Online and Offline Insulation Monitoring for Waterworks

Technical reference · English

1. Project Introduction

The insulation status of water pumps, fans and their power supply cables is related to the operation of the water plant. Insulation monitoring and alarm information can help operators detect changes, narrow down troubleshooting, and schedule inspections and maintenance. Combining online monitoring, shutdown inspections and regular preventive tests can supplement information under different operating conditions.

2 Principles of online insulation monitoring

The online insulation monitoring device observes the ground insulation of the ungrounded system through corresponding measurement signals. The selection needs to consider the AC, DC or hybrid circuit, system voltage, distributed capacitance and load characteristics; the adaptive measurement function and monitorable range should be based on the corresponding device data.

Figure 1

Transmit alarms, measured values ​​and fault loop information to the corresponding monitoring terminals to help operators check status and arrange inspections. The communication method, information reachable range and alarm process should be configured according to the on-site system.

Technical requirements:

The following is an example configuration for a water-plant pump system. Set each monitor’s range, interfaces and alarms according to the selected model and actual system requirements.

This solution is designed for insulation monitoring of AC, DC or AC-DC hybrid systems. The measurement method and its applicable conditions should correspond to the equipment model and power supply structure, and the protection cooperation should be determined according to the applicable requirements and project design.

Maximum distributed leakage capacitanceMax. Capacitance1000µF

Offline insulation monitoring

Offline insulation monitoring is used for loads such as water pumps and blowers in a non-energized state to observe the ground insulation of the motor windings. Monitoring devices, display units and switching coordination should be configured according to the equipment description and power supply drawings.

When the motor starts running, the offline insulation monitoring and protection device is disconnected by disconnecting the auxiliary contactor; and when the motor stops running, the offline insulation monitoring and protection device automatically resumes work. The offline insulation monitoring and protection device always monitors the power-off load when the entire motor is in a static state. Through the motor windings, the offline insulation monitoring and protection device can evenly monitor each phase circuit.

When the insulation resistance is lower than the set value, the monitoring device outputs an early warning or fault alarm according to the configuration. Response time, alarm classification and subsequent handling should be determined according to the selected model and project protection plan.

The offline insulation monitoring and protection device can upload the measured value to the background monitoring in the form of 4-20mA analog quantity, or transfer it to the specialized person in charge, so that problems can be discovered early and dealt with in a timely manner.

The following are the offline monitoring configuration requirements in this water plant solution and are not used as general specifications for all devices:

ØSystem voltage: 3-phase AC 400V (line voltage)

The monitoring object is the insulation between the three-phase windings of uncharged motors and the ground. This solution uses a two-level alarm setting, the alarm value range is 100 kΩ ~ 10 MΩ, and the alarm error is required to be within 15%; the actual configuration should match the selected device and system conditions.

ØThe withstand voltage of the offline insulation monitoring and protection device is not less than 2.2kV

ØThe voltage of the insulation measurement circuit of the offline insulation monitoring and protection device (not the supply voltage Us, nor the system voltage Un) is not greater than 25V, and the insulation measurement current is not greater than 10μA

ØThe internal DC resistance (internal resistance) of the offline insulation monitoring and protection device is not less than 1.2MΩ

ØThe leakage capacitance of the applicable system for the offline insulation monitoring and protection device is not greater than 20μF.

Displays insulation resistance in real time and provides 4–20 mA analog output and alarm dry contact output.

Benefits and implementation options of insulation monitoring applications

We know that the high return on the safe operation of water plants depends on: reasonable process production planning, reliable quality equipment, meeting corresponding standards, safety protection measures and high-quality components or components, as well as comprehensive and adequate equipment condition monitoring and protection measures. Through monitoring systems and safety features that meet corresponding standards, operators can detect impending or emerging faults and dangers early to improve the reliability of equipment operation and support the safety management of personnel and equipment. At the same time, hazardous operating conditions and incorrect equipment installations are diagnosed early to reduce the risk of equipment failure and interruption of operation, and effectively manage equipment operating conditions. Make the water plant system operation safer and more reliable. Based on the drawings and requirements provided by Changqiao Water Plant, we provide the following product monitoring solutions. Low-voltage loads adopt offline insulation monitoring and protection measures, while medium-voltage 6kV loads adopt online insulation monitoring and protection methods.

Medium-voltage pump configuration example: two 6 kV bus sections on feeders 601HP–630HP, each connected to two or three medium-voltage pumps and equipped with online insulation monitoring. Confirm the actual feeder layout and equipment quantities against the project single-line diagram.