Series Compensation of lines

It is also possible to change power flows by injecting variable voltage in series with a line through a transformer as shown below.

This can be achieved by using power electronics based voltage source converters -VSC - (dc/ac converters). The schematic of this device is shown on the right. When this device injects three phase reactive power in series with the line (no active power interchange), what is the nature of current on the dc side if the VSC is lossless? Does that explain why one can work with a capacitor (as shown) instead of a battery on the DC side?

Devices like a TCSC or the device shown above are often called "Flexible AC Transmission Systems" controllers, because they increase the flexibility of AC system operation by allowing additional control.

Series connected inductors can be connected to thermally limited lines to reduce power flow through them.