Couplings for ripple control systems
Working principle

Basic circuits for parallel coupling and serial coupling

The audio-frequency can be fed into the 50 Hz network in serial or parallel mode. The type of coupling is determined by the level of the control-frequency and the structure of the network onto which this frequency is superimposed.

In the case of control-frequencies below 200 Hz, the serial coupling is the preferred option. Above 200 Hz, the parallel coupling is preferred. If both types of coupling are feasible, it is common practice to opt for parallel couplings since these can be disconnected from the network and they can be set up at an independent location from the 50 Hz power source.

Serial coupling

20 kV serial coupling for 40 MVA

With serial couplings, the audio-frequency is fed in by means of transformers which are linked to the 50 Hz supply lead. The substitute circuit diagram illustrates that the inductivities of the supplying transformer and of the superimposed network are in series with the prevailing network load. This leads to a relatively high level of voltage drop at the induction resistors, especially at high audio-frequencies. For this reason serial couplings are most commonly chosen for low audio-frequencies.

The use of supply transformers means that there is no need for expensive 50 Hz resonance circuits used in converters for creation of short circuits. Thus only capacitors are required to compensate the audio-frequency magnetization current.

The benefits of serial couplings are:

  • high resistance to short circuits
  • high resistance to overload
  • low 50 Hz losses
  • insignificant levels of 50 Hz voltage drop
  • high trap resistance
  • low audio-frequency losses.

Parallel coupling

20 kV-parallel coupling for 40 MVA

With parallel couplings, the control-frequency is superimposed to the 50 Hz voltage as a three-phase signal by means of tuned sequential resonance circuits comprising coupling capacitors and a coupling inductivity.

The 50 Hz voltage is stepped down in the coupling capacitors while the audio-frequency passes unimpeded from the transmitter to the 50 Hz network.

The inductivity of the feed transformer and the superimposed network are parallel to the network load. The ohmic impedance of this auxiliary connection is low at low frequencies and requires a correspondingly large audio-frequency input. This is why parallel couplings are used predominantly at medium and higher frequencies.

coupling inductivity, three-phase, with voltage transformer

During the transmission intervals, the coupling acts as an absorption circuit, absorbing audio-frequency voltages of the same frequency as they emerge from the superimposed network. The capacitive reactance of the coupling effective at 50 Hz often improves the cos φ (Phi) of the network.

The benefits of parallel couplings are:

  • high electric strength
  • good absorption characteristics
  • low losses
  • low power level for transmission.

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