Our distribution grid is a Y-Y multigrounded system, so we plan to replace the few existing and old delta transformers feeding delta loads with standard Grounded Y-Y distribution transformers (500kVA to 3000kVA), with the star points solidly grounded on both side; a three wire circuit will feed...
We have a 24940GRDY system, (14.4kV L-G) and in order to avoid installing a MV single phase metering unit to feed a small 37.5kVA single phaseload requiring 2400V, we plan to use some units we have in stock, ie a step down 100kVA 14,4kV-120/240, and connect its 240V to the 240V LV of a 75kVA...
Our main distribution system is multigrounded Y ( 24940GrdY/14400V) but to feed our older 4160GrdY/2400V downtown system we typically use some 5MVA Delta-Wye transformers (24940D-4160Y). The 4160V system is typically well balanced. However, we need to remove some of the loads from it, and...
For mgtrp.... thanks! I can see the voltage collapse in the LV ground faulted phase and the corresponding shift of the neutral in the primary. I taught there would be some zero sequence flowing in the ground, but with no voltage there can be no current. It makes sense. But then, can you help...
Stevenal and mgtrp, will there really be no zero sequence currents flowing in the LV grounded neutral? The Primary will be an ungrounded wye, but the secondary will be solidly grounded. With no zero sequence current, I am increasingly worried that this may lead to ferroresonnance if one of the...
An industrial customer is currently fed via a 2000kVA 13860V delta-600 GrdY 347V. The medium term plan is to replace the 13860V system by a 24940GrdY system, and install a new 2500kVA transformer connected GrdY-GrdY
What would be the drawbacks to remove the present transformer (delta primary)...
cuky,
I see indications showing the %Iz of the 300kVA bank as being 3 times the % Iz of the individual single phase. This is really not the case..... eg if all 3 single phase units had a 1.6% IZ (on a 100kVA base), then the 3 ph bank has a 1.6% IZ as well (on a 300kVA base).
Bill, David and Jghrist, thanks for the answers, we did indeed suggest to the consultant to use the lowest %Iz on a 300kVA base as the worst case scenario for a 3 ph short circuit... but he insisted! go figure!
With regards to the fact that the Nameplate %Iz is obtained using the LV winding...
I have a three phase pole mounted bank made out of 3 single phase transformers 100kVA 12470GrdY/7200-120/240V. The %Iz of each transformer is respectively 1.9%, 1.8% and 1.6% on the 100kVA base. The transformers are connected as a 12470GrdY-208GrdY bank (The 2 LV windings of each single phase...
These transformers are wound core, shell type ie the HV and LV windings are wound concentrically on the same bobbin.
In this case the HV winding is sandwiched between the 2 - 120V windings. This is a very common design practice to obtain a low % Iz for these lower distribution kVA sizes.
The...
There is an application for a GrdY-GrdY bank using 3 single phase transformers 14.4kV-120/240 into a 24.9kVGrdY-208GrdY bank.
2 of the units are 100kVA, with an 1.8% Iz and both will have their 120V LV winding paralleled; the third transformer is a 200kVA, same voltages, 2.2% Iz but only 1 of...
David Beach
Thanks for the answer! the medium voltage system feeding the bank of 3 - 167kVA is a solidly multigrounded 24.9kV.
So for this appliaction (ie a NGR in the LV) the H0 must be solidly grounded (as Waross indicates, tied to the system ground).
The LV X0 must then be an insulated...
We are looking at installing a three phase transformer bank made out of 3 single phase oil filled transformers rated 167kVA each, to feed an small industrial load. The HV is a 4 wire system 24940GrdY14400V
The LV neutral will be connected to ground via an NGR
If we choose a Y-Y connection, I...
Bill,
Absolutely correct... and the same goes for Voltage Regulation as it is a function of %Ir and %Ix
There are many good articles on paralleling, but do you (or anyone) have any article that develops single phase banking in a three phase bank, specifically for GrdY application? The idea is...
PRC, thanks for this note; the IEC document is quite detailed for parallel operation indeed. I have found some data on paralleling in IEEE C57.105-1978. I wish there was as well a detailed document for banking single phase in a three phase bank
Of course when replacing a failed 75 kVA in a 3 ph...
PRC, thanks for this note; the IEC document is quite detailed for parallel operation indeed. I have found some data on paralleling in IEEE C57.105-1978. I wish there was as well a detailed document for banking single phase in a three phase bank
Of course when replacing a failed 75 kVA in a 3 ph...
Looking at the ABB Distribution Guide page 63 and 64 and applying it to a GrdY-GrdY bank, it appears that a balanced three phase load will split evenly between the 3 phases. Am I correct in assuming that the currents in each phase will be different and proportional to the Iz of the individual...
magoo2 -
You are correct on all points; its just that dealing in % can be misleading. I have the Distribution Tx Guide and it goes back indeed to the Westinghouse days.
Bill and Magoo2
Thanks..
A few more precisions... the avg voltage was calculated as the avg of the three line to ground voltages; the 0.5 is the absolute Iz spread that would still allow 2 pu overloading; for a 1.3 pu overload they would use a spread of 0.77.
Magoo2, the 0.5 is a plain...
