1. Fundamental Objectives of CT Configuration in Relay Protection

• Ensure Selective Fault Isolation: CTs must accurately measure fault currents to enable relays to trip only the faulty circuit.

• Guarantee Protection Sensitivity: CTs should provide sufficient current transformation ratio (CT ratio) to detect low-magnitude faults.

• Maintain Protection Speed and Reliability: Proper CT placement and rating minimize protection misoperation or refusal.

2. Key Configuration Principles

2.1 Placement and Location Principles

2.2 CT Rating and Technical Parameter Selection

• Current Transformation Ratio (CT Ratio)

• Matched to the maximum continuous operating current (120% of rated current) to avoid saturation during normal operation.

• For fault conditions, CTs must withstand short-circuit currents (e.g., 20–50 times rated current) without excessive error.

• Accuracy Class and Burden

• Class 5P/10P: For general protection (e.g., overcurrent, earth fault), allowing 5%/10% error at specified fault currents.

• Class TP (Transient Performance): For high-voltage systems with significant transient components (e.g., generator or EHV line protection).

• Accuracy Class: Selected based on protection type:

• Burden Calculation: CT secondary burden (impedance) must not exceed the rated burden to maintain accuracy, considering cable length, relay input impedance, and parallel loads.

2.3 Redundancy and Backup Configuration

• Multiple Secondary Windings:

• Each CT should have 2–4 secondary windings to:

1. Serve primary protection and backup protection separately.

2. Support measurement and metering functions without interfering with protection.

3. Provide spares for future system upgrades.

• Dual CT Sets for Critical Systems:

• Ensure protection continuity if one CT fails.

• Enable independent protection zones (e.g., main and backup differential protection).

• In 220 kV+ substations or primary protection of generators/transformers, dual CT sets (primary and backup) are installed to:

2.4 Polarity and Connection Principles

• Polarity Consistency:

• CTs in differential protection systems (e.g., transformer or busbar differential) must have consistent polarity to avoid false differential currents.

• Polarity marking (e.g., P1 to S1) must align with relay requirements to ensure correct phase comparison.

• Wiring Configurations:

• Star (Y) Connection: Used for phase and ground fault protection, providing a neutral point for ground current measurement.

• Delta (Δ) Connection: Mitigates zero-sequence currents in transformer differential protection or voltage compensation.

• Series/Parallel Connections: Adjust secondary current for relay input compatibility (e.g., 1 A or 5 A ratings).

2.5 Anti-Interference and Grounding Measures

• Shielding and Grounding:

• CT secondary cables must be shielded (e.g., copper braid) and grounded at one end (usually the control room) to prevent electromagnetic interference (EMI).

• Unused secondary windings should be short-circuited and grounded to avoid high voltage induction during open-circuit operation.

• Stray Capacitance Control:

• Long secondary cables may require impedance matching or twisted-pair wiring to minimize signal distortion.

2.6 Compliance with Industry Standards

• IEC Standards:

• IEC 60044-1: Specifications for CTs, defining accuracy classes, transient performance, and testing methods.

• IEC 61850: Digital substation standards for CTs in merged unit (MU) configurations for digital protection systems.

• IEEE Standards:

• IEEE C57.13: Requirements for CTs in power systems, including ratings, tests, and application guidelines.

3. Application Scenarios and Case Studies

• EHV Substation (220 kV–1,100 kV):

• CTs on each transformer winding with 4 secondary windings: 2 for main differential protection, 1 for backup overcurrent protection, 1 for metering.

• Dual CT sets (primary and backup) for busbar differential protection to meet redundancy requirements.

• Distribution Substation (10 kV–35 kV):

• Single CT set per feeder with 2–3 windings for overcurrent protection, earth fault protection, and revenue metering.

• Class 10P20 CTs (10% error at 20x rated current) suitable for medium-voltage fault levels.

4. Advanced Configuration Trends

• Digital CTs (Electronic CTs):

• Use optical fiber or low-power electromagnetic sensors to replace traditional CTs, reducing size, weight, and magnetic saturation risks (e.g., IEC 60044-8 compliant).

• Intelligent Electronic Devices (IEDs) Integration:

• CT secondary outputs merged with IEDs for digital protection, enabling data sharing and adaptive protection schemes.

• Condition Monitoring of CTs:

• Embedded sensors for CT temperature, partial discharge, and secondary burden monitoring to predict failures and optimize maintenance.

5. Conclusion