CHAPTER FOURTEEN
STABILITY, RECLOSING AND LOAD SHEDDING
POWER SYSTEM FREQUENCY CONTROL
1.0
INTRODUCTION
The quality of electric power supply is defined in terms of permissible
variation in the statutory requirements of frequency (1%) and voltage
(6%).
System instability has a direct effect on the quality and
security of supply; as such, it is of interest to the protection engineer.
This chapter, therefore, aims at examining the phenomenon vis-a-vis
the associated protection in an inter-connected system such as ours.
1.1
System Instability/Frequency Control
Any large inter-connected power system is composed of several
generators synchronously connected. A perfect real or active power
balance (active generation = active demand including losses) ensures
constant speed and frequency of operation.
Unfortunately, the load
impressed on the system does fluctuate; more so in a random fashion.
Thus, it is virtually impossible to accomplish equilibrium of active
generation and active demand.
An excess or deficiency in active
generation will always be present. This mismatch normally results in
frequency fluctuation.
If active generation, PG > system demand including losses, machines in
the system will increase in speed and frequency will rise. On the other
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hand, if active generation, PG < system demand including losses,
machines will
decelerate and frequency will fall.
A nominal frequency of 50Hz is
obtained when active power generation in the system equals the total
demand including losses.
In practice, this is achieved by manual load shedding/generation
scheduling or by the appropriate application of frequency relays.
1.2
Interconnected Power System
System response following an instantaneous loss of generation is a
function of many factors; such as stored energy, governor action,
system voltage, spare capacity and demand response to frequency and
voltage.
The change in active power for a given change in frequency in an interconnected system is known as the STIFFNESS in the system. Thus, the
smaller the changes in frequency for a given load, the stiffer or more
stable the system.
Assume a system operating at steady state, i.e.
PG
Where
PD
PD
PG
demand
generation
Let there be an increase in demand, dPD followed by an increase in
generation, dPG. Then the out of balance power, dP is given by:
dP
dPG
dPD
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dP affects the system in three ways namely:
a) Changing the energy potential of the generators
b) Changing the load demand
c) Changing the export of power via the tie lines.
System stiffness is defined by:
K
dP
df
dPG
df
dPD
df
The unit of K is MW/Hz.
Power generation, PG = f (PT); where PT is the turbine input power.
Hence, K may be re-defined as
K
K1PT
K2PD =
Stiffness.
K1 and K2 are coefficients associated with the turbine and load
respectively.
Quite often K1 and K2 are taken as being approximately equal to 0.8
and 0.6 respectively.
Where 2500 K 10000 MW/Hz depending on the system load.
Considering the two limits of stiffness K, a loss of 500 MW will lead to
frequency change of:
500MW____
2500MW/HZ
0.2 Hz at light load
500MW_____
10000MW/HZ
0 .05 Hz at heavy load
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The stiffness figures reveal the importance of having spare capacity
running (or otherwise) immediately available to offset the frequency
change.
The response of the units involved is also important in
controlling the frequency.
In small power systems, the change in
frequency for a reasonable load change is
relatively large; as such, control measures must be introduced to
improve the power frequency (P-f) characteristics.
1.3
Illustrations/Case Study
Consider two separate systems A and B. Power is transferred from A to
B. An extra load in B - dPD, causes an extra input dPT from system A.
dPT
SYSTEM A
KA
SYSTEM B
KB
dPD
Drop in system A frequency due to extra input, dPT
=
dPT Hz
KA
Drop in system B frequency due to extra load dPD and extra input dPT
or
(dPD dPT)
KB
dPT
KA
dPD dPT
KB
dPT
(KA) dPD
KA + KB
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Suppose A and B are operating at a common frequency f with A
exporting power (dPT) to B.
dPT
f
Consider the link (or tie-line) between A and B broken.
System A will have excess generation corresponding to dP T
Therefore
frequency in A will rise.
