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Multiband Power System Stabilizer

Implement multiband power system stabilizer

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Machines

Description

    Note   This block requires that you have a Control System Toolbox™ license. Otherwise, trying to simulate a model containing this block produces an error.

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's 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 the figure called Conceptual Representation). The outputs of the three bands are summed and passed through a final limiter producing the stabilizer output Vstab. 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.

Conceptual Representation

Internal Specifications

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

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

  1. 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 FL, FI, and FH. The peak magnitude of the frequency responses (see the figure called Conceptual Representation) can be adjusted independently through the three gains KL, KI, and KH. Only six parameters are therefore required for a simplified tuning of the MB-PSS.

  2. 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, multi unit 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 the figure called Internal Specifications have to be specified in the dialog box.

Dialog Box and Parameters

Simplified Settings Mode

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 pu.

Plot frequency response

If selected, a plot of the frequency response of the stabilizer is displayed when you click the Apply button.

Detailed Settings Mode

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 KL of the low-frequency band, in pu.

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:

[TB1 TB2 TB3 TB4 TB5 TB6 TB7 TB8 TB9 TB10 TB11 TB12 KB11 KB17]

Set KB11 to 0 in order to make the first block of the positive filter branch a washout block. Set KB11 to 1 in order to make the block a lead-lag block.

Set KB17 to 0 in order to make the first block of the negative filter branch a washout block. Set KB17 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 KI of the intermediate-frequency band, in pu.

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:

[TI1 TI2 TI3 TI4 TI5 TI6 TI7 TI8 TI9 TI10 TI11 TI12 KI11 KI17]

Set KI11 to 0 in order to make the first block of the positive filter branch a washout block. Set KI11 to 1 in order to make the block a lead-lag block.

Set KI17 to 0 in order to make the first block of the negative filter branch a washout block. Set KI17 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 KI of the high-frequency band, in pu.

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:

[TH1 TH2 TH3 TH4 TH5 TH6 TH7 TH8 TH9 TH10 TH11 TH12 KH11 KH17]

Set KH11 to 0 in order to make the first block of the positive filter branch a washout block. Set KH11 to 1 in order to make the block a lead-lag block.

Set KH17 to 0 in order to make the first block of the negative filter branch a washout block. Set KH17 to 1 in order to make the block a lead-lag block.

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 pu.

Plot frequency response

If selected, a plot of the frequency response of the stabilizer is displayed when you click the Apply button.

Input and Output

dw

Connect to the first input the synchronous machine speed deviation dw signal (in pu).

Vstab

The output is the stabilization voltage, in pu, to connect to the vstab input of the Excitation System block used to control the terminal voltage of the block.

Example

See the help text of the power_PSSpower_PSS example model.

References

[1] Grondin, R., I. Kamwa, L. Soulieres, J. Potvin, and 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), August 1993, pp. 954-963.

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

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