- Transformers
- Transformer Selection and Sizing
- Ratings of Circuit Breakers
- Ratings of Isolators
- Voltage Transformer
- Current Transformer
- Low Power Current Transformers
- Standard Ratings
- Design & Calculations
- Busbar Size Calculation
- Short Circuit Current Calculation
- Capacitor Bank Size Calculation
- Cable Size Calculation
- Voltage Drop Calculation
- Useful Resources
- Substation Design - Useful Resources
- Substation Design - Discussion
Current Transformer for Metering and Protection
In an electrical substation, a Current Transformer (CT) is a type of instrument transformer used to reduce high currents of transmission lines to a lower standard value of typically 1 A or 5 A. Current Transformers are crucial components in a substation as they play an important role in metering and protection applications.
In this chapter, we will understand the process of selecting a CT for metering or protection in substation design, so let's start with the basic definition of current transformer.
What is a Current Transformer?
A current transformer is specially designed electrical transformer used in electrical power system to reduce high values of electric current in transmission lines to a small and standardized values, typically 1 A or 5 A. These small currents are then used in measuring instruments like ammeters or energy meters or protective relays like overcurrent relay, differential relay, etc.
Construction wise a current transformer consists of the following components −
- Primary Winding − It is a single turn of wire or a busbar passing through the current transformer. It is connected to the system or line whose current to be measured.
- Secondary Winding − It is a coil of fine wire having multiple turns and it is connected to the measuring device or protective relay.
- Magnetic Core − This component provides a low reluctance path for magnetic flux to flow from primary winding to secondary winding.
It is clear that construction wise, a current transformer is similar to an ordinary electrical transformer, but it has slightly different design in terms of electrical.
Important Safety Note About CT
The secondary winding of a current transformer must never be left open-circuited while the transformer is in ON state, i.e., when its primary winding carries electric current, this is because a dangerously high voltage will appear across the secondary winding and causes insulation failure and core saturation.
Therefore, to ensure safety, the secondary winding of the current transformer must always be short-circuited when no burden is connected to it.
Functions of Current Transformer
In substation design, a current transformer performs the following key functions −
- Measurement of electric current
- Protection through protective relays
- Isolation between high-current power system and low-current control system
Accuracy Class of CT
The accuracy class of a current transformer can be defined as the measure of precision that the CT provides in scaling of electric current from primary to secondary winding, under specified operating conditions.
Accuracy class of CT provides a standard method for classifying CTs as metering CT or protection CT.
The following are accuracy classes defined for metering and protection CTs −
- Accuracy classes of metering CT − 0.1, 0.2, 0.5, 1.0, and 3.0
- Accuracy classes of protection CT − 5P, 10P, PX, PS, PR
These accuracy classes are explained in the following table in detail −
| Accuracy Class | Error Limit | Application in Substation |
|---|---|---|
| 0.1 | These CTs have an error of less than or equal to 0.1% at the rated current and burden. | They are mainly used where high-precision is required such as revenue metering, laboratory testing, etc. |
| 0.2 | The error limit for class 0.2 CTs is less than or equal to 0.2% at rated current. | These CTs are used for energy billing meters and utility metering points. |
| 0.5 | CTs of this accuracy class has an error of less than or equal to 0.5% at rated current. | Class 0.5 CTs are mainly used in general measurement and operational metering applications. |
| 1.0 | These CTs has an error of less than or equal to 1.0% at rated current. | These CTs are employed to use with indicating instruments like ammeters. |
| 3.0 | The error limit for these CTs is less than or equal to 3.0% at rated current. | These are used for rough indication only, and cannot be used for billing purposes. |
| 5P | These protection CTs have a composite error limited within 5% at rated burden and accuracy limit factor. | They are used for overcurrent and earth fault protection purposes. |
| 10P | These CTs have a composite error of less than or equal to 10% at rated burden and ALF. | They are used where less accurate protection required and error tolerance is acceptable. |
| PX / PS / PR | These are special types of CTs having some additional parameters defined such as low leakage reactance, matched characteristics, and knee-point voltage. | These CTs are used in differential and distance protections. |
Parameters for Selecting a Current Transformer in Substation Design
From the above discussion, it is clear that a current transformer in a substation is used for scaling down high values of system currents to standard low values suitable for metering, protection, and control applications.
Selecting the correct current transformer is important for accurate measurement, reliable protection, and long-life span of equipment and overall system. Selecting an incorrect current transformer in substation design can lead to inaccurate metering of current or energy and protection failure.
Given below are some of the important parameters for CT selection in substation design −
System Voltage and Insulation Level
The current transformer to be selected for a substation application must be rated for the maximum system voltage and capable to withstand with transient overvoltage. The insulation level of a CT is generally expressed in terms of −
- Rated system voltage (in kV)
- Power frequency withstand voltage (kV for 1 min)
- Impulse withstands voltage (kVp)
For example, in a substation of 400 kV system voltage, the insulation level can be 420/630/1425 kV, as rated voltage/power frequency withstand voltage/impulse withstand voltage.
