The IS200EDEXG1A is an Exciter De-Excitation Control Board designed by General Electric (GE) for its EX2100™ Excitation Control System. This board is the version within the EDEX series specifically used for SCR de-excitation applications and serves as the core control board in the de-excitation module. The primary function of the IS200EDEXG1A is to rapidly remove stored energy from the generator field during an emergency shutdown when the field breaker or contactor opens, ensuring safe unit shutdown.
In the EX2100 system, the EMIO board initiates the de-excitation command via the EXTB board. The EXTB board opens the 41dc contactor (MDA/MDB) or breaker, and then transfers the de-excitation signal from the auxiliary contacts to the SCR firing circuit on the EDEX board. The EDEXG1A board provides SCR firing, conduction sense feedback, and voltage hold-up functionality, ensuring reliable de-excitation even in the event of a complete system power failure.
The IS200EDEXG1A board includes an SCR firing circuit, a self-firing circuit (composed of a Break-over Diode network), Hall Effect current sensors, a voltage hold-up circuit, multiple interface connectors (J1, J2, J8, DEPL, EPL1, EPL2), stab-on terminals (for anode voltage setting), and configuration jumpers (JP1-JP6). The board design meets industrial-grade standards and is suitable for critical protection applications in power plant excitation systems.
II. Main Functions
The primary functions of the IS200EDEXG1A include, but are not limited to, the following:
1. SCR Firing
The IS200EDEXG1A board receives two firing commands (from the M1 and M2 controllers) via the EXTB board, input through the J8 connector. Upon receiving the command, the firing circuit generates a high-frequency square wave signal, which is connected to the SCR gate via the DEPL connector, causing it to conduct. The firing circuit features redundant design to ensure reliability.
2. Self-Firing Protection
If the main firing circuit fails or is delayed, the self-firing circuit on the EDEXG1A board takes over the firing task. This circuit consists of a Break-over Diode (BOD) network controlled by the SCR anode voltage and connected to the SCR gate. When the anode voltage exceeds the BOD's break-over voltage, the BOD conducts and fires the SCR. This design ensures reliable de-excitation even if the main firing circuit fails.
3. Voltage Hold-Up
The IS200EDEXG1A board features a voltage hold-up circuit that maintains positive 24 V dc and negative 24 V dc voltages even during a complete system power failure, ensuring the SCR can be fired. This circuit also provides voltage for wetting the external de-excitation contacts, ensuring the de-excitation command remains effective after a power loss.
4. Conduction Sensing
The IS200EDEXG1A board uses Hall Effect sensors to detect current in the field circuit. A mechanical ring (flux concentrator) couples magnetic flux to the sensors located at the top edge of the board. Two sensor circuits (1 and 2) can detect bi-directional current flow. The detected current signals are sent to the EXTB board and then to the control system. Two red LEDs indicate the status of each current sensing circuit, lighting up when current is detected.
5. Multi-Board Parallel Configuration
For high-current applications, multiple de-excitation SCRs are required. Each SCR is controlled by one EDEX board configured in a Master-Follower arrangement. The Master board's J1, J2 power and commons are connected to the Follower boards via EPL cables; Followers do not have connections to J1 and J2. Jumpers JP1 and JP2 configure the EDEX board as Master or Follower.
III. System Applications
1. Application in EX2100 Excitation Control System
The IS200EDEXG1A is the core board of the de-excitation module within the EX2100 Excitation Control System. Its roles within the system include:
Emergency De-excitation: Rapidly fires the SCR during an emergency shutdown to discharge the stored energy in the generator field into the de-excitation resistor, protecting the generator and excitation system.
Redundant Firing: Receives redundant firing commands from the M1 and M2 controllers, ensuring reliable de-excitation even if one controller fails.
Self-Firing Backup: Incorporates a built-in Break-over Diode network that automatically fires the SCR if the main firing circuit fails, providing a final layer of protection.
Current Monitoring: Continuously monitors the de-excitation current using Hall Effect sensors and feeds the status back to the control system for diagnostics and recording.
Power Loss Ride-Through: Includes a built-in voltage hold-up circuit to ensure de-excitation can be completed even during a complete system power failure.
