The IS200EDEXG1B is a critical protection component within the General Electric (GE) EX2100e digital excitation control system, belonging to the Exciter De-Excitation Control Board (EDEX) family. This board is the core circuit board of the de-excitation module, specifically designed to safely and rapidly dissipate the substantial energy stored in the generator field winding during an emergency shutdown or fault trip. The IS200EDEXG1B corresponds to the Group 1 type of EDEX board, which is the SCR (Silicon-Controlled Rectifier) de-excitation configuration, suitable for static excitation systems requiring fast field suppression.
During a normal generator shutdown, the excitation system feeds the field energy back to the AC source through the inverting bridge. However, during an emergency trip, the field breaker or DC contactors (41A/41B) open immediately. If the energy stored in the field inductance is not rapidly dissipated, it will generate an extremely high induced voltage, potentially damaging the generator rotor insulation, the power bridge thyristors, and associated equipment. The role of the IS200EDEXG1B is to precisely trigger the de-excitation thyristor (SCR) at this moment, establishing a low-impedance freewheeling path through a discharge resistor or inductor. This allows the field current to decay at a controlled rate, safely releasing the field energy as heat.
System Positioning and Working Principle
Within the EX2100e excitation control architecture, the IS200EDEXG1B is mounted on the de-excitation module inside the auxiliary cabinet. Its control commands originate from the EAUX (Exciter Auxiliary Interface Board) or the EXTB board. When a trip occurs, the EAUX/EXTB first opens the 41DC contactor. The change in state of the contactor's auxiliary contacts is converted into de-excitation firing commands, which are sent to the IS200EDEXG1B board via connector J8. Upon receiving the firing command, the board immediately generates a high-frequency square wave pulse train applied to the gate of the de-excitation SCR, causing it to conduct. At this point, the polarity of the generator field winding has reversed, making the SCR anode positive with respect to its cathode. Once the SCR conducts, the field current forms a loop through the SCR and the discharge resistor, causing the energy to be dissipated rapidly.
The IS200EDEXG1B is designed with two completely independent firing control circuits (corresponding to the M1 and M2 control sections), each capable of independently driving the SCR gate. Furthermore, a third firing channel is integrated on the board—the self-firing circuit—consisting of a break-over diode (BOD) network connected between the SCR anode and gate. When the anode-to-cathode voltage exceeds a preset value, the BOD network automatically breaks over, directing the anode voltage to the gate and forcing the SCR into conduction. This "self-firing" mechanism serves as the ultimate backup protection, ensuring that the de-excitation function operates reliably even under extreme conditions of total control power loss and failure of both normal firing circuits, embodying a defense-in-depth design philosophy.
Core Functional Features
1. Dual-Channel Redundant Triggering and Self-Triggering Protection
The IS200EDEXG1B receives two independent firing commands (Firing Command for M1 and M2) from the M1 and M2 controllers via connector J8. Each command activates its corresponding firing circuit, which generates the high-frequency gate drive pulses. The two circuits are independent of each other, sharing a single SCR gate, but the activation of either one is sufficient to trigger conduction. Simultaneously, the break-over diode network continuously monitors the SCR anode voltage. When the voltage exceeds the anode firing voltage setting (selectable from 700 V to 3500 V via wire jumpers on the board's stab connectors), the BOD network self-triggers, directing anode charge to the gate and forcing the SCR to fire. This triple-redundant firing design ensures the absolute reliability of the de-excitation action.
2. Voltage Hold-Up Circuit
The IS200EDEXG1B board integrates a Voltage Hold-Up Circuit. During normal operation, the board is powered by +24 V DC and -24 V DC supplies. In the event of a complete station power failure or loss of control power, the voltage hold-up circuit maintains the voltage at key nodes on the board long enough to ensure the firing circuits can generate the necessary gate pulses to complete de-excitation. Additionally, this circuit provides the wetting voltage for external de-excitation auxiliary contacts, guaranteeing that the firing command can be correctly captured and executed at the instant of power loss. This feature is crucial for ensuring the safe shutdown of the generator under extreme conditions.
3. Hall Effect Conduction Detection
To verify that the de-excitation SCR has actually turned on and is conducting discharge current, the IS200EDEXG1B integrates two sets of non-contact current sensors based on the Hall Effect. A conduction ring (mechanical ring) is mounted on the top edge of the board, encircling the Hall sensors. When the SCR conducts and discharge current flows, the magnetic field generated by the current is collected by the conduction ring and focused onto the Hall sensors, which output a voltage signal proportional to the current. The two sensor circuits are processed independently and can both detect bipolar current flow (for Group 1 SCR de-excitation applications, it is bipolar detection; Group 2 diode de-excitation is configured for unidirectional detection). The detected current status signals are sent back to the EAUX/EXTB through the M1 and M2 channels respectively, from where the controller can confirm the discharge circuit has operated successfully. Two red LED indicators on the board correspond to the two detection circuits; they illuminate whenever current flow is detected, providing intuitive visual indication of the de-excitation status for field maintenance personnel.
