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 Post subject: motor contribution
PostPosted: Sun Dec 21, 2014 7:41 pm 

Joined: Sun Dec 21, 2014 6:44 pm
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Is there any literature indicating the appropriate duration of motor contribution into an arcing fault? Normal power system analysis for a solid 3 phase short circuit at the motor terminals assumes that induction motor contributions decays from locked rotor current to near zero within 100 ms. Is there any study or test that has been done to quantify the time that should be entered for the duration of motor contribution into an arcing fault?


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 Post subject: Re: motor contribution
PostPosted: Mon Dec 22, 2014 12:28 pm 
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The motor contribution to arcing fault current will decay faster comparing to the short circuit scenario due to added arc resistance. The actual duration depends on motor size and arc resistance. The 100 ms assumption seems a reasonable approximation in most cases. Arc flash software program featuring IEEE 1584 empirical model provides for specifying an amount of short circuit current through protection device when determining arc duration as a function of the upstream protection device time-current characteristic. I would recommend using minimum available short circuit current coming from utility only while ignoring contribution from motors and generators upstream when determining the duration (values from Figure 1 column Min Isc3, KA below). I would also recommend using maximum available short circuit current when calculating arcing current and incident energy (values from Figure 1 column Max Isc3, KA below). The above approach results in the worst case incident energy scenario associated with the longest arc duration and the largest energy flux effectively compensating for uncertainty in fault current contribution and current decay from all different type and size of motors being a part of power system.

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Fig1. Screen from Short Circuit Analytic V1.0 software showing calculated short circuit current values.

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Fig 2. Screen from Arc Flash Analytic V5.0 software showing controls for Available Short Circuit Current, Part of ASSC thru Protection Device, predicted arcing current values and arc duration as a function of predicted arcing current thru protection device and the device time-current characteristics.


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 Post subject: Re: motor contribution
PostPosted: Thu Dec 25, 2014 11:35 pm 

Joined: Sun Dec 21, 2014 6:44 pm
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Thank you for your reply. In regards to the statement, "The motor contribution to arcing fault current will decay faster comparing to the short circuit scenario due to added arc resistance", is there any way of quantifying how much faster? The Siemens Review XXXVII (1970) No 11 has a very useful paper entitled "Tests to Determine Transients in Industrial Networks containing a Large Number of AC Induction Motors" [by Goly, Schultz & Webs] which shows that the short circuit contribution from a group of unloaded HV induction motors will decay to approximately 18% of their locked rotor current 100 ms after being closed onto a solid three phase fault. For your information the short circuit was created by closing a feeder breaker on the HV MCC supplying the motors onto a three phase bolted fault.

I wish to add another question, "How will motor pre-loading effect decay time of the transient current contribution of an AC induction motor into an arcing fault?"


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 Post subject: Re: motor contribution
PostPosted: Fri Dec 26, 2014 8:14 am 
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To quantify motor contribution into an arcing fault, I would examine two stages. During first stage, starting the moment the arc has been initiated and up until the moment of time the upstream protection device clears the fault, the motor contribution will be more or less limited due to voltage drop across the arc resistance produced by current coming from the source. During this stage, the motor current contribution will be determined mainly by the difference between motor counter-motive force and the arc voltage drop.

During second stage that starts right after the upstream protection device has disconnected an arc from power supply, only motor(s) downstream from the fault will be left to feed power to arc through the arc resistance. It is during the stage two, the motor contribution to arcing fault current will decay faster comparing to the short circuit fault scenario. Variables such as arc resistance, motor inductance, current, horse power rating and motor load would be considered when modeling the transient current contribution of an AC induction motor into an arcing fault.


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 Post subject: Re: motor contribution
PostPosted: Mon Jan 19, 2015 10:01 pm 
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Analytical answer depend on motor parameters beyond the simplified model and woul require dynamic effects not normally given or measured that are present in the extended model used mostly for vfd design. However as incident energy itself is an estimate and an inexact one at best, very simple estimates of parameters that can be estimated from a minimum of information (usually just horsepower) are used to model the motor as a complex impedance, possibly as sequence impedances. It is close enough in practice.


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