The source and damping of local (cc. 1.2 Hz) and inter area (0.02…0.6) Hz oscillations

Our ten years' experience has showed that a PSS realized by first order controlling stages is not efficient in 0.02...0.6 Hz frequency domain. In addition, these instruments have a disadvantage: damping the active power oscillations are the source of reactive power oscillations, which are exported into the network. So a PSS tuned 0.25 Hz are rejected by experts of power plants. This paper describes our method, revealing the source of the cc. 0.25 Hz frequency oscillations, and damping these by using a supplementary stage installed into the turbine governor. Damping oscillations in a lower frequency range we propose an appropriate setting of the turbine Static Load Frequency characteristics.

I. Nomenclature

:

turbine power [p.u.];

:

Time constants of PSS [sec],

Laplace operator ,

gain of PSS [p.u.];

T:

transfer function of PSS [p.u.];

:

network frequency [Hz or mHz],

:

turbine controller output [p.u.],

:

servo motor output (position) [p.u.],

:

main valve position of turbine [p.u.],

:

power of High Pressure Part of turbine [p.u.],

:

set value of the frequency [p.u.],

:

set value of the turbine power [p.u.],

:

Laplace transformed function of time functions respectively,

:

speed droop of turbine [0.02 … 0.06 p.u.],

:

turbine controller time constant [up to 0.1 sec],

:

turbine servo motor time constant [0.1–0.3 sec],

:

time constant of turbine HPP steam chest [0.1–0.3 sec],

:

turbine reheater time constant [cc. 8 sec],

k:

height factor of HPP in the turbine power [0.3 p.u.],

:

the modified value of by Band Stop Filter=BSF,

damping coefficient of BSF [sec⁻¹], $$\Omega=2\cdot\pi\cdot {\rm f}_{{\rm T}}\sec^{-1} \eqno{\hbox{(1)}}$$

:

tuning frequency of BSF [sec⁻¹].