Rated Primary Current
The primary winding current carrying capacity must be equal to or greater than the maximum load current of the transformer or feeder or transmission line.
The value of current can be calculated by using the following formula −
$$\mathrm{I \:=\: \frac{S}{\sqrt{3} \:\times\: V}}$$
Where, "I" is the system load current, S is the apparent power of the system, and V is the system voltage.
For example, if a power transformer has an MVA rating of 500 MVA and the line to which it is connected is of 400 kV, then its load current will be,
$$\mathrm{I \:=\: \frac{500 \:\times\: 10^{6}}{\sqrt{3} \:\times\: 400 \:\times\: 10^{3}} \:=\: 722\:A}$$
Therefore, for measuring the current of this transformer, we can use a CT of rated primary current equal to 800 A.
Rated Secondary Current
The standard secondary current ratings available are 5 A or 1 A. The selection of secondary current rating of CT depends on the distance between CT and metering device or protective relay.
A 5 A CT is typically used when meters or relays are located near the CT, i.e., short cable runs. While 1 A CT is used for long cable runs where the meter or relay is installed at a far distance from the CT, this is done to reduce copper losses in cable.
However, modern electrical substations are generally using 1 A CTs as a standard.
Burden/VA Rating
In the context of CT, burden is nothing, but the total load connected including meters, relays, and connecting cables to the secondary winding of the CT and it is expressed in VA (Volt Amperes).
While selecting a correct current transformer in substation design, it must be ensured that the CT has at least the same VA rating as the connected load/burden. This is crucial for maintaining the accuracy in measurement and protection. Therefore,
$$\mathrm{connected\:VA \:\: \leq \:\: Rated\:CT\:Burden}$$
For example, a meter of 5 VA, a relay of 10 VA, and wiring of 2 VA, then total connected VA is 17 VA. Hence, we can select a CT with rated burden of 20 VA.
Accuracy Class
Accuracy class of CT defines the limit of error in its performance. For example, high accuracy class CTs like 0.1, 0.2, 0.5 are used for energy/revenue metering while CT accuracy class of 5P or 10P are used to ensure correct relay operation under fault conditions.
Short-Time Thermal Current
The current transformer must be capable in handling fault currents for at least 1 second. The short-time thermal current of a CT has a value of typically 25-50 times of the rated current. For example, a CT with 800 A rated current can have 50 x 800 = 40 kA system fault current.
Standard Compliance
The current transformer being selected for measurement and protection in substation design must comply with national and international standards like IEC 60044, IEC 61869, IS 2705, or IEEE C57.
Frequency
The rated frequency for which the CT is design must match the system frequency, like in India it is 50 Hz and in some other regions, it is 60 Hz.
Step-By-Step Procedure for Current Transformer Selection
One can follow this stepwise procedure for selecting a correct CT in substation design −
Step 1 − First of all define the application for which CT to be used, such as metering, protection, or both.
Step 2 − Determine the system parameters of the substation like voltage, MVA, fault level, etc.
Step 3 − Calculate the system load current to determine the rated current of primary winding of CT.
Step 4 − Select the secondary current rating, either 1 A for long cable runs or 5 A for short cable runs.
Step 5 − Estimate the burden to be connected to the CT by adding total VA power rating of meters, relays, and wirings.
Step 6 − Select the accuracy class depending on the application, like 0.2 for metering or 5P20 for protection.
Step 7 − Then, verify that the CT's short-time thermal current rating matches with the system fault level.
Step 8 − Finally, confirm that the CT's design complies with the regulatory standards such as IEC, IS, IEEE, etc.
Practical Example of Current Transformer Selection in Substation Design
Consider a substation of 400 kV having a load current of 1500 A under normal operating conditions. We will perform the following procedure to select the correct CT for this substation application −
- Determine the primary current rating, which is 1500 A under normal operating conditions, and 10 times (it could rise) of the rated current under fault condition, i.e., 15000 A.
- Select secondary current rating. In India, it is common to take 5 A for protection CT and 1 A for metering CT.
- Then, select accuracy class for metering and protection, which is recommended 0.2 or 0.5 for metering CTs, while 5P10 and 5P20 for protection CTs.
- Select burden or VA rating of CT. For protection CTs, burden is taken between 10 VA and 15 VA. While for metering CTs, it can be up to 30 VA depending on the metering system.
Finally, we can select the correct CT for the given 400 kV substation as follows −
| Parameter | Metering CT | Protection CT |
|---|---|---|
| Primary current rating | 1500 A | 1500 A |
| Secondary current rating | 1 A | 5 A |
| Accuracy class | 0.5 | 5P10 |
| Burden | 15 VA | 15 VA |
Conclusion
Current Transformers are an important component of an electrical substation. In this chapter, we covered the fundamental theory of Current Transformers and the specific methods of selecting a correct Current Transformer, for either metering or protection in substation design.