**What is Power System Transient Stability Study?**

Transient stability study is studies and analysis of the response of a system to disturbances such as loss of generation, line-switching operations, faults, and sudden load changes in the first few seconds after a disturbance. After the disturbance, the synchronous machine frequency experiences a transient deviation from the synchronous frequency. The purpose of the transient stability study is to determine whether the machine will return to synchronous frequency after a disturbance – **Omazaki Engineering** is a consultant serving the transient stability studies and analysis of power systems. Contact Omazaki Engineering if you need a power system transient stability analysis study consultant by sending an email to cs@omazaki.co.id or filling in the form in contact. We conduct transient analysis study using ETAP software.

The ability of the power system to return to normal or stable conditions after a disturbance is called stability. System disturbances can be of various types such as sudden load changes, short-circuit between line and ground, line-to-channel interference, third-channel interference, switching, etc.

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**Power System Stability Classification **

System stability mainly depends on synchronous machine behavior after a fault occurs. The stability of the power system is mainly divided into two types depending on the magnitude of the disturbance. The power system stability is classified as shown in the figure below.

**Stability Steady-State**

Ability to maintain synchronization between the engine in the system and the external tie line after minor disturbances (load fluctuation, turbine regulator, voltage regulator). The steady-state stability limit refers to the maximum power that can be transferred through the system without losing stability.**Transient Stability**

Ability to maintain synchronization between the engine in the system and the external tie line after minor disturbances (load fluctuation, turbine regulator, voltage regulator). The steady-state stability limit refers to the maximum power that can be transferred through the system without losing stability.**Dynamic Stability**

By using one of the many short circuit analysis software available, the system data is input and the short circuit current at various points in the system is calculated as output.

**Transient Stability**

Traditionally, transient stability is determined taking into account only the inherent mechanical and electromagnetic characteristics of synchronous machines and the impedance of the circuits connecting them. The response of the excitation or control system to changes in generator speed or electrical output caused by system disturbances is neglected. On the other hand, dynamic stability takes into account the automatic voltage regulator and the response of the regulating system.

The traditional definition of transient stability is closely related to the ability of a system to remain in sync for interference. Transient stability studies are usually carried out with the assumption that the excitation time constants and the prime mover are longer than the duration of the disturbance causing the instability.

**System Fault That Can Cause System Instability**

The most common disruptions that result in instability in industrial power systems are (not necessarily in order of probability):

- Short-circuit
- Loss of a tie circuit to a public utility
- Loss of a portion of on-site generation
- Starting a motor that is large relative to a system generating capacity
- Switching operations
- Impact loading on motors
- Abrupt decrease in electrical load on generators

The effect of each of these disturbances should be seen from the previous discussion of the fundamentals of stability.

**The Factors Can Affect Transient Stability**

- Generator WR
^{2}X rpm^{2 –}The greater this quantity the lower the acceleration factor .^{– } - System Impedance — which must include the transient reactance of all generating units. This affects phase angles and the flow of synchronizing power.
- Duration of the fault — chosen as the criterion for stability. Duration will be dependent upon the circuit-breaker speeds and the relay schemes used.
- Generator loadings — prior to the fault which will determine the internal voltages and the change in output.
- System loading — which will determine the phase angles among the various internal voltages of the generators.

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**Importance of Transient Stability Analysis**

The power system is designed to provide a continuous power supply that maintains voltage stability. However, due to undesirable events, such as lightning, accidents or other unforeseen events, a short circuit between the phase wires of the transmission line or between the phase and ground wires that may occur is called a fault. Due to an error, one or more generators may be severely disrupted causing an imbalance between generation and demand. If the problem persists and is not resolved within the predetermined time period, it can cause serious damage to the equipment which in turn can lead to loss of power and blackout. Therefore, protective equipment is installed to detect faults and clean / isolate the faulty part of the power system as quickly as possible before the disturbance energy is distributed throughout the system.

