The document discusses various aspects of power system reliability including adequacy, security, and stability. It defines adequacy as relating to having sufficient generation and transmission facilities to meet customer demand. Security pertains to how the system responds to disturbances like loss of generation or transmission. Stability refers to generators staying synchronized during disturbances. The document also discusses reliability assessment techniques like loss of load probability and expectation indices used to evaluate generation adequacy. Distribution reliability is assessed using indices that consider customer interruptions and outage times.

1. Outlines
4.1 power system reliability and quality
4.2 Reliability assessment techniques
4.3 Reliability indices
1

2. 2
4.1. power system reliability
 The concept of power system reliability, i.e. the overall ability of the
system to satisfy the customer load requirements economically &
reliably, is extremely broad.
 For the sake of simplicity, power system reliability can be divided into
the two basic aspects of
• system adequacy, and system security.
 Adequacy relates to the existence of sufficient facilities within the
system to satisfy customer load demands. These include the facilities
to generate power, and the associated transmission and distribution
facilities required to transport the generated energy to the load points.
Adequacy, therefore, relates to static system conditions.

3. ……Conty’
• Security pertains to the response of the system to the
disturbances it is subjected to.
• These may include conditions associated with local and
widespread disturbances and loss of major
generation/transmission.
• Most of the techniques presently available are in the domain
of adequacy assessment.
3

4. 4
An operator’s view of “security”
Security
Overload
Security
Voltage
Security
Angle/
Frequency
security
Trans-
former
Overload
Line
Overload
Voltage
out of
limits
Unstable
Voltage
Frequency
instability
Transient
instability
Oscillatory
instability
Static security Dynamic security

5. 5
Other Views of “Security”
Security = dynamic security
Adequacy = static security
Protection engineers use the word “security” to refer to the
“degree of certainty that a relay or relay system will not
operate incorrectly.”

6. 6
System Dynamic Performance
In designing and operating the interconnected power network,
system dynamic performance is taken into account because:
• The power system is subjected to changes (small and large). It is
important that when the changes are completed, the system
settles to new operating conditions such that no constraints are
violated.
• Not only should the new operating conditions be acceptable (as
revealed by steady-state analysis) but also the system must
survive the transition to these conditions. This requires
dynamic analysis.

7.  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” OR NOT TO “GO OUT OF
STEP.” THIS IS THE PROBLEM OF
“POWER SYSTEM STABILITY”
7

8. 8
Importance of Power System Stability
• Generators must be kept in synchronism; if their relative
motion begins to change too much, uncontrollable
oscillations may appear in the grid causing damage to
equipment, particularly generators.
• Therefore, relays are used to detect this condition and trip
generators before the damage occurs. Although tripping
prevents the damage, it results in under-frequency, and
possibly load interruption, and in the worst case, cascading
outages and blackout.

9. 9
• Power System: A network of one or more electrical
generating units, loads, and/or power transmission lines,
including the associated equipment electrically or
mechanically connected to the network.
• Operating Quantities of a Power System: Physical
quantities, measured or calculated, that can be used to
describe the operating conditions of a power system.
• Operating quantities include real, reactive, and apparent
powers, & rms phasors of alternating voltages &
currents.

10. ………..Cont’
• Steady-State Operating Condition of a Power System:
 An operating condition of a power system in which all the
operating quantities that characterize it can be considered to be
constant for the purpose of analysis.
• Synchronous Operation:
 A machine is in synchronous operation with a network or
another machine(s) to which it is connected if its average
electrical speed (product of its rotor angular velocity and the
number of pole pairs) equals the angular frequency of the ac
network or the electrical speed of the other machine(s).
10

11. 11
………..Cont’s
 A power system is in synchronous operation if all its
connected synchronous machines are in synchronous
operation with the ac network and with each other.
• Asynchronous or nonsynchronous operation:
Asynchronous Operation of a Machine: A machine
is in asynchronous operation with a network or
another machine to which it is connected if it is not
in synchronous operation.

12. 12
……Cont’
 A power system is in asynchronous operation if one or more
of its connected synchronous machines are in asynchronous
operation.
. Disturbance in a Power System: A disturbance in a power
system is a sudden change or a sequence of changes in one or
more parameters of the system, or in one or more of the
operating quantities.

13. 13
… cont’
 Small Disturbance In a Power System: It is a disturbance
for which the equations that describe the dynamics of the
power system may be linearized for the purpose of accurate
analysis.
 Large Disturbance In a Power System: It is a disturbance
for which the equations that describe the dynamics of the
power system cannot be linearized for the purpose of
accurate analysis.

14. 14
Differences between reliability, security, and stability
• Reliability is the overall objective in power system design and
operation. To be reliable, the power system must be secure most
of the time.
• To be secure, the system must be stable but must also be secure
against other contingencies that would not be classified as
stability problems e.g., damage to equipment such as an
explosive failure of a cable, fall of transmission towers due to ice
loading or sabotage. As well, a system may be stable following a
contingency, yet insecure due to post-fault system conditions
resulting in equipment overloads or voltage violations.

