INTRODUCTION. The Bulk Power Supply Reliability criteria The 2 components Security assessment from operators view Requirements of a reliable electric service System dynamic performance Power system stability Definitions Reliability criteria How are they used
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1. The Bulk Power Supply
- Many users are affected by these interruptions
- They can be costly
- Much effort is spent to do that
Power Systems are built and operated with the following goal in mind:
TO ACHIEVE A RELIABLE and ECONOMIC ELECTRIC POWER SUPPLY.
For the consumer to have a reliable and economic electric power supply, a complex set of engineering analysis and design solutions need to be implemented.
Security is the ability of the electric systems to withstand sudden disturbances such as electric short circuits or unanticipated loss of system elements.
Adequacy is the ability of the electric systems to supply the aggregate electrical demand and energy requirements of their customers at all times, taking into account scheduled and reasonably expected unscheduled outage of system elements.
Security = dynamic security
Adequacy = static security
This view is strongly held by engineers that have been deeply involved with “reliability”assessment tools for planning such as TRELLS, Tplan, etc., which compute probabilistic indices based on static security assessment. They will tell you that their tools are concerned with adequacy, not security.
You must note the person’s background who uses the term “Security” in order to understand the meaning being implied.
NERC, North American Electric Reliability Council: Its purpose is to augment the reliability of the bulk power supply in the electricity systems of North America. It is composed of ten regional reliability councils and encompasses virtually all the power systems in the United States and Canada.
NERC sets the reliability criteria for planning and operating the interconnected network of North America, which play an essential role in preventing major system disturbances following severe contingencies.
Ref: NERC Operating Standards, 2/00
In designing and operating the interconnected power network, system dynamic performance is taken into account because:
ONE ASPECT OF SYSTEM SECURITY IS THE ABILITY OF THE SYSTEM TO “STAY
TOGETHER.” THE KEY IS THAT THE GENERATORS CONTINUE TO OPERATE “IN
SYNCHRONISM,” OR NOT TO “LOSE SYNCHRONISM.” THIS IS THE PROBLEM OF
“POWER SYSTEM STABILITY”
A) In System Planning or Design
B) In System Operation
(you will find an expanded table in the NERC criteria)
captured by the following statement taken from the WSCC criteria for transmission system planning, which describes its disturbance-performance table:
“The table is based on the planning philosophy that a higher level of performance is required for disturbances generally having a higher frequency of occurrence.”
Or stated another way,
“The table is based on the planning philosophy that a LOWER
level of SEVERITY is required for disturbances generally
having a higher frequency of occurrence.”
Considering risk=frequency severity, we see that the criteria
suggests a constant maximum risk for different contingencies.
VIOLATED IN DESIGNING THE SYSTEM.
RELIABILITY CRITERIA SHOULD NEVER BE
INTENTIONALLY VIOLATED IN OPERATING THE SYSTEM.
SOMETIMES, VIOLATIONS OCCUR IN OPERATIONS.
When the alert state is entered following a contingency, operators can take actions to return the system to the normal state.
The following example of design and operating criteria related to system stability is based on those of the Northeast Power Coordinating Council (NPCC)
Normal Design Contingencies
The criteria require that the stability of the bulk power system be maintained during and after the most severe contingencies specified below. These contingencies are selected on the basis that they have a significant probability of occurrence
1) All facilities in service.
2) A critical generator, transmission circuit, or transformer out of service, assuming that the area generation and power flows are adjusted between outages by use of a ten minute reserve.
The extreme contingency assessment recognizes that the interconnected bulk power system can be subjected to events that exceed in severity the normal design contingencies. The effect of these contingencies on system performance should be determined in order to obtain an indication of the system strength and to identify the extent of a widespread disturbance. After the analysis and assessment of extreme contingencies, measures are developed to reduce the frequency of occurrence of such contingencies or to mitigate the consequences that are indicated by the simulations of such contingencies:
The reliability criteria should serve primarily as a means to measure the strength of the systems to withstand the entire spectrum of contingencies that may or may not be readily
visualized, rather than comprising detailed listing of probable disturbances.
The selection of reliability criteria is based not on whether the specific contingencies for which the system is being tested are themselves highly probable but rather on whether they constitute an effective and practical means to stress the system and thus test its ability to avoid uncontrolled, area-wide power interruptions under adverse credible conditions.
A. Steady-state stability
Use of linear system analysis techniques to study the modal system response
I use “simulation”(transient stability) and “calculation” (steady-state stability) for this.
I use the word “study” to denote a set of simulations/calculations from which one draws a relevant conclusion.
A. Stability is now considered a problem for system operations in many systems
Here is a quote from a recent paper by B.C. Hydro Engineers about their on-line dynamic
security assessment scheme:
“Conventionally, dynamic security assessment has been performed using off-line calculations. In this process, detailed stability analysis is conducted for each credible contingency under a variety of operating conditions. In real system operations, conditions frequently do not match those studied off-line. Consequently, the guidelines and limits produced are usually provided on the conservative side. Power system networks nowadays operate more and more in a stressed state where conservative operation often results in significant financial consequences. A more effective approach is to assess only those contingencies likely to cause dynamic violations for the operating condition encountered in real time. The new B. C. Hydro EMS system offered an opportunity for implementation of an on-line transient stability assessment (TSA).
It is not enough to just say a particular contingency is
acceptable or not – we also need to know the “limit.”
Thus we need to:
The challenge is to obtain a clear understanding of the physical aspects of each phenomena, use the appropriate representation of the power system to analyze the phenomena, and apply new or existing tools in an original and creative manner to provide needed answers.
My aim in this course is to motivate and channel your thought process towards this challenge and provide the various building blocks, tools, and relevant reference material you will need.