

Some of The Ground Detectors:

Bender

Esterline (Paper type)

Honeywell

Yokogawa

Lamps

& Others

Administrative Controls for DC Grounds

In order to determine the threshold resistance of a ground fault that, if followed by a solid ground, could initiate operation of a normally de-energized load or could inhibit dropout of a normally energized load; the most sensitive dc loads (devices) should be identified and their minimum pickup current and maximum dropout current should be evaluated (IEEE 946)

Based on that proactive steps and priorities could be assigned to isolate/troubleshoot the ground. Example: Alarm set point: 125K Ohm Level I: ≥ 125K Ohm Alarm or close to 125K Ohm. Evaluate and trend it. Level II: 20 – 125K Ohm Evaluate and schedule to troubleshoot in

timely manner Level III: ≤ 20K Ohm 14-day time clock to locate, isolate and repair the

ground. Engineering review and OP Eval. 14 days to repair otherwise JCO

Procedures to be in place for actions to be taken for each level of DC ground.

Procedure for circuits isolation – with low and high-risk circuits (fuses, circuit breakers) for sensitivity to plant operation.

Ground busting to narrow down the ground if it is safe to do so based on risk to the plant operation using equipment craft is skilled with or contractors’ services.

Conclusion:

Ways to manage & minimize DC grounds:

• Look at the most likely areas and causes of grounds based on plant history and take proactive actions for replacing degraded components

• Seal openings for water intrusion and in outdoor equipment (junction & pull boxes etc.)

• Good housekeeping of batteries • Training of craft for good workmanship and in ground busting

Questions & Answers

?????????

Sal Salgia 2-10-2020

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Battery Ground FaultAndrew Sagl & Volney Naranjo

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Battery Ground Fault

• What is DC ground fault?• Stationary battery - designed

and operated as ungrounded• Ground faults - A conductor

makes contact with ground.

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Battery Ground Fault• What are the causes of DC ground fault?

– Wires soaking in water– Degraded cable or wire insulation caused by aging– Animals– Sharp objects piercing cable and wire insulation– Wires that have pulled out of their terminations– Failed components

• Capacitors• Surge suppressors• Pressure Relief Valves Insulation• Leaking batteries

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Battery Ground Fault

• Consequences of DC ground fault • Equipment (Relays) fails to work• Failed Batteries• Breaker trip• Multiple ground can lead to high current flows• Damage of equipment• Safety Hazards

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Battery Ground Fault• General concept• Apply a signal across

the side of the sting with the ground fault and earth.

• Drives a current through the fault.

• Trace the signal.• Can be performed on

either energized or de-energized lines.

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Battery Ground Fault

• Applied Signal• AC• Pulse

– Must be synchronized between transmitter and receiver.

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Simple Concepts are not often simple

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Battery Ground Fault

• Problems that can arise– Tripping Breakers– Stray Capacitance– Parallel Paths– Noise– High Impedance Faults

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Battery Ground Fault• Tripping Breakers• Inject too high of a signal and trip

a breaker.

• Ground Fault Monitors– Parallel connection

• Too high of a signal can unbalance the ground fault monitor– Which can trip the relay.

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Battery Ground Fault

• Tripping Breaker Solutions• Disconnect Ground Fault Monitor

– When tracing fault

• Use Low frequency signal – Relays less susceptible.

• Limit signal amplitude.– Higher amplitudes are only

required on high impedance faults.

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Battery Ground Fault

• Stray Capacitance• Due to long cable runs• Can present false paths• Can swamp the trace signal

A cable is like a capacitor. It has two conductors and an insulator. The longer the cable the greater the surface area of the conductors. This is the same as increasing the plate size in a capacitor; which increases the capacitance.

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Battery Ground Fault• False paths• As the capacitance goes up

the capacitive reactance goes down.

• As the capacitive reactance goes down the impedance goes down.

• As the impedance goes down the current goes up.

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Battery Ground Fault

• Swamp the trace signal• AC gets shifted due to

capacitance• Pulse get dampened.

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Battery Ground Fault

• Solution to capacitance• Measure capacitance

– Differentiate• Resistive path• Capacitive path

• Capacitive compensation

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Battery Ground Fault

• Parallel paths• Multiple faults• Measure fault current

on multiple lines

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Battery Ground Fault• Parallel path solution• Methodology• First locate all paths

that are due to stray capacitance.

• Then trace the lowest impedance fault first.

• Correct• Re-test

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Battery Ground Fault

• Noise• Low frequency• AC tracing uses low

frequencies• Low frequency noise can

cause signals not to settle.• Pulses can lose their sync

altogether.

Scneck SB0227Y

Time04/16/18 - 10:12:58 AM 04/20/18 - 11:32:58 PM 04/25/18 - 12:52:58 PM 04/30/18 - 02:12:58 AM 05/04/18 - 03:32:58 PM

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2.2

3.3

4.4

Ia #

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(%) Ia #3 Interharm

(%)

0

1.1

2.2

3.3

4.4

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Battery Ground Fault

• Noise Solution• Place the CT around

the return line as well as the line you are measuring. When the CT is placed

around the return line the low frequency noise on the system will be cancelled out. Leaving only the transmitter signal.

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Battery Ground Fault

• High Impedance / water ingress

• Require a greater signal amplitude to trace

• The cable can dry out• The fault disappears.

52

Battery Ground Fault

• Water ingress solution• Use minimal signal.• Enough to get a stable reading• If it does dry up

– Mark the spot– It will come back when it rains.

53

Battery Ground Fault• Plus / Minus Connections• Loads can be connected

– Positive to Ground– Negative to Ground– Positive to Negative

• Positive to Negative loads with faults can draw fault current on both the positive and negative.– Only supply test current

on one side.

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Battery Ground Fault• Characterization• Un-faulted system• Measure the capacitance• Measure the resistance• On all circuits

• When a fault occurs use these as a baseline to locate the circuit with the fault.

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Finding That Elusive Ground Fault

George PedersenCarrann Associates LLC

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So why are ground faults to hard to find?

58

One possible way is to switch off each load circuit in turn until the fault disappears.

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When a ground fault exists the way to identify which leg of the battery it exists on is to measure from each leg to ground

The leg which shows the lowest voltage is the one with the ground fault

This method using a single meter can’t be used to check if a ground fault exists

If no ground fault exists the meter will have no reference and the readings will be erratic

61

To positively identify a ground fault condition using a multimeter it is necessary to have a high value balanced resistive circuit to ground to provide a reference.

This can be achieved using two multimeters

The internal resistance of the multimeters provides that balanced circuit to ground

62

Make sure you have access to the drawings showing the DC power distribution that can be critical in tracing the fault.

63

If a ground fault detector is part of the DC power system and is a balanced resistor type, it will have to be disconnected from the DC bus.This can require:• Isolating the sensor board within the charger.• Disconnecting the charger from the bus

If that is necessary, then check that the battery will be able to sustain the load while the fault is traced.

False Grounds

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Pseudo Grounds

You will appear to be following a ground path, but it isn’t You are following capacitance in the circuitThis is typically of the EMI filtering in power supplies of the electronic controls

Power In Power Out

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When the 120 Volt station battery also supports the -48V Communications battery.This may seem like a good idea, but it has its problemsIt’s called a -48V supply for a reason, the Positive of the system is groundedAnd all the equipment that works from that supply is designed to have a grounded supplySo if the 120V to 48V converter is not fully isolated, you can have problems

120V Charger

120V to -48VDC-DC Converter

Control Circuits

Communications Circuits

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Any Questions?

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