Battery and DC Backup System Protection and Coordination

7/28/2019 Battery and DC Backup System Protection and Coordination

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BATTERY & DCBATTERY & DC BACKUPBACKUPSYSTEM PROTECTIONSYSTEM PROTECTION

COORDINATIONCOORDINATION

Hassam Nasrat P.Eng.

8/28/2011 Primax Technologies Inc. 1


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Protection Coordination AGAIN?

Its been already well covered by many

studies and books...


IntroductionIntroduction


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Very simple:

99% of all available literature and

standards are about AC applications.

Very few cover DC faults.




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In this presentation, Well focus on

Fault (short circuit) coordination

assuming that overload protection is much

easier to define and manage.Challenge

Better understand the whole DC backup

system fault behaviour in order to get to aconclusion for safe and reliable solution.




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RationalRational

In typical DC backup systems multiple sourcesof energy are:

connected in parallel simultaneously contributing into the faultEach has different time constant and

amplitude

Fault effect reflected on AC mains must betaken in consideration.

Ex. 2 chargers, 1 or more batteries, inductiveloads, fully charged capacitors....



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Once breaking current of each source isdefined, one has to refer to the country

standard to specify the requiredcomponents

IEC std ratings and requirements mayvary from CSA, UL, IEEE, NFPA...


RuleRule


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BottomBottom lineline

Fuses, circuit breakers, switches, bus bars,cables and other equipment need to

operate safely and reliably during fault:

Breakers and Fuses

Need to open SELECTIVELY

Cables, switches, bus bars...Need to WITHSTAND the fault energy



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BasicsBasics

For the purpose of this discussion we define

the following: Fault: refers to a very low impedance path

created when both polarity conductors are

shorted.

Failure: refers to over-current: Ex. Failingload, leakage in battery or DC source has lost

regulation

As always, Ohms law applies: I = V / Z

Z: individual component impedance including interconnectingcable resistance.



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Steady state faultSteady state fault

During fault evolution:

System voltage decays gradually.The decay rate is related to impedance

rate-of-changeof all parallel sources over the period of

time before that protection devices open.

This calculation is less accurate than theinitial fault current.



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EffectsEffects

At the occurrence of a fault, a badly coordinated protection

device may lead to:

Device failure: Circuit breaker contacts may weld duringopening, so the fault is not interrupted

Explosion: the device will explode due to the energyavailable during the fault arcing.

Fire: due to the arc ignition of material during the contactopening

Injury or death: Operators will be operating in unsafeenvironment.

Note: Arc flash hazard level is directly proportional to theduration of the arc fault.



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Energy sourcesEnergy sources

To define minimum protection fault currentratings, we need to evaluate the

contribution in the fault of each of the DCcomponents



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TypicalTypical layoutslayouts

Lets looks first to typical DC backupsystem layouts:



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TypicalTypical layoutlayout8/28/2011 Primax Technologies Inc. 13


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RedundantRedundant layoutlayout8/28/2011 Primax Technologies Inc. 14


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RedundantRedundant complexcomplex layoutlayout8/28/2011 Primax Technologies Inc. 15


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TypicalTypical nuclearnuclear layoutlayout8/28/2011 Primax Technologies Inc. 16


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TypicalTypical batterybattery chargercharger layoutlayout8/28/2011 Primax Technologies Inc. 17


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BatteryBattery Charger outputCharger output underunder faultfault



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Contribution of different DCContribution of different DCcomponentscomponents



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BatteryBattery

Depending on the battery technology, plate

thickness and number, specific gravity (inthe case of Lead acid batteries) available

fault current may vary.

Check with battery manufacturers for theexact fault current.

Time constant: consider 10ms



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BatteryBattery exampleexample

Typical 350AH battery fault current at itsterminals:

VLA 0.33 thick Flat calcium plate battery with1.215 SG: 3200A

Tubular VLA 0.35 thick plate battery with 1.215SG: 3400A

AGM VRLA : 4361A

Gel VRLA: 3750A

Ni-Cd Medium performance: 3200A



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FilterFilter CapacitorsCapacitors

Capacitors are a source of large instantaneouscurrents

Consider time constant 10 ms

Ex. 8 x 10,000uF-200VDC- ESR:20m

with line impedance of 0.1



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Capacitor discharge caseCapacitor discharge case8/28/2011 Primax Technologies Inc. 23

C (uF) R() Vo RC(s) Vc Imax(A) Q (uC) I(A)

Vo . e(-t/RC) Vo/R CVo(e-t/RC) oe(-t/RC))/R

80 000 0,1 136 0,008 1360 10880000 1360

at t=0 1360 At=RC(s) 0,008 I at 1 RC 500 A

t=2 RC(s) 0,016 I at 2RC 184 A

8 x 10,000uF-200VDCESR:20m

with lineimpedance of0.1


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Charger/rectifierCharger/rectifier

Magnitude of faults depends mainly on:

AC mains impedance Power transformer impedance Inductive filter design Other series components

As a rule of thumb take 15-20 times the full current ratingtime constant 25ms

Ex. 500A battery charger with a 3.5% Z transformerimpedance may deliver up to10 000A during a short circuit until its own protection getsinto action to interrupt.



