The UNS 2880a-P, known as the Control Board (COB), is the undisputed central processing unit of the ABB UNITROL® 5000 excitation system. As the "brain" of the entire system, the COB is responsible for executing all key control, regulation, protection, and monitoring tasks. It hosts the system's core software, which is adapted to specific application needs through parameterization, managing everything from basic excitation control to complex grid support functions.
In UNITROL 5000 dual-channel systems, two COB boards (Channel 1 and Channel 2) are typically configured for redundancy, thereby enhancing system availability and reliability. The COB plays a critical role in both the stable operation of the generator and rapid protection during fault conditions.
2. Key Features & Functions
1. Central Processing and Control System
The COB integrates high-performance microprocessors and Application-Specific Integrated Circuits (ASICs) to run the excitation system's control software. This includes:
Sequence Control: Manages operational sequences such as excitation system start, stop, grid connection, and channel switching.
Closed-Loop Regulation: Contains all control loops, such as the Automatic Voltage Regulator (AVR), Power System Stabilizer (PSS), and manual current regulator, ensuring precise and stable control of generator terminal voltage, reactive power, and other controlled variables.
Protection and Monitoring: Integrates comprehensive protection functions (e.g., overvoltage, overcurrent, under-excitation, loss of excitation) and continuous condition monitoring functions to ensure the safety of the generator and the excitation system itself.
2. Gate Control Unit
The COB contains a critical Gate Control Unit, whose core function is to generate firing pulses to drive the thyristors in the power converters. This unit calculates and generates phase-shifted firing pulses based on the output of the control regulators and the synchronization voltage signal, thereby controlling the converter's output voltage and enabling precise control of the generator's field current.
3. Communication and Data Exchange Hub
The COB is the central node for internal and external system communication, featuring multiple communication interfaces:
ARCnet Fieldbus Controller: Acts as a master node on the ARCnet network (with fixed node addresses: Channel 1 = 1, Channel 2 = 2), responsible for high-speed data exchange with other intelligent devices within the system (e.g., CIN, FBC, AIO).
Service Panel Communication: Connects to the UNS 0874 Service Panel via a dedicated interface for local commissioning and monitoring.
Optical Communication with CMT Tool: Provides an optical interface for connecting the commissioning and maintenance tool, facilitating parameter setting, fault diagnosis, and data logging by engineers.
Optical Communication with Fieldbus Couplers: Supports interfaces with various upper-level control system interface modules (e.g., Modbus, Profibus, Advant), enabling seamless integration with plant DCS or PLC systems.
4. Data Storage and Management
Operational Memory: During normal operation, all parameters and temporary data are stored in volatile memory (RAM).
Permanent Storage: Equipped with Flash-PROM for the permanent storage of validated parameter settings and system configurations, preventing data loss during power outages.
Fault Logger: The volatile memory also serves as a fault recorder. Fault information is retained for up to 2 days after a complete power failure, providing valuable data for post-event analysis.
5. Status and Diagnostic Display
The COB features an integrated seven-segment display for:
Optical Board Status Indication: Provides a visual indication of the board's operating status.
Last Error Code Display: In the event of a fault, it displays a specific error code (format Axxx), greatly facilitating rapid on-site diagnosis and maintenance.
3. Working Principle & System Integration
1. Role in System Architecture
Within the UNITROL 5000 system hardware layout, the COB occupies a central position. It is directly connected via ribbon cables to key devices like the MUB, FIO, PSI, CIN, and EGC, forming a high-speed, reliable local control network. Simultaneously, it communicates with a wider range of I/O devices via the ARCnet coaxial cable. This structure ensures that control commands are issued and field data is acquired with low latency and high determinism.
2. Software and Parameterized Control
The entire excitation system's control logic and functional algorithms are embedded as software within the COB. Users do not need to modify the underlying code but configure and optimize the system through parameterization. These parameters cover everything from basic voltage setpoints to complex PID regulator parameters and protection settings. Using dedicated engineering tools like CMT or SPT, users can easily access and modify these parameters to precisely adapt the standard UNITROL 5000 system to specific generator and grid applications.
