Capacitive voltage transformer is mainly used for measurement of voltage and for protection purpose. It is also used for power line communication between different substations. In this article we are going to have details of the parameter which are mentioned on a nameplate of capacitive voltage transformer. So, to get these details read the following article.
In most of the countries capacitive voltage transformers are manufactured as per IEC 61869 – 5. This IEC also gives us the parameters which should be marked on the name or rating plate of the capacitive voltage transformer. The parameters are given below. we’ll see these parameters one by one.
- Name of the manufacturer
- Insulation level
- Primary capacitance
- Secondary capacitance
- Highest system voltage
- Rated Voltage factor
- Rated frequency
- Simultaneous burden
- Thermal burden
- Total weight of CVT
- Total weight of oil
- Year of manufacturing
- Primary and secondary voltage
- VA burden
- Accuracy class
- Fuse rating
1. Name of the manufacturer.
This will show you the name of the manufacturer.
2. Insulation level.
Now, here you can see there are two values mentioned one is in RMS and other is in Peak. So, let’s understand what are these values. The first value which is mentioned in kV rms, is called as Power frequency withstand voltage, and the second one mentioned in Kv peak is called as Lighting impulse voltage. These are the highest system voltages use to check the insulation properties of the equipment. It can also be called as insulation levels, if combined, which you can seen on the nameplate.
Power frequency withstand voltage can be caused by these reasons
- Phase to earth faults
- Load rejection
- Ferro resonance
- Ferranti effect
Lighting impulse voltage is generally caused due to lighting strokes. Voltage generated due to these reasons can cause insulation failure of CVT or any other equipment, and hence it is important to check the insulation properties the equipment. And to do so, power frequency withstand and lighting impulse test is performed as per IEC and based on the values defined by IEC. So, on the nameplate you can see for 245kV voltage level the values defined are 460kV rms for power frequency and 1050 kV peak for lighting impulse.
This will give information about the standard as per the equipment is manufactured.
CVT is a device made up of capacitors. It is the combination of two series capacitance C1 & C2, as shown in the following figure. And the equivalent capacitance of C1 & C2 is given on the nameplate which is 4400pf. Values mentioned in bracket is the allowed tolerance in the equivalent capacitance. Standard values of equivalent capacitance are 4400pf, 6600pf & 8800pf.
As we know CVT is the combination of two capacitance C1 & C2. C1 is called as primary capacitance whose value we can find on the nameplate. Similarly C2 is called as secondary capacitance whose value is also mentioned on the nameplate. So, if you calculate equivalent capacitance of these two capacitors you’ll get the value equal to 4432.8pf which is within the tolerance. But remember one thing values of C1 & C2 can be different for different manufacturer based on their design because the ultimate aim is to get the equivalent capacitance equal to 4400. So, I can play with values of C1 & C2.
7. Highest system voltage.
This is the highest system voltage for which the CVT is designed. This is also called as rated voltage. So, on the nameplate it is mentioned as 245kV.
8. Rated Voltage factor.
On the nameplate you can see it is mentioned as 1.2 continues & 1.5 for 30 seconds. What does it indicate? Let’s see that. First thing is the number mentioned here are in per unit system i.e 1.2 PU & 1.5 PU. Voltage factor indicate highest voltage CVT can carry for specific time. So, it is mentioned as 1.2 continues this indicate that the CVT can carry 120% of rated voltage continuously. 120% of 245kV is 294kV and the CVT can carry 294kV voltage continuously. Similarly, it is also mentioned that CVT can carry 150% of rated voltage i.e. 367.5kV for 30 seconds. This feature is required when the system is undergoing over voltages or voltage swells.
9. Rated frequency.
This is power frequency on which the power is generated transmitted and distributed. In some countries it is 50 Hz and in some it is 60.
10. Simultaneous burden.
This indicates how much apparent power we can take at the same time from all the provided windings without affecting the accuracy of metering core I.e. 0.2 class. Secondary windings of CVT is generally connected on a same core and hence the burden connected on the one winding may affect the accuracy of other winding. And hence it is specified that, how much amount of total VA burden you can connect on all the windings of CVT without affecting the accuracy of 0.2 class. On our nameplate it is mentioned as 100VA, which means we can connect total 100VA on all the windings put together and still get the accuracy of 0.2.
11. Thermal burden.
This indicates the total amount of apparent power we can take form CVT at a same time without increasing the temperature of CVT above the specified limits by IEC. On the nameplate it is mentioned as 300VA, which indicate we can take up to 300VA power without increasing the temperature of CVT.
12. Total weight.
This will give us the total weight of CVT in kilograms.
13. Total weight of oil.
This will give us the total amount of oil used in CVT in kilograms.
14. Year of manufacturing.
This will show the year in which the CVT is manufactured.
Now on the nameplate you can see there is table given on the right hand side of the name plate. Let’s see the details of parameters given in this table.
15. Primary and secondary voltage.
Primary voltage you can see mentioned as 220000V / root 3. Remember one thing, instrument transformers are single phase devices, which are connected across phase and neutral in case of CVT, and in series in case of current transformer. And that’s why the voltage is divided by root 3. Similarly the secondary voltage is given as 110V / root 3. This CVT has 3 different windings and hence you can see secondary voltage is mentioned 3 times, which indicates 3 windings. As per IEC 100V / root 3 or 110V / root 3 are the standard values of secondary voltages.
16. VA burden.
Now, whatever circuit you are going to connect to the secondary winding of CVT, it is called as burden. This includes the equipment which we are going to connect to the secondary like relay or meter, and also the wires we are going to use to connect these equipment to the secondary winding. So as per the name plate we can connect burden up to 50VA on all the three windings. Standard values of secondary burdens as per IEC are 1VA, 2.5VA, 5VA, 10VA, 25VA, 50VA, 100VA.
17. Accuracy class.
As we know CVT can be used for measuring as well as protection purpose. And hence different windings are used for different purposes. There will be a separate winding provided for metering purpose and separate for protection purpose, as providing separate windings offers many advantages. So, for winding used for metering purpose there are some accuracy classes given by IEC which you can see on the screen. These numbers have specific meaning to it. So, for example, accuracy class of 0.2 indicates voltage error equal to plus or minus 0.2%. Similarly, 0.5 class indicates voltage error equal to plus or minus 0.5%. Class 1 indicate error of 1% and class 3 indicate error of 3%. so, the lesser is the voltage error the more accurate CVT is. This was about the accuracy classes of metering CVT. There are accuracy classes of protection CVT also. 3P & 6P. 3P class means, even if we apply 2% of rated voltage to CVT, the voltage or ratio error should be within plus or minus 6%. Similarly, 6P class means, even if we apply 2% of rated voltage to CVT, the voltage or ratio error should be within plus or minus 12%. So, of course 3P class is more accurate and better than the 6P class. So, as you can see on the nameplate out of three, two windings are used for protection purpose with accuracy class of 3P, and one winding is used for metering purpose with the accuracy class of 0.2.
18. Fuse rating.
On the secondary winding fuses are provided for protection purpose which you can see here. F1, F2 & F3 indicate the different fuses used for three windings. And the rating of the fuse is 10A. Instead of fuses one can also use MCB.
I hope you found this article helpful. If you like the article do share it. Thanks for reading.
You can also watch my video on the same.