The time scales of power systems refer to the different time frames at which various processes and events occur within the system. These time scales can range from fractions of a micro seconds to several hours or even days, and they are important for understanding the behavior and operation of the power system.
Well known time scale in power systems is the Transient Time Scale, which refers to events that occur within fractions of a second to a few seconds. These events include faults, switching operations, and other disturbances that can cause significant changes in voltage and current levels. The next time scale is the Dynamic Time Scale, which covers events that occur over several seconds to a few minutes. These events include changes in power demand, generator output, and other system parameters that can affect the stability and operation of the power system. The slowest time scale is the Steady State Time Scale, which covers events that occur over several minutes to hours or even days. These events include changes in load demand, scheduled maintenance, and other long-term changes that can affect the overall operation and planning of the power system.
Understanding the different time scales of power systems is important for designing and operating the system effectively and efficiently. It allows for the proper selection & coordination of protective devices, control systems, and other equipment to ensure reliable and stable operation.
Power system stability has been recognized as one of key issues to be dealt with in order to achieve a secure power system operation. For assessment of diverse stability issues, study of power system time scales of different phenomena analysis is important. The time scales range cover the wide spectrum of wave phenomena, electromagnetic transients, electromechanical transients, and up to thermodynamic phenomena. Depending on the nature of the transients considered for particular stability studies, dedicated assumptions on component modeling are to be made.
Today power systems mostly dominated by Synchronous Generators (SG), the main focus for stability assessments has traditionally been on the time scale of electromechanical or slower phenomena. The time scale of electromechanical phenomena is shown to range from several milliseconds to seconds. The oscillation frequencies of the associated transients are typically between 0.1 Hz and 5 Hz. The concentration on those frequencies has allowed several modeling simplifications, which in turn led to the well-known stability assessments based on quasi-static phasor calculus.
Increase in the penetration of converter interfaced generation (CIG) has considerable changed the nature of transients in power systems due to the fast response times, where the time scale related to the inverter-based controls varies from few microseconds to several milliseconds. This time scale is actually related to electromagnetic phenomena, thus being much faster than the electromechanical phenomena of oscillations of SG.
Accordingly, the transients of power systems dominated by CIG become faster, thereby leading to new control interactions and stability issues. Among the key instability drivers in CIG are their control loops with fast response times such as the Phase Lock Loop (PLL) Controllers and
Although recent studies have shown that these control loop instabilities are more likely to arise in Weak Networks with low levels of Inertia.
Smart grid initiative to promote and transform the traditional power systems to modern and automated power grids. An electrical power system is a set of many elements such as transmission lines, transformers and generators connected together into a larger system, that can generate, transmit and distribute electric power. Different kinds of electrical elements imply a large variety of dynamic actions or responses to disturbances. Some power system disruptions can affect single element, others can affect larger fragments. Some failures can spread and affect the system as a whole.
As each dynamic effect reflects certain unique feature of power system dynamics, some of them can be grouped according to their cause, consequence, time frame, physical character or the place in the system that they occur.
Based on the physical character of the disturbance, different power system dynamics can be divided into FOUR GROUPs, defined as:
- WAVE,
- ELECTROMAGNETIC,
- ELECTROMECHANICAL,
- THERMODYNAMIC.
This classification also corresponds to the time frame involved.
WAVE: The fastest transients are related to the wave effects or surges in high voltage transmission lines and correspond to the propagation of electromagnetic waves caused by lightning strikes or switching operations. This time scale is associated with the propagation of electromagnetic waves in the power system.
It operates on a time scale of picoseconds, nanoseconds, microseconds (µs) to milliseconds (ms) as associated with lightning and switching transients.
ELECTROMAGNETIC: Much slower are the electromagnetic dynamics, that take place in the machine windings following a disturbance, such as a short-circuit, operation of the protection devices like the distance or overcurrent protection as well as the interaction between the electrical machines and the power system. This time scale is associated with the interaction of electric and magnetic fields in the power system. It operates on a time scale of microseconds to milliseconds and is associated with protective relaying and fault detection.
ELECTROMECHANICAL: Due to the oscillation of the rotating masses of the generators and motors that occurs usually after disturbance like a short-circuit or switching off large amount of generation, the electromechanical dynamics are even much slower. Electromechanical transients are also caused by operation of the protection system such as underfrequency or undervoltage protection.
This time scale is associated with the mechanical motion of generators, transformers, and other rotating equipment in the power system. It operates on a time scale of seconds to minutes and is associated with control and stability.
THERMODYNAMIC: The slowest dynamics are the thermodynamic transients, related to the active power generation control in power plants (boiler-turbine-generator control) or transmission line wires temperature variations due to varying weather conditions and line current flow. (The line current flow is a consequence of weather conditions or disturbances like long-term line overloading because of high ambient temperatures, short-term line overloading due to power swings in the system or very short-term high overcurrent due to short-circuit.)
This time scale is associated with the thermal characteristics of the power system. It operates on a time scale of minutes to hours and is associated with planning and scheduling of power generation and transmission.
In today’s electric power systems, sensing and monitoring equipment is employed in a limited number of critical assets, such as power transmission lines and substations. Generally, this sensing equipment is not interconnected, and transmits the collected data to a central location using wire communications (such as Ethernet or fiber-optic), where a supervisory control system and dispatchers at the utility headquarters manage grid operations.
Wired monitoring systems require expensive communication cables to be installed and regularly maintained. Additionally, the use of wire communication in harsh environments is very often uneconomical or even impossible. Hence, there is an urgent need for cost-effective wireless monitoring and diagnostic systems that can improve system reliability and efficiency by optimizing the management of electric power systems.
Credit: https://www.youtube.com/@PowerSystemsOperation
Time Scales of Power System | Power System Time Scales | Electromagnetic & Thermodynamics Phenomena
https://youtu.be/rHG1uHfxBrE