What is the difference between onload tap changer and an offload tap changer?

With off load tape changer you must isolate the transformer to change tapes where as the on load tape changer you can change the tapes while the transformer is servece

An on-load varies the transformer ratio while the transformer is energized and carrying load. A off-load is a tap changer that cannot be moved while the transformer is energized.

On Load Tap Changer we can change the tape on load. But in Off Load we can only operate on no load.

There are two types: 1- ON load ( changing while in service ) the process describe in the phrase: Make before break. Two resistors used in the process. 2- OFF load ( must be switched off from the network to change the tap ). If the tap changed while energized, a short circuit current will be generated between the coils.

In OLTC, The tap changing operations can be done while the transformer is in operation/service.

But in Transformers with Off Line tap changer, Tap changing operations are done while the Transformer is OFF /de energized condition.

 

 On Load if you change tap position of Trafo, called ON-Load Tap Changer,is basically used in HV Generation & Transmission purpose, mostly of Power Transformer

But in case of OFF-Load,Tap can be changed,when Transformer is in OFF Condition,Mostly used in LT Distribution & Auxiliary-Transformer.

ON-Load Tap Change: Most of the Generating Station, Substation system having Power Transformer with ON-Load Tap Changer. Not only in Generating Station Transformer also in Distribution Class Transformer too. 

but OFF-Load Tap Change: Mainly Used in Solar Power Project, Wind Projects. Here Generating voltage level is LT at Conversion Level.[i.e Inverter Level]Solar Inverter Duty oil type Transformer & Dry Type Transformer having OFF-Load Tap Change.

So the Main difference is Tapping will be done only When Transformer is in OFF-Switch Condition.

In off load tap changing transformers tap is changed after discharging of load on LT side where as in on load tap changing transformers tap can be changed with out discharging load on transformer.