15. … Cont’
• System security may be further distinguished from stability in
terms of the resulting consequences. For eg., two systems may
both be stable with equal stability margins, but one may be
relatively more secure b/c the consequences of instability are less
severe.
• Security & stability are time-varying attributes w/c can be
judged by studying the performance of the power systems under a
particular set of conditions.
• Reliability is a function of the time-average performance of the
power system; it can only be judged by consideration of the
15

16. 16

17. 4.2. Power Quality
The term “unprotected power” refers to electric power, voltage
or current waveforms containing other than desired components.
These voltage and current components contaminate and distort
the ideal waveform. They can be caused by:
• Electric switching (digital) devices including switch-mode
power supplies.
• Arcing devices including fluorescent lamps and welders.
• Deteriorated wiring, connections or ground currents.
• Weak, overloaded power supplies. 17

18. ……Cont’
. Various system design problems including transformer saturation
from geomagnetically induced currents.
Electrical distortion is typically characterized as:
 Noise, Harmonics, Transients and various other short-term
and long-term disturbances.
How we minimize or eliminate some distortions? Using
Surge arresters, electromagnetic shields, isolation transformers,
constant voltage regulators, tap-changing regulators, low-
impedance power conditioners, proper grounding and electrical
maintenance programs
18

19. Electrical distortion
1. Noise:
• Unpredictable, random, unwanted signals superimposed on data
signals or on power waveforms is called noise.
• In a power system, electrical noise is generally a long series of
relatively high-frequency impulses having varying amplitudes and
riding on the power waveform.
 Interference affects data signals;
 Static affects radio signals. B/c noise can affect equipment operation,
noise makes power and control systems, such as programmable logic
controllers, less reliable.
• In technical terms, noise is electromagnetic interference (EMI) and
19

20. 2. Harmonics
• Electrically, a harmonic is a voltage/current whose waveform cycles
at a frequency that is an integer-multiple of the desired or
fundamental frequency (odd harmonics especially third order).
• Harmonics are caused by the use of variable-speed drives,
rectifiers, inductive heaters, personal computers, saturated
transformers (fluorescent lighting ballasts), copying machines,
solid state switches, fluorescent and high intensity discharge arc
lamps & other nonlinear/arcing devices.
• Harmonics contaminate the sinusoidal power supply waveform.
• The distorted waveform caused by harmonics is a repeating, non-
20

21. • Harmonic analyzers are test sets that determine all necessary
measurements and calculate waveform distortion.
• Less expensive instruments reading only voltage for the selected
harmonics can be used along with manual calculations to determine
the same information.
• Total Harmonic Distortion (THD) is the overall level of harmonic
distortion of a voltage waveform.
• The THD of a voltage waveform can be calculated by the formula:
21

22. Even harmonics usually do not appear in a properly operating
power system.
Linear Voltage / Current
No Harmonic Content
voltage
current

23. Non-Linear Voltage / Current
Harmonic Content
voltage
current

24. Non - Linear Load
"A load where the waveshape of the steady-
state current does not follow the waveshape
of the applied voltage."
voltage
current

25. 3. Transients
• Transients are surges/changes in steady-state voltage/current that
disappear as a new steady state condition is reached. Typically these
rapid electrical changes, usually lasting milliseconds or microseconds,
transfer readily b/n circuits through electromagnetic coupling.
• A pulse (impulse/spike) usually lasts less than 2 milliseconds and is a
short duration transient where the initial & final steady-state
conditions of the system are usually the same.
• The causes of transients include lightning, restoring power following
an interruption, routine switching, loose wires & electrical breakdown
and other fault conditions.
25

26. 4. Other Electrical distortion
• Power conditioning equipment combines voltage regulation with
power system isolation and shielding.
• UPSs are generally an excellent way to control power
distortions, but their primary purpose is to improve power
reliability not to clean poor power quality.
• Short-term power quality disturbances include dropouts, sags
and swells.
 Dropouts are momentary power losses typically occurring
within one electrical cycle.
26

27. RMS Voltage Variations
-200
-150
-100
-50
0
50
100
150
200
Sag Swell Interruption

28. ………..Cont’
 Sags are depletions of power caused when heavy loads are
started or connected.
 Swells are power surges caused by sudden load disconnects.
 Sags and swells are typically seen as voltage changes lasting 2.5
seconds (150 cycles for a 60 Hz system or 125 cycles for a 50
Hz system ) or less upon sudden changes in power demand.
28

29. 4.3. Reliability assessment techniques
1. Generating System Reliability Assessment
 Generating capacity reliability is defined in terms of the
adequacy of the installed generating capacity to meet the
system load demand.
 Outages of generating units and/or load in excess of the
estimates could result in “loss of load”, i.e., the available
capacity (installed capacity - capacity on outage) being
inadequate to supply the load.
29

30. ….Cont’
 In general, this condition requires emergency assistance
from neighboring systems and emergency operating
measures such as system voltage reduction and voluntary
load curtailment.
 Depending on the shortage of the available capacity, load
shedding may be initiated as the final measure after the
emergency actions. The conventional definition of “loss of
load” includes all events resulting in negative capacity
margin or the available capacity being less than the load.
30

