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Nightfox1925, just to answer your question, I am dubious of having capacitance in circuit for motor starting and concur with waross. I believe that for your application if you went for an MV motor - captive transformer combination you would draw somewhat less current during starting because the...

Nightfox1925, Technical parameters can vary a lot depending on manufacturer and excitation system. You would be best to approach a steam turbine or generator vendor and request information from them (and be sure to tell them of your application if it includes large motor starting). They...

Nightfox1925, Your biggest problem is that the system is weak. You can get excessive voltage dip whilst starting and quite often a voltage spike when the motor is shut down. In an ideal world you would be able to start your motor unloaded. This would allow setting the soft starter to a low...

The most economical solution to power factor correction is to connect the capacitor downstream of the motor contactor so that the capacitor is energised at the same time as the motor is (and you use only one item of switchgear). You must be careful in sizing such a capacitor. Its capacity...

The performance of a three-phase STAR - STAR transformer (YNyn) with primary star un-grounded and secondary star grounded during an earth fault (or phase to neutral load) on the secondary side will depend upon the transformer type construction; shell or core. For shell type, the zero sequence...

Had a very similar problem with 2500A Merlin Gerin ACBs in 97-98. These ACBs were lightly loaded and every now and then one would trip on earth fault (they would take turns doing this). Testing showed there was no earth fault and primary injection testing confirmed that for each ACB the earth...

A good approximation is that the pu voltage dip at the motor bus is motor starting kVA divided by the available 3-phase fault level kVA at the bus. This considers not only the affect of the transformer impedance but also that of the source impedance. Draw a simple model of the power system and...

innomation, If your motor has three sliprings and a secondary rating is stated then it sounds like it is not a synchronous type; it may actually be a wound rotor induction motor. The key thing with a synchronous motor is that the rotor winding is DC (you apply a DC voltage to it) so there...

PAP, Undervoltage protection is a good thing to have but it should not be instantaneous; it needs to be time delayed to allow time graded overcurrent protection to operate. You will get a dip in voltage whenever you get a short circuit fault and the extent of the dip will depend on the...

If your two incoming 13.8kV supplies are in phase then the two 400V supplies will be out of phase with each other and you will not be able to parallel them. The 400V supply derived from the 13.8/6.3kV - 6.3/0.4kV will be in phase with the 13.8kV supply. The other 400V supply, derived from the...

Reactive, There is not sufficient data to be able to predict the performance. The problem will be voltage dip during start-up. From "Diesel Generator Handbook" by LLJ Mahon, the initial dip is calculated as follows: Vd(%) = 100Xd'/(Xd'+c) where: Xd' = generator unsaturated...

Many plants have switchgear that is rated for the normal plant configuration, which is generally for non parallel operation of incoming power supplies. There are strong economic arguments for doing this. When you do a supply changeover as you propose, you parallel incomers for a very short...

It is all to do with acceptable voltage dip caused by the impact load of your motor starting. The main reason for using a star-delta starting system or any other reduced voltage starting system, as opposed to DOL starting, is to reduce the impact load and allow larger motors to be started than...

Deansharafi, I have read (somewhere) that earthing the secondary middle phase VT, for star-star VTs, is normal for systems where very limited earth fault current is available on the primary side (such as with unearthed or high resistance earthed systems). The thought is that even on the...

HiSet, The impedance values are quoted from Knable and are not a percentage of Z1, but actual percentage impedance on the transformer base, where 100% impedance = 1 per unit (pu). For core type, Zo = 50%-200% (i.e. 0.5pu - 2pu) and for shell type Zo = 3000% (30pu).

jghirst, It is possible to get the text second hand through Abe books, they currently have two or three available. It is surprisingly expensive though, in the $US mid 40's. Not a bad book. I have come across wye-wye transformers where these have been used in preference to delta-wye...

davidbeach, you are spot on. jghirst, If you are able to get a hold of the text I referenced (Alvin H Knable "Electrical Power System Engineering", McGraw-Hill 1967) he explains it quite well, the tank of the transformer acts as a tertiary winding of sorts. Sorry but I do not have my...

HiSet, For a YN/yn configuration, with no tertiary winding, and the YN starpoint unearthed the zero sequence impedance will depend on the type of transformer construction. For shell type, the zero sequence impedance will be very high (some 100's %) and the earth fault current available on the...

Ahr35181, I think that your responses to jbartos are inappropriate and uncalled for. I like your advice though (but not in the context in which it was applied) and intend to follow it myself (in part at least) after work tonight. As far as EEx d motors go, you have a couple of issues I guess...

If you know hated horsepower (HP), full load current (If) and rated voltage (V) then at full load: power factor = HP*0.7457/(sqrt(3)*If*V) The above will give you the power factor at full load. Motor power factor varies with load, power factor tends to get lower with reducing load. If you do...