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Backup power automatic input device (BZT)
Abstract: Most chemical plants require a high level of power supply reliability due to their continuous production processes. Sudden power outages and rapid restoration can lead to explosions, causing personal injuries, equipment damage, and production losses. Polymerization and other phenomena may result in significant waste products, leading to full-line shutdowns and substantial economic losses. Therefore, it is essential to implement a reliable automatic standby power supply system based on engineering requirements to enhance power supply reliability and minimize major financial losses.
Keywords: reliability, automatic backup power supply, interlocking
Most chemical plants have stringent power supply requirements. Many electrical devices are classified as secondary load, with some being primary loads. Since chemical companies operate continuously, sudden power cuts followed by rapid restoration can cause explosions, resulting in injuries, equipment damage, and difficulties in repair or production. This can also lead to polymerization and the generation of large amounts of waste products, ultimately causing full-line parking and significant economic losses. Thus, there is a higher demand for power supply reliability.
To address this issue, chemical equipment typically employs dual-circuit power supply. Under normal conditions, both circuits are simultaneously powered, serving as backups for each other. The power supply unit (BZT) automatically switches to the other circuit when one power source fails. This ensures that all loads, including primary and secondary loads, are restored, allowing for an automatic restart to meet continuous production needs. For critical loads requiring higher reliability, UPS, EPS, or diesel generators are used to ensure uninterrupted power supply.
The continuous development of modern power technology necessitates the installation of various automatic devices in substation equipment. These devices serve several key 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. Ensure the safe and reliable operation of electrical equipment, enabling operators to promptly detect and handle abnormal situations.
Automatic devices must be reliable, selective, sensitive, and quick-acting. The focus of this article is how to use electrical interlocks to achieve these requirements 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. A normal working line and a hot standby line. The BZT device controls the incoming switch of the hot standby line. When the normal working line fails and is cut off, 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. The BZT device controls the busbar segmentation switch. When normal, the segmentation switch is disconnected. If any working circuit fails and is cut off, the segmentation switch is automatically engaged, allowing the other segment to carry the full load or primary and secondary loads. This is known as two-way BZT and is commonly 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 activated only 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 its 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 the automatic input device should only operate once.
5. After the standby power supply is automatically activated, if a fault occurs, protection should be accelerated if necessary.
6. The current blocking circuit of the working power source can be set in the standby power supply self-injection device.
7. A standby power supply self-injection device equipped with synchronous check or low-voltage start should include a signal or blocking device to prevent BZT from being triggered by the secondary side fuse of the voltage transformer or other reasons.
8. To inform operating personnel about the status of the working and standby power supplies, necessary signals such as no-voltage signals for the standby power supply should be provided.
9. For a BZT device with low-voltage start, when the operating power supply voltage drops or disappears, and the voltage on the power busbar also 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 differences from previous systems, such as 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 voltage and trigger the standby power supply automatic input device through low-voltage detection. The detailed interlocking principle of the system is shown in the following logic requirements for the main switch operation principle.
Compared to the previous system, this paper introduces two additional selection switches. One is an automatic/manual selection switch. When in the automatic position, the BZT device is activated. When in the manual position, the BZT is deactivated. The other is a trip position selector switch with three positions: Incoming Line A Trip, Incoming Line B Trip, and Bus Trip. The specific applications are described below.
Fourth, the action time and selectivity of BZT
The action time of BZT must be considered with lower-level issues. If a higher-level substation BZT exists, the time setting of the higher-level substation BZT should be met first. The device with automatic motor restart must also shorten the BZT setting time to facilitate motor automatic 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 comprehensively considered, taking into account electrical selectivity and the special requirements of the process equipment, and conducting a specific analysis 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.