Application of Ankerui motor protection device in a chemical enterprise

1

introduction

The motors for the boiler induced draft and blower fans of a chemical enterprise's thermal power station are all 10kV high-voltage motors with a power of 450kW. Once the motors are damaged, the entire line will be shut down, and the maintenance cycle is long and the cost is high, which has a great impact on production. The boiler induced draft and blower motors have been running in a harsh environment with high temperature and a lot of dust for a long time, which is prone to bearing damage, stator-rotor phase friction failure, mechanical overload causing motor winding insulation aging, stator winding phase short circuit, turn-to-turn short circuit and single-phase grounding. If not discovered and handled in time, once the motor is damaged, it will directly threaten the company's safe production and cause great losses to the company. In order to ensure the safe and stable operation of high-voltage motors and improve the sensitivity and reliability of protection, high-voltage motors all use Ankerui AM5SE-M motor protection and measurement and control devices to achieve the purpose of quickly cutting off faults.

2

Overview of Motor Protection Devices

AM5SE-M motor protection and control device can be applied to high-voltage motors with a power of 2000kW and below. The motor protection and control device has online monitoring functions such as protection, measurement, display, and communication. Through its own perfect protection function, it can play a good protective role against the heat generated by bearing damage, stator-rotor phase friction, inter-turn short circuit, etc. of the motor. It can be installed on the high-voltage switch cabinet and connected to the power monitoring system Acrel-2000 through serial port or Ethernet communication. The monitoring system can monitor and remotely control in real time, and it is easier to realize automatic control and online monitoring.

3

Protection configuration and principle

AM5SE-M motor protection and control device has a full set of motor protection, including overcurrent protection, overload protection, overheat protection, stall protection, long start time protection, phase loss protection, current imbalance protection, low voltage protection and overvoltage protection. It also has measurement, monitoring, operation, fault recording and communication functions. Users can choose to start or stop according to their needs.

Overcurrent protection

The overcurrent protection in the AM5SE-M motor protection and control device is mainly used for short-circuit protection inside the motor and the incoming line. It is mainly divided into one-stage overcurrent protection during startup/operation, two-stage overcurrent protection, and inverse time overcurrent protection.

The current of asynchronous motors is very high during the starting process, usually reaching 5 to 8 times the rated current (Ie), and the starting time can be as long as tens of seconds. The device is set with two overcurrent settings. During the starting process, the "starting overcurrent setting" is used. This value is set according to the motor starting current. After the motor starting process is completed, the "running overcurrent setting" is automatically used. This value is based on the motor's self-starting current and the motor feedback current when the outlet is short-circuited outside the zone, and the larger of the two currents is taken.

Overheat protection

Overheat protection protects the motor from shortening its life or damage due to overheating by tracking and calculating the thermal capacity of the motor during operation. Motor overload, long start time, stall, etc. will generate large positive sequence current; while phase failure, asymmetric short circuit, and asymmetric input voltage will simultaneously generate large positive and negative sequence currents. Based on the heating characteristics caused by the positive and negative sequence currents of the motor stator, thermal overload protection can be provided for the above faults.

The positive and negative sequence comprehensive measurement value Ieq is used as the equivalent current to simulate the heating effect of the motor, that is:

Where: Ieq: equivalent current

I1: Positive sequence current

I2: Negative sequence current

K1: positive sequence current heating coefficient, during motor starting, K1 = 0.5, after starting, K1 = 1

According to the inverse time characteristics of the motor heating model, in order to reasonably protect the motor, the relationship between the protection action time t and the equivalent current Ieq has the following two curves to choose from:

（1）

Where: τ: overheating time constant

I∞: The current value that allows the motor to run for a long time, which can generally be set to 1.1

（2）

Where: τ: overheating time constant

I∞: The current value that allows the motor to run for a long time, which can generally be set to 1.1

Ip: load current before overload. If it is in cold state before overload, Ip=0

Select one of the above two curves for calculation. When the heat accumulation value reaches τ, the device will send out an alarm signal or the protection will trip.

Figure 1 Overheat protection logic

Stall protection

The stall protection is to prevent the motor from burning out due to a sudden increase in current caused by rotor blockage during operation. This may occur when the rolling bearing is broken or the load torque suddenly increases. The stall protection is locked during the motor startup process, and the stall protection is reasonable after the motor enters the running state. In a sense, the stall protection can be used as a backup protection for short-circuit protection during the operation of the motor. When any phase of the three-phase current IA, IB, IC of the motor exceeds the stall current setting value and there is a low speed signal input, the protection trips after the set delay time is reached.

Figure 2 Stall protection logic

Startup time too long protection

After a normal start, the motor's operating current will be lower than the rated value or close to the rated value, while if the start time is too long, the motor's operating current will remain at a relatively high value after the start time (generally due to mechanical reasons). The AM5SE-M motor protection and control device can automatically identify whether the current process is a start process. If so, when the set start time is reached and the motor's three-phase current is still greater than the start current setting, this protection will trip.

Phase failure protection

Phase failure protection is to prevent the motor from being disconnected during operation, which may cause the two phases of the motor to overheat and burn out. The phase failure protection function detects the circuit voltage and determines the phase failure fault in the motor circuit. After detecting the phase failure fault, the phase failure protection function starts and triggers the delay, and issues a trip command after the set delay ends.

Current unbalance protection

The current unbalance protection is to prevent the motor from overheating due to three-phase current imbalance. When the current imbalance exceeds the current imbalance setting value and the motor is in the running state, the device protection trips or alarms after a delay.

Figure 3 Current unbalance protection logic

Low voltage protection

When the motor is running at low voltage, the torque drops sharply, which will cause serious overload of the motor. Therefore, low voltage protection is set. When the three line voltages are all less than the low voltage trip/alarm setting value, after a delay, the device trips or alarms. In order to prevent the protection from malfunctioning due to PT disconnection, a PT disconnection lock is set. When a PT disconnection occurs, the device will send an alarm signal and lock the pressure loss protection. The lock condition can be selected to be put into or out. The device can be set to add the closing position as a condition for judging the pressure loss. In addition, the device can choose when to release the low voltage fault information according to the user's use occasion. If the low voltage threshold is put into the withdrawal, the device voltage is less than the no-pressure setting, and the protection action can be returned. If the low voltage threshold is withdrawn, the device voltage needs to be restored to the normal voltage before the fault information can be released. Low voltage protection opening condition: one of the three line voltages is greater than 1.05 times the low voltage setting, and the delay is 500ms. Once this condition is met, the low voltage protection is reasonable.

4

Problems and modifications of the device

Although the AM5SE-M motor protection and control device has a full set of protection functions for high-voltage motors, when the fault protection is activated, the fault has already occurred and the machine can only be stopped for processing. When the device is operating normally, it only monitors the operating voltage and current of the motor. It is difficult to find hidden faults based on these alone. In order to discover potential accidents as early as possible and eliminate accidents in the bud, after sufficient research and demonstration, it is believed that the damage to the motor is mostly caused by heating of the windings and bearings. The overheating protection only comprehensively calculates the thermal effects of the positive and negative sequence currents of the motor. If the bearing continues to run after heating, the motor may be damaged. Therefore, a PT100 resistance temperature sensor is embedded inside the motor to measure the temperature of the windings and bearings inside the motor, and the PT100 is connected to the ARTM-8 temperature inspection instrument. The ARTM-8 is used to monitor the temperature in real time. At the same time, the temperature data is also transmitted to the Acrel-2000 power monitoring system through the RS485 communication interface, which is convenient for operation and maintenance personnel to view the temperature in real time, providing technical support for the safe and stable operation of the equipment.