Relay Curves GEC CDG-23 1

[bfdeleen]

Is there, on the internet, manuals for older electromechanical relay, GEC type CDG-23, in order to make the settings. Also, if approximate equations have been made, I would like to find them.

 

[peterb]

You could try the Alstom website -

http://www.tde.alstom.com;

it doesn't have the CDG relay listed, but you can get info on say the KCGG series which have duplicated the curves of the obsolete electromechanical relays.
The GEC Measurements Protective Relays Application Guide, 3rd edition, gives the formula for the CDG13 very inverse relay as follows -
t=13.5/(I-1)
Where:
t= operating time (at 1.0 time dial)
I= current (multiple of plug setting)

 

[bfdeleen]

Thanks peterb. I also received a brochure on the CDG relay in pdf electronic format from Alstom which is very helpful. I started at their 24 hour contact centre:

http://www.tde.alstom.com/p-c/en/contcent.htm

 

[Nilsen]

I do not know of any web site for the curve information. Please let me know if you find one. We would have the information on file and could scan and email the curves if required.

Regards Simon Morgan
Nilsen Electric
Engineering Services Queensland Australia
State Manager
Ph +61 (07) 38998866
Fax +61 (07) 38998766
Email: simonmorgan@nilsen.com.au

 

[Sajith]

First of all I confess that I don't have the data for CDG 23, but I think the equation t = 13.5 / (i-1) seems to be too simplified. The equations I derived from the data for CDG 12 and CDG 14 differs very much from this.
1) CDG 12 (Inverse)
t = 111.82 i ^ -0.796
2 <= i <= 20 , i is multiple of the setting.
2) CDG 14 (Extremely Inverse)
t = 73.82 i ^ -2.229
2 <= i <= 6
For CDG 14, the characteristic curves go upto 40 times the setting, but the equations hold good only for the initial portion of the curve, upto 6 times the setting. Curve Fitting techniques failed to find an equation for the remaining part. But the equation shown above is very accurate upto 6 times setting

Once again, I admit that I don't have the data for CDG 23. If some one can send me the curves, I will try to find an equation.

Expecting comments,

Sajith Kumar
sajkumar@yahoo.com

 

[peterb]

Sajith -
The formulas given in the GEC PRAG are as follows (they are also given by ABB, for the DPU2000R relay with IEC curves, and for the Alstom KCGG relays - see appropriate websites for relay manuals) -

Standard Inverse: t=0.14/((I^0.02)-1)
Very Inverse: t=13.5/(I-1)
Extremely inverse: t=80/((I^2)-1)
Long time inverse: t=120/(I-1)

You will notice the differences from your derived formulas- I would regard these as being definitive.

 

[Sajith]

Restricted earth-fault (REF) protection is used to overcome the difficulties inherent in the provision of adequate earth fault sensitivity in the overall Differential protection of the transformer. (definition from Power System Protection - Vol 3 Application from Peter Peregrinus Ltd). This is applied to both windings of the transformer. Separate CTs can be used or those used for the Differential scheme can be employed. Essentially, CTs from the Three phases and earthed Neutral (for Star side) are taken and their leads connected in opposition (as for differential scheme). If the side is delta, CTs from the three phases and earthing transformer neutral are used.

So much for theory. What is implemented in our system is as follows. We have a 1.6 MVA, 11 kV/440 V Dy11 transformer that is protected with REF on the secy side only (star solid earthed) and no differential protection. We use 30% of the Full load current as the setting for the relay. We use a CAG 14 relay with range from 20%-80% of FLC).

The above referred book mentions that &quot;for solidly earthed star-connected transformer windings, an effective setting of greater than 50% but less than 100% of rated current is usually specified. For transformer delta windings connected to resistance earthed systems, the setting specified should lie between 20 and 25% of the neutral resistor current setting.&quot;

If any further clarification or diagram is required, please feel free to contact me at sajkumar@yahoo.com

Sajith

 

[peterb]

Further to Sajith's post above on REF settings, note that a stabilizing resistor is usually used with the CAG relay. The purpose of this is to minimize any CT saturation effects for external faults (saturation of the phase CTs could lead to unbalance of the REF scheme & consequent incorrect operation)
Setting the scheme requires knowledge of the maximum & minimum fault levels, the CT ratio, kneepoint voltage and secondary resistance, secondary wiring loop resistance. With a current based (CAG type) scheme, the pickup current is set as a percentage of the circuit rating (as noted by Sajith), but not more than say 30% of the minimum fault level. The scheme operating voltage is set by the stabilizing resistance to the value of the maximum voltage developed at the relay location due to the maximum external fault (Vs=Isec*(Rct+Rwiring). This value should not exceed 50% of the CT kneepoint voltage.
See the relay instruction manual for details.