0 Introduction
"Power dips" refer to power quality events caused by lightning strikes, short circuits to ground, reclosing, equipment startup, power plant failures, and other reasons, resulting in short-term voltage loss, large fluctuations in grid voltage, or brief power outages lasting several seconds. Chemical enterprises have high requirements for the reliability of their power supply systems. Once a power supply system dip occurs, it can cause undervoltage protection devices to malfunction, production equipment to shut down unexpectedly, leading to production line paralysis, escalating accidents, significant economic losses, and even threatening the safety of operators.
1. Commonly used measures to prevent power fluctuations
1.1 UPS Anti-Power Flicker System
The operating power of control systems such as DCS and PLC is supplied by a UPS to achieve the purpose of resisting power fluctuations. The working principle of an online UPS is shown in Figure 1. When the power grid is working normally, it supplies power to the load and charges the energy storage battery at the same time. When the mains voltage is low or suddenly drops, the UPS starts to work and supplies power to the load from the energy storage battery.
Figure 1 Working principle of online UPS
When a power outage occurs in the system, the contactor coil continues to operate normally with the UPS power supply, maintaining the main contacts engaged to prevent motor shutdown caused by the power outage. If the busbar loses power for a certain period, the output is disconnected according to the time set by the secondary control unit to prevent accidents after voltage recovery. Its control wiring diagram is shown in Figure 2.
Figure 2 UPS control wiring diagram
1.2 DC-BANK Anti-Power Flicker System
The following methods can be used to protect frequency converters from power fluctuations:
(1) Cancel the low-voltage protection setting of the frequency converter and set fast restart. The disadvantage of this method is that the stopping and restarting of the key motor will affect the continuity of production and increase the number of defective products. In addition, low voltage often manifests as the overcurrent protection of the frequency converter, and canceling the overcurrent protection will increase the risk of damage to the frequency converter itself. Therefore, this method is rarely used in petrochemical enterprises with high requirements for continuous production.
(2) The DC-BANK system is mainly used in variable frequency motors and PLC/DCS power supply systems. When the power grid is normal, the frequency converter is powered by the AC bus, and the DC-BANK system is in hot standby mode. When the power grid voltage drops or the standby automatic transfer switch is activated, the power grid voltage drops, and the power supply is switched from the DC-BANK to the DC bus of the frequency converter, and the frequency converter continues to operate normally. Its working mode is shown in Figure 3, and the single-unit control logic diagram is shown in Figure 4.
Figure 3 DC-BANK System Operating Mode
Figure 4. Single-unit control logic of DC-BANK system
1.3 Anti-voltage fluctuation measures for electric motors
AC contactors are widely used in low-voltage motor control systems. A common motor control circuit is shown in Figure 5. When a voltage drop occurs, the contactor disconnects, causing the motor to stop.
Figure 5 Motor control circuit
Motor anti-voltage fluctuation mainly relies on contactor anti-voltage fluctuation. Methods for anti-voltage fluctuation of AC contactors include:
(1) An anti-slip contactor (a contactor with delayed release/avoidance of the bounce zone) is used. When a slip occurs, the contactor does not release immediately, nor does it operate in the critical bounce zone. Its installation wiring is shown in Figure 6.
Figure 6 Wiring diagram of anti-slip contactor
(2) Add a delay module to the existing AC contactor. The control circuit is shown in Figure 7.
Figure 7 Control circuit with added delay mode
(3) Add a restart controller. The start control circuit is shown in Figure 8.
Figure 8 Control circuit with added restart controller
(4) Use a motor protector with anti-voltage fluctuation function. The motor protector has protection functions such as overload, phase loss, imbalance, locked rotor, blockage, start-up timeout, overvoltage, undervoltage, grounding, and leakage; measurement functions such as current measurement, voltage measurement, frequency measurement, and power measurement; control functions such as start-up control, anti-voltage fluctuation function, and undervoltage restart; and 4-20mA DC transmitter output, MODBUS, and PROFIBUS communication functions. The anti-voltage fluctuation control circuit (protection mode) of Acrel ARD series motor protector is shown in Figure 9.
Figure 9. ARD series motor protector anti-voltage fluctuation control circuit (protection mode)
The working principle is as follows: Contacts 95-96 are protection contacts. After the protector is powered on, they are normally closed. When a fault occurs or the auxiliary power supply is cut off, they become normally open. Contacts 7-8 are connected in parallel with the start button SB2. When a voltage flicker occurs, contact 7-8 is engaged to keep the control circuit in the start state. After the voltage moment is instantaneous, the restart function can be executed.
