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  • Users: Cerkit
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  1. Cerkit

    Transformer rating and impedance

    Hi, I am looking a transformer which has different ratings based on it being naturally cooled VS forced cooling. The nameplate states the impedance of the unit based on the transformer naturally cooled MVA value. How do I treat this unit if I wanted to model it, will the impedance change if I...
  2. Cerkit

    Ynyn transformer with open circuit fault

    Hi, Wondering if anyone is aware on if a single phase open circuit fault on an overhead line feeding the HV side of a Ynyn transformer, 33/11kV for example, can be detected by protection on the 11kV side of the transformer? Would the 11kV side current distribution be able to identify the...
  3. Cerkit

    Distributed generation and network voltage

    Hi. Can someone explain why generators on distribution networks result in the increase of voltage on the network?
  4. Cerkit

    Generator Power Factor

    Hi, Can someone help me understand, in really basic terms how does a synchronous generator change the power factor at which it exports at? I think with inverter based generation this is done by thyristors switching the current at different angles along the voltage waveform is this correct...
  5. Cerkit

    Current and Voltage in Circuit

    Hi, Can anyone advise, for the attached circuit, is the current and voltage in the metrosil going to be in phase with that of the capacitor? Thankshttps://files.engineering.com/getfile.aspx?folder=e1ad7afc-7826-490b-bcc2-73cbd284b532&file=Circuit.JPG
  6. Cerkit

    Commissioning VT

    Hi, In commissioning a 110V VT with an earthed neutral, if I am I getting 50V across the fuse holders with the fuses out what might be the cause of this? The cables and instruments have already had insulation tests. Could it be an interconnection via another earthed VT on site? Thanks
  7. Cerkit

    Load Power Factor

    Hi, I am struggling to see how I can plot a vector of a current for which the load is stated to have wattful and wattless components of 87% MW export and 50% MVAr import respectively. Does anyone understand this? Thanks
  8. Cerkit

    Using 33kV line for 11kV distribution

    Hi, Does anyone know what the main issues (if any) are associated with using higher voltage (such as 33kV) lines or cables for 11kV power distribution? Thanks
  9. Cerkit

    Summation Transformer

    Hi, I am looking at using a 1:1:3 summation transformer for a phase comparison system. For a blue-ground internal fault is it correct that if I find what the fault values are entering from each side of the circuit I can then take 33.3% of each value to see if it is above the threshold of the...
  10. Cerkit

    L-L-G VS L-G Fault Level

    Hi, Between a L-L-G fault and a L-G fault which would you expect to give a higher MVA? I am getting a study with a higher L-G fault level, but I though that the L-L-G fault level should be higher? Thanks
  11. Cerkit

    Pilot wire impedance

    Hi, I am calculating the voltage and current flowing in pilot wires in a voltage balance protection system. The literature I am reading says that for an external fault the impedance seen is half the open circuit impedance of the pilots. Does this mean that no current flows for an external fault?
  12. Cerkit

    CT Ratio Selection

    Hi, I am looking at a 22/400kV 600MVA transformer. I am looking at CT sizes for the LV side of the transformer. For full load current the current is 15745A. Is it possible to use a 16000/1 CT or does this ratio of CT not exist? Thanks
  13. Cerkit

    Interposing CTs

    Hi, I am trying to calculate what interposing CT I will require. For a delta/star transformer with the star side solidly earthed, the transformer 600MVA 22/400kV. I am proposing to have Ct ratios of 200/1 on HV side and 4000/1 on LV side assuming a 5A rated relay with the following: HV side...
  14. Cerkit

    Star Delta CT

    Hi, Please see the current distribution I am proposing in the attached as I need to verify which relays will be tripping. Is this correct, for a phase to phase fault at F1 I am getting the attached fault current in the CTs...
  15. Cerkit

    Restricted Earth Fault

    Hi, For a restricted earth fault relay with an effective setting of 0.2A. The CT ratio is 2000:1 and the maximum earth fault current is 800A through an earthing transformer. If there is an earth fault at the end of an LV winding (so covers the entire winding) is it correct that the primary...
  16. Cerkit

    Current Distribution in CTs

    Hi, Can someone please comment if I have done the current distribution in the CTs at the bottom correctly? Thanks https://files.engineering.com/getfile.aspx?folder=187dbb62-8428-438b-955d-05e3a66ecd38&file=Bias_Differential.JPG
  17. Cerkit

    SNE Earthing

    Can anyone explain why we would ever need to use SNE earthing instead of CNE? Is it because if we lose the neutral then there is still a return path for fault current? In CNE if you lose the neutral you will know as the voltages will deviate. With SNE you could lose your earth and you would not...
  18. Cerkit

    HV and LV fuse discrimination on transformer

    Hi, I am looking at the fuse characteristic graph for an HV and LV fuse to assess suitable discrimination between the two. I am a bit unsure as to what the impact of changing the voltage ratio of the transformer is when the impedance of the transformer stays the same. I have attached the...
  19. Cerkit

    90o - 30o directional overcurrent for transformer feeder

    Hi, Does anyone know why a 90 - 30 relay mal operates for a phase-phase fault when a transformer is on the feeder? I have read that a 90-45 relay will operate correctly for all faults. Thanks
  20. Cerkit

    Transformer Current Distribution

    Hi, I am looking at a star-delta (HV/LV) transformer configuration. Please can you correct me if wrong on the following: For a phase-phase fault external to the transformer on the LV side (delta side) with the source being on the HV side, the current distribution in the windings should be as...

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