Differential protection is a very sensitive protection scheme for large electrical machines. Ordinary current transformers (CT) are not enough for this service and we employ Class 'PS' type of CT for this. The following is a very nice excerpt on what is PS and why is it used for differential protection. This is taken from GlobalSpec forum. The full thread may be accessed
here.
Normally protection CTs like 5P, 10P or 15P are used in almost all
protection schemes. But, for Unit Protection Schemes like Differential,
REF, etc., these CTs are not preferred. Why?
In unit protection schemes, it is very very important that the
scheme operates only and only for the internal faults and must remain
stable for all external faults. That is, when the unit protection scheme
operates, one can be pretty sure that something is wrong within the
protected equipment.
Also, unit protection schemes are employed for very critical
equipment in the network. As such, whenever any unit protection scheme
operates, all hell breaks loose. And one cannot put back the equipment
into service, without conducting an array of tests and ensuring that the
equipment is fit to be put back to service. But, this will take time
and effort. And until such time, the plant will be shut down.
So, it is all the more imperative that the unit protection scheme operates only for genuine internal faults and NOT for any external faults.
Now, if we employ conventional protection class CTs like 5P or 10P
for this application, let us see what happens. Lets us assume that one
has selected 5P10 Class CTs for a Unit Protection Scheme. Let us say,
the relay setting is 10%; this means that any differential current of
10% will operate the relay. Now, a 5P10 CT means that the CT will
maintain its accuracy at least up to 10 times the rated current. This
means that the CT will not saturate at least up to 10 times the rated
current.
This also means that the CT may saturate anywhere after 10 times
its rated current. This level will differ for different CTs. Among the
same two 5P10 Class CTs, one may saturate at 12 times and the other may
saturate at 13.5 times. In such a condition, during a through fault
condition, there will be differential current and the relay will operate
for external faults too. Even when both CTs are identically
manufactured, the deterioration of its core properties over time may
differ and yet they may behave differently over time.
Also, even when the CTs may be supplying to unit protection scheme
of the same equipment, it is highly impossible that all the CTs of the
scheme will be located at the same place. The incoming side CTs or the
outgoing side CTs may have to be located far away from the relay
location, thereby incurring extended lead lengths, thus imposing
additional burden on the CTs. This increased burden will also shift the
saturation level, as we have already seen.
Thus again, during a through fault condition, there will be
differential current and the relay will operate for external faults too. There are many other similar factors contributing to the
maloperation of unit protection schemes, when conventional protection
class CTs are employed. Thus, it has called for a special class of CTs
for such applications. That Special Class is called Class PS. (PS is the
abbreviation of the French Word "Protection Speciale")
Here, instead of generalising on the minimum saturation level of
the CT, the users have to exactly specify the saturation level of the
CT. This is called the Knee Point Voltage (VKP), as it
appears as a human-knee in the CT Magnetisation Characteristics. This
specification will take into account the maximum through fault current,
the actual lead burden, the relay burden & the resistance of the CT
secondary winding, as also a factor of safety.
The minimum Knee Point Voltage for a given PS Class CT is calculated by:
VKP = K * I(f)s (RCT + RB), where,
If(s) = Maximum thro fault current as reflected at the CTsecondary terminals ( = If(P) / CT Ratio)
RCT = CT Secondary Winding Resistance
RB = Connected Burden, includes the relay burden & the burden of the connecting leads
K = Factor of Safety, normally taken as 2
VKP = Knee Point Voltage of the CT
As can be seen from the above formula, here the customer is
specifying the level of saturation, duly taking into account the maximum
possible fault current in his network, the actual burden connected to
the CT, etc. If the factor of safety is taken as two, this means that at
least up to two times the maximum possible fault current the CTs will
not saturate. Which also means that at the maximum possible fault
current, both the incoming and outgoing side CT characteristics would
exactly coincide. That is, their secondary currents would match exactly
and the scheme would not operate for any external fault.
