New Overview and Classification of Power System Stability

Advances in Energy Engineering, Volume 4, 2016 www.seipub.org/aee doi: 10.14355/aee.2016.04.002

New Overview and Classification of Power System Stability Dr. Mohammed Hammed Yasen *1 Team Leader in Iraqi Ministry of Electricity, Electrical Engineering ‐ Power System,Department of Power Systems and Networks,Institute of Electrical Power Engineering,Faculty of Electrical Engineering,Warsaw University of Technology, Poland *1

electronic_boy1979@ieee.org; *1 eng.mohammed.hammed@gmail.com.

Abstract This work shows the classification of power system stability. It is important to understand the electrical system operation under all circumstances without losing the stability of system. Planing to develop the power system reliabilty needs to review and understand the power system stabilty in order to find soultion to all problems expected to accure in the stability to opearate the power system in all its parts. Several papers talked about power system stability, but they did not collected all parts of the tobic in the same paper while collecting all aspects of the topic is important in order to be easer to the reader to find out all the information needed.

Keywords Power System, Voltage Stability, Frequency Stability, Rotar Angle Stability

Introduction Every system includes many parts. that is working together to get stable system. the Stable system means: Economic, Security, Reliability, Friendly...etc. The Power System can be classified into four parts, as main parts.

The proposed definitions will be shown as follows:‐ A. Power Generation part.

B. Power Transmission Part.

C. Power Distribution Part.

D. Power Control Part.

Stability of Power System The Proposed Definitions: when the System is able to provid the Demand from Supply after the Problem occurs.

Currently the Stability of power System is a very Important thing when you want it to be Strong, Secure, Economic ...etc. You have to think about stability.[1] The Stability is usually classified into three Parts they are:‐ 1‐Transient Stability.

2‐Steady state Stability.

3‐Dynamic Stability.

And the Power System Stability can be Classified into three main parts, shown as the following Parts:‐ 1‐Voltage Stability.

2‐Frequency Stability.

3‐Rotor angle Stability.[1] Classification of Stability Power system stability is a single problem; however, it is impractical to study it as such. the instability of a power system can take different forms and can be influenced by a wide range of factor. Analysis of stability problems,

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identification of essential factors that contribute to instability, and formation of methods of improving stable operation is greatly facilitated by classification of stability into appropriate categories. This is based on the following considerations: 

The physical nature of the resulting instability;



The size of the disturbance considered;



The devices, processes, and time span that must be taken into consideration in order to determine stability;



The most appropriate method of calculation and prediction of stability.

Power System Stability

‐ Ability to remain in operating equilibrium ‐ Equilibrium between opposing forces

Angle Stability

‐ Ability to maintain synchronous ‐ Torque balance of synchronous machine

Transient Stability

‐ Large disturbance ‐ First‐swing a periodic drift ‐ Study period up to 10 s

Voltage Stability

Frequency Stability

*Mid‐term Stability

‐ Ability to maintain steady acceptable voltage ‐ Reactive power balance

*Long‐term Stability

Large‐Disturbance Voltage Stability

‐ Uniform system frequency ‐ Slow dynamics ‐ Study period to tens of minute

‐ Severe upsets; large voltage and frequency excursions ‐ Fast and slow dynamics ‐ Study period to several minute

‐ Large disturbance ‐ Switching events ‐ Dynamics of ULTC, loads ‐ Coordination of protections and control

Small‐Signal Stability

Non‐oscillatory Instability

‐ Insufficient synchronizing torque

Local Plant Modes

Small‐Disturbance Voltage Stability

Oscillatory Instability

‐ Steady‐state P/Q‐ V relation ‐ Stability margins, Q reserve

‐ Insufficient damping torque ‐ Unstable control action

Interarea Modes

Control Modes

Torsional Modes

* With availability of improved analytical techniques providing unified approach for analysis of fast and slow dynamics, distinction between mid‐term and long‐term stability has become less significant.

FIGURE 1 CLASSIFICATION OF POWER SYSTEM[1]

Figure 1 gives an overall picture of the power system stability problem, identifying its classes and subclasses in

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term of the categories described in the previous section. As a practical necessity, the classification has been based on a number of diverse considerations, making it difficult to select clearly distinct categories and to provide definitions that are rigorous and yet convenient for practical use. For example, there are some overlaps between mid‐term/long‐term stability and voltage stability. With the appropriate models for loads, on‐load transformer tap changers and generator reactive power limits, Mid‐term/long‐term stability simulations are ideally suited for the dynamic analysis of voltage stability. Similarly, these are overlaps between transient, mid‐term and long‐term stability: all three use similar analytical techniques. Although the three categories are concerned with different aspects of the stability problem in terms of analysis and simulation. they are really extensions of one another without clearly defined boundaries.

