Surge Suppression for Drilling Rigs
Two different methodologies are deployed on Drilling Rigs for Surge Suppression, but which is the better?
Why Surge Suppression?
Noise Spikes on AC Supply
The purpose of the surge suppression circuit as fitted to Ross Hill and Hill Graham SCR systems for drilling rigs was to limit damaging voltage spikes on the AC busbars to a level well below the voltage rating of the SCR devices themselves. SCR devices with a voltage rating of 2400V are now commonplace and relatively cheap, but in the 1970's and 1980s devices were expensive and not so well rated.
The surge suppressor is NOT intended to provide any form of harmonic filtering, and it is NOT intended to provide protection for any equipment other than the SCR devices fitted in the system.
Simplified Capacitor Surge Suppression
The early surge suppressors were designed as a three-phase rectifier with large reservoir capacitors connected across the DC side of the bridge. Under normal operation the capacitors charge up through the rectifier diodes to the busbar peak voltage (about 850V for a 600V system). Any spikes which appear on the busbars have to have sufficient energy to deliver enough current to further charge the capacitors in order to be able to force the busbar voltage to rise. Since the spikes usually are high frequency, and have a sharp rise time (requiring more current to charge the capacitor) the voltage rise is limited.
Note that the diode bridge can not feed current back on to the busbars so there is no mechanism for 'filling' and notches in the AC supply.
Simplified MOV Surge Suppression
The diode bridge and capacitor type surge suppressor is expensive in terms of components and assembly, so an economy version was created using MOV (or BOV) devices. An MOV works by breaking down when the voltage across its terminals exceeds its rated value. This breakdown has consequences in that over a period of time the MOV wears out, at which time they are commonly known to fail catastrophically, i.e. explode. It is advisable to replace MOVs regularly.
Selecting a suitable MOV voltage is fairly easy - it has to greater than the busbar working voltage but less than the SCR device voltage rating. However, the tricky part is anticipating what energy rating is required. Too small and the devices will last no time at all. Too large means that cost goes up. Either way, an MOV surge suppressor is a cheaper solution in the short term
Which is better?
It really is a case of you pay your money, you take your choice.
Both units, if properly designed, will do a good job, although the components in an MOV type suppressor will require replacement on a more regular basis than those in the diode/capacitor version.
Our feeling is that the capacitor type goes a little further to protect other equipment because it acts just above the 600VAC level whereas MOVs are selected to operate (typically) at over 1600V simply to protect the SCR devices. By fitting higher rated devices this voltage level could be even higher
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