Power System Stabilizer Models with Emphasis on PSS4B

Mohammad Umar Rehman

Research Scholar, EED

IIT Delhi

15th March 2013

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Outline Review Various PSS Models Comparison of PSS2B and PSS4B Alternative types

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Review Power System Stabilizers are an essential

component of modern interconnected systems Provides damping of power system oscillations

through excitation control leading to enhancement of transient stability and transmission capacity

Needs to produce a torque component in phase with rotor speed deviations (Δω)

Consists of a lead-lag compensator as the basic functioning block

Common input signals are shaft speed, terminal frequency, and power or their combination

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Evolution of PSS Models

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PSS1A, Single input PSS

Transducer Washout

Lead Lag compensators

Account for LF signals and Characteristics shaping

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PSS2B, Dual-input PSS

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PSS2B…contd When using speed as an input signal, torsional

oscillations may occur creating disturbance in measurement

The solution lies in using several sensors &/or a torsional filter

Therefore, the Integral of accelerating power-based stabilizer or PSS2B was developed to overcome this

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PSS2B…contd Represents two distinct types of dual-input stabilizer

implementations:

a) Stabilizers that, in the frequency range of system oscillations, act as electrical power input stabilizers. These use the speed or frequency input for the generation of an equivalent mechanical power signal, to make the total signal insensitive to mechanical power change

b) Stabilizers that use a combination of speed (or frequency) and electrical power. These systems usually use the speed directly (i.e., without phase-lead compensation) and add a signal proportional to electrical power to achieve the desired stabilizing signal shaping.

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PSS3B, Dual-input PSS

PE

Δω

Transducer Washout

Phase compensation

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PSS4B

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PSS4B

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PSS4B…contd Also known as multi-band stabilizer Motivation: lead/lag compensating filters in the older

structures could not give an accurate compensation over a wide range of oscillation frequencies

If the network suffers from low and high frequency oscillations, the tuning procedure of the single-band stabilizers have to compromise and will not achieve optimal damping in any of the oscillations

The multi-band stabilizer has three separate signal bands, which can be tuned individually to handle different oscillation frequencies

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PSS4B…contd The low band is typically associated with the power

system global mode, the intermediate with the inter-area modes, and the high with the local modes

The PSS4B measures the rotor speed deviation in two different ways. Δω L-I feeds the low and intermediate bands, while ΔωH is dedicated to the high-frequency band.

Next we consider performance evaluation of PSS2B and PSS4B

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SMIB Scenario Gain margins and phase margins were evaluated for

the following benchmark system [5]

The stabilizers have a robust performance satisfy the conventional criteria of a minimum of 6 dB and 30° for gain and phase margins, respectively

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Multi-machine scenario Damping performance was evaluated for the well

known Kundur Test system [6]

PSS4B outperforms PSS2B by providing better damping under all oscillatory modes

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Pros & Cons of PSS2B

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Pros & Cons of PSS4B

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Alternative Types of PSS

1. Based on shaft speed signal (Δω) Earliest PSS (1960s) to be used with hydraulic units Measurement is prone to noise and can lead to

inaccuracies Needs several measurement points and a torsional

filter thus adding to complexity and cost

2. Combination of shaft speed and power (Δ-P-ω) Based on the equation

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…contd The integral of mech power is related to shaft

speed and elec power as:

The overall TF is:

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…contd Major advantages:

1. The ΔPe signal a high degree of attenuation towards torsional oscillations eliminating the need for a torsional filter

2. Measurement becomes simple with the use of end of shaft speed sensing arrangement and also allows used of standard design for all units irrespective of torsional charactersitics

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References[1] Kamwa, I. et. al. “ IEEE PSS2B Versus PSS4B: The Limits of

Performance of Modern Power System Stabilizers”, IEEE Transactions on Power Systems, vol. 20, no. 2, May 2005.

[2] IEEE, IEEE Recommended Practice for Excitation System Models for Power System Stability Studies, IEEE Std 421.5™-2005.

[3] Hammer, A., “Analysis of IEEE Power System Stabilizer Models”, M. S. Thesis, Department of Electric Power Engineering, Norwegian University of Science and Technology, 2011.

[4] Guanghui, Hua, “Research and Implementation on Power System Stabilizer PSS4B Model”, China International Conference on Electricity Distribution, 2010.

[5] G. N. Taranto, J. H. Chow, and H. A. Othman, “Robust redesign of power system damping controllers,” IEEE Trans. Contr. Syst. Technol., vol. 3, no. 3, pp. 290–298, Sep. 1995.

[6] Prabha Kundur, “Power System Stability and Control”, McGraw Hill, 1994.