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Backup power automatic input device (BZT)
Abstract: Most chemical plants require a reliable power supply due to their continuous production processes. Sudden power outages and rapid restoration can lead to explosions, causing personal injuries, equipment damage, and significant economic losses. Polymerization and other phenomena may result in large amounts of waste products, leading to full-line shutdowns. Therefore, it is essential to implement a reliable automatic standby power supply device according to engineering needs to enhance power supply reliability and minimize major economic losses.
Keywords: reliability, automatic backup power supply, interlocking
Most chemical plants have high requirements for power supply. Many electrical devices are classified as secondary load, with some being primary loads. These companies operate continuously, so sudden power interruptions followed by quick restoration could cause explosions, resulting in personal injury, equipment damage, or the production of large quantities of waste products. This leads to full-line parking and substantial economic losses. Hence, ensuring power supply reliability is crucial.
To address this issue, chemical equipment typically uses dual-circuit power supply. Normally, both circuits are active, serving as backups for each other. When one power source fails, the Automatic Power Supply Unit (BZT) automatically switches to the other circuit. This ensures that all loads, including primary and secondary ones, are powered, allowing for automatic restarts to meet continuous production demands. For critical loads requiring higher reliability, solutions like UPS, EPS, or diesel generators are used.
The development of modern power technology also necessitates the installation of various automatic devices in substation equipment. These devices serve several main functions:
1. Ensure reliable power supply and eliminate errors that may occur during manual operations.
2. Reduce the workload of operating personnel and increase productivity.
3. Guarantee the safe and reliable operation of electrical equipment, enabling timely detection and handling of abnormal situations.
Automatic devices must be reliable, selective, sensitive, and quick-acting. The focus of this article is on how to use electrical interlocks to achieve these goals and prevent power outages caused by human error in operational sequences.
First, the basic method of BZT
In the power supply system of chemical enterprises, BZT generally has two basic methods:
1. One normal working line and one hot standby line. BZT controls the incoming switch of the hot standby line. When the normal working line fails, the standby line is automatically put into operation, known as single-direction BZT. This method is commonly used for security power supplies in chemical plants.
2. A substation with two independent working lines. BZT controls the busbar segmentation switch. When one of the working circuits fails, the segmentation switch is automatically activated, allowing the other segment to take over, known as two-way BZT. This method is widely used in chemical plant substations.
Second, the wiring requirements for the standby power supply automatic input device
1. Ensure that the backup power supply is put into the standby circuit after the voltage and working circuit are disconnected.
2. When the voltage on the working circuit disappears for any reason, the automatic input device should delay the action.
3. When the working circuit is manually disconnected, the automatic input device should not activate.
4. The closing pulse of the standby power circuit breaker should be short, and only one operation of the automatic input device is allowed.
5. After the standby power supply is automatically put into operation, if there is a fault, protection should be accelerated if necessary.
6. The current blocking circuit of the working power source can be set in the standby power supply automatic input device.
7. A standby power supply self-injection device equipped with synchronous check or low voltage start should be provided to prevent BZT from being blown by the secondary side fuse of the voltage transformer of the working power source or other reasons.
8. Necessary signals should be set to allow operating personnel to know the working status of the working power supply and the standby power supply in time, such as the standby power supply no voltage signal.
9. For a BZT device with a low-voltage start, when the operating power supply voltage drops or disappears, and the voltage on the power busbar drops or disappears, but the standby power supply bus maintains a certain voltage value, the automatic device is allowed to start.
Third, the composition of the 6kV automatic switching system
The system consists of a low-voltage detection and starting device and a standby power supply automatic input device. This article focuses on the difference from the previous system, which includes adding a voltage transformer on the power supply side and the bus line. The voltage transformer on the power supply side can monitor the power supply side voltage and start the standby power supply automatic input device as a low voltage detection. The detailed interlocking principle of the system is shown in the following logic requirements for the main switch operation principle.
Different from the previous system, there are two more selection switches in the system introduced in this paper. One is an automatic and manual selection switch. When the switch is in the automatic position, the BZT device is activated. When the switch is in the manual position, the BZT is disabled. Another selector switch is the trip position selector switch, which has three positions: Incoming Line A Trip, Incoming Line B Trip, and Bus Trip. The detailed application is described below.
Fourth, the action time and selectivity of BZT
The action time of BZT must be considered in relation to lower-level issues. If there is a higher-level substation BZT, the setting time of the lower-level BZT must be adjusted accordingly. Devices with automatic restart motors must also shorten the BZT setting time to facilitate motor restart. Some chemical devices cannot tolerate long power outages, as they are prone to explosions and other accidents. When there is a conflict between the two, the safety production requirements of the process equipment should be prioritized, and the BZT setting time should be shortened. When the power supply is restored, the power supply is switched to the dual power supply.
In summary, the BZT setting time should be considered comprehensively, taking into account electrical selectivity and the specific requirements of the process equipment. A specific analysis should be conducted for each power supply device to develop an effective power supply scheme.
Reference material
1. The third edition of the Manual for Industrial and Civil Power Distribution Design;
2. "Code for design of relay protection and automatic devices for electric installations" GB50062-92;
3. Electrical Engineering Electrical Design Handbook.