HV Power Quality Monitoring and IT limitations

[marks1080]

I know... another power quality monitoring thread. But please bear with me.

IT = Instrumentation Transformer (ie: CT, PT, VT, CVT, etc...)

So let’s assume we want to add some power quality/disturbance monitoring equipment at a HV station. This implies we are interested in getting information about harmonics, over voltage transient spikes, etc… that are happening on the primary of our power system. Now we are going to connect some kind of monitoring equipment to secondary IT circuits. Also, let’s assume we have both protection rated and metering rated IT circuits available to ‘hack’ into.

Our metering ITs will have a high level of accuracy, but are not robust and will quickly saturate. Our protection ITs are very robust but do not have a high level of accuracy. Given these considerations which circuits are best to use for power quality monitoring?

As I ask myself this question I start to contemplate the following:

1. Can metering ITs saturate for sub-cycle transient spkies?

2. What is the response time for transient and sub-transient disturbances of both metering and protection ITs? Is there a difference in response time between the two? Can this limit the monitoring resolution?

3. Knowing that during a fault situation, the protection IT is probably best suited to obtain information about the primary power system, as the metering IT is susceptible to early saturation, does this imply that I should use protection ITs for disturbance monitoring?

4. Can a secondary IT impose any limitations of Power Quality Monitoring?

After a quick search of previous threads on Power Quality Monitoring I haven’t seen these issues discussed. I apologize if these questions have been asked before. I would appreciate any insight people may have on this topic.

Thanks,
Mark

 

[pscheshki]

You are right.
I am also meditated on the subject, and soon I was facing the same dilemma. The question is what you want to measure and what information you need? You say you want to measure the harmonic components of the voltage, over voltage transient spikes ...
Once you want to measure over voltage transient spikes is appropriate to use the protection IT.
If your power quality monitoring device has enough current and voltage inputs, you can connect both metering and protection ITs. This way you'll be on your own conclusions.

 

[waross]

Be aware that by increasing the burden of CT circuits you may be compromising the existing design.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[marks1080]

Thanks marincheshki and waross;

I have been finding it difficult to find information on this topic - specifically the transient/sub-transient effects on secondary transformers (CTs, PTs, CVTs, etc...) and the impact of measurments for power quality and disturbance monitoring functions.

To me, it makes sense that any sudden change of a transient nature on the primary side can result in a mis-transformation of the signal to the secondary side. I believe these influences would depend on the response time of the IT itself, which is probably a function of the physical properties of the IT. I imagine a scenario where voltage surges on the primary side might look 'clipped' on the secondary side. If the clipping of the voltage wave is only due to the transformation of the VT/CVT this could incorrectly lead to someone to conclude harmonics are present.

Any more insight, objective or subjective would be appreciated.

Thanks,
Mark

 

[catserveng]

This question came up at a seminar that SEL was holding in my area last year, according to the sales engineer at the seminar, SEL has done quite a bit of study on the accuracy of measurements thru PT's and CT's and there are supposed to be some technical documents they have published on the subject. I never followed up at that time and a quick search of SEL's website has a number of application notes and white papers on power quality monitoring.

Maybe a call to one of their application engineers could get you pointed in a better direction? I would also imagine that folks like ABB and Areva also probably have looked pretty hard at the subject as well, as power quality seems to be a more discussed topic these days.

Mike L.

 

[marks1080]

Thanks Mike! I'll have a search for these papers.

Mark