Multiband Power System Stabilizer (SimPowerSystems)

Implement a multiband power system stabilizer

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Machines

Description

The disturbances occurring in a power system induce electromechanical oscillations of the electrical generators. These oscillations, also called power swings, must be effectively damped to maintain the system stability. Electromechanical oscillations can be classified in four main categories:

Local oscillations: between a unit and the rest of the generating station and between the latter and the rest of the power system. Their frequencies typically range from 0.8 to 4.0 Hz.
Interplant oscillations: between two electrically close generation plants. Frequencies can vary from 1 to 2 Hz.
Interarea oscillations: between two major groups of generation plants. Frequencies are typically in a range of 0.2 to 0.8 Hz.
Global oscillation: characterized by a common in-phase oscillation of all generators as found on an isolated system. The frequency of such a global mode is typically under 0.2 Hz.

The need for effective damping of such a wide range, almost two decades, of electromechanical oscillations motivated the concept of the multiband power system stabilizer (MB-PSS).

As its name reveals, the MB-PSS structure is based on multiple working bands. Three separate bands are used, respectively dedicated to the low-, intermediate-, and high-frequency modes of oscillations: the low band is typically associated with the power system global mode, the intermediate with the interarea modes, and the high with the local modes.

Each of the three bands is made of a differential bandpass filter, a gain, and a limiter (see Figure 4-1). The outputs of the three bands are summed and passed through a final limiter producing the stabilizer output V_stab. This signal then modulates the set point of the generator voltage regulator so as to improve the damping of the electromechanical oscillations.

To ensure robust damping, the MB-PSS should include a moderate phase advance at all frequencies of interest to compensate for the inherent lag between the field excitation and the electrical torque induced by the MB-PSS action.

Figure 4-1: Conceptual Representation

Figure 4-2: Internal Specifications

The MB-PSS is represented by the IEEE Std. 421.5 PSS 4B type model [2], illustrated in Figure 4-2, with built-in speed transducers whose parameters are fixed according to manufacturer's specifications.

Generally, only a few of the lead-lag blocks in Figure 4-2 should be used in a given PSS application. Two different approaches are available to configure the settings in order to facilitate the tuning process:

Simplified settings:

Only the first lead-lag block of each frequency band is used to tune the Multiband Power System Stabilizer block. The differential filters are assumed to be symmetrical bandpass filters respectively tuned at the center frequency F_L, F_I, and F_H. The peak magnitude of the frequency responses (Figure 4-1) can be adjusted independently through the three gains K_L, K_I, and K_H. Only six parameters are therefore required for a simplified tuning of the MB-PSS.

Detailed settings:

The designer is free to use all the flexibility built into the MB-PSS structure to achieve nontrivial controller schemes and to tackle even the most constrained problem (for example, multiunit plant including an intermachine mode in addition to a local mode and multiple interarea modes). In this case, all the time constants and gains appearing in Figure 4-2 have to be specified in the dialog box.

Dialog Box

Mode of Operation: Simplified Settings

Global gain: The overall gain K of the multiband power system stabilizer.
Low frequency band: [FL KL]: The center frequency, in hertz, and peak gain of the low-frequency bandpass filter.
Intermediate frequency band: [FI KI]: The center frequency, in hertz, and peak gain of the intermediate frequency bandpass filter.
High frequency band: [FH KH]: The center frequency, in hertz, and peak gain of the high-frequency bandpass filter.
Signal limits: [VLmax VImax VHmax VSmax]: The limits imposed on the output of the low-, intermediate-, and high-frequency bands and the limit VSmax imposed on the output of the stabilizer, all in p.u.
Plot frequency response: If selected, a plot of the frequency response of the stabilizer is displayed when you click the Apply button.
Magnitude in dB: If selected, the magnitude of the frequency response is plotted in dB. The Magnitude in dB parameter is not visible in the dialog box if the Plot frequency response is not selected.
Frequency range (Hz): The frequency range used to plot the frequency response of the stabilizer. The Frequency range (Hz) parameter is not visible in the dialog box if the Plot frequency response is not selected.

Mode of Operation: Detailed Settings

Low frequency gains: [KL1 KL2 KL]: The gains of the positive and negative branches of the differential filter in the low-frequency band and the overall gain K_L of the low-frequency band, in p.u.
Low frequency time constants:: The time constants, in seconds, of the lead-lag blocks in the positive and negative branches of the low-frequency filter. You need to specify the following twelve time constants and two gains:; [T_B1 T_B2 T_B3 T_B4 T_B5 T_B6 T_B7 T_B8 T_B9 T_B10 T_B11 T_B12 K_B11 K_B17]; Set K_B11 to 0 in order to make the first block of the positive filter branch a washout block. Set K_B11 to 1 in order to make the block a lead-lag block.; Set K_B17 to 0 in order to make the first block of the negative filter branch a washout block. Set K_B17 to 1 in order to make the block a lead-lag block.
Intermediate frequency gains: [KI1 KI2 KI]: The gains of the positive and negative branches of the differential filter in the intermediate-frequency band and the overall gain K_I of the intermediate-frequency band, in p.u.
Intermediate frequency time constants:: The time constants, in seconds, of the lead-lag blocks in the positive and negative branches of the intermediate-frequency filter. You need to specify the following twelve time constants and two gains:; [T_I1 T_I2 T_I3 T_I4 T_I5 T_I6 T_I7 T_I8 T_I9 T_I10 T_I11 T_I12 K_I11 K_I17]; Set K_I11 to 0 in order to make the first block of the positive filter branch a washout block. Set K_I11 to 1 in order to make the block a lead-lag block.; Set K_I17 to 0 in order to make the first block of the negative filter branch a washout block. Set K_I17 to 1 in order to make the block a lead-lag block.
High frequency gains: [KH1 KH2 KH]: The gains of the positive and negative branches of the differential filter in the high-frequency band and the overall gain K_I of the high-frequency band, in p.u.
High frequency time constants:: The time constants, in seconds, of the lead-lag blocks in the positive and negative branches of the high-frequency filter. You need to specify the following twelve time constants and two gains:; [T_H1 T_H2 T_H3 T_H4 T_H5 T_H6 T_H7 T_H8 T_H9 T_H10 T_H11 T_H12 K_H11 K_H17]; Set K_H11 to 0 in order to make the first block of the positive filter branch a washout block. Set K_H11 to 1 in order to make the block a lead-lag block.; Set K_H17 to 0 in order to make the first block of the negative filter branch a washout block. Set K_H17 to 1 in order to make the block a lead-lag block.
Output limits: [VLmax VImax VHmax VSmax]: The limits imposed on the output of the low-, intermediate-, and high-frequency bands and the limit VSmax imposed on the output of the stabilizer, all in p.u.

Input and Output

dw: Connect to the first input the synchronous machine speed deviation dw signal (in p.u.).
Vstab: The output is the stabilization voltage, in p.u., to connect to the vstab input of the Excitation System block used to control the terminal voltage of the Synchronous Machine block.

Example

See the help text of the psbPSS demonstration file.

Reference

[1] Grondin, R., I. Kamwa, L. Soulieres, J. Potvin, R. Champagne, "An approach to PSS design for transient stability improvement through supplementary damping of the common low frequency," IEEE Transactions on Power Systems, 8(3), Aug. 1993, pp. 954 -963.

[2] IEEE recommended practice for excitation system models for power system stability studies: IEEE Std 421.5-2002 (Section 9).

See Also

Generic Power System Stabilizer

MOSFET Multimeter