4. Configurable Anode Voltage and Jumper Settings
The IS200EDEXG1B offers extensive anode firing voltage configuration capabilities to accommodate different voltage ratings and sizes of SCRs. Through wire jumpers on the board's stab connectors (E1B through E12, E7A through E11, etc.), the break-over voltage of the BOD network and the current limiting resistor values can be set. The documentation provides typical configuration schemes for 53 mm and 77 mm SCRs:
Anode Firing Voltage | BOD Jumper Connection (53 mm) | Current Limiting Resistor Jumper (53 mm) | BOD Jumper Connection (77 mm) | Current Limiting Resistor Jumper (77 mm) |
700 V ± 50 V | E7A – E11 | E1B – E5 | E7A – E11 | E1B – E6 |
1400 V ± 100 V | E7A – E10 | E1B – E3 | E7A – E10 | E1B – E5 |
2100 V ± 150 V | E7A – E9 | E1B – E1C | E7A – E9 | E1B – E4 |
2800 V ± 200 V | N/A | N/A | E7A – E8 | E1B – E3 |
3500 V ± 250 V | N/A | N/A | E7A – E7B | E1B – E2 |
This flexible jumper configuration approach allows the same IS200EDEXG1B board to adapt to various excitation parameters for generators ranging from small to large, reducing the variety of spare parts and enhancing system maintainability and flexibility.
5. Master-Follower Multi-Board Parallel Configuration
For large generators, the current capacity of a single SCR may be insufficient to carry the entire discharge current. The IS200EDEXG1B supports a Master-Follower multi-board parallel operating mode, allowing multiple de-excitation SCRs to share the discharge current. In a parallel configuration, one IS200EDEXG1B board is set as the Master by selecting the "M" position on jumpers JP1 and JP2; the remaining boards are set as Followers by selecting the "S" position. The Master board's control commands and power are transmitted to the Follower boards via EPL1/EPL2 ribbon cables. The Follower boards receive only the firing pulse and power from the Master, and no connections should be made to their J1 and J2 connectors. When the Master board receives a firing command and generates the gate pulse, it simultaneously triggers all parallel SCRs, ensuring even distribution of the discharge current. The Follower boards also possess their own conduction detection capabilities, and their status signals are sent back to the Master via the EPL cables for aggregation and reporting.
Master and Follower Jumper Settings:
Jumper | Board Type | Jumper Position | Configuration |
JP1 | Master | M | Select as Master |
JP2 | Master | M | Select as Master |
JP1 | Follower 1 | S | Select as Follower |
JP2 | Follower 1 | S | Select as Follower |
JP1 | Follower 2 | S | Select as Follower |
JP2 | Follower 2 | S | Select as Follower |
Interfaces and Connector Definitions
The IS200EDEXG1B board provides multiple connectors for power input, receiving firing commands, outputting SCR gate pulses, and Master-Follower interconnection. The connector definitions are as follows:
J8 Connector (6-pin locking connector) — Connects to EAUX/EXTB board:
Pin | Signal Name | Description |
1 | Firing Command for M2 | Firing command from M2 controller |
2 | De-excitation status A | De-excitation status feedback A |
3 | Ret_48VM2 | 48 V return for M2 |
4 | Firing Command for M1 | Firing command from M1 controller |
5 | De-excitation status B | De-excitation status feedback B |
6 | Ret_48VM1 | 48 V return for M1 |
DEPL Connector (2-pin locking connector) — Connects to SCR gate and cathode:
Pin | Signal Name | Description |
1 | Fire Signal Return | Firing signal return (connects to SCR cathode) |
2 | Firing Signal | Firing signal (connects to SCR gate) |
EPL1/EPL2 Connector (25-pin D-sub connector) — Used for Master-Follower interconnection:
Pin | Signal Name | Description |
1 | Master Pulse 1 from Control Circuit 1 | Master pulse from Control Circuit 1 |
2-3 | Common from M1 | Common from M1 |
4-5 | Positive 24 V dc from M1 supply | +24 V DC from M1 supply |
6-7 | Negative 24 V dc from M1 supply | -24 V DC from M1 supply |
8-9 | Common from M1 | Common from M1 |
10 | Positive 24 V dc supplied from EXTB for Conduction Sense Circuit 1 | +24 V DC from EXTB for Conduction Sense Circuit 1 |
11-15 | No Connection | Not connected |
16 | Master Pulse 2 from Control Circuit 2 | Master pulse from Control Circuit 2 |
17-18 | Common from M2 | Common from M2 |
19-20 | Positive 24 V dc from M2 supply | +24 V DC from M2 supply |
21-22 | Negative 24 V dc from M2 supply | -24 V DC from M2 supply |
23-24 | Common from M2 | Common from M2 |
25 | Positive 24 V dc supplied from EXTB for Conduction Sense Circuit 2 | +24 V DC from EXTB for Conduction Sense Circuit 2 |
J17M1 and J17M2 Power Connectors (4-pin locking connectors) — Supply power for M1 and M2 control circuits respectively:
Pin | Signal Name | Description |
1 | +24 V dc | Positive 24 V DC Supply |
2 | 24 V return | 24 V Return |
3 | 24 V return | 24 V Return |
4 | -24 V dc | Negative 24 V DC Supply |
Safety Precautions and Installation/Maintenance
The IS200EDEXG1B board is directly connected to the generator field high-voltage circuit, and high-voltage safety procedures must be strictly followed during installation and maintenance. Before replacing the board, full de-energization and Lockout/Tagout (LOTO) procedures must be completed to ensure the control and auxiliary cabinets are completely powered down. The board is sensitive to electrostatic discharge; an anti-static wrist strap must be worn during handling, and spare boards must be stored in anti-static bags.
Key replacement steps: Disconnect all cables from the board (retaining the wire jumper connections and removing only the E1A jumper), remove the conduction ring located above the Hall Effect sensors, open the PEM Snap Top fasteners, and remove the old board. Set the jumpers and JP1-JP6 of the new board identically to the old board, install and secure the board, replace the conduction ring, reconnect all cables, and finally close the cabinet door.
