What role does a microcomputer integrated protection device play in high-voltage switchgear, and how to select it?

Role and Selection of Microcomputer Integrated Protection Devices in High-Voltage Switchgear

In recent years, the application of microcomputer integrated protection devices in medium- and high-voltage power distribution system projects has increased significantly. These devices are user-friendly and overcome the drawbacks of traditional relay protection, such as complex wiring, low reliability, and cumbersome setting and debugging procedures. Microcomputer integrated protection devices feature comprehensive self-diagnostic functions, making detection and commissioning highly convenient.

Once an anomaly is detected, the central processing unit (CPU) commands the signal generator to issue corresponding audible and visual alarm signals. Additionally, various auxiliary functions are easily implemented, such as printing fault information and recording the time of protection actions after an event. Numerous manufacturers produce these devices, each offering products with differing functionalities and hardware configurations, making it challenging to choose the most suitable integrated protection device.

I. Selection of Microcomputer Integrated Protection Devices

To ensure that microcomputer integrated protection devices correctly and accurately fulfill their relay protection tasks, selection during the design phase should be based on a comprehensive evaluation of reliability, response time, maintenance and commissioning ease, and additional functions.

1.1 Reliability of Microcomputer Integrated Protection Devices

The signal input for microcomputer integrated protection devices is the same as for traditional relay protection: voltage and current signals are introduced from voltage transformers (VTs) and current transformers (CTs), converted by transducers into standard signals required by the protection device, and filtered to remove low- and high-order harmonics and other interference signals. Analog-to-digital (A/D) converters then transform the analog signals into digital signals. The CPU performs calculations on the digital input, compares them with preset values, makes judgments, and decides whether to trigger an alarm or trip.

To meet reliability requirements, the measurement and protection input signals are processed and output by independent processing units within the device. This ensures high measurement accuracy and provides ample margin during severe faults. The device should not experience A/D overflow or saturation when the fault signal current reaches 20 times the normal value, which generally satisfies reliability requirements for typical engineering applications.

1.2 Response Time of Microcomputer Integrated Protection Devices

During design and selection, the quality of a protection device can only be judged based on three indicators: computational accuracy, response time, and computational load. These three factors are mutually contradictory: lower computational accuracy and smaller computational load lead to faster response times, while higher accuracy and larger load result in slower response times. Generally, for end-users of the power grid, the computational load should be greater than 3 times, the computational accuracy should be higher than 0.2%, and the maximum response time should be less than 30 ms to meet typical engineering requirements for response time.

1.3 Selection of Other Functions of Microcomputer Integrated Protection Devices

Integrated protection devices contain numerous integrated chips, requiring high-level technical expertise for maintenance. During selection, devices with modular and universal hardware should be preferred, allowing hardware faults to be resolved by simply replacing modules, thereby improving work efficiency.

Additionally, the protection device should have a built-in EPROM module, enabling all setting values to be stored digitally. Field personnel can readily recall these settings for equipment commissioning without needing to rewrite data. To integrate with the overall project's automated monitoring system, the protection device should have communication capabilities, allowing easy network formation via data buses and enabling the transmission of action information to the higher-level automated monitoring system.

2. Relationship Between Integrated Protection Devices and the Plant-Wide Automation Control System

Based on the configuration and communication requirements of the plant automation control system, the automation system for microcomputer integrated protection devices is typically divided into three layers: the switchgear layer, the substation layer, and the central control room.

2.1 Switchgear Layer

The switchgear layer consists of various types of microcomputer integrated protection devices, directly installed on the switchgear. Each device directly handles measurement, protection signals, and control functions for its respective cabinet. Specific functions are as follows:

(1) Incomer Cabinet

• Protection Functions: Instantaneous overcurrent tripping, time-delayed overcurrent tripping.

• Measurement Functions: Three-phase current, three-phase voltage, active and reactive power, active and reactive energy.

• Monitoring Functions: Circuit breaker open/closed position.

• Control Functions: Manual open/close (on cabinet), remote control open/close.

• Alarm Functions: Trip due to fault, warning signals, open/close, device fault, fault recording, etc.

(2) Transformer Cabinet

• Protection Functions: Instantaneous overcurrent tripping, time-delayed overcurrent tripping, inverse-time overloading, single-phase ground fault, heavy gas trip.

• Measurement, Monitoring, and Control Functions: Same as incomer cabinet.

• Alarm Functions: Trip due to fault, light gas, temperature alarm, warning signals, open/close, device fault, fault recording, etc.

