What is balanced power?
When 120-volt AC power is balanced, one side of the circuit has +60 Volts to ground while the other has -60 Volts to ground. (Across the circuit, the usual 120 Volts is still present. Fig. 1) A European 230 Volt balanced power system has +115 Volts and -115 Volts to ground on the conductors.
Standard unbalanced AC power systems have a "hot" conductor and a "neutral" conductor. In the US, the "hot" conductor nominally has 120 Volts to ground and the "neutral" conductor has 0 Volts to ground. (Fig. 2) Europe has a similar system but with 230 volts on the "hot" and 0 Volts on the "neutral."
In a balanced power system, the voltages on the system's two output terminals are 180 degrees out of phase to each other with respect to ground. The system reference (ground) originates at the output center tap of an AC isolation transformer. In other words, the system's grounding reference (zero position) is located at the system's mean voltage differential or zero crossing point of the AC sinewave. This is a far more effective way to establish a reference potential for an AC system. The center tap is then grounded to Earth for electrical safety and for referencing shields.
There is never any voltage or current present on the ground reference in a balanced power system. Transient voltages and reactive currents which normally would appear on the neutral and ground wires are also out of phase and likewise, sum to zero at the ground reference thereby canceling out AC hum and noise.
A balanced AC Power system works the same way as a balanced audio circuit but with a higher amplitude. Both balanced audio and balanced AC incorporate phase cancellation or common mode rejection to eliminate noise.