What causes the phase shift in a delta-wye transformer, is it the way the transformer is connected or the way the transformer is built? Will varing the voltages in each of the windings give a different phase shift? What would a manufacturer have to do to build a transformer with a special...
Does anyone know why it is a good practice to turn off a generator's excitation system as soon as the generator breaker is opened and the driver is still running? It seems like it would make no difference if excitation remains on since there is no load.
We have 3 generators rated 30MW running in parallel not connencted to the grid. They are run in reaactive droop mode and not in speed droop. What does reactive droop actually mean. If one generator would trip, the load, both real power and var would increase on the other two generators. What...
Does anyone know how the primary taps ratio on a transformer work. Is a +2.5% tap apply to the full voltage accros the primary or to the number of turns on the primary? To raise the secondary voltage does one connect to the + or the (-) taps?
We have 4-13MW, 13.8kV generators that are cabled to a 13.8kV switchgear. Each generator has its own breaker. The generators are grounded via a resistor rated at 200a for 10s. The problem comes in sizing the cable from the star point at the generator to the resistor located outside the...
I posted this in the NFPA forum and no one replied! Any takers here?
Vessels in a refinery which are tall and slim are usually not required to have lightning protection since they are constructed of metal of at least 3/16" in thickness and are electrically conductive. However, what if the...
Vessels in a refinery which are tall and slim are usually not required to have lightning protection since they are constructed of metal of at least 3/16" in thickness and are electricall conductive. However, what if the vessel is covered with insulation, top to bottom. Normally the cladding...
Can someone explain why an LCI drive topology can be used with synchronous motors only. I understand a VSI type drive can be used with both induction and synchronous.
Does anyone know which NEMA, ANSI, IEEE or UL standard applies to UPS system design that covers EMC requirements in the US. In the IEC there is 62040-2, but does the US have one?
I work for a company that has two main air compressors powered by 2 36KW synchronous motors. They are soft started by a medium voltage VFD and placed on an 11KV run bus that has vacuum type contactors. Over the past several years lightning strikes has caused two of the surge packs in the motor...
Does anyone know of any manufacturer of a variable voltage/ variable frequency soft starter in the medium voltage range, 5.5, 6.6kV? Are there more advantages in using a 50% rated VFD for soft starting duties than using a real soft starter that varies the voltage and limits the current?
Does anyone know where I can get the IEC recommended cable derating factors for underground feeders and cables in cable racks? I would also like to know which IEC spec in particular addresses this. I've read that 60287 addresses the topic but it consists of several different parts. I do not...
At our facility we have an unwritten rule that mandates that all shields of MV cable be grounded at both ends in order to ensure the the voltage to ground on the shield is zero. I have asked my superintendents and engineers is there an IEEE, NEMA or any standard that backs this practice. No...
Does anyone know what the IEC requirements are concerning the use of earthing tags on cable glands. Are they to be used in non-metallic enclosures or on medium and high voltage cables entering panels? Do they have to be used at all?
Our construction people want to install 15kV feeders underground, direct buried and leave them for approximately 9 months untill the equipment arrives. The cables consist of corrgated aluminum armor with an outer PVC jacket. The cables would be stubbed out at both ends and sealed. They will...
