Previous in Forum: Surge Arrestor Lead Length   Next in Forum: IR Test Voltage
Close
Close
Close
11 comments
Participant

Join Date: Nov 2010
Posts: 4

Why to use class PS CT's for Differential Protection?

11/17/2010 1:25 AM

For differential protection, we use class PS CT's on both sides of equipments(suppose for a transformer).

If i know that Maximum Fault Current(under Worst case) that will flow through transformer is 12 times the rated current, then i can use CT's with class: 5P15 or 5P20.Yet they will not saturate at time of fault.

Then why to use class PS CT's only?

Register to Reply
Pathfinder Tags: class ps ct differential protection
Interested in this topic? By joining CR4 you can "subscribe" to
this discussion and receive notification when new comments are added.

Comments rated to be Good Answers:

These comments received enough positive ratings to make them "good answers".

Comments rated to be "almost" Good Answers:

Check out these comments that don't yet have enough votes to be "official" good answers and, if you agree with them, rate them!
8
Guru

Join Date: Oct 2009
Posts: 2061
Good Answers: 169
#1

Re: Why to use class PS CT's for Differential Protection?

11/17/2010 7:06 AM

I reproduce below, excerpts from my training sessions on CTs:

"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 CT

secondary 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".

Register to Reply Good Answer (Score 8)
Power-User

Join Date: May 2010
Location: Cambridge, MA
Posts: 390
Good Answers: 82
#2
In reply to #1

Re: Why to use class PS CT's for Differential Protection?

11/17/2010 7:48 AM

You didn't provide a reference, who wrote that excerpt?

__________________
Thanks, Win
Register to Reply
Guru

Join Date: Oct 2009
Posts: 2061
Good Answers: 169
#3
In reply to #2

Re: Why to use class PS CT's for Differential Protection?

11/17/2010 8:50 AM

I have authored the presentation on CTs, from which the above excerpts are reproduced. My presentation is based on my experience in the field and from discussions with experts in the field of CT Design.

Register to Reply
Power-User

Join Date: May 2010
Location: Cambridge, MA
Posts: 390
Good Answers: 82
#4
In reply to #3

Re: Why to use class PS CT's for Differential Protection?

11/17/2010 9:12 AM

Oh, I'm sorry to have misunderstood, you wrote it. Excellent. At the very least, here's a GA for you.

__________________
Thanks, Win
Register to Reply
Anonymous Poster
#5
In reply to #1

Re: Why to use class PS CT's for Differential Protection?

11/17/2010 9:40 AM

It looks a good answer, but could you explain what is:

1) differential means ?

2)what is class PS means ?

Thanks

Register to Reply
Guru

Join Date: Feb 2009
Location: Houston, USA
Posts: 947
Good Answers: 244
#6
In reply to #1

Re: Why to use class PS CT's for Differential Protection?

11/17/2010 9:41 AM

It is excellent explanation. GA from me.

- MS

__________________
"All my technical advices in this forum must be consulted with and approved by a local registered professional engineer before implementation" - Mohammed Samad (Linkedin Profile: http://www.linkedin.com/in/msamad)
Register to Reply
Associate

Join Date: Dec 2010
Location: Chennai
Posts: 40
#7
In reply to #1

Re: Why to use class PS CT's for Differential Protection?

12/25/2013 12:31 PM

Very useful explanation Sir. Thank You. Request you to kindly clarify that

1. When using PS Class for Differential or for REF protection, do we need to use CT's of same PS Class i.e same Imag and Knee Point Voltage for both Ct's even when we are using Numerical relays GE T60.

Though I believe that different levels of fault will have different levels of Vkp requirements in the Ct and CT will be designed accordingly. Hence it may not be necessary to have same Vkp on both sides. But still if you could clarify the same, I will be Thankful.

2. What should be the maximum allowable Imag value at Vk/2? In one of the previous thread, I found that it should be <30mA for Vk/2. Our vendor is giving 100 mA at Vk. Is it acceptable. Please Clarify.

Thank You,

__________________
"Engineering is Passion"
Register to Reply
Guru

Join Date: Oct 2009
Posts: 2061
Good Answers: 169
#8
In reply to #7

Re: Why to use class PS CT's for Differential Protection?

12/26/2013 11:28 AM

1. Not necessary to have same Vkp & same Imag. But, the minimum Vkp shall be defined for both. If minimum I mag is 300V, then one can be, say, 350V and the other can even be 500V.

2. The Imag shall be as minimum as could possibly be achieved for the given CT. The more the Imag, the more of the CT secondary current during an in-zone fault would go to excite the CT Core(s) in the circuit and thus relay sensitivity would be affected.

Register to Reply Score 1 for Good Answer
Associate

Join Date: Dec 2010
Location: Chennai
Posts: 40
#9
In reply to #8

Re: Why to use class PS CT's for Differential Protection?

12/26/2013 12:13 PM

Thank You Very Much Sir. That was really helpful.

__________________
"Engineering is Passion"
Register to Reply
Participant

Join Date: Apr 2015
Posts: 1
#11
In reply to #1

Re: Why to use class PS CT's for Differential Protection?

04/23/2015 12:04 AM

Hello sir,

I have 1 question

in our power plant, 5P20 & PS class both CT are used for overall differential schemes.

5P20 are used in HV side

PS are used in LV side i.e. generator side

why ??

Register to Reply Off Topic (Score 4)
Participant

Join Date: Feb 2015
Posts: 1
#10

Re: Why to use class PS CT's for Differential Protection?

02/11/2015 5:30 AM

Sir/Madam,

Can any one please clarify my problem.

For one transformer, I need to provide 50/51RYB (Inst & IDMT Over current protection), 87T (Transformer Differential) and 64REF(Restricted Earth Fault Protection). I am using SEPAM T87 relay which can handle all these faults. But my doubt is, do I need to put individual CT cores for the above mentioned 3 protections or a single CT core will do?

Please clarify my doubt at earliest.

Register to Reply
Register to Reply 11 comments
Interested in this topic? By joining CR4 you can "subscribe" to
this discussion and receive notification when new comments are added.

Comments rated to be Good Answers:

These comments received enough positive ratings to make them "good answers".

Comments rated to be "almost" Good Answers:

Check out these comments that don't yet have enough votes to be "official" good answers and, if you agree with them, rate them!
Copy to Clipboard

Users who posted comments:

Anonymous Poster (1); electricalexpert65 (3); hotcool030 (1); msamad (1); Raghavendran R (2); Winfield Hill (2); ykuakhi (1)

Previous in Forum: Surge Arrestor Lead Length   Next in Forum: IR Test Voltage

Advertisement