Connectors PositivePlug and NegativePlug are nearly identical.
The only difference is that the icons are different in order
to identify more easily the plugs of a component.
Usually, connector PositivePlug is used for the positive and
connector NegativePlug for the negative plug of an electrical component.
Connector Plug is a composite connector containing m Pins (Modelica.Electrical.Analog.Interfaces.Pin).