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1. Introduction
The growth of large interconnected power systems demands a high degree of security for normal
operation. The primary aim of an electric power system is to provide adequate uninterrupted
supply of electrical power to customer premises within the set limits of frequency and voltage
levels. The current trend towards deregulation and the participation of many players in the power
market are contributing to the decrease in the security margin.
Fig 1. On-Line and Off Line functions in an EMS
Figure 1(a) shows the here main important entities of power systems, EMS
and SCADA. EMS and SCADA are two important entities in the real time
monitoring, operation control of power systems. Figure 1(b) shows the
information flow between the three modules.
Power flows from Power
Systems through SCADA to EMS. SCAD forms the interface between Power
Systems and EMS.
Power System Security is defined as the ability of the power system to remain secure without
serious consequences to any pre-selected list of credible contingencies. The most common
operational problems are transmission equipment overloads and inadequate voltage levels at
system buses. The process of detecting, whether the system remains in secure (normal) or
insecure (emergency) state, is called power system security assessment. Secure state implies that
the load is satisfied and no limit violations will occur under present operating conditions and in
the presence of unforeseen contingencies. Violation of any security related inequality constraints
pushes the system to emergency or insecure state, thereby initiating corrective actions to be
taken to bring the system back to secure state. Power system security problems are classified as
static and dynamic. Static security problem evaluates the system steady state performance for all
possible postulated contingencies, whereas dynamic analysis pertains to long term behavior of
the system of the order of few minutes under transient disturbances.
A power system at any time can never be totally secure. It is always possible to devise a
sequence of events that will lead to a total or partial collapse of the system. Single contingencies
are more frequent than multiple contingencies. Power system security poses an important issue in
planning and operation stages of a power system. Security assessment, basically, deals with
evaluating the ability of the system to continue to provide service in the event of an unforeseen
contingency. Conventional method of security evaluation involves solving full AC load flow
equations and transient stability analysis of the current system state by time domain simulation
Security assessment is a major concern in planning, design and operation stages of electric power
systems. Security assessment consists of three modes, static, transient and dynamic. The
traditional method used in static security analysis involves solving full AC load flow equations
for each contingency scenario. This is highly time consuming and inadequate for real time
applications. Security assessment is the analysis performed to determine whether, and to what
extent, the system is reasonably safe from serious interference to its operation. Occurrence of
certain severe disturbances may cause the system to go to an undesirable emergency state, if the
system security is not well defined beforehand. Hence, effective control of power systems
demands a quick security evaluation of their operating states. Conventional method of security
evaluation, performed by simulation program, involves long computer time and inadequate for
real time applications.
Security assessment is the analysis performed to determine whether, and to what extent, the
system is reasonably safe from serious interference to its operation. It is duty of the system
operator to maintain the system in a normal state. Under certain conditions, occurrence of some
severe disturbances may cause the system to go to emergency state. Under such situations, the
operator must take immediate control actions to retain the system in normal operating state,
wherein system security needs to be well defined. Hence, effective control of power systems
demands a quick security evaluation of their operating states.
Security analysis may be broadly classified as Static Security Assessment (SSA) and Transient
Security Assessment (TSA). Static security analysis evaluates the post contingency steady state
condition of the system neglecting the transient behavior and other time dependent variations.
Transient security analysis evaluates the performance of the system as it progress after a
disturbance. Analysis of power system stability, namely, rotor angle stability, is an essential
component in TSA. This has made the security evaluation more important and demands the
investigation of fast and reliable techniques to allow on-line transient security assessment (TSA).
Static Security Assessment
The main goal in security analysis is to increase the power system’s ability to run safely and
operate within acceptable economic bounds [6]. Most of the Energy Management Systems todate performs only the static security analysis and hence the focus of this work is on static
security assessment. Static security is defined as the ability of the system to reach a state within
the specified secure region following a contingency [7]. Static security assessment evaluates the
post contingency steady state of the system neglecting the transient behavior and other time
dependent variations due to changes in load generation conditions.
Under normal operating conditions of power systems, the following constraints must be satisfied:
∑ P=
i =1
PD + PLoss
≤ PGi ≤ P
1, 2...NG
Vkmin ≤ Vk ≤ Vkmax k =
Pkm ≤ Pkmmax
for ∀ branch k − m
where P Gi represents real power generation at bus i, P D is the total system demand; P LOSS is the
total real power loss in the transmission network; V k is the voltage magnitude at bus; P km
represents the real power flow at branch k-m; NG is number of generators and NB is the numbers
of buses in the system.
Constraints (1) and (2), when referred to post contingency scenarios, are referred to as Security
Constraints [2]. The system operating state is classified as secure if constraints (1) and (2) are
satisfied for a given operating condition under contingencies, such as line outages, transformer
outages, etc. If constraints (1) and/or (2) are violated for any post-contingency scenarios, the
system operating state is classified as insecure. In conventional practice, security assessment is
obtained by analytically modeling the network and solving the load flow equation repeatedly and
checking the security constraints for all the prescribed outages, one contingency at a time [8].
This traditional approach is not entirely satisfactory because a huge number of simulations need
to be carried out. Hence, a new promising technique called pattern recognition is suggested for
online security evaluation.
Operating States of a Power System:
The operation of EMS is based on the working of the operating states. The operating state of a
power system determines the security of the system. The power system operates in two important
areas, namely normal and abnormal states.
Normal operating state
Abnormal operating state
Restorative state
Normal or secure state
In the normal operating state, the system is said to be secure and all constraints like voltages at
nodes, real and reactive power generation, real and reactive power flows are satisfied. The aim
of the power system is to keep the operating state of the power system to lie in the normal state.
Even though this is a stable operating state, any slight disturbance will take it to the abnormal
Abnormal or insecure state:
In the event of a disturbance, like generator outage or line outage, the operating conditions
change and the variables like nodes voltages and powers (real and reactive); real and reactive
power flows violate the operating limits or constraints.
The abnormal state or insecure state is further classified in to the following states;
a. alert
b. emergency
c. in-extermis ( or islanding)
Restorative state:
The power system disturbance, based on its nature, can lead the power systems to a blackout or
brownout state. In the blackout state, the entire load is separated from the generators, through
either the tripping of the generators or the transmission lines. No load is supplied. In the
brownout state, partial load is supplied through the transmission network. The blackout state is
more severe than the brownout state and requires several stages for restoring in back to the
normal operating state.
After the disturbance has occurred, the operator in an EMS tries to bring back the power system
to normal operating state through measures known as restorative strategies. In this process the
generators and lines which have tripped will be bought back to service through a sequence of
steps known as restorative measures
This section describes the security assessment in power systems.