GE IS200VAICH1C Analog Input/Output Board

The IS200VAICH1C is an Analog Input/Output Board designed by General Electric (GE) for its Mark VI™ Control System. This board is an early version within the VAIC series (including all versions up to and including VAICH1C), primarily used for processing analog signal acquisition and output. It is an indispensable I/O module in gas turbine and steam turbine control systems.

The IS200VAICH1C board accepts 20 analog inputs and controls 4 analog outputs. It works in conjunction with two TBAI terminal boards, each providing 10 inputs and 2 outputs. Connected via cables to the VAIC processor board located in the VME rack, input signals are conditioned, multiplexed, and A/D converted. They are then transferred over the VME backplane to the VCMI board and finally to the controller. For outputs, the VAIC converts digital values to analog currents, driving external loads through the terminal board.

The IS200VAICH1C supports both simplex and triple modular redundant (TMR) applications. In a TMR configuration, input signals on the terminal board are fanned out to three VME racks (R, S, T), each containing a VAIC. Outputs are driven using a proprietary circuit where all three VAICs contribute to create the desired current. If one board fails, the remaining two boards continue to produce the correct output current.

As the final version of the original generation VAIC, the IS200VAICH1C differs from subsequent versions (such as VAICH1D) mainly in output drive capability: when driving 20 mA outputs, it supports a maximum load resistance of 500 Ω (using 1000 ft of #18 wire). It requires operation with TBAIH1B or earlier versions of terminal boards and is also compatible with all revisions of DTAI terminal boards.

The board incorporates comprehensive diagnostic functions, including input limit checks, output monitoring, and relay status checks, with status indication via front panel LEDs. Additionally, the VAIC firmware integrates gas turbine compressor stall detection algorithms, monitoring stall precursors in real-time at a 200 Hz scan rate and triggering shutdown protection within 30 ms.

II. Main Functions

The primary functions of the IS200VAICH1C include, but are not limited to, the following:

1. Analog Input Acquisition

The board accepts 20 analog inputs via two TBAI terminal boards. Input types are selectable via jumpers on the terminal board, supporting:

• 4-20 mA current input

• ±1 mA current input

• ±5 V dc voltage input

• ±10 V dc voltage input

Input circuits feature noise suppression, including hardware filters and configurable software low-pass filters to protect against surges and high-frequency noise. The first 10 inputs (via J3 connector) have a single-pole hardware filter with a break frequency of 500 rad/s. The second 10 inputs (via J4 connector) have a two-pole hardware filter with break frequencies of 72 rad/s and 500 rad/s.

2. Analog Output Control

The board provides 4 analog outputs. Two are fixed at 4-20 mA, and the other two can be configured for 4-20 mA or 0-200 mA. Output circuits also feature noise suppression and utilize a patented redundant drive technique: in TMR systems, each output is driven by three VAICs together. The actual output current is measured with a series resistor, which feeds back voltage to each VAIC. The resulting output is the median value of the three currents. If a board fails, its output is disconnected, and the remaining two boards continue to produce the correct output.

3. Compressor Stall Detection

The VAIC firmware includes gas turbine compressor stall detection functionality, scanning the first four analog inputs at 200 Hz. Two selectable algorithms are available:

• Small LM Gas Turbine Algorithm: Uses two pressure transducers.

• Heavy-Duty Gas Turbine Algorithm: Uses three pressure transducers.

The algorithm monitors compressor discharge pressure (PS3) and its rate of change (dPS3dt), comparing them with configurable stall parameters (KPS3 constants) to determine if a stall is imminent. Once triggered, the VAIC issues a trip signal within 30 ms, causing the controller to shut off all fuel shut-off valves (FSOVs).

4. Input/Output Signal Conditioning

All input and output channels are equipped with signal conditioning circuits, including filtering, amplification, and isolation. Inputs utilize a 16-bit A/D converter with 14-bit resolution, achieving measurement accuracy better than 0.1% of full scale. Outputs utilize a 12-bit D/A converter with 0.5% accuracy.

5. Fault Diagnostics and Self-Test

The board features comprehensive diagnostic functions:

• Each analog input has hardware limit checking (preset non-configurable high/low levels near the ends of the operating range). Exceeding these limits stops scanning that channel and generates a composite diagnostic alarm, L3DIAG_VAIC.

• Each input has configurable software system limit checks, which can be used to generate alarms and configured as latching or non-latching.

• In TMR systems, each input signal is monitored for deviation from the median value. Exceeding a preset limit generates a fault, providing early warning of potential channel problems.

• D/A outputs, output currents, total current, suicide relays, and 20/200 mA scaling relays are monitored for reasonableness.

• The ID device on the terminal board is interrogated by the VAIC. A mismatch generates a hardware incompatibility fault.