While the classification of power system stability is an effective and convenient means to deal with the complexities of the problem, the overall stability of the system should always be kept in mind. Solutions to stability problems of one categories should not be at the expense of another. It is essential to look at all aspects of the stability phenomena and at each aspect from more than one viewpoint. This requires the development and wise use of different kinds of analytical tools. In this regard, some degrees of overlaps in the phenomena being analyzed are in fact desirable [1],[2].

Mid-term and Long-term Stability The terms including long‐term stability and mid‐term stability are relatively new to the literature on power system stability. They were introduced as a result of the need to deal with problems associated with the dynamic response of power system to severe upsets [2]. Severe system upsets result in large excursions of voltage, frequency, and power flows that thereby invoke the action of slow processes, controls, and protections not modeled in conventional transient stability studies. The characteristic times of the processes and devices activated by the large voltage and frequency shifts will range from a matter of seconds (the responses of devices such as prime mover energy supply system and load‐voltage regulators) [3].

Long‐term stability analysis assumes that inter‐machine synchronizing power oscillations have damped out, and the result should be uniform system frequency [4]. The focus is on the slower and longer‐duration phenomena that accompany large‐scale system upsets and on the resulting large, sustained mismatches between generation and consumption of active and reactive power. These phenomena include: 1) Boiler dynamics of thermal units.

2) penstock and conduit dynamics of hydro units.

3) Automatic generation control.

4) Power Plant and Transmission system Protection/Controls.

5) Transformer Saturation.

6) Off‐nominal frequency effects on loads and the network.

The mid‐term response represents the transition between short‐term and long‐term responses. In mid‐term stability studies, the focus is on synchronizing power oscillations between machines, including the effects of some of the slower phenomena, and possibly large voltage or frequency excursions.

Typical ranges of time periods are shown as follows: 1) Short‐term or transient (from 0 to 10 seconds).

2) Mid‐term (from 10 to a few minutes).

3) Long‐term (from a few minutes to 10ʹs of minutes).[3] It should, however, be noted that the distinction between mid‐term and long‐term stability is primarily based on the phenomena which is analyzed and used by the system representation, particularly with regard to fast transients and inter‐machine oscillations rather than the time period involved.

Generally, the long‐term and mid‐term stability problems are associated with inadequacies in equipment responses,

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poor coordination of control and protection equipment, or insufficient active/reactive power reserves.

Long‐term stability is usually concerned with system response to major disturbances that involve contingencies beyond the normal system design criteria. This may entail cascading and splitting of the power system into a number of separate islands with the generators in each island remaining in synchronism. Stability in this case is a question of whether or not each island will reach an acceptable state of operating equilibrium with minimal loss of load. It is determined by the overall response of the island as evidenced by its mean frequency rather than the relative motion of machine. In an extreme case, the system and unit protections may compound the adverse situation and lead to a collapse of the island as a whole or in part [4].

Other applications of long‐term and mid‐term stability analysis include the dynamic analysis of voltage stability requiring simulation of the effects of transformer tap‐changing, generator over excitation protection and reactive power limits, and thermostatic loads. In this case, inter‐machine oscillations are not likely to be important.

However, care should be exercised not to neglect some of the fast dynamics.

There is limited experience and literature related to the analysis of Long‐term and mid‐term stability. As more experience is gained, analytical techniques for simulation of slow are improved and fast dynamics become available, the distinction between mid‐term and long‐term stability becomes less significant. [3],[4],[5].

Conclusion The classification of power system stability is important for understaniding the electrical system operation under all circumstances without losing the stability. Planing to develop power system relabilty needs to review and understand the power system stabilty to find soultion to all problems expected to accure in the stability of operating the power system in all its parts.

ACKNOWLEDGMENT

To my teachers , my family and my counrty.

REFERENCES

[1] Kundur, P., “Power System Stability and Control”, McGraw Hill, 1994.

[2] Michigan, ʹRenewable Energy Reportʹ, USA,2015.

[3] Mohammed Hammed Yasen, ʹEnhancing the Control of Iraqi Power System Using FACTS Devices and Renewable Energy with Matlab Simulationʹ, India, 2014.

[4] H.M. Rustebakke, ʺElectric Utility System and Practicesʺ, John Wiley & Sons, 1983.

[5] C. Concordia, D.R. Davidson, D.N. Ewart, L.K. Kirchmayer, and R.P. Schulz. ʺLong Term P ower System Dynamics ‐A new Planning Dimensionʺ, CIGRE paper 32‐13, 1976.

Author’s Details MOHAMMED HAMMED YASEN Team Leader in Iraqi Ministry of Electricity Belongs to Kirkuk/Iraq DOB is 18/12/1979, Received his Bachelor of engineering degree from Kirkuk collage Technology (Iraq) in 2007, And Master of Technology from SHIATS / India /Electrical and Electronic Engineering (Power System), Currently PhD student in Warsaw University of Technology /Power System/ Poland, His field of interest includes power system operation & control, Artificial Intelligent control Load forecasting, Power Electronics and Energy management system.

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