Insulation Monitoring Solutions for Low-voltage Motors in Large Power Plants
Insulation Monitoring Solutions for Low-voltage Motors in Large Power Plants
I. Project Overview
The stator of an electric motor is generally composed of conductive copper wires, a magnetic silicon steel core and an insulation system. As the most vulnerable part of the motor, the reliability of the insulation system's operation determines the safety and stability of the motor's operation. For motors that have been shut down or are not frequently operated, according to relevant regulations, it is necessary to ensure that the insulation of the motor is good before it is put into operation. Therefore, the traditional manual shaking of insulation has become indispensable. The insulation resistance of the high-voltage motor before starting shall not be lower than the relevant standard (1MΩ/KV), and for motors with a voltage lower than 1000V, it shall not be lower than 0.5MΩ. To meet the above requirements, the common approach is to remove the power cable before starting the motor and manually rotate the motor with a megohmmeter to determine whether the motor's resistance to ground is qualified. If the insulation resistance is low, measures such as restarting the heater will be taken, which consume a lot of manpower, time and resources and pose safety hazards.
There are currently multiple low-voltage motors in a large power plant in Zhuzhou (An industrial city in Hunan Province, China). At present, the insulation of the motors is still measured manually. Insulation monitoring is of great significance for ensuring the safe and stable operation of the motors. It can detect potential safety hazards in advance and deal with them in a timely manner. If insulation monitoring is not carried out, a series of serious consequences may be triggered.
For example:
Motor damage risk: Problems such as insulation aging and damage cannot be detected in time. As the insulation performance continues to decline, the motor windings are prone to short circuits, causing the motor to burn out, seriously shortening the motor's service life and increasing the cost of equipment replacement.
There is a safety hazard: Insulation faults may cause the motor to leak electricity, making the motor casing electrified. Once operators come into contact with it, there is a risk of electric shock, which endangers their lives and may also cause electrical fires and other safety accidents, causing serious damage to personnel and property.
Production interruption: Sudden insulation failure of the motor can lead to forced production interruption, disrupt the production plan and affect product delivery. Not only will it cause direct production halt losses, but it may also face breach of contract compensation due to delayed orders, damaging the company's reputation and market competitiveness.
High maintenance costs: If motor insulation faults are not detected and dealt with in time, the faults may deteriorate further. What was originally a simple insulation repair may evolve into a complex motor overhaul, and even require the replacement of the entire motor, significantly increasing maintenance costs and time.
To improve the above problems, the customer plans to install insulation online monitoring devices and residual current monitors RCMX200 on several low-voltage motors.
2. Insulation monitoring scheme
The low-voltage motor is equipped with the SKIM500 intelligent insulation monitor. This model of insulation monitor can automatically detect the overall insulation level of the windings and coils of the motor when it is shut down, and trigger a lockout or remote alarm signal when the insulation is poor, to prevent the operator from putting the motor with insulation defects into operation. And it is integrated into the existing in-operation monitoring system to achieve local and remote automatic monitoring of the insulation condition of the motor.
Installing an insulation monitor can meet the following requirements:
1. Automatic collection, remote monitoring, real-time reporting of raw data, analyzed data, etc;
2. When insulation occurs in the equipment, the specific motor can be located in time;
3. When the status of the power outage equipment is abnormal, the system will promptly issue an alarm and notify relevant personnel;
4. Easy to install and flexible to connect;
5. It can be integrated with other production and operation management systems in the factory;
6. It can completely replace manual insulation monitoring without opening the cabinet or removing the wires;
7. Achieve remote manual measurement and permanently save the data;
8. Two sets of outputs for early warning and alarm are provided to detect problem motors in advance;
9. Enhance production efficiency and reduce costs;
10. Comply with industry standard requirements;
11. Achieve automated management.
The SKIM500 online monitoring device is shown in Figure 1 below: SKIM500 circuit diagram
The wiring of the SKIM500 low-voltage insulation monitor is shown in Figure 1. When the motor switch cabinet opens, the signal from the auxiliary contact of the circuit breaker is fed back to the insulation monitoring device. After the device determines that the motor has stopped, the main unit automatically applies a DC test voltage (48Vdc) to the motor winding after a delay to test the insulation state of the motor. When the insulation monitor detects that the insulation resistance of the motor winding to ground is lower than the set value, it will output an alarm. When the motor starts, the SKIM500 insulation monitor exits the insulation monitoring mode. The data generated throughout the monitoring process will be uploaded to the background via RS485 and recorded for preservation.
The SKIM500 low-voltage motor insulation monitor can monitor the insulation resistance of the motor winding according to the pre-set shutdown cooling time, measurement time, measurement stop time, early warning and alarm parameters when the motor is shut down. When measuring the motor insulation resistance, it automatically applies a 48V DC voltage. The main unit of the instrument adopts a standard rail installation method. The display interacts with high voltage. The instrument can withstand high voltage impact without applying voltage. The instrument adopts an RS485 communication interface and is connected to the DCS through the standard Modbus_RTU communication protocol.
The product is equipped with three independent relay insulation resistance early warning and alarm output functions, amd the product can monitor the insulation performance of up to three low-voltage motors.
As shown in the above figure, one insulation monitor can simultaneously monitor three AC380V low-voltage motors.
When the motor is powered off (or the backup motor) :
When the motor is powered off and not in operation, the circuit breaker opens, and the auxiliary contacts of the circuit breaker send an enable signal to the insulation monitor. The insulation monitor starts to work, injecting a measurement voltage into the main circuit of the motor to measure the motor windings. The measurement continues for one minute. The measurement data is uploaded to the upper computer for display and storage through the standard RS-485. The upper computer can permanently store the data.
The insulation monitor can set insulation alarm values and insulation early warning values. When the insulation measurement data is lower than the pre-alarm value, an alarm signal will be issued. Early warning and alarm signals can be output as switch quantities through two sets of relays and can be connected to DCS systems, alarmers, and motor interlocking switches.
The measured voltage of the power frequency motor is DC48V.
When the motor is running:
When the motor is running, the circuit breaker closes, and the auxiliary contacts of the circuit breaker send an enable signal to the insulation monitor. The insulation monitor stops outputting the measurement signal and stops measuring the insulation of the motor.
Residual Leakage Monitor Relays RCMX200
The RCMX-200 multi-channel residual current monitor relays, when matched with CT and KCT precision residual current transformers, is used to precisely measure the residual current values of each branch to ground in AC grounded power grids and identify faulty branches. It can also transmit the residual current values to the upper computer via the RS485 communication interface. The RCMX200 residual current monitor supports 12-channel residual current monitoring. The product features strong anti-interference ability, high detection accuracy and flexible networking.
The main functions of RCMX200
1. It can be connected to 12 precision residual current transformers to collect residual current from 12 branches;
2. It is capable of setting fault current alarm values for each branch;
3. Measurement display range: 10mA⁓10A;
4.Modbus RTU and RS485 communication protocols are adopted, which can be connected to various monitoring systems;
5. Two sets of relays can be set with two sets of alarm values;
6.LCD high-definition display screen