6. Status Indication

The front panel features three LED indicators:

• RUN (Green): Flashes during normal operation.

• FAIL (Red): Steady on indicates board failure.

• STATUS (Orange): Illuminates steady when a diagnostic alarm condition exists (normally off).

  
  

III. System Applications

1. Application in Mark VI Control System

The IS200VAICH1C is the core analog I/O processing board in the Mark VI control system, widely used in gas turbine and steam turbine process control, protection, and monitoring. Its roles within the system include:

• Process Parameter Acquisition: Acquires analog signals from sensors measuring temperature, pressure, flow, speed, etc., converting them to digital values for use by the controller.

• Actuator Control: Outputs 4-20 mA or 0-200 mA signals to drive valve positioners, servo valves, variable frequency drives, and other actuators.

• Stall Protection: Monitors compressor discharge pressure in real-time, rapidly detects stall precursors, and triggers emergency shutdown to protect turbomachinery.

• Redundant I/O: In TMR systems, provides triple-redundant analog inputs and outputs, ensuring system operation unaffected by single point failures.

2. Simplex and TMR System Configurations

Simplex System: Uses one IS200VAICH1C board and two TBAI terminal boards. Input signals are directly connected, and outputs are driven by the single VAIC.

TMR System: Uses three IS200VAICH1C boards (located in R, S, T racks respectively) and two TBAI terminal boards. Input signals are fanned out in parallel on the terminal board to the three racks. Outputs use median-select redundancy, with each VAIC contributing a portion of the current. The total output is the median of the three. If one board fails, the remaining two automatically take over.

3. Typical Application Scenarios

• Gas Turbine Control: Compressor inlet guide vane control, fuel control, exhaust temperature monitoring, stall protection.

• Steam Turbine Control: Governor valve control, steam pressure/temperature monitoring.

• Generator Excitation: Excitation voltage/current monitoring.

• Industrial Process Control: Acquisition and control of pressure, flow, level, etc.

  
  

IV. Detailed Interface Description

1. Terminal Board Interface

The IS200VAICH1C connects to two TBAI terminal boards via two 50-pin cable connectors (J3 and J4). Each terminal board provides 10 inputs and 2 outputs. Jumpers on the terminal board select the input type (current/voltage) and output range (20/200 mA).

2. Backplane Interface

The board communicates with the VCMI board and other cards via the VME backplane connector, transmitting digitized input data and receiving output commands. The backplane also provides 24 V dc power for powering external transmitters (via the terminal board).

3. Front Panel Indicators

4. Diagnostic Function Interface

Detailed diagnostic information, including the limit status of each input channel, output current values, relay status, and TMR deviations, can be read via the Toolbox software.

V. Diagnostics and Maintenance

1. Diagnostic Functions List

2. Maintenance Recommendations

• Regularly check diagnostic alarms via Toolbox and address anomalies promptly.

• Ensure terminal board jumpers match the actual sensor types.

• In TMR systems, monitor the status LEDs on each board and replace faulty boards promptly.

• Pay attention to compatibility when replacing boards: VAICH1C must use TBAIH1B or earlier terminal boards.

  
  

VI. Compressor Stall Detection

1. Algorithm Overview

The VAIC firmware includes two stall detection algorithms, scanning the first four inputs at 200 Hz:

• Small LM Gas Turbine Algorithm: Uses two pressure transducers (e.g., PS3A, PS3B).

• Heavy-Duty Gas Turbine Algorithm: Uses three pressure transducers (e.g., PS3A, PS3B, PS3C).

The algorithm monitors compressor discharge pressure (PS3) and its rate of change (dPS3dt), comparing them with configurable stall parameters. Parameters include:

2. Triggering and Protection

When the algorithm detects a stall precursor, the VAIC issues a compressor stall trip signal (CompStall) within 30 ms. This signal is sent to the controller to shut off all fuel shut-off valves (FSOVs), implementing emergency shutdown. The sensitivity of stall detection can be adjusted via configuration parameters.

VII. Compatibility and Configuration

1. Terminal Board Compatibility

The IS200VAICH1C belongs to the original generation VAIC and must be used with the following terminal boards:

• TBAIH1B or earlier (e.g., TBAIH1A)

• DTAI any revision

Incompatible terminal boards: TBAIH1C and later (these are designed for VAICH1D, supporting higher loads).

2. Configuration Methods

• Input type selected via jumpers on the terminal board.

• Output range (20/200 mA) selected via terminal board jumpers.

• Stall detection parameters, system limits, TMR deviation limits, etc., configured via Toolbox software.

• Filter parameters, alarm enables, etc., also configured via software.

3. Firmware and Hardware Matching

The ID chip on the board stores board type and revision information, read by the system to ensure hardware compatibility. When replacing a board, verify the model number matches and update the configuration file (tre file) to match the firmware version.