vltage drop in CVT cable 2

[sadgol]

Hi everybody
I need to know the standard value of voltage drop in CVT cable calculation(is it 1% or 0.5%)

 

[7anoter4]

If by "CVT cable" did you mean indeed triplex cable of copper conductor Cross-linked polyethylene insulated and PVC sheathed, the voltage drop depends upon the conductor cross-section area, length of the cable and the value of the electric current flowing through it.
This drop will be limited by total voltage drop from the source voltage up to the supplied system: motor, switchgear, MCC and other.
If CVT cable a Low-Voltage cable and-for instance-is dedicated to supply a Induction Motor, the voltage drop is limited by motor minimum permitted voltage in steady state[usually 95% from motor rated voltage] and in start position[usually up to 80% from motor rated voltage].
If CVT is a Medium-Voltage cable, usually the voltage drop is negligible as conductor cross-section is big .The conductor cross-section is determined by Short-Circuit Current Thermal Stability [up to 250oC permitted temperature] or by Steady State current [for unlimited time, 90oC maximum temperature].

 

[magoo2]

I think it may be a problem with acronyms that may not be common to all.

There's a capacitive voltage transformer (CVT) that can be used as a VT since it's cheaper at higher voltages than oil-filled VTs. It has a limited load capability, so the load is fairly light.

Why to you presume it is around 1% or 0.5%? Can you share some details?

 

[sadgol]

Dear 7anoter4
CVT is not used to supply the equipment,motor,...is supplied from the DC supply of the substation or if they need AC supply,this provided from AC auxiliary supply of the substation.As you know CVT is used for protection relays and meters.e.g for input of distance relay,measuring center,....When we calculating DC or AC supply cable for equipment,permissible voltage drop is defined by the equipment e.g for motor is 15%,...but CVT cable is not same as AC supply cable.When we want to calculate the cross section of the cable we should know the permissible voltage drop & I want to know this.
best regards

 

[scottf]

The voltage drop across the secondary wiring of a CVT should be no where near 1% or 0.5%.

Keep in mind the point of a CVT (or VT) is to provide voltage sensing to instruments accurately. Having a 1% or 0.5% voltage drop would nearly double the accuracy rating of most CVTs/VTs.

In a practical example, modern relays and meters draw less than 1VA each...for discussion, we'll 1 VA per device. In a typical substation perhaps 6 devices would be connected to the secondary of a CVT, so that would be approx. 6 VA. For a 115 V rated secondary, that's 52 mA in the secondary circuit. In the US, typical cabling size is #10 or #12AWG. We'll use #10 for this example, which is close to 1 ohm/1000 ft. If we assume 200 ft of cable, that's 0.2 ohms, which works out to a 10mV drop in the cabling, or 0.009%. That is the kind of drop you should be expecting in a modern measuring circuit.

Now, if you're running electro-mechanical relays or using the CVT for some type of powering application, then the drop will be a bit higher.

Bottom line use #10 or #12AWG (or equiv) and you will be fine.

 

[sadgol]

Dear scottf
I'm so grateful for your reply.
Do you mean that 1% or o.5% voltage drop is very identical and this can be more?
we define the cross section of the cable as e.g 4mm2 and I didn't undrestand the 10AWG,IF it's posible please explane it more.
thanks a million

 

[slavag]

Hi.
10AWG=6mm^2
12AWG=4mm^2
14AWG=2.5mm^2
16AWG=1.5mm^2

 

[scottf]

Sadgol-

I'm saying that 1% voltage drop on this type of application is NOT acceptable, or normal.

12AWG (4 sqmm) has about 1.5 ohms per 1000 feet rating.

How long is your cabling run and what kind of burden is being placed on the CVT to come anywhere close to a 1% voltage drop?

Using the above example with this wire size, 200 feet would be 0.3 ohms. For a 120 V secondary CVT, 1% drop is 1.2 V, which at 0.3 ohms would be 4A, or 480VA. No way you have that kind of burden on a CVT or VT, unless you're using it for a powering application, in which case, the voltage drop is not really a concern.

 

[sadgol]

Dear scottf
our case is as follow:
400kv substation,400kv/110v CVT,burden of CVT is 10VA,secondary is connected to distance relay(7SA522)& metering equipment,total load is 6VA.lenght of cable is 300meter.We will use cable with 2.5mm^2 cross section.is it correct or not?

 

[scottf]

Sounds fine to me.

Do the calculations and you can calculate voltage drop.

 

[sadgol]

voltage drop is 0.7%.it was my question,is it acceptable or not?should I use 4mm^2 cable or 2.5mm^2 is suitable?

 

[scottf]

I'm not working out to the same voltage drop you are.

6 VA at 110 V is approx. 5.5 mA. 300m of 2.5sqmm cable is approx. 2 ohms. 2 ohms x 5.5 mA is 110 mV or 0.1% voltage drop.

How did you calculate 0.7%?