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30 years ago I saw false trips on similar "W" units when large chiller motors with wye-delta reduced voltage starters transitioned from wye to delta. Another installation had unsymmetrical arrangement of conductors in the zero sequence CT. It looked like large currents would saturate a portion...

Mbrooke- My 1956 edition of Mason's "The Art & Science of Protective Relaying" only mentions a breaker's failure to trip should be protected by backup relaying and implies this is the upstream overcurrent protection or the Zone 2 protection of a remote relay. Blackburn's "Protective...

Great discussion gentlemen! Lots of food for thought. On the Astoria "BLue Sky" failure, I was involved with the design/build construction of the Astoria-1 and Astoria-II independent power plants tying into the ConEd system. Plant 1 tied into that bus that had the spectacular failure. Plant II...

"If the produced secondary voltage from any of the parallel CTs is higher than the other, it would supply the excitation currents for the remaining CTs in parallel - Both for Internal and External faults. Is it correct? " For an external fault, one CT will have current out of the zone and at...

Thank you for the input. We looked at using an undervoltage release, but the Chinese supplier (Tianshui Chancheng Switchgear Co.) has not provided information on second trip coils or under voltage release. We have had poor experiences with undervoltage release coils on distribution 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...

David- Slightly off topic, -"Astoria Blue Fire" event- I was involved with two new power plants that tied into ConEd at the Astoria Substation, one at 345 kV and one at 138 kV (?). I saw the late night news about the arcing and immediately started trying to find out everything I could. I was...

Like David said, add a single phase 240/120V panel for the single phase loads. Leave the three phase loads on the 3-phase. While some of the paralleling schemes will work, they don't provide very much additional KVA as Waross pointed out and the complexity is a recipe for disaster...

I include the differential relay on the coordination curve, graphing the total tripping time including relay actuation time, lockout relay contact time and breaker arc clearing time. It helps to coordinate any backup overcurrent functions.

When I was in the breaker testing business we had 10 kA, 20ka, and 50 kA "portable" test sets. With good bus bars from the set to the breaker we could test 2,000A breakers in place. Pushing 6ka for the 20 minutes on each phase was a pain. We didn't find that many problems except loose joints...

DLRO cables have fine strands to make them flexible, but it also makes them difficult to terminate reliably in large sizes. The strands don't compress well, leading to loose joints. It doesn't take much of difference in impedance for a single conductor to carry a lot more than its share of...

Add a zig-zag transformer with a neutral grounding resistor sized to limit fault current to 5-10 amps, making this a high resistance grounded system. The first ground fault will only alarm, not trip. Use a standard pulsing current fault location system to locate & correct the fault while the...

If the cable sheath is metallic, the serrated washers may be intended as a grounding/earthing connection between the cable sheath and the enclosure with the serrations digging into the metal. An HDPE/nylon washer still might work for sealing the exterior if the serrated washer or nut on the...

There are some good IEEE papers that explore how the Generator Step UP (GSU) Transformer's tap range and impedance affect the ability of the generator to provide or absorb MVAR's without exceeding generator or station service voltage limits. IEEE C57-116 "IEEE Guide for Transformers Directly...

Check IEEE Standard 3000.2-8-2018 " Recommended Practice for Conducting Harmonic Studies and Analysis of Industrial and Commercial Power Systems" Table 2 in the standard shows the characteristic line harmonics for 6, 12, 18 & 24 pulse inverters. For 6 pulse harmonics are: 5th: 20.0%, 7th...

"Raising DC voltage" means the input rectifier was changing its firing angle and allowing more current flow to the batteries. (Or were the batteries disconnected?) Noise could be from the increased current in the windings or from harmonics in the current waveform or just the natural increased...

I've seen problems with this type of relay false tripping a main at moderate load when a large HVAC unit was started with a wye/delta reduced-voltage starter. The wye-delta transition surge current would randomly trip the GF. I don't recall how the client fixed that issue, replaced relay...

I'm assuming you are talking about a reduced voltage starter for an electric motor that runs a concrete mixer. the reason for reduced voltage start is to limit the starting current. Wye-delta starters start the motor in wye configuration to apply 57% voltage across each winding. At the...

M Brooke- Some small plants can also island and pick up radial loads. About thirty years ago a severe snow & ice storm took out 60 kV lines in the Northern California mountains putting the area around Susanville in a blackout. 20 miles from Susanville, the 30MW Honeylake geothermal and...

A commissioning engineer told me one reason for the soak on a new unit is to vibrate any remaining gas bubbles out of the windings. I agree with DPC that the thump and noise on first energization of a new, large transformer is loud and a little scary. You can feel the magnetic thump from...