The anti-power fluctuation control principle using direct starting is shown in Figure 10. The functions of contacts 95-96 are the same as in Figure 9. Contact 7-8 is a starting control relay. After the protector receives the jog start signal from SB1, 7-8 is energized and self-holding is activated, and the contactor KM coil is energized to connect the motor main circuit. After the protector receives the jog stop signal from SB2, the output of 7-8 is disconnected, the contactor KM coil is de-energized, and the motor main circuit is disconnected. After a power fluctuation occurs, the coil KM is de-energized, the motor main circuit is cut off, and the protector automatically selects to execute "immediate restart", "batch delayed self-start" or "prohibit starting" according to the duration of the power fluctuation.
Figure 10. Anti-power fluctuation control principle using direct starting method
The industry standard for motor protectors (JB/T 10736) describes the function of anti-voltage fluctuation (undervoltage restart) as follows: "If a protector with undervoltage (undervoltage) restart protection function stops due to an undervoltage fault or undervoltage in the main circuit, and the voltage recovers to normal (above the allowable restart setting value) within the 'immediate restart time,' the protector can immediately restore the motor to its operating state before stopping (without starting delay, voltage reduction, etc.); if the voltage recovers to above the undervoltage (undervoltage) restart setting value within the 'delayed restart delay time' set time after the 'immediate restart undervoltage time,' the motor will start after a delay according to the 'delayed restart delay time' (the same process as normal starting), with an allowable error of ±10% for the delay time; if the voltage recovers after the 'delayed restart delay time,' the motor will not automatically restart, and the error of the recovered voltage value will not exceed ±10%."
2. Working principle of motor protector against shaking and setting of related parameters
Taking Acrel ARD series motor protection as an example. To achieve the anti-voltage fluctuation function, the motor protector needs to be equipped with an anti-voltage fluctuation module. The AC input is connected to the input terminal of the anti-voltage fluctuation module, and the output of the anti-voltage fluctuation module is connected to the auxiliary power input terminal of the motor protector. The voltage measurement signal of the motor protector is taken from the upper stage of the contactor to prevent the contactor from disconnecting during a voltage fluctuation, thus ensuring the recovery voltage can be measured. When the line is normally powered, the internal energy storage device of the anti-voltage fluctuation module is in an energy storage state. During a voltage fluctuation, the energy storage device of the anti-voltage fluctuation module supplies power to the motor protector to maintain its normal operation. When the system voltage recovers to the "restart voltage", the motor protector judges the duration of the voltage fluctuation. If the duration is less than the "immediate restart power loss time", the "output relay 7-8" is immediately activated to start the motor; if the voltage fluctuation time is longer than the "immediate restart power loss time" but less than the "allowable power loss time", a delayed restart is performed; if the voltage fluctuation time is longer than the "allowable power loss time", starting is not performed.
The motor protector can achieve immediate restart after power fluctuation, delayed restart after voltage loss, and lockout start due to excessive voltage loss time. It also has comprehensive protection functions such as overload, phase loss, locked rotor, blockage, overvoltage, and underpower protection, which can ensure the stable operation of the motor and reduce system investment. It has practical significance in the chemical industry. The relevant parameter settings are shown in Table 1.
Table 1 Relevant Parameter Settings
parameter | scope | illustrate |
Restart voltage | (75%-95%)Ue | Ue is the system rated voltage. The voltage fluctuation is considered to have ended and the voltage has recovered once the system voltage returns to this set value. |
Immediate restart power outage time | (0.1-0.5)s | The term "power outage time" usually refers to a short power outage time, during which the circuit can be restarted immediately after the power outage is resolved. |
Allowable power outage time | (0.5-10)s | Maximum power outage/voltage loss time: if the time exceeds this limit, the start-up will not be executed. If the time exceeds this limit, the start-up will be executed in batches. This prevents the motor from causing a simultaneous impact on the power grid. |
Restart Delay | (1-60)s | The staggered restart delay time is set for different motors to prevent simultaneous restarts and potential grid disruption. |
3. Conclusion
This paper introduces several common methods for dealing with voltage fluctuations, with a focus on the method of using motor protectors to resist voltage fluctuations.
Source: Automation Applications, Issue 6, 2014
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