**Stability Problems In The Power System**

Power system stability is a very important aspect to supply power continuously. It is defined as a property of the power system that allows it to remain in a state of operating equilibrium under normal operating conditions and to regain an acceptable state of equilibrium after a fault. Power system instability can occur in many different situations depending on the system configuration and operating mode. One of the stability problems is maintaining synchronous operation or synchronization, especially power systems that rely on synchronous machines. This aspect is influenced by the dynamics of the generator rotor angle and the power angle relationship. Another instability problem you may encounter is the voltage collapse which is mostly related to the load behavior and not the synchronous speed of the generator.

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**Purpose and Objectives of Transient Stability Analysis**

The power system operates closer and closer to its limits which makes instability problems more likely. With this gift, it is very important to detect any disturbances that can cause instability. Instability can occur during steady-state; However, this happens more frequently after a short circuit which makes the time for cleaning up major annoyances very short. That is, it is very important to determine whether the system will be temporarily stable or will lose synchronization. Therefore, transient stability analysis requires very fast computation and decision making.

Stability studies are helpful for determining the critical clean time of circuit breakers, voltage levels, and system transfer capabilities.

The Transient Stability Study seeks to increase the reliability of the systems under study by minimizing the damaging effects of transients that often occur in power systems with the help of industry standard mitigation methods.

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**Transient Stability Analysis Methods**

Methods in transient stability analysis include swing equation, equal-area criterion, numerical integration methods, and direct transient stability analysis methods.

**Swing Equation**

The swing equation describes the rotational dynamics of synchronous machines and is used in stability analysis to characterize those dynamics. During normal operation, the relative positions of the rotor axis and the resultant axis are fixed. During a disturbance in the engine, the rotor accelerates or decelerates in relation to the MMF of the synchronous rotating air gap. The swing equation illustrates this relationship. The power system swing equation is given as:

**Equal-Area Criterion**

Consider a single machine single-machine infinite bus (SMIB) in the Figure below. For the system model considered in Figure below, it is not necessary to formally solve the swing equation to determine whether the rotor angle is increasing indefinitely or oscillating at an equilibrium position.

**Numerical Integration Methods**

The most commonly used method for solving swing equation 2.3 is numerical integration. The initial condition for the differential equation to be solved is the swing angle 0 (SEP) of Figure 2.8.

**Direct Methods Transient Stability Analysis**

The method directly determines stability without explicitly solving the system’s differential equations. This approach has received a lot of attention since Magnusson and Aylett’s early work using the transient energy function for stability assessments.

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**Transient Stability Analysis Using Computer Software**

**Commonly Used Software**

There are many software (software) available to conduct transient stability analysis studies of electric power systems, including:

- ETAP
- SKM
- EasyPower
- DigSILENT
- PSS®

**Methodology**

- Data collection & verification
- Site visit
- System modeling
- Model verification and validation
- Simulation
- Analysis
- Reporting

**Data Requirements**

The data required to carry out a provisional power system transient stability study and the recommended formats for organizing and presenting the information for the most convenient use are discussed in detail in the application guide for specific stability programs. The following is a summary of the generic class of data required. Note that some of the more esoteric information is unimportant; eliminating them only limits the accuracy of the results, especially at times greater than five times the duration of the studied interruption.

- System data
- Impedance (R + jX) of all critical transmission lines, cables, reactors and other series components

**For All Significant Automatic Transformers and Transformers**

- kVA rating
- Voltage ratio
- Impedance
- Voltage ratio
- Winding connection
- The tap is available and the tap is in use

**For Load Tap Changing Regulators and Transformers**

- Load data: real and reactive electrical loads on all buses significant load on system
- Short circuit capacity (steady-state basis) of the utility supply, if any
- kVAR from all significant capacitor banks
- Description of normal and alternative switching settings
- Rotating engine data