(3) Busbar Cabinet

• Protection, Monitoring, and Control Functions: Same as incomer cabinet.

• Alarm Functions: Trip due to fault, device fault, fault recording, etc.

(4) Motor Cabinet

• Protection Functions: Instantaneous overcurrent tripping, time-delayed overcurrent tripping, overload, single-phase ground fault, low voltage, overheat.

• Measurement Functions: Three-phase current, three-phase voltage, active and reactive power, active and reactive energy.

• Monitoring Functions: Circuit breaker open/closed position.

• Control Functions: Manual open/close (on cabinet), remote control open/close.

• Alarm Functions: Trip due to fault, warning signals, open/close, device fault, fault recording, etc.

After data acquisition within their respective switchgear, the protection devices transmit data via a bus to the monitoring computer at the substation layer. This system significantly reduces control cables, shortens on-site commissioning time, and improves work efficiency.

2.2 Substation Layer

Many signals from the substation need to be transmitted to the central control room via the plant's industrial Ethernet, and the substation receives signals from the central control room to issue control commands to the protection devices. The substation layer typically consists of industrial control computers, printers, and monitors. Its main functions include configuring and managing the switchgear integrated protection devices, monitoring system operation, establishing and managing the substation database, and communicating with the central control room.

Due to manufacturers keeping their protection device software and electrical calculation methods confidential, the substation layer must also handle communication protocol conversion to facilitate signal transmission and reception between the central control room and the protection devices.

2.3 Communication Network

Communication between the switchgear and substation can use a MODbus bus network, supporting up to 64 slave stations. Optical isolation is used between the communication network and the devices to prevent external interference. Communication between the substation and the central control room uses an industrial Ethernet with a fiber-optic medium, with a communication rate greater than 1 Mbps.

2.4 Software

System software can use mainstream platforms with international standard architectures, such as Windows NT. Software modules should include: main control software, graphics software, database management software, report generation software, and communication software.

When selecting software, the main control software should have a high degree of modularity. High modularity allows field personnel to readily call up software based on site conditions without additional programming, greatly reducing the operational and maintenance workload for dispatchers and maintenance personnel and improving work efficiency.

3. Issues to Note When Selecting Hardware for Microcomputer Integrated Protection Devices

Additionally, the following issues should be noted when selecting hardware for microcomputer integrated protection devices:

• Use a sealed, reinforced chassis that is resistant to strong vibration and interference, with a compact installation size suitable for harsh environments and cabinet mounting.

• Adopt an industrial-grade dual-CPU structure, with each device containing a main CPU and a communication CPU. The two CPUs work in a mutual-checking mode, improving the device's response time and accuracy, preventing maloperation or failure to operate, and enhancing stability and reliability.

• Full-range temperature automatic compensation allows the device to operate long-term in environments from -20°C to +60°C.

• Measurement and protection signals are processed separately within the device, meeting both accuracy requirements and protection range and reliability requirements.

• Use a dedicated frequency sampling circuit to precisely track grid frequency, making electrical quantity calculations more accurate.

• Use optical isolation for digital input and output signals, and shielded cables for internal cabinet wiring, effectively preventing external interference signals and improving the device's anti-interference capability.

• Use a large-screen LCD display and soft keyboard for clearer numerical display and easier operation.

• After commissioning and operation, the setting values for various protection modes are stored digitally in EPROM, allowing for easy recall after debugging or circuit fault repair.

• Include a comprehensive circuit breaker control circuit suitable for controlling various types of circuit breakers, facilitating substation upgrades.

• Have comprehensive accident analysis capabilities, including protection action event records, electrical quantity signal limit exceedance records, and fault recording.

4. The Role of Microcomputer Integrated Protection Devices in High-Voltage Switchgear

Microcomputer protection devices safeguard circuits against abnormal conditions. Their roles in high-voltage switchgear are as follows:

Microcomputer protection devices possess strong data processing, logical operation, and information storage capabilities, featuring an advanced internal architecture. They offer the complete protection functions of conventional relay protection. By receiving signals from measurement components such as current transformers and voltage transformers, the device can monitor, control, and protect the circuit state. This includes protection against short circuits, overloads, single-phase ground faults, etc. Without a protection device, these functions in a high-voltage switchgear are achieved using relays. With microcomputer protection, additional functions are available, such as easy acceptance of remote control, communication with the upper-level system to transmit current, voltage, power, and energy signals from the circuit, and convenient adjustment of protection settings.