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Inductive loadsInductive loads

Main concern is DC motors:They operate switchgear, turbine lube pumps

During deceleration, motors act as a generatorscontributing in the short circuit.

- Consider that all motors are connected and running atthe time of the fault

- Typical motor armature has large L/R time constant 20-60ms lower di/dt longer time to clearprotection

- Rule of thumb: Use 4 times of running FLA



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NoteNote

Its not bad to have high fault availability from thesource to efficiently perform selective clearance.

So if the protection fuse or breaker has larger I2t thanwhat the source can provide, then this protection

cannot clear its load fault.

Ex. Switchmode power supplies provide fast currentlimiting and current foldback feature leading to achallenge in clearing faults.



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ProtectionProtection

Fuse or circuit breaker?

Both provide over-current, short circuitprotection, selective coordination and arc

flash protection.

Breakers can provide remote monitoring,

adjustability, reset and control.Semiconductor fuses for ex. can provide sub-cycle fault protection and long term overload

capacity



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Fuse or circuit breaker?Fuse or circuit breaker?

Fuses Circuit Breakers

DC Interrupting KA ++ -

Flexibility (optional features) + +++++

Adjustable - +++

Resettable - ++++

Reliability (maintenance) ++ -

Arc flash protection +delay trip must be at minimum

++++



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Fuse dataFuse data


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FuseFuseblockblockdatadata


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Fuse coordinationFuse coordination curvescurves


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CircuitCircuit BreakerBreaker DataData


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CircuitCircuit BreakerBreaker DataData


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Case:Case:DC protectionDC protectioncoordinationcoordination


HKD400

HJD250

At first, one

can thinkthat

protection is

perfect:

250A CB isfed from

400A main

CB!!


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BreakersBreakers coordinationcoordination8/28/2011 Primax Technologies Inc. 35


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CaseCase


HKD400HKD400

7250ASC 7250ASC

3500ASC

Total Fault at X:

2x3500A+2x7250A=

21500A+ inductive

22kADC breakers

X

HJD250

To

Inverter


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Other protection considerationsOther protection considerations

Soft startIf fault occurs during the soft start period of the rectifiers, available currents can

be limited to lower value and chargers might shut down.

Inherent current limitDepending on the fault evolution over time, rectifiers may or may not limit their

output current during faults

Ground Leakage alarmsin a floating DC system, this alarm will alert user for 1st polarity ground leakagewhich may not cause operation failure although it can be a safety hazard. A 2nd

polarity ground leakage may cause high impedance fault through the ground.

Connection cablesInterconnecting cable impedance reduces the fault current. This depends on their

material, cross section and length. In general, battery interconnecting cablevoltage drop is accounted in the short circuit capabilities of the batteries.

Ex. 4/0-259 strands copper cable has a DC resistance of 0.15 /km.



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Few applicable standardsFew applicable standards

IEEE 946: Recommended Practice for the Design of DC Auxiliary Power

Systems for Generating Stations

IEEE 1375: Guide for the Protection of Stationary Battery Systems

IEEE 1584 empirical equations are used to calculate arc-flash levels

ANSI/IEEE C37.40 (1993), "Standard Service Conditions and Definitions forHigh-Voltage Fuses, Distribution Enclosed Single-Pole Air Switches, FuseDisconnecting Switches, and Accessories"

IEC 60909: Short-circuit currents in three-phase-a.c. systems

IEC 60947: Low-voltage switchgear and control gear

IEC 60127 family: requirements applicable to fuses

UL 489 and CSA 22.2-5-09 Harmonized Standards: Molded-Case CircuitBreakers, Molded-Case Switches and Circuit-Breaker Enclosures



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ConclusionConclusion

During faults, DC system componentsresponse will be non linear, resulting into

complicated behaviour.Consequently, rule of thumb and past

experience might be needed to definebreaking currents.



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However,in special or large applications,Rules of thumb might lead to

overestimated values affecting the sizeand cost of the installation.

So, in this case, more accurateassumptions and calculations must be

made.



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Take all precautions to properly define your

equipment protection and selectivity:ASK FOR

Breaking capacity of protection device

Fault capabilities of all connected energysources

Require professionals with relevant

experience and background to help



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QUESTIONS?QUESTIONS?



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ThankThank you !you !


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Battery and DC Backup System Protection and Coordination