I am currently working on a protection system using high impedance differential relay MFAC34 on a busbar. I have carried out analysis and calculate the plug setting voltage and the knee point voltage of CT is fine.

Also, how can we know the primary current for operation is large enough for the relay to operate.

I have also heard that this relay can be used to protection HV/MV cable with short length. Will there be any difference in calculation?

Normaly you need to know the primary fault current to calculate the relay setting. How did you calculate the setting?

There is no difference to the calculation when using this protection on a short cable, just make sure you use the longest pilot cable for the CT resistance loop

I calculated the short circuit current through per unit system. The maximum fault current is calculated to be around 5kA.

Then i use the technical manual to calculate if the CT knee point voltage is enough and the require setting voltage. I get stuck in the final step to find if the primary current is enough for operation. I don't know how to know if hte operating current is enough and if a shunt resistor is needed.

The primary current for relay operation is in relation to number of CTs and exciting current as specified in the manual. When the primary current is too low the shunt resistor is used so that the primary operating current can be increased. The relay operating current is only 38mA. Primary current will not be very high to operate it. The relay is in fact more like a voltage operating relay. This kind of relay is therefore appropriate for through fault currents when stability is required.

Regards
Chris Chew

Hi Chris, thanks for your answer. I also found on the technical manual that the operating current at different setting voltage.

The equation shows that primary current = turn ratio of CT x ( operating current + no of CT in parallel x current transformer exciting current)

So how do we calculate if the current is enough? we just use the promary fault current and current transformer exciting current to calculate the operating current

What is the value of primary current that you have? What is the CT ratio?

Regards
Chris Chew

The fault current will be 15kA and the CT ratio is 2000/1A.

As you are aware the primary current for fault operation depends on the number of CTs in the whole busbar protection scheme. The multiplication of CT and exciting current together with CT ratio will give you the primary current. Do you have the exciting current values?

Regards
Chris Chew

there are 4 CT parallel, exciting current is 10mA.

Each circuit will have 3 CTs and there will be quite a number of circuits in a busbar. Therefore you will have many CTs to consider in the determination of primary current.

If the primary current is too small i.e. less than 10% of CT ratio you can put in the shunt resistance.

Regards
Chris Chew

kss32, I don't know if you have sorted your problem, but the question is not very clear. I haven't got any information on these relays, and Chris's comments fit in with what I remember, but I'll add a comment so people can advise me if things have got more technical.

A differential relay has 2 important points - minimum operation (setting current), and through fault stability. If the relay will trip at 38mA, add 10mA, any primary current imbalance larger that 96 A is likely to trip the relay. This is probably less than the accuracy of the CTs. When you say you have 4 CTs, do you mean there are 4 circuits connected to the busbar? The worst case is that all the others are at the opposite limit of the accuracy band.
Exciting current in the other CTs may or may not affect the issue, depends on the current flowing on the primary side in those circuits.

If you need to avoid tripping at such a low current, add shunt resistance, which has the effect of splitting the relay current, so increasing the imbalance needed on the primary side before a fault is registered.

If your electrical system has a high impedance connection for the neutral- earth reference, and earth fault current is limited, this scheme may not work.

If the secondary current is more than 10% of the CT secondary side rated current(e.g. the secondary current is 139mA and the CT is a 800/1A, voltage setting is 125V), do we have to use the metrosil and stabilizing resistor? Moreover, do we have to set the stabilizing resistor anyway?

one more thing regarding MFAC34/MFAC14, do anyone know the resistor setting range, i have seen setting of 1000 and 500. The calculated resistor value is different from the resistor setting, can anyone explain this?