In power systems, dual power supplies are generally configured to guarantee uninterrupted power supply to critical loads. When the main power source fails, loads need to be quickly transferred to the standby power supply.
This function is commonly realized with the help of digital protection relays, and two typical devices are widely applied: the conventional Backup Auto Transfer (BZT) device and the high-performance fast power transfer device.
While they share the same operational goal, the Backup Auto Switch and automatic fast bus transfer device differ fundamentally in technical principles, switching speed and application scenarios.
Backup Auto Transfer (BZT) vs Fast Transfer: Core Principles and Differences
Overview of Core Differences: Backup Auto Switch and Automatic Fast Bus Transfer
The fundamental difference between BZT and Fast Transfer lies in their switching philosophy and operating speed.
| Comparison Dimension | Backup Auto Transfer (BZT) | Fast Transfer Device |
|---|---|---|
| Core Philosophy | “Switch after outage”: Operates only after busbar voltage loss is detected. | “Disturbance-free switching“: Completes switching early in the fault, before residual voltage disappears. |
| Switching Speed | Slow (typically 700 ms ~ 1000 ms or longer). | Fast (ideally within 100 ms). |
| Switching Direction | One-way: Only from working power to standby power. | Two-way: Supports fault transfer and manual/automatic switching in normal operation. |
| Impact on Equipment | Severe inrush: Low busbar voltage at closing; motor inrush current 4–7 times rated. | Minimal impact: Synchronized closing at residual voltage; nearly no shock to equipment. |
| Technical Complexity | Low: Simple logic, based mainly on busbar undervoltage. | High: Real-time calculation of voltage, frequency and phase angle to capture optimal closing time. |
Principle of Backup Auto Transfer (BZT)
The logic of BZT is based on “break first, then close” sequential control.
Starting Criteria: Three-phase voltage loss on the working busbar, and no current in the working power supply.
Operation Process: After confirming the voltage loss with a short delay, the device trips the working power switch first. Once the switch is confirmed open, it closes the standby power switch.
Characteristics: The process usually takes a relatively long time (in seconds), by which time the busbar voltage has almost disappeared or dropped very low. When the standby power is closed, it is equivalent to directly starting all motors on the entire busbar, generating a huge inrush current (group starting effect), which may cause the standby power overload protection to operate incorrectly.
Principle of Fast Transfer Device
Different from static automatic bus transfer, the logic of the fast transfer device is to capture the optimal closing moment, leveraging the back EMF characteristic of motors.
Starting Criteria: In addition to busbar voltage loss, it also uses predictive criteria such as protection initiation and reverse power initiation, starting to prepare for transfer before the switch trips.
Operation Process: When the working power supply fails, the fast transfer device immediately trips the working switch. At this time, numerous motors on the busbar continue rotating due to inertia, feeding voltage back to the busbar like generators and forming residual voltage. The fast transfer device monitors the frequency difference, phase difference and voltage difference between the residual voltage and the standby power supply in real time.
Transfer Modes: Based on real-time data, the device automatically selects the optimal transfer path:
- Fast Transfer: Closes rapidly when the phase difference between residual voltage and standby power is still small (e.g., <20°), with almost no impact—this is the preferred mode.
- Synchronization Capture Transfer: If the fast transfer window is missed, the device predicts the moment when the phase difference is about to coincide (e.g., 360° or 0°) and closes.
- Residual Voltage Transfer: If both previous modes fail, it closes when the residual voltage decays to a safe value (e.g., 20%–40% of rated voltage). This is equivalent to BZT and serves as the last line of defense.
Fast transfer suits switching of two operating modes.
Single Bus Configuration
Single Bus with Sectionalizer
Backup Auto Switch (BZT) Product Introduction
Functions and Features of Static Automatic Bus Transfer Switch
Main Function: The static automatic bus transfer switch realizes automatic switching of the standby power supply. Common logics include incoming-line backup auto transfer and section (bus-coupler) backup auto transfer.
Technical Features:
- Fixed Logic: The operating logic is based on typical primary wiring schemes, offering strong adaptability.
- Integrated Design: Modern BZT devices integrate protection, measurement & control, and communication, supporting protocols such as IEC 61850.
- Charge/Discharge Logic: Equipped with comprehensive charge and discharge condition checks, ensuring only one valid operation under correct working conditions.
Application Scenarios
Widely used in 110kV and below substations and distribution rooms where power supply continuity requirements are not extremely stringent. Examples include:
- Power incoming cabinets in ordinary factories and buildings.
- Auxiliary loads insensitive to voltage sags and motor speed drops.
Fast Transfer Device Product Introduction
Functions and Features
Main Functions: In addition to fast transfer under fault conditions, it supports multiple initiation modes, including manual bidirectional transfer in normal operation, false trip transfer, and transfer upon abnormal voltage/frequency.
Technical Features:
- Predictive Algorithm: Equipped with an advanced synchronization capture algorithm that dynamically predicts the rate of phase difference change, improving transfer success rate.
- Fault Identification: Effectively distinguishes between bus faults and voltage dips caused by large motor starting, preventing false transfer.
- Multiple Transfer Modes: Provides three switching sequences—parallel transfer, series transfer, and simultaneous transfer—to adapt to different operating conditions.
Application Scenarios
Primarily used in high‑voltage auxiliary power systems (6 kV, 10 kV, 35 kV) of power plants and continuous‑process industries such as petroleum, chemical, and metallurgical plants. These sites host a large number of high‑voltage motors and are highly sensitive to power interruption; even millisecond‑level outages can trigger process interlocks and shutdowns.
Summary
Both Backup Auto Switch (BZT) and Fast Transfer devices are fundamentally designed to ensure power supply reliability.
If your loads have low requirements for power interruption time, or the standby power capacity is limited and cannot withstand large inrush currents, the Backup Auto Switch is fully sufficient.
If your production process is highly sensitive to voltage, and any brief power outage may cause significant economic losses or safety incidents, a Fast Transfer device with disturbance-free switching capability is the only choice instead of the conventional Backup Auto Switch.
For some retrofitting projects with extremely high requirements, a dual configuration of Fast Transfer + Backup Auto Switch can also be adopted as the final safeguard.
