The DARREX AIR™ Waterless Diode Bridge is a direct replacement for the existing water cooled diode bridges commonly found in Alterrex™ Excitation Systems.

DARREX AIR™ replacement bridges are convection cooled and mount on the same back plane of the original Alterrex™ bridge.

DARREX AIR™ is a trademark of Darrah Electric Company.
Alterrex™ is a trademark of General Electric Company.

COST COMPARISON
Typical 4 Bridge Alterrex™ Excitation System

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DARREX AIR™

Background of Water Cooled Alterrex™ Systems

Water cooled excitation systems were developed in the early 1970's to provide DC to the generator field. The system consisted of an AC rotating exciter providing a three phase output connected to multiple remote stationary diode rectifier bridges.

Larger generators requiring higher amperage often had three or more water cooled bridges, connected in parallel. Typically, an additional bridge was connected to provide redundancy ( N+ 1 ) in the event of a failure of one of the bridges. To isolate a defective bridge, and allow maintenance, each bridge included a five pole manual disconnect switch to isolate the AC input and DC output.

The limited semiconductor technology used for excitation in the 1970's forced systems to be water cooled to meet amperage capacities required. These systems were designed with a sophisticated array of plumbing, pumps, valves and copper tubing networked to cool the diodes.

As the existing systems age, numerous problems with water cooled diodes have developed. With time, corrosion and erosion have deteriorated the copper tubing and fittings, resulting in leaks. The water cooled systems often caused costly machine shutdowns at critical time periods when generation was at a premium. Leaks have developed to a point where most systems are unrepairable or repairs would be cost prohibitive. The safety of existing systems is also a prime consideration.

The DARREX AIR™ is a "FORM, FIT, AND FUNCTIONAL" replacement three phase diode bridge to be used with existing generator field excitation systems, such as General Electric's Alterrex™ system. DARREX AIR™ eliminates the maintenance, reliability and safety issues associated with existing water cooled diode rectifier bridges while meeting and exceeding voltage and DC current requirements.

The three phase diode rectifier bridge of the DARREX AIR™ incorporates only six diodes. The original design used 24 lesser amperage and voltage rated diodes to form the diode bridge. Since the original diodes had lower current and voltage ratings, it was necessary to parallel and series multiple diodes to meet the required DC output amperage and voltage rating for the field. Each original diode bridge had a continuous rating of 1200 amps dependant upon water temperature and flow rate. The DARREX AIR™ bridge has a continuous output rating up to 1800 amps operated in an ambient to 50°C with natural convection cooling. Higher ratings can be achieved in a forced air cooled situation. The DARREX AIR™ diode bridge voltage rating is 1000 volts DC.

Each diode of the DARREX AIR™ is compressed between specifically designed aluminum extrusion. This high efficiency extrusion is drawn with 6063-T5 aluminum. The extrusions are cut, and contact surfaces are machined to the recommendations of the diode manufacturer. The entire heat sink has a gold irridite finish. For accurate conductivity each diode is evenly compressed with a specifically designed clamp between two finished extrusions. Each clamp is pre-calibrated and has a built in force indicator. The clamps are double insulated and have a dielectric rating exceeding 8000 volts.

In the DARREX AIR™ AC line fuses are provided to each input phase of the diode bridge. These fast acting semiconductor fuses were selected to limit short circuit peak currents and to clear faults in the bridge assembly. The I²t let through of the fuse will clear before the diode shorts. Individual blown fuse indicators are mounted to each fuse and wired to a terminal strip for annunciation.

The five pole disconnect switch simultaneously isolates the three phase AC lines and both the negative and positive DC outputs. The switch is manually operated and is supplied with a kirk key interlock and auxiliary contacts for annunciation. The switch is a bolted pressure contact type rated at 2000 amps continuous and has a voltage rating of 1000 volts DC.

2000 Amp, 50 milli volt (mv) calibrated shunts are provided in the negative and positive DC output legs from the bridge. The 0 to 50 mV signals from the shunts are wired to a terminal strip for current monitoring the output.

Thermostats and/or Resistive Temperature Detectors (RTD's) are strategically located on each heat sink extrusion and wired to a terminal strip for annunciation. These devices are selected to annunciate an alarm before a failure mode is reached.

Resistor and capacitor snubber circuits supplied across each diode with values chosen to protect the diodes and operate to 1000 volts DC.

Safety barriers are located in front of the switch to protect personnel. Control and data terminal blocks are provided for auxiliary contacts from the five pole switch, blown fuse indicators, RTD's or thermostats and DC current monitoring shunts.

Other options include a replacement door mounted interface panel housing AC and or DC meters, temperature indicators, and pilot light is also available if required.

DARREX AIR™ WIRING DIAGRAM
Showing the three phase diode bridge rectifier and five pole switch

For pricing or application related questions, please visit Basler Electric.