CT Selection Guide in Motor Protection Relay

In motor protection systems, current transformers (CTs) play a key role. They are used to monitor the current of the motor and trigger protection devices to prevent motor overload, short circuit and other faults.

Proper CT selection and installation is critical to ensuring the reliability and safety of your motor system. Here are some guidelines on how to choose CT among motor protectors:

Wiring method

The CT selection in Bluejay’s PR series motor protection relay mainly includes two aspects: 1. CT (current transformer) 2. Zero sequence transformer (ZCT). Let’s talk about the first aspect of CT (current transformer) selection.

1.CT (current transformer)

The CT options provided by Bluejay are CT306, CT305, and CT304. The CT selected by Bluejay by default is CT306 with a rated current of 10A, the rated current of CT305 is 100A, and the rated current of CT304 is 200A.

For CT provided by Bluejay company, the setting ratio of CT Ratio is 1:5. The default setting value of CT ratio is 0.001.

When the rated current of the motor protector is greater than 200A, you need to connect an external expand CT to expand the measurement range. When you need to connect an external expand CT, you need to pay attention to a problem, because the default configuration of PR series motor protection relay in Bluejay is CT306. CT306 The rated current is 10A, and the setting ratio of CT Ratio is 1:5, so the current measured by expand CT should also be multiplied by 5 accordingly.

For example, when the rated current of the motor protector is 300A, in addition to the CT306 with a rated current of 10A, an additional expand CT needs to be configured. When the measured value is 60, it needs to be multiplied by 5. . The CT Ratio needs to be set to 0.005 in the settings.

2. Zero sequence transformer (ZCT)

The zero-sequence current transformer (ZCT) in the motor protector is a special current transformer used to monitor the zero-sequence current in the motor system. Zero-sequence current refers to the current component generated when there is an imbalance between the three-phase currents in a three-phase power system. The application of ZCT can help detect possible faults in the motor system and trigger protective devices to prevent serious damage.

The zero-sequence current transformer (ZCT) in Bluejay’s PR series motor protection relay can be selected according to large and small current categories to meet application scenarios with different needs.。

Some of the main roles of zero sequence current transformers in motor protectors include:

Ground fault detection:

When a ground fault occurs in a motor system, a zero-sequence current usually occurs. ZCT can monitor the zero-sequence current in the motor system. Once an abnormal zero-sequence current level is detected, the protection device can be triggered to isolate the fault and prevent the fault from spreading and damaging other equipment.

Insulation condition monitoring:

ZCT can also be used to monitor the insulation status of motor systems. When there is an insulation fault or insulation damage in the motor system, it may cause an increase in zero sequence current. By monitoring changes in zero-sequence current, insulation faults can be detected in time and necessary maintenance measures can be taken to ensure the safe operation of the motor system.

Load imbalance detection:

Zero sequence current can also be used to detect load imbalances in motor systems. When the loads in the motor system are unbalanced, an increase in zero-sequence current may result. ZCT can monitor changes in zero-sequence current and alert operators by triggering protection devices to take appropriate measures to adjust load balance.

PR 260 Electric Motor Protection Relay

December 26, 2023 No Comments

The PR 260 electric motor protection relay, known as the motor management relay, is designed for motor protection and control, which integrates protection, control, and monitoring functions for electric motors. It is a complete and reliable solution for low-voltage electric motors (Motor voltage below 0.66 KV).

This electric motor protective relay offers up to 11 programmable switching active and passive node inputs and 6 programmable relay outputs. Alarm values and action values can also be set for all motor protection functions at the same time. Just Get the right motor protection solution to reduce downtime, extend your motor lifespan, and improve safety.