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  1. rasevskii

    Rebar and DC current

    In the early days of electric streetcars (trams) which run on 600V DC supply, it was found that corrosion of iron gas and water pipes, telephone cable sheaths, and other metal structures occured close to the DC substations (or DC generating stations) where the DC current returned to the...
  2. rasevskii

    Breakers Charged/Discharged

    Charged and discharged usually refer to the springs that operate the breaker. This so that a maintenance man wont get hurt if during removal/racking out of the breaker there is no mechanical energy storage inside the breaker. Usually the springs are recharged automatically by the spring charging...
  3. rasevskii

    Synchronous Generator speed

    That powerhouse was the subject of a thread in Eng-Tips some couple years ago. It is an historic intallation, and has a web page. I will not reveal more, only the OP can do that. The 6 40HZ generators are coupled to the 60Hz grid by two synchronous/synchronous frequency changer sets, or that...
  4. rasevskii

    DC Generator Contactors

    @doubledecker: Please dont be offended by the discussions. We need some kind of circuit diagram or sketch to see where the problem may be.Is this possibly a drum-type control switch rather than DC contactors? Was there always a problem or is this something new? Background info needed. Is it a...
  5. rasevskii

    DC Generator Contactors

    @doubledecker: This sounds like a Ward-Leonard system since you mention a MG set. Therefore likely an older system. Why are you replacing contacters at all? You must use DC contactors as stated above by others. Was there always a problem or is this something new? More background info is needed...
  6. rasevskii

    Trip Contact Not Rated to Trip Coil

    @thepunisher: Perhaps you ought to listen to your client. As was said above, it is more likely that the trip coil of the CB draws more like 10A on a 125V system. Do not play games with tripping circuits. Use a man-sized interposing relay after the Phoenix (such as the old ABB type RXMH or RXMVB...
  7. rasevskii

    Trip circuit supervision relays, anyone have experience using these? Are these common?

    You might want to consider also a breaker-fail relay that will trip one or more upstream breakers if all else fails. That would be better than a meltdown and the smoke and lawsuits... rasevskii
  8. rasevskii

    Trip circuit supervision relays, anyone have experience using these? Are these common?

    In the classic design, the high-ohmic coil of the trip supervision relay is connected in parallel with the final tripping contact. The current is only some mA if the main trip coil circuit is intact. The trip coil itself has to be low ohmic. If the trip coil circuit is for some reason open (such...
  9. rasevskii

    Same 87T for 2 transformer not paralleled

    There should be a separate 87T for each transformer, in principle, that trips the HV and LV breakers for its own transformer only and not the other one. This assumes that there is actually a breaker on the HV side of each transformer that can be tripped. If you have fuses only on he HV side or...
  10. rasevskii

    Generators and furnaces

    The OP said induction and ladle furnaces, not arc furnaces. We know that arc furnaces are a very rough load but what about induction furnaces that evidently have a frequency converter system which possibly can regulate the power level in a smooth controllable manner. But what exactly are the...
  11. rasevskii

    Operation of the SF6 Circuit Breaker with a gas leak

    Of course you cannot. The SF6 is the insulation and arc-quenching medium. At low gas pressure, in two or more stages, there is an alarm and a trip or blocking of operation. The very concept of SF6 gear is that it is possible, due the high density compared to air, to build such compact gear and...
  12. rasevskii

    Hydro Generators at speed no load.

    It does not make much sense to run the unit at SNL, unsynchronized for extended periods. Why not run it on line at a small loading like a few percent of rated, safely out of the cavitation zone. With 10% gate opening to get SNL. this sounds like a large slow speed Francis or Kaplan unit. If...
  13. rasevskii

    Micom 633 trafo Diff Relay

    @bdf5526: You should do a primary test. By injecting on the HV side with the LV side shorted, using a 400V 3 phase supply from a genset, you will get about 20% of rated current (assuming 10% impedance of the trafo) That would be around 140A. With all CTS and the relay in service postion, the...
  14. rasevskii

    Wind farm step-up transformers - excessive gas problem

    Hello, Ters. Where are these measurements being taken? At he 690v secondary of each trafo or at the 33kV level via PTs? I would imagine that at the 690V level the distortion would be much greater than at the 33Kv level inasmuch due to the high capacitance of the long 33KV system cables, which...
  15. rasevskii

    Bad Start or Run Capacitor

    Yes, a TEFC motor would be the better choice. But still a small heater would be desirable. rasevskii
  16. rasevskii

    Bad Start or Run Capacitor

    Sounds like a moisture problem in the windings. If the motor is not used for long periods of time (months) and is in an unheated space, or there is a lot of humidity around, then such a problem may occur. As the motor warms up, the moisture will be driven off, but a ground fault may occur inside...
  17. rasevskii

    Shaft current transformer for hidrogenerator

    @nelvex: The original patents for the shaft current transformer and associated relay belonged to ASEA of Sweden. Very likely this system is still available from ABB. Do a search of the ABB library and you may find what is currently offered. Or google shaft current protection. rasevskii
  18. rasevskii

    Core Balance CT

    When using core balane CTs it is important that the cable sheath (armor) conductor passes backward through the CT window to its ground bus connection. THis to avoid sheath currents from causing a false operation of the protection. Cable sheath currents can be caused by third harmonic currents...
  19. rasevskii

    Small power plant vs grid-powr flow

    @hansforum: It depends...if you are in fact on a small island system where the frequency can vary noticably, then you might have a problem getting full power output at times of low load on the system. But that would be quite a picturesque bush operation indeed. On large utilities in North...
  20. rasevskii

    Small power plant vs grid-powr flow

    You do not seem to understand the basics of AC generation. If this is (likely) an induction generator, the power output will be determined by the turbine gate opening. IN other words the turbine will try to run the generator at a higher than normal speed, but the grid frequency will keep the...

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