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  1. rcw retired EE

    "Fail Safe" Trip Circuit for MV Motors Fed By Circuit Breakers

    What does your plant do if a motor controlled by a MV circuit breaker is running and the DC trip circuit fails defeating the normal trip? 1. Automatically trip immediately using a back up source and accept the process upset? 2. Alarm the failure and let operators determine the best course of...
  2. rcw retired EE

    2 MVA transformer Excitation Current Issue

    One of two new, identical, 2 MVA, 4.16kV-480/277V delta-wye, KNAN transformers has unusual excitation current tests results prior to energization. Both transformers passed factory tests and most of the usual NETA field tests: insulation power factor (Doble), insulation resistance, TTR, winding...
  3. rcw retired EE

    245 kV SF6 Circuit Breaker and Air Insulated Bus Field Testing

    Is an AC overpotential test common for air insulated 230 kV substation commissioning? A utility's spec calls for "AC Hi-pot Test of Circuit Breaker and Applicable Bus Sections" in a checklist attachment to their generic circuit breaker installation specification that is used for breakers from...
  4. rcw retired EE

    Definitions for Standard Chinese Abbreviations for Aux Relasy

    I'm trying to commission a large sytem designed and built by a Chinese contractor for a power plant in Central America. The owner severed the relationship with the EPC contractor so I can't get answers from the design engineers. The protection relay logic and all the breaker controls use...
  5. rcw retired EE

    ANSI/IEEE vs. IEC Generator Temperature Ratings

    On a combined cycle power plant proposal, a vendor provided steam turbine and gas turbine generator ratings versus ambient air temperature charts that had different curves for "IEEE Class B Rise, 0.85 pf" and "IEC Class B Rise, 0.85 pf". It looked like the ANSI/IEEE curve had about 30C...
  6. rcw retired EE

    NEC Required Access at Equipment. Is it ALL equipment.

    NFPA 70 requires a 30" wide x 36" deep by 78" tall clear working space in front of electrical equipment operating at <150V to ground where it is "likely to require examination, adjustment, servicing, or maintenance while energized".. (The depth is 42" for...
  7. rcw retired EE

    "k" factors for Medium Voltage Fuses?

    Is there a "k' factor for MV fuses to allow increasing the rated KAIC for operating voltages lower than the rated voltage rated? My co-worker is checking a design of fused 4.16 kV motor starters. Short circuit levels under some unusual conditions are very close to the fuse interrupting rating...
  8. rcw retired EE

    400 kV Transformer neutral reduced insulation.

    We are bidding a spec for a plant in the Mid-east that requires a disconnect switch in the neutral ground of the 400 kV transformers, 375 MVA, 400 kV-18.5kv, YNd11. The intent is to allow 400 kV system operation grounded or ungrounded. No reason given. Can the neutral insulation level be...
  9. rcw retired EE

    Singel Phase Operating Conditon for a 3-Phase Transformer

    A client's RFP specifies that the "480 Volt Secondary unit substation shall be rated to supply the total 480 volt auxiliary load plus 20 percent under all single Phase Operating Conditions." How do we size/specify the transformer to continue carrying three-phase loads when the primary is...
  10. rcw retired EE

    Common Motor Control Power Transformers -Grounded or floating.

    I'm helping on an IEC project in Africa and want to know if ungrounded control power is typical for IEC MCC's. MCC's were built in China. Each 690V MCC has a 690V-230V, single-phase control power transformer that feeds a distribution panel with single pole breakers, one breaker per MCC bucket...
  11. rcw retired EE

    GIS Bus - Trip On Low-Low Gas Pressure?

    Our client has a 350 kV class design Gas Insulated Substation (GIS) operating at 230 kV. It is a breaker-and-half arrangement with three diameters, two outgoing lines to the utility and three GSU transformer feeders to the generating plant. The local utility added a requirement for...
  12. rcw retired EE

    Acid Fumes from Corona or Partial Discharge?

    Can corona or partial discharges generate acid fumes? If it is in an enclosed space can enough acid fumes build up to be noticeable? After an inspection of a 21 KV generator terminal enclosure, a client sent this comment. "The generator termination box door was opened, a strong smell of ozone...
  13. rcw retired EE

    Strange Transformer Turns Ratio Test Results

    It is a new 250/333/416/450 MVA, 23kV delta - 240 kV Wye Generator Step Up Transformer getting ready to energize. All looks good except for (TTR) test(s) on one phase. H1: X1-X2. Tap 1 is 1% low, Tap 2 OK, Tap 3 1% high, 4 1.5-2.5% high and 5 is 2.5% to 3.5% high. Other phases look OK. We...
  14. rcw retired EE

    Conduit Sealing in Hazardous Locations - IEC Style

    We are building a project in the MidEast that has underground PVC raceways stubbing up in areas classified as Class I Division 2 due to the posibility of natural gas leaking from piping and valves. North American codes require the stub ups to be rigid conduit capped with a conduit sealing...
  15. rcw retired EE

    VFD Efficiency Measurement Problems

    Our client's VFD specification for some 125 HP, 600V cooling tower fan drives requires a factory load test to demonstrate the VFD full load efficiency is 90% or better. (Motor not included.) The vendor's fabricator has set up a test system with a 125HP AC motor driving a DC motor as a variable...
  16. rcw retired EE

    Method C IEC Short Circuit Calculations- 20 Hz?

    A co-worker stumped me with an IEC question. ETAP software provides three methods for calculating short circuit currents. Method A gives approximate results, Method B provides more detailed results and requires more information. Method C appears to calculate the impedances at 20 Hz. Our client...
  17. rcw retired EE

    CO2 Fire Supression Inside Switchgear & MCC?

    Our client is requiring each "switchgear cubicle and MCC cabinet" to have automatic CO2 fire suppression. That means a detector and a discharge nozzle in each "cubicle". Has anyone ever seen 15 kV or 600V switchgear or MCC's with CO2 protection? Not an electrical room with a Halon or CO2...

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