31. …….Cont’
 The basic methodology for evaluating generating system
reliability is to develop probability models for capacity on
outage and for load demand, and calculate the probability
of loss of load by a convolution of the two models.
 This calculation can be repeated for all the periods (e.g.,
weeks) in a year considering the changes in the load
demand, planned outages of units, and any unit additions or
retirements, etc.
31

32. Probabilistic Criteria and Indices
An understanding of the probabilistic criteria and indices used in
generating capacity reliability studies is important. These include
1. loss of load probability (LOLP)
2. loss of load expectation (LOLE)
3. loss of energy expectation (LOEE)/expected energy not supplied
(EENS)
4. frequency & duration (F&D) indices
5. energy index of reliability (EIR)
6. energy index of unreliability (EIU), and
7. system minutes (SM). 32

33.  loss of load probability (LOLP):
• This is the oldest and the most basic probabilistic index. It is defined
as the probability that the load will exceed the available generation.
Its weakness is that it defines the likelihood of encountering trouble
(loss of load) but not the severity; for the same value of LOLP, the
degree of trouble may be less than 1 MW or greater than 1000 MW or
more. Therefore it cannot recognize the degree of capacity or energy
shortage.
• This index has been superseded by one of the following expected
values in most planning applications because LOLP has less physical
significance and is difficult to interpret.
33

34. • Loss of load expectation (LOLE):
• This is now the most widely used probabilistic index in deciding
future generation capacity.
• It is generally defined as the average number of days/hours on which
the daily peak load is expected to exceed the available capacity.
• It therefore indicates the expected number of days (or hours) for
which a load loss or deficiency may occur.
• This concept implies a physical significance not forthcoming from
the LOLP, although the two values are directly related.
• It has the same weaknesses that exist in the LOLP.
34

35. Loss of energy expectation (LOEE)
 This index is defined as the expected energy not supplied
(EENS) due to those occasions when the load exceeds the
available generation.
 It is presently less used than LOLE but is a more appealing index
since it encompasses severity of the deficiencies as well as their
likelihood.
 It therefore reflects risk more truly and is likely to gain
popularity as power systems become more energy-limited due to
reduced prime energy and increased environmental controls.
35

36. Energy index of reliability (EIR) and Energy index of
unreliability (EIU)
 These are directly related to LOEE which is normalized by
dividing by the total energy demanded.
 This basically ensures that large and small systems can be
compared on an equal basis and chronological changes in a
system can be tracked.
36

37. Frequency & Duration (F&D) Indices
• The F&D criterion is an extension of LOLE and identifies
expected frequencies of encountering deficiencies and their
expected durations.
• It therefore contains additional physical characteristics but,
although widely documented, is not used in practice.
• This is due mainly to the need for additional data and
greatly increased complexity of the analysis without having
any significant effect on the planning decisions.
37

38. 38
Distribution System Reliability Assessment
Load Point Indices
• failure rate, 
• average outage time, r
• average annual unavailability, U = .r
• average load disconnected, L
• expected energy not supplied, E = U.L

39. 39
n
i
i 1
=



Interruptionfrequencyf [interruptions/year]
s
n
i i
i=1
n
i
i=1
r
=




Interruptiondurationr [hours/interruption]
s
n
i i
i=1
r
=


U
navailabilityqs 8760
Distribution System Reliability Assessment

40. 40
System Oriented Reliability Indices, Number of
Interruptions
• Weighting by number of customers
– System Average Interruption Frequency Index SAIFI:
tot
n
i i
i=1
n
i
i=1
f N
N


S
A
IFI= (interruptions/year)
fi = number of interruptions at load point i
Ni = number of customers connected to load point i
n = number of load points interrupted
ntot = total number of load points

41. 41
System Oriented Reliability Indices, Annual Interruption Time
• Weighting by number of customers
– System Average Interruption Duration Index SAIDI:
tot tot
n n
i i i i i
i=1 i=1
n n
i i
i=1 i=1
U N f r N
=
N N
 
 
S
A
ID
I= (hours/year)
Ui = firi = annual outage time for load point i
ri = Average outage duration for load point i
Distribution System Reliability Assessment

42. 42
System Oriented Reliability Indices, Average Interruption Duration
• Weighting by number of customers
– Customer Average Interruption Duration Index CAIDI:
tot
n
i i
i=1
n
i i
i=1
U N
f N


C
A
ID
I= (hours/interruption)
SAIFI  CAIDI = SAIDI
tot tot tot tot
n n n n
i i i i i i i i i
i=1 i=1 i=1 i=1
n n n n
i i i i i
i=1 i=1 i=1 i=1
f N U N U N f r N
= =
N f N N N

   
   
Distribution System Reliability Assessment

43. 43
System Oriented Reliability Indices, Unavailability, Energy Not
Supplied
• Average Service Unavailability Index
SAID
I
ASU
I=
8760
• Energy Not Supplied
n
av(i) i
i=1
P U

E
N
S= (kW
h/year)
P =A
verageloadconnectedtoloadpointi
av(i)
• Average Energy Not Supplied
tot
n
i
i=1
N

E
N
S
A
E
N
S= (kW
h/custom
er.year)
Distribution System Reliability Assessment

44. 44