Pendulum damping device

The pendulum damping device (PDG, Power System Stabilizer, PSS) is used to dampen rotor angle oscillations in three-phase synchronous generators . It is part of the generator control circuit and is mostly integrated in the generator's controller software.

Rotor angle oscillations

The rotor angle (also known as the "slip angle") of a synchronous machine is a measure of the real power delivered by the machine to the electrical network or to the consumer .

${\displaystyle {P}_{\mathrm {el} }=3\cdot {\frac {U\cdot U_{\mathrm {p} }}{X_{\mathrm {d} }}}\,{\cdot \sin \vartheta }}$

_{(Equation is only valid for an ideal full pole machine )}

in which:

${\displaystyle {U}}$ Mains voltage (conductor-earth),

${\displaystyle U_{\mathrm {p} }}$ Magnetic wheel voltage of the generator (voltage induced in the stator windings by the rotating rotor magnetic field),

${\displaystyle X_{\mathrm {d} }}$Synchronous reactance ( reactance ) of the d-axis of the generator

${\displaystyle \vartheta }$ Rotor angle of the synchronous machine,

Course of the generator active power and rotor angle after a load jump on the generator

The line voltage and the reactance can be regarded as constant apart from slight changes in the line operation, and in the case of permanently excited synchronous machines also the pole wheel voltage . ${\displaystyle U}$ ${\displaystyle X_{\mathrm {d} }}$ ${\displaystyle U_{\mathrm {p} }}$

In all cases, the machine's rotor angle changes with its active load.

If a synchronous generator with a sudden, abrupt change in the active load, z. B. acted upon by switching operations or short circuits, it can lead to pole wheel oscillations during the compensation process. The rotor angle is accelerated to its new value, which results from the new load condition. However, it does not adjust to it immediately, but rather oscillates around its new end value with decreasing amplitude due to the inertia of the rotor. The oscillation takes place with the natural frequency of the generator rotor. It can have a negative effect on the stability of the generator.

Oscillation damping via the excitation system of the synchronous machine

Control loop of a synchronous generator with a sway damping device

Synchronous machine with exciter power converter, voltage control loop and sway control device
symbols: , Generator voltage and current , excitation voltage and current electrical active power angular frequency control angle for Power
${\displaystyle {U}}$ ${\displaystyle {I}}$
${\displaystyle U_{\mathrm {f} }}$ ${\displaystyle I_{\mathrm {f} }}$
${\displaystyle P_{\mathrm {e} }}$
${\displaystyle \omega }$
${\displaystyle \alpha }$

In order at different loads of the generator its clamping voltage to keep constant (generator in island operation ), or to a particular resistive load of the generator to deliver a variable reactive power ( AC power generator), most generators include a voltage or cos - regulator . It is integrated into the generator's control circuit and influences its terminal voltage or power factor via the excitation current . Depending on the operating mode (voltage control / reactive power control), a voltage setpoint for the control circuit is generated and specified for the controller. ${\displaystyle {\mathit {\varphi }}}$

Depending on the version, the PDG generates an additional signal from the generator speed and / or generator power, which is added to the voltage setpoint. It reacts to dynamic changes in its input signals, which mean that the generator is compensating for a new operating point, and changes the voltage setpoint briefly. The generator voltage setpoint, the voltage regulator, the subordinate excitation current regulator and the voltage induced in the machine ultimately influence the course of the active power of the generator during a compensation process. The rotor oscillations are dampened.

The real power that the generator delivers to the grid comes from the drive power of the turbine to which the rotor is coupled.

In order to have an optimal influence on the generator, the PDG must be designed for the respective machine and its operating point .

Different PDG structures

Various PDG structures are listed in the international standard IEEE 421.5. They also differ in the number and type of their inputs. There are single and dual input PDGs. The speed and / or the active electrical power of the generator are used as input signals.

literature

• Valentin Crastan , Electrical energy supply 1 , 2nd edition, Springer Verlag 2007 ( ISBN 978-3-540-64193-3 )
• Valentin Crastan, electrical energy supply 2 , 2nd edition, Springer Verlag 2009 ( ISBN 978-3-540-41326-4 )
• Dietrich Oeding , Bernd R. Oswald , Electrical power plants and networks , 6th edition, Springer Verlag 2004 ( ISBN 978-3-642-19245-6 )
• IEEE 421.5-2005, IEEE Recommend Practice for Excitation System Models for Power System Stability Studies , IEEE 2006