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The Role of Current Transformers (CTs)

Time:2024-11-28   Author:As Beam   Browse:

In power systems, measuring large currents directly from high-current conductors is often impractical. This is where current transformers (CTs) play a critical role. CTs proportionally reduce large currents to manageable levels, enabling accurate measurement and providing necessary power to protective relays and automation devices.

Current Transformer.jpg

 

Applications of Current Transformers

Imagine dealing with a high-current cable with a massive conductor. Measuring its current would theoretically require cutting the cable and connecting it in series with an ammeter. However, conventional ammeters are typically rated for currents below 5A, making such direct measurement impossible.

Using a CT, the cable simply passes through the transformer. The CT induces a scaled-down current proportional to the original, which can then be measured with standard instruments. By multiplying the measured value by the CT's ratio, the actual current can be determined.

Example:
A 500/5 CT reduces current by a factor of 100. If the ammeter reads 4A, the actual current is: 400A.

 

 

Connection and Usage of CTs

1.Wiring Principle

CTs must be connected in series.

The primary winding is connected in series with the circuit to be measured.

The secondary winding is connected in series with all load instruments.

 

2.Proper Selection

Choose the appropriate CT ratio based on the current to be measured to ensure accuracy.

Ground one side of the secondary winding to prevent high-voltage intrusion into the low-voltage side in case of insulation failure, protecting personnel and equipment.

 

3.Avoid Open Circuits in the Secondary

The secondary winding must never be open-circuited.

If it opens, the entire primary current becomes a magnetizing current, causing core saturation, overheating, and possible coil damage.

If an open circuit occurs, immediately disconnect the load and address the issue safely.

 

4.Multi-Winding Configuration

To support multiple devices (e.g., meters and protective relays), CTs in generator and transformer circuits often feature 2–8 secondary windings.

 

5.Placement for Protection

To minimize unprotected areas in primary protection zones, install CTs in overlapping protection areas whenever possible.

CTs are typically positioned on the circuit breaker’s outgoing side or the transformer side to avoid busbar faults caused by CT bushing flashovers.

 

Principle and Fault Handling

CTs perform current transformation and electrical isolation, converting various current levels into standardized ones for secondary instrument measurements. Additionally, they provide safety by isolating high-voltage circuits from secondary measurement devices.


Common Faults Include:

· Open circuits in the secondary winding.

· Overheating.

· Smoke or unusual odors.

· Loose coil screws.


Inspection and Maintenance:

· Check for overheating and abnormal smells.

· Periodically test insulation.

· Ensure the three-phase ammeter readings are within permissible limits.

· Keep porcelain surfaces clean and intact.

Critical Note:
Never allow the secondary winding to open. High potential may develop, damaging the insulation and posing safety risks. Address open circuit issues with insulated tools after reducing the load to zero.


Overload Prevention

Operating CTs under prolonged overload can saturate the core, increasing measurement errors and leading to incorrect readings. Ensure the CT operates within its rated capacity to maintain accuracy and protect the device.

 


New industry Technology regarding to Bussmann fuse, ABB breakers, Amphenol connectors, HPS transformers, etc. 


TAG:   Current Transformers CTs in Power Systems CT Electrical Power Cabinet