Types of Power System Stabilizers
8.6 Conceptual explanation of the action of the pre-filter in the IAP PSS
8.6.3 Dynamic performance of the complete pre-filter
Sec. 8.6 Action of the pre-filter in the IAP PSS 425
Figure 8.15 The variable names and their locations in the pre-filter are defined for use in Figures 8.16 to 8.18.
For the three disturbances the responses of the variables in the pre-filter are shown in Fig- ures 8.16 to 8.18. The left- and right-hand plots in each figure show the relevant responses when torsional modes are absent or present, respectively. So that the responses to the changes in reference voltage are clearly discernible, the damping gain of the PSS in Case C, Section 5.10.6, is reduced from 20 to 10 pu.
From Figure 8.16 the following are noted:
• In (a)-(i) the nature and timing of two of the input disturbances are shown.
• In (a)-(ii) the decaying oscillatory responses in true speed due to the step changes in reference voltage are observed; the output of the second speed washout fil- ter (not shown) is identical for practical purposes.
• In (a)-(ii), as predicted by (8.16), the output of the pseudo-integrator (IPE) contains both the oscillatory rotor speed component and a component associated with the ramp in mechanical power. Importantly, it is observed that the true speed component is eliminated from the signal IPM which is input to the RTF.
• However, in (b)-(i) the signal IPM at the input to the RTF contains a component associ- ated with the ramp in mechanical power as well as the torsional mode, . As men- tioned, the true speed component seen in IPE is absent from IPM.
In (b)-(ii) is shown , the torsional mode modulated by the true speed component.
t
in out
Pe
IPE WO1
WO1
WO2
WO2 IPE
PW2 INT
IPM RTF
Vrtf
W2
C
in
=
Pseudo-Integrator RTF: Ramp-tracking filter Washout filters, WO#
in
W2
t
C
Figure 8.16
(a) Torsional mode absent (b) Torsional mode present (i) Disturbances: Ramp in mechanical power
and step changes in reference voltage, (i) Responses of internal variables IPE & IPM
(ii) Inputs: & . Responses of
internal variables IPE & IPM (ii) Inputs: &
IPE IPM Δωin ΔωC
12 13 14 15
1.5 1.6 1.7 1.8 1.9 2 2.1 2.2
Units of Speed (%)
IPE (i)
IPM
12 13 14 15
−0.5
−0.4
−0.3
−0.2
−0.1 0 0.1 0.2 0.3 0.4 0.5
Time (s)
Units of speed (%)
(ii) Δωin Δωc
ΔPm ΔVref IPE IPM Δωin ΔωC
0 5 10 15 20
0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16
Mech.Power, Ref.Voltage (pu) (i)
ΔP
m
ΔV
ref
0 5 10 15 20
−0.5 0 0.5 1 1.5 2 2.5
Time (s)
Units of speed (%)
Δωin ΔωC (ii) IPE
IPM
(a) (b)
Pm
Vref
in C in C
Sec. 8.6 Action of the pre-filter in the IAP PSS 427
Consider Figure 8.17 in which are shown the responses of internal and external variables.
Figure 8.17
(a) Torsional mode absent (b) Torsional mode present (i) Responses of internal variables IPM &
; input & output
(i) Responses of internal variables IPM &
(ii) Responses: terminal voltage and reactive power
IPM ΔV ΔV rtf
t ΔQ
12 13 14 15
1.5 1.6 1.7 1.8 1.9 2 2.1 2.2
Units of speed (%)
ΔVrtf IPM (i)
12 13 14 15
−2 0 2 4 6 8 10 12 14 16 18
Time (s)
Change in Vt and Q (%)
ΔVt (ii) ΔQ
IPM ΔV
rtf Δωout Δωin ΔV
t ΔQ
0 5 10 15 20
−0.5 0 0.5 1 1.5 2 2.5
Units of speed (%)
Δωout Δωin (i) ΔVrtf
IPM
0 5 10 15 20
−2 0 2 4 6 8 10 12 14 16 18
Time (s)
Change in Vt and Q (%)
(ii) ΔV
t
ΔQ
(a) (b)
Vrtf in out Vrtf
Vt Q
In Figure 8.17 it is observed that:
• in (a)-(i) the output of the RTF, , follows the mechanical power component related to the input signal to the RTF, IPM, with zero tracking error;
• in (a)-(i) the speed output signal from the pre-filter is identical to the ‘true’ speed input signal ; the associated rotor mode is clearly evident in the terminal voltage and reactive power responses in (a)-(ii).
• in (b)-(i) the torsional mode present at the input to the RTF, IPM, is not evident in the heavily attenuated output of the RTF, .
In considering Figure 8.17(a)-(ii), it should be remembered that, as the electrical power out- put increases while following the mechanical power ramp, the reactive output of the gener- ator will also ramp in order to supply the additional I2X losses. Moreover, from the figure it is noted that there is also a step increase/decrease in reactive power output associated with the step changes in terminal voltage; this is superimposed on the reactive power ramp. In Figure 8.17(b)-(ii) there is no evidence of the heavily attenuated torsional mode in terminal voltage and reactive power responses.
The output of the second washout filter in the electrical power signal path is dis- played in Figure 8.18(a)-(ii), together with the output of the pseudo-integrator (IPE). The ef- fect of the mechanical ramp change can be observed in both signals.
The responses of the speed output signal from the pre-filter and associated response of the PSS are seen in Figure 8.18(a)-(ii). Note that there is negligible off-set in both these signals from their zero values. Therefore, as a consequence, the offset in the output of the pseudo-integrator (IPE) due the ramping of mechanical power will not be manifested as an offset either in the PSS output, the terminal voltage, nor in the reactive power output of the unit. When the torsional mode is present, due to amplification by the PSS, there is evidence of the attenuated torsional mode in the PSS output in the expanded display of Figure 8.18(b)- (ii). Bear in mind, however, the amplitude of the torsional mode, seen in Figure 8.18(b)-(i), and the ramp rate of mechanical power have been exaggerated for illustrative purposes.
Figures 8.16 to 8.18 confirm that, due to the action of a properly designed pre-filter, the ef- fects of neither the ramping of the mechanical power output of the turbine, nor of torsional oscillations, are manifested in the output of the PSS. Furthermore, the swinging of terminal voltage and reactive power output due to ramping of power is not observed.
Vrtf
out
in
Vrtf
PW2
out
Vpss
Sec. 8.6 Action of the pre-filter in the IAP PSS 429
Figure 8.18
8.6.4 Potential causes of degradation in performance of the pre-filter of the IAP