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  • Users: cesgibis
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  1. cesgibis

    IEEE 80 to low voltage 400 V

    Does IEEE 80 apply to transmission line voltages only ? Can it be used for medium and low voltage networks ? The way the ground current is calculated in chapter 15.3 of IEEE 80-2013 does not take into account the resistance of the ground grid and if applied to 400 V networks for example, it will...
  2. cesgibis

    CT on the buried delta of Y-Y transformer

    Hi, In continuation to thread thread238-372076, I am working on a project where we are buying a 120 MVA Y-Y 220/33 kV transformer with a buried delta winding. The primary is solidly earthed, while the sencondary is earthed through a resistance to limit the earth fault current to 400A. I have...
  3. cesgibis

    Motor Diff with Variable Speed Drive

    Hi We are specifying medium volatge induction motors (6.6kV) with biased differential protection above 1500kW. However some of these motors will be supplied by Varibale Speed Drives. A Supplier is saying that motors driven by VSDs should not be protected by differential protection because at...
  4. cesgibis

    Transformer Buried delta protection

    Hi, We are specifying a 220/33kV transformer as Y-Y with a tertiary buried delta for stabilisation purposes only. The transformer is 115MVA, the primary solidly earthed and the secondary is resistance earthed to limit the current to 400A. I am preparing a protective diagram and I am not sure how...
  5. cesgibis

    CT saturation in a high impedance scheme

    Hi, When an internal fault occurs in a high impedance differential protection (whether REF or bus bar),the voltage across the relay can reach high levels, because the CTs are supposed to force their secondary currents into the high impedance branch. 1) But does this push the CTs into early and...
  6. cesgibis

    Switching transformer neutral to earth link ?

    Hi, I have a 33kV distribution system with three bus bars and two bus couplers (oeprating normally closed). Each bus bar is supplied from a transformer and in normal operation the three transformers operate in paralllel. The client specifies the NER to limit earth fault current to 400A. Th earth...
  7. cesgibis

    OLTC on HV or LV side of transformer

    What are the advantages of having OLTC on the HV or LV winding of a transformer ? Why are they always installed on the HV side ? HV side: current is lower but voltage is higher Thanks.
  8. cesgibis

    EMTP Insulation Coordination

    Hi, I have an indutrial network with a 132 kV GIS. Incoming lines are overhead and are owned and protected by the utility. All outgoing feeders are by XLPE insulated cables to step down transformers. I have been requested to do an insulation coordination study. Not being a specialist of I.C., I...
  9. cesgibis

    AC or DC motor control voltage

    Hi, I have specified a 110V DC for motor control , our client prefers 230V AC. Could anyone please tell the advantages of DC to AC control. I've been told about problems with using AC: when local control (stop/start) is located at a large distance (say >400m) from the MCC, the cable capacitance...
  10. cesgibis

    transformer REF HI protection

    Hi, Client wants transformer 30MVA 33/6.6kV DY to be protected by REF High Impedance on sec. side. earth fault is limited by resistance to 300A. CTs ratio 3000/1A (no additional details for now) I think that REF is not sensitive enough and operating primary current will be close to the 300A and...
  11. cesgibis

    Current transformer sensitivity

    Hi, 1) I've been told that CT (for protection) should not be used to detect a fault current which is less than 10% of its nominal current because acuracy decreases. Is this true ? if yes Is this applicable for toroid CTs (CBCTs) also ? 2) I have to select a CT (toroid) to install on the neutral...
  12. cesgibis

    EHV cable directly buried

    HI, I am trying to specify the installation of a 132 kV XLPE cable to be buried directly in the ground. Can anyone give me information about the method of laying, type of backfill, size of trench (depth, width) , laying arrangment, bonding, etc.... I will have also to lay in the same trench a...
  13. cesgibis

    ControLogix vs SLC5/05

    I have to replace an existing SLC500 with another AB PLC. The new PLC will have additional Ethernet, DH and Modbus comunication capabilities. I would go for a ControlLogix, but I am not sure it will integrate with existing PLC-5 and SCADA, especially for the Ethernet communication . Should I...
  14. cesgibis

    PLC vs RTU

    I need to chose a control system for a power substation. both RTU and PLC have digital/analogue I/O. Both can comunicate through serial networks with upstream SCADA and downstream protective relays. Could anyone please tell me what is the difference between RTU and PLC and what are the crireria...

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