Compounded means that the excitation is derived from both the generator’s output voltage and its output current.

• On no-load, excitation to the generator is provided by the PRI.1 winding of the excitation transformer.
• On load, the generator current contributes an additional excitation current via the PRI.2 winding of the transformer. This helps maintain a constant output voltage.

If the excitation components are carefully designed, the output voltage of a compounded generator can be kept virtually constant across all load conditions, without the use of an AVR or manual voltage trimmer.

However, some generator manufacturers include:

• AVR (Automatic Voltage Regulator), and
• Manual trimmer rheostat

even in such compounded static excitation systems. These additions:

• Allow finer voltage regulation over the load range, and
• Enable manual voltage control, useful during synchronising and kVAr load sharing between generators.

A practical three-phase static excitation system typically includes additional components such as:

• Reactors, and
• Capacitors.

The circuit shown in the referenced figure contains no AVR or manual trimmer regulator. In such a system, any surge in load current feeds back automatically to adjust the field excitation. This correction happens so rapidly that the output voltage remains practically constant.

Important: Compound excitation systems must have their static components precisely matched to the generator they are designed to operate with.