3. Signal Processing and Control Flow
Signal Acquisition: The COB acquires raw electrical quantities (e.g., generator stator voltage, current) from the MUB and relevant signals from the excitation circuit (e.g., field voltage, current) from the PSI.
Data Processing: Internal microprocessors and DSPs perform high-speed calculations on the acquired signals to compute derived quantities like active power, reactive power, frequency, and RMS values, which are then compared to setpoints.
Regulator Calculation: Control algorithms (e.g., PID regulators) compute an output based on the deviation, which determines the firing angle for the thyristors.
Pulse Generation and Distribution: The Gate Control Unit generates high-frequency (e.g., 62 kHz) firing pulse trains based on the regulator output and synchronization signals. These pulses are sent via ribbon cables to the CIN and then amplified by the GDI to drive the thyristor bridges.
Continuous Monitoring: Throughout this process, protection modules continuously monitor all critical variables. Upon detecting an out-of-limit condition or fault, they immediately execute appropriate protective actions (e.g., alarm, field forcing, trip).
4. Redundancy and Reliability Mechanism
In a dual-channel system, two COBs operate in parallel. Typically, one is in "Active" mode, controlling the output, while the other is in "Standby" mode, tracking the system state in real-time. If a failure in the active channel is detected, the system can perform a bumpless transfer, where the standby channel takes over control, ensuring maximum continuity of excitation system operation.
4. Device Configuration & Settings
While most COB functions are configured via software parameters, some fundamental and crucial hardware settings must be made directly on the board.
1. ARCnet Node Address Setting
This is the most critical setting for ensuring proper system communication. Incorrect addresses will cause communication failure.
Channel 1 (COB 1): High Byte=0, Low Byte=1 (Decimal Address: 1)
Channel 2 (COB 2): High Byte=0, Low Byte=2 (Decimal Address: 2)
Set using the hexadecimal rotary switches on the board.
2. Frequency and Synchronization Settings (via Jumpers)
For special applications, such as 16⅔ Hz railway power supply systems, hardware configuration is required:
S2:7-8: Used for 16⅔ Hz excitation systems, must be set to OPEN to receive an external synchronization pulse from the EGC.
S2:9-10: Used for normal 50/60 Hz applications, a jumper must be installed.
Resistors R6246/R6249: Used to adapt to three-phase (50/60Hz) or single-phase (16⅔ Hz) field voltage measurement.
3. Temperature Measurement Configuration (via Jumper S3)
S3:1-2: If using UNS 2861b for temperature measurement, this jumper should be OPEN.
S3:3-4: Used for shunt measurement analog input on the PSI, this jumper should be CLOSED.
4. Factory Test and Internal Use Jumpers
Some positions of jumpers S1 and S2 (e.g., S2:1-2, S2:5-6, S1:1-10) are marked "For factory tests only." In normal applications, these must remain in their default open or closed state (usually open) and should not be altered by the user.
5. Application Scenarios & System Advantages
1. Core Applications
As the central controller of UNITROL 5000, the COB is suitable for all synchronous machine excitation systems using this platform, including:
Large thermal power plants and hydroelectric power stations
Nuclear power plants
Large synchronous motors for industrial drives
Pumped storage power plants
2. System Advantages Delivered
High Integration: Integrates control, protection, and monitoring functions into a single unit, simplifying system architecture.
Superior Performance: Powerful processing capability ensures fast and precise control response, meeting the high demands of modern power grids for power quality.
Excellent Flexibility: Easy adaptation to various generator characteristics and grid specifications through software parameterization.
High Reliability: Redundant design, fault self-diagnostics, and permanent data storage mechanisms ensure long-term stable system operation.
Ease of Maintenance: Rich communication interfaces and status display functions make commissioning, monitoring, and troubleshooting more efficient.
