Turn Ratio Testing

How to Choose the Right Turns Ratio of CT/PT?

How to choose turns ratio of current transformer(CT)?

Abstract: Correctly selecting the transformation ratio for Current Transformers (CT) and Voltage Transformers (PT) is fundamental to ensuring the accuracy of electrical measurements and the reliability of protective relaying in power systems. This guide will detail the standardized principles for ratio selection, based on the International Electrotechnical Commission (IEC 61869 series) and industry standards, differentiating between requirements for measuring and protection-class instrument transformers.

Part I: Selection of Current Transformer (CT) Transformation Ratio

The CT transformation ratio Ki is defined as the ratio of the rated primary current I_1n to the rated secondary current I_2n （Ki= I_1n/ I₂_n）=

9). Internationally, the rated secondary current I₂_n is typically standardized at 5 A or 1 A.

1. Core Principles for Selecting the Rated Primary Current I_1n

The selection of I_1nmust balance the requirements for thermal stability and measurement accuracy.

(1) Thermal Stability and Current Carrying Capacity (Upper Limit)

The CT's rated primary current I_1n must be greater than or equal to the maximum continuous operating current I_max expected in the circuit to ensure the long-term temperature rise and stability of the CT winding.

Design Rule: Select a standardized value for I_1n that is slightly higher than I_max .

$I 1n \geq I max$
Engineering Practice: It is common to select I_1n to be about 1.2 to 1.5 times I_max to provide a safety margin.

(2) Measurement and Metering Accuracy (Lower Limit)

To ensure measurement accuracy, the minimum load current I_min must fall within the CT's specified accuracy operating range.

Requirement: Metering CTs (e.g., Accuracy Class 0.2 S) must maintain their accuracy down to a low limit, such as 1% or 5% of the rated current I_1n. Selecting an I_1n that is excessively large compared to I_max will cause the secondary current I₂ to be very small during light load conditions, potentially dropping below the CT's accuracy limit and leading to increased metering errors.

2. Special Requirements for Protection-Class CTs

In addition to the basic principles, protection-class CTs must be able to perform reliably under fault conditions.

Accuracy Limit Factor (ALF) / Saturation: Protection CTs are primarily concerned with their saturation characteristics during a fault. It must be ensured that the Accuracy Limit Current I_ALF= ALF x I_1n covers the system's maximum possible short-circuit current I_scmax. This prevents severe CT saturation during a fault, ensuring the protective relaying equipment receives an accurate current signal for reliable operation.

3. Warning on Ratio Selection Risks

Risk Scenario

Consequence Analysis

Standard Practice

Ratio Selected Too Small

( $I 1n < I max$ )

High Risk. Leads to chronic CT overheating and insulation degradation. Under fault conditions,

severe saturation occurs, potentially causing protection failure to trip (refusal) or false tripping (misoperation).

Strictly adhere to the

$I 1n \geq I max$ principle.

Ratio Selected Too Large

(( $I 1n ＞ I max$ )

Causes the light-load current $I min$ to fall outside the CT's optimal accuracy range, increasing metering errors.

May also reduce the sensitivity of protective relays.

Ideally, maintain

I max

within the

70% ~ 90%

range of

I 1n

Part II: Selection of Voltage Transformer (PT) Transformation Ratio

The PT transformation ratio K_u is defined as the ratio of the rated primary voltage U_1n to the rated secondary voltage U_2n（K_u = U_1n / U_2n).

1. Determining the Rated Primary Voltage U_1n

U_1nmust strictly match the rated system voltage U_systemat the connection point and the PT's wiring configuration.

Phase-to-Ground Connection: Primarily used in high-voltage systems with directly grounded neutrals (e.g., 110 kV and above).

$U 1n = U system / \sqrt3$
Phase-to-Phase Connection: Primarily used in medium-to-low voltage systems with ungrounded or Petersen coil grounded neutrals (e.g., 6 kV, 10 kV).

$U 1n = U system$

2. Standardized Rated Secondary Voltage U_2n

The rated secondary voltage U_2n is typically a standardized value to be compatible with secondary equipment.

Application	Winding Type	Rated Secondary Voltage U2n
Measurement & Protection (Main Winding)	Connected Phase-to-Ground	$100 / \sqrt 3 V$
Measurement & Protection (Main Winding)	Connected Phase-to-Phase	$100 / \sqrt 3 V$
Ground Fault Protection (Auxiliary Winding)	Connected in Open-Delta for Zero-Sequence Voltage $3 U 0$	$100 / \sqrt 3 V$

3. Calculation Example for Transformation Ratio K_u

The ratio K_u is a dimensionless number, while the PT rating is typically specified as U_1n / U_2n.

Example: 110 kV System (Phase-to-Ground Connection)

Rated System Voltage U_system = 110 kV
PT Primary Voltage U_1n = 110 kV/ √3
PT Secondary Voltage U_2n = 110 kV/ √3
Calculation of Ratio K_u:
The PT specification would be indicated as: 110000 / √3 / 100 / √3/ 100 V (referencing the main and auxiliary windings).(referencing the main and auxiliary windings).

Conclusion: Key Design Considerations

Instrument Type

Core Selection Criterion

Key Principles

Applicable Standards

Primary Current

I 1n

1. Satisfy Max Load ( $I 1n \geq I max$ );

2. Ensure Min Current is within Accuracy Range.

IEC 61869-2

Primary Voltage

U 1n

1. Strictly match System Voltage $U system$ ;

2. Match Wiring ( $U/ \sqrt 3$ for Phase-to-Ground or $U$ for Phase-to-Phase).

IEC 61869-3

Recommendation: In engineering design, one should always refer to the latest national and international standards (such as IEC 61869 series) and select the accuracy class and rated ratio based on the specific application (measurement/metering or protection) of the CT and PT.

CT& PT Analyzer JYH-C from Kingrun