2. Typical Application Scenarios
Emergency shutdown protection for synchronous generator excitation systems
De-excitation circuits for steam turbine generators
De-excitation circuits for hydro turbine generators
Protection for gas turbine excitation systems
Multi-SCR parallel de-excitation in high-current excitation systems
IV. Detailed Interface Description
1. Power Connectors J1 and J2
J1 and J2 are 4-pin locking connectors for ±24 V dc power input:
| Pin | J17M1/J17M2 Signal Description |
| 1 | +24 V dc |
| 2 | 24 V Return |
| 3 | 24 V Return |
| 4 | -24 V dc |
2. De-excitation Signal Connector J8
J8 is a 6-pin locking connector for connection to the EXTB board:
| Pin | Signal Description |
| 1 | M2 Fire Command |
| 2 | De-excitation Status A |
| 3 | Ret_48VM2 |
| 4 | M1 Fire Command |
| 5 | De-excitation Status B |
| 6 | Ret_48VM1 |
3. SCR Firing Connector DEPL
DEPL is a 2-pin locking connector for connection to the SCR gate:
| Pin | Signal Description |
| 1 | Fire Signal Return |
| 2 | Firing Signal |
4. Master-Follower Interconnect Connectors EPL1 and EPL2
EPL1 connects the Master board to the first Follower board. EPL2 is used for daisy-chain connections between Follower boards. Both are 25-pin D-type connectors with identical pin assignments:
| Pin | Signal Description |
| 1 | Master Pulse 1 from Control Circuit 1 |
| 2 | Common from M1 |
| 3 | Common from M1 |
| 4 | Positive 24 V dc from M1 supply |
| 5 | Positive 24 V dc from M1 supply |
| 6 | Negative 24 V dc from M1 supply |
| 7 | Negative 24 V dc from M1 supply |
| 8 | Common from M1 |
| 9 | Common from M1 |
| 10 | Positive 24 V dc supplied from EXTB for Conduction Sense Circuit 1 |
| 11-15 | No Connection |
| 16 | Master Pulse 2 from Control Circuit 2 |
| 17 | Common from M2 |
| 18 | Common from M2 |
| 19 | Positive 24 V dc from M2 supply |
| 20 | Positive 24 V dc from M2 supply |
| 21 | Negative 24 V dc from M2 supply |
| 22 | Negative 24 V dc from M2 supply |
| 23 | Common from M2 |
| 24 | Common from M2 |
| 25 | Positive 24 V dc supplied from EXTB for Conduction Sense Circuit 2 |
5. Anode Voltage Setting Stab Terminals
The IS200EDEXG1A board features a series of stab-on terminals (E1A-E12) used for setting the SCR anode firing voltage via wire jumpers. Specific settings are detailed in Section VI below.
V. Configuration and Setup
1. Anode Voltage Setting
Depending on the specifications of the SCR used, wire jumpers must be connected to the appropriate stab-on terminals to select the correct Break-over Diodes and current limiting resistors, matching the required anode firing voltage. The table below lists the jumper connections for different voltage levels (applicable for 53 mm SCR cells):
| Anode Firing Voltage | Break-Over Diode Jumper | Current Limiting Resistor Jumper |
| 700 V ± 50 V | E7A – E11 | E1B – E5 |
| 1400 V ± 100 V | E7A – E10 | E1B – E3 |
| 2100 V ± 150 V | E7A – E9 | E1B – E1C |
| 2800 V ± 200 V | E7A – E8 | E1B – E3 |
| 3500 V ± 250 V | E7A – E7B | E1B – E2 |
Important Warning: If a break-over diode's break-over voltage is exceeded and the BOD is not connected to the gate of the SCR, the EDEX board will be destroyed. Before attempting to fire the SCR by exceeding the break-over voltage of the BOD, make sure that the firing control circuit will fire the SCR.
2. Master/Follower Configuration
Jumpers JP1 and JP2 configure the EDEX board as Master or Follower:
| Jumper | Board Type | Jumper Position | Description |
| JP1 | Master | M | Select as Master |
| JP2 | Master | M | Select as Master |
| JP1 | Follower | S | Select as Follower |
| JP2 | Follower | S | Select as Follower |
Note: JP3-JP6 are always connected and require no configuration.
VI. Installation and Replacement
1. Mounting Location
The IS200EDEXG1A board is mounted within the de-excitation module in the EX2100 exciter auxiliary cabinet. It is secured to an intermediate mounting panel via Snap Top fasteners. A removable flux concentrator ring is installed over the Hall Effect sensors.
2. Replacement Procedure
Safety Warnings:
WARNING: To prevent electric shock, turn off power to the exciter and follow the complete de-energizing procedures outlined in the EX2100 Installation and Startup Guide (GEH-6631). Adhere to all local Lock-out/Tag-out practices. Allow time for capacitors to discharge.
CAUTION: To prevent component damage caused by static electricity, treat all boards with static sensitive handling techniques. Wear a wrist grounding strap and store boards in anti-static bags.
Replacement Steps:
Verify Power Off: Before opening the auxiliary cabinet door, test electrical circuits to ensure power is off.
Label Cables: Verify all cables are correctly labeled for easy reconnection.
Disconnect Cables:
Remove Flux Concentrator Ring: Carefully remove the flux concentrator ring located over the Hall Effect sensors.
Remove Old Board: Open the Snap Top fasteners securing the EDEX board and remove the old board.
Configure New Board:
Place all wire jumpers (including stab-on jumpers) on the replacement EDEX board in the exact same positions as on the old board.
Verify that jumpers JP1-JP6 are in the same positions as on the old board.
Install New Board: Orient the replacement EDEX board in the same position on the mounting panel and close the Snap Top fasteners.
Replace Flux Concentrator Ring: Reinstall the flux concentrator ring.
Reconnect Cables:
Reconnect all cables to the new board as labeled.
Reconnect the wire jumper to E1A.
Ensure all connectors are properly seated.
Restore Power: Close the auxiliary cabinet door, restore power to the excitation system following procedures, and test operation.