**For Main Synchronous Machines**

- Mechanical and / or electric power rating (kVA, hp, kW, etc.)
- Speed
- The inertia constant H or inertia Wk2 of the rotating engine and the connected load or prime mover
- Speed torque curve or other load torque description, if motor
- Real and reactive loading, if the generator is basic charged
- Sub-transient, transient, and direct axis synchronous reactance
- The quadrature axes of sub-transient, transient, and synchronous reactance
- Sub-transient time constant and transient time on the direct and quadratic axes
- Saturation information
- Potier reactance
- Data attenuation
- Excitation system type, time constant, and limitation
- Regulator and steam system or other types of prime mover, time constant and limit

**For Small Synchronous Machines**

- Inertia
- Speed
- Direct axis synchronous reactance
- For main induction machines or engine groups
- Mechanical and/or electrical power rating
- Positive-sequence equivalent sequence data (for example, R1, X1, XM)
- Negative sequence equivalent data (e.g., R2, X2)
- Load speed torque curve
- Description of low voltage or other starting arrangement

**For Small Induction Machines**

- Interference data
- An overview of the disturbance to be studied, including (as applicable) the initial transition status; error type, location, and duration; changing operations and timing; manufacturer, type and arrangement of protective relays; and the clearing times of the associated breakers
- Study parameters
- Study duration
- Limits on acceptable voltage, current, or power swing
- Integration interval
- Output printing interval
- Data output is required

**Results (Deliverables) of the Transient Stability Study**

Most stability programs provide the user with a large selection of outputs for printing. This program can compute and print the following information as a function of time in time domain analysis:

- Voltage and voltage angles on all buses
- The rotor angle, torque and engine speed are synchronous
- Real, reactive power flows throughout the system
- Bus frequency
- Torque and slip of all induction machines

This user-selected result combination can be printed for each printing interval (also user selectable) during the study period.

The study value is strongly influenced by the selection of the appropriate printing interval and the total duration of the simulation. Typically, a printing interval of 0.01 or 0.02 s is used; Longer intervals reduce solution time slightly, but increase the risk of losing the rotor’s fast angular swing. The time it takes to get a solution is proportional to the length or period studied, so this parameter must be strictly controlled for the sake of the economy.

Avoiding long transient stability study periods is very important especially if the systems and machines have them roughly or incompletely represented as errors will accumulate and render the results meaningless after some point. A time limit of five times the duration of the major disruption studied is generally long enough to indicate whether the system is stable (in the sense of temporary stability) or not, while keeping the solution time requirements to a reasonable level.

Frequency domain analysis will calculate the eigenvalues to determine the stability characteristics of the system. For large utility systems, most programs can only provide the dominant eigenvalues of the system. This information is sufficient for most stability studies except for a multi-dominant eigenvalue situation.

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Contact **Omazaki Engineering** if you are looking for power system transient stability study and analysis consultant to assist your new or existing project in Indonesia and South East Asia.

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**Related Articles:**

- Power System Study & Analysis
- Load Flow Study & Analysis
- Short-Circuit Study & Analysis
- Protection Coordination Study
- Motor Starting Study and Analysis
- Harmonic Study and Analysis
- Arc Flash Study & Assessment
- Power Quality Study & Assessment
- Voltage Drop Study & Analysis
- Voltage Imbalance (Unbalance) Study

**References:**

- IEEE 1110-2019 – IEEE Guide for Synchronous Generator Modeling Practices and Parameter Verification with Applications in Power System Stability Analyses
- Hussain Hassan Al Marhoon, A Practical Method for Power Systems Transient Stability and Security, Thesis Master of Science in Engineering Electrical of the University of New Orleans.
- Velimir Lackovic, Char. Eng., Power System Transient Stability Study Fundamentals, Continuing Education and Development, Inc. 9 Greyridge Farm Court, Stony Point, New York.
- Factors affecting transient stability – Faults analysis, Electrical Engineering Portal

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