EHC Mark I Power System Upgrade for GE EHC Mk I Turbine Control Systems

Outdated power supplies, which are no longer available directly from the manufacturer, have become expensive to maintain and replace. In addition, these power monitor systems have experienced an undesirable number of failures. As a result, MD&A Control Systems Division has designed a complete power system upgrade for the EHC MKI system. This upgrade improves the reliability and availability of the overall system, increases the long-term integrity of analog adjustments and delivers substantial cost savings over the remaining life of the unit.

Circuit Design:

The MD&A upgrade replaces of all six DC power supplies with three new rack assemblies. These assemblies each contain two state-of-the art switching power supplies which reduce heat build-up in the system cabinet and overall power consumed by the EHC system. Each rack contains a redundant power supply-pair for each of the DC voltage levels. One power supply is powered by the PMG and one by house power, retaining the dual AC input power feed from the original system. Each power supply has its own built-in failure-isolated contact output that connects to the power malfunction bus, eliminating the need for the alarm function of the original meter relays.

Additional features include current-sharing and hot-swap ability. Current-sharing has a distinct advantage not available in the original design, where the primary supply carried the entire load and the back-up supply carried none. The former caused a noticeable drop in the system voltage when transferring to the backup supply. Now, remote voltage sensing is used to minimize load related voltage changes. The hot-swappable feature of these new supplies makes replacement simple, even when the EHC has been reset and the system is on-line. The new power supply easily plugs into a power supply slot in the rack adapter.

In addition, modifications have been made to the power distribution system that increase the current-carrying capabilities of the high quality analog power and ground system. This adds card rack power busses, similar to those used in later control systems and minimizes potential differences found on these busses, from one cabinet to another, thereby increasing the integrity of calibration adjustments.

This upgrade also includes a new power monitor panel that replaces all the old power monitor analog meter relays with more reliable and accurate digital models that display readings in volts. The new relays connect directly to each of the power distribution busses on the rear of the card rack. The original system did not have a voltage monitor connected to the distribution bus. This new power monitor does not have a voltage monitor connected to each supply ahead of the auction diode as each power supply has it own built in failure output. If the power supply is in the failure mode an LED will indicate on the front of the supply. The EHC system oscillator output monitors read in true RMS, to 3-1/2 digits. High and low alarm settings are made from the digital display on each meter. Alarm outputs interface directly to the cabinet alarm busses as do the current meter relays.

Installation and Documentation:

All installation packages include a complete circuit schematic showing all components and system interconnections. Also included are step by step installation instructions and line-up instructions. When the upgrade package is installed by personnel familiar with EHC circuitry, the complete installation and testing can be accomplished in approximately five, eight-hour shifts.

For more information on MD&A Control Systems' products and services, please call (970) 224-2223 Ext. 140 or email: amcneill@mdaturbines.com