Summation Cts

[peetey]

I need to get some info on summation cts and their application. Is there anyone who has used these type of cts to sum up more than one secondary to one output? I was wondering which one I will need if I have 1000:5 cts in a powerhouse generator environment with coupling capacitors as a voltage source. The secondary of these cts never see more than 5 amps on the secondary. I need to sum up same size cts, they are all 1000:5. I also need some info on which one to use. I need the type that works with same size cts, I need it to have two seperate h1 & h2 and output to one x1 & x2. I have one technical hurdle, this is a ring bus configuration and the two cts that will feed each summation ct will be on the same phase of the bus but will be oriented in opposite directions to allow for poer flow. I need to know if a summation ct can take say 5 amps + (positive) and 5 amps - (negative) and as a result put out 0 amps on the secondary and have this be within .3 accuracy class for revenue metering. When these are summed,is the addition done strictly thru magnetism or is this a paralleled method (which will not meet my needs)? Any comments on this topic would be appreciated.

peetey

 

[peetey]

There has to be someone who can answer this question or at least point me in the right direction. Is there someone who is an expert on current transformers or is even a little familiar with interposing cts or summation cts?

--peetey

 

[niallnz]

Peetey

Go to you local transformer winding shop and ask for a summation CT with a 5amp secondary. The primary will be 5+5+5 if it is the case that you want to sum three 5Amp secondary CT's.

If only one of you 1000:5 CT's is fully loaded (the other 2 are at zero) the output of a 5+5+5:5 summation CT will be one third of 5Amp. Your measuring device will have to be scaled such that it thinks it is connected to a 3000:5 CT.

All the same CT accuracy classes apply to summation CT's.

Cheers
Niallnz

 

[peetey]

I'm glad I'm speaking to someone who is familiar. I actually want to sum up two currents and output the combined to a meter element. We have a ringbus breaker scheme at one of our powerplants and it uses a 6 element meter. It is a very old mechanical type meter and it has to be replaced with a solidstate meter because of FERC guidelines. We have an IN and OUT meter. There is "NO" manufacturer that currently makes a 6 element meter that does electrically and magnetically what these old meters do. We have entertained the idea of parallelling these cts into one current but there are errors associated with this. Unlike a standard meter that measures power consumed or delivered this meter measures the "net" power. All the power that is generated is fed into a ring configuration and sent over various transmission lines. Here is an example: If I generate 10 megawatts of power and feed it into the ring bus and only 3 megawatts are transmitted out of the bus then there remains 7 megawatts of capacity in the bus. Lets look at this as only one of the phases. There are two CTs on each bus and they are oriented in oppostite direction with respect to polarity. When the 10 megawatts is fed into the bus it is seen by one of the CTS and all 10 megawatts is seen. When the 3 megawatts are fed out of the bus it is seen by the opposing ct and subtracts from the scheme. The IN or OUT meter will register this load but instead of measuring the load directly it nets the power in the ring bus. I need to take the two currents on this single bus and net them with a summation ct. Any comments on this?

--peetey

 

[niallnz]

Hello Peetey,

I understand what you want to do and can see that a 5+5:5 summation CT is what you need on each phase of your generator feeder and your transmission feeder to/from the ring main. But, just as an alternative why not have metering on all circuits on the ring main and monitor them over a comms system and have a computer/PLC calculate the net power.

Cheers
Niallnz

 

[peetey]

We need this meter to be put on phone lines for an MV90 system and we need it to put out one pulse per mega watt either delivered or received. The meters we use cost around $3000 each so they will probably want only one that utilizes 4 quadrants. I just need to know that a summation ct achieves the summation magnetically and can I have two currents that are 180 degrees out and have them net within the .3 accuracy class required for revenue metering.

--peetey

 

[Electrifier]

Correct me plz, if i got it wrong;
Generally, there are three possible ways to get summation of 02 circuits for metering or protection purpose;
• DIRECT SUMMATION: Combine the CT secondary circuit of similar phases at the end of your existing secondary circuit (Scan over Partial differential over current scheme in any standard book. Ratio of phase CTs is 1000:5 and input of metering device will be configured as same. Make sure that continuous full load secondary current does not exceed the input rating of metering device.
• SUMMATION BY INTERPOSING CTs: These CTs are used in conjunction with main CTs to alter/balance the ratio of main CT or to provide isolation to meters or relays from main CTs secondary circuit. Install an interposing CT with a 5:5 ratio in each CT secondary circuit and get the direct summation of secondary of interposing CTs for required phases. Ratio of phase CTs is 1000:5, interposing CTs 5:5 and input of metering device will be configured for 1000:5 input as well. Make sure that continuous full load secondary current does not exceed the input rating of metering device. If summation current is more than 5A, you can select interposing CT with a lower secondary current.
• SUMMATION BY A SUMMATION CT: When current flowing in more than one feeder is required to be metered, summation current transformer are used. These CTs are provided with more than one primary winding and one common secondary. The standard primary & secondary currents are 5 or 1 amp. If you have a summation CT with two primary windings rated for 5A and secondary rated for 5A as well, primaries you can directly connect to the secondary of your main CTs(1000:5A) and secondary of summation CT to your metering device. Input of metering deivce will be configured same as main CTs(1000: 5A)

Hope this help