HPS On Load Tap Changer (OLTC) Systems

In the April 2014 Article, “How Taps Work“, HPS discussed how power systems have a rated nominal voltage, but often operate slightly above or below that rated voltage. A fluctuating voltage supply to the primary (HV) side of the transformer will also be seen on the secondary (LV) or load side of the transformer.  For example, a 600V supply operating at +5% will cause the load voltage to be 218V.  For this reason, most transformers are designed to have primary taps to adjust the turns ratio to compensate for this supply voltage variance.  By connecting the supply voltage to one of the Full Capacity Above Normal (FCAN) or Full Capacity Below Normal (FCBN) taps, the load voltage will be closer to the rated load voltage when the supply voltage is higher or lower than the rated voltage.

What happens when the power supply is constantly fluctuating?  The 600V supply was operating at a fixed +5% in the simple example above, but in real life that 600V supply is operating at +5% on Monday, +1% on Tuesday and -4% on Friday.  This unpredictable shift in voltage can impact the secondary electrical system.  Manually monitoring the voltage, shutting down the system, adjusting the bolted tap and then re-energizing the system is impractical for most applications.

In systems were these voltage inconsistencies cannot be tolerated, an On Load Tap Changer (OLTC) can be integrated onto a medium voltage dry-type ‘power’ transformer, HPS Dry-Type Medium Voltage Catalog.  A typical dry-type transformer equipped with OLTC would range from 1MVA to 25MVA capacity with a 40kV to 5kV class supply and a 15kV to 1.2kV class load.   The function of the OLTC is to continually monitor the load voltage and automatically adjusts the tap setting, while the transformer is still energized, to consistently maintain the rated load voltage. OLTCs can operate manually, fully automatic or even remotely.

HPS has integrated OLTCs on some of the largest and demanding dry-type ‘power’ transformers in the world.  For more information regarding HPS’ integration capabilities please contact your local HPS Sales Representative, HPS Rep Locator.