System B will have extra load corresponding to dP T
Therefore
frequency in B will fall.
dPT
fA, KA
fA fB
OR
dPT
excess
deficiency
fA
f + dPT
KA
fB
f dPT
KB
dPT + dPT
KA
KB
dPT__ =
B
fB, KB
KA KB__
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fA fB
KA + KB
Hence, opening the tie line and measuring the resultant changes in
frequency in the two systems f A and fB, the values of KA and KB may be
obtained.
Problem
Two power systems A and B are inter-connected by a tie line and have
P-f constants KA and KB. An increase in load of 500MW on system A
causes a power transfer of 300MW from B to A. When the tie line is
open, the frequency of system A is 49Hz and of system B is 50 Hz.
Determine the values of KA and KB.
Solution
fA
49Hz
fB
50Hz
A
excess
fB, KB
49 KA
fA
f dPT
KA
49
50 500
KA
50 KA 500
deficiency
fA, KA
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50 KA 49 KA
500
KA
500MW/Hz
dPT
fB fA
KA KB _
KA + KB
300___
50 49
500 KB__
500 + KB
500 KB__
500 + KB
300
1.4
150000 + 300 KB =
500 KB
200 KB
15 x 104
2 KB
1500
KB
750MW/Hz
established
Load Shedding and Under Frequency Relay
Load shedding is the attempt to match load to the available generation
after a disturbance that has left a deficiency in the generation relative
to the connected loads.
It is carried out either manually by system operation personnel or by
the control action of under frequency relays deployed in circuit. Thus,
the primary application of under frequency relays is to detect system
over load and, thereby, save the system from failure resulting from
instability due to excessive frequency decay.
Normal load changes can be absorbed by the spinning reserve in the
system, as all the generators are usually not operating at full capacity.
Moderate over loads result in small increments of speed and frequency
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which activate the governors to increase the prime mover input.
Transient changes such as those that result from faults involve the
exchange of kinetic energy of the rotating masses to the system until
the system can re-adjust to equilibrium. Load shedding is especially
useful when the spinning reserve is inadequate or not available to
compensate for increase in demand.
When the load requirements significantly exceed the generation
capabilities, the frequency of the system decreases.
The system
survives only if enough load is dropped until all the generator outputs
equal or are greater than that of the connected loads. This imbalance
often results from the loss of a key or major transmission line or
transformers which are involved in a major transfer of power either
within the system or between two inter-connected systems. This could
be the consequence of faults cleared without high-speed reclosing,
undesirable relay operation or other situations which interrupt large
power flows.
A veritable means of checking this unhealthy trend of excessive
system demand (i.e. Pdemand > PGeneration) is by the appropriate use of
under frequency relays.
These relays are set at different frequency levels to switch off quickly,
varying amounts of load to restore system equilibrium.
The application and setting of under frequency relays is not
standardised and is based - for a large system - on a study of the most
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probable
and
worst-case
possibilities
seasoned
experience, factual knowledge and judgement.
with
general
In the NEPA system,
three-stage under frequency relays are in use. Their settings are as
given hereunder:
Stage 1
49.8Hz
trips approximately 250 MW load
Stage 2
49.5Hz
trips approximately 300 MW load.
Stage 3
49.2Hz
trips approximately 600 MW load.
They all operate in 0.3 seconds.
1.5
Load Scheduling
Under normal operating condition, it is ensured that the current plant
availability
scheduling.
is
reviewed
under
carefully
planned
generation
Accurate knowledge of the generation status of the
various stations facilitates proper matching of generation with demand
to obtain a stable generation - demand profile over a period of twenty
four hours.
The production of a workable generation schedule is usually derived
from a reliable hour by hour demand forecast as prepared by the
System
Planning
Department
of
the
National
Control
Centre.
Generation scheduling entails reviewing of plant availability at all
power stations in the grid. The reliability of generation schedule is a
function of the accuracy and dependability of plant status reports
normally supplied to the N.C.C. by various power plants on daily basis.
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The objective of generation schedules is to obtain economic water and
fuel usage by the hydro and thermal plants taking into consideration
the following:
1. Actual unit cost of thermal fuel type.
2. Heat rate of steam turbine/gas turbines.
3. Efficiency curves of hydro turbines.
4. Spinning reserve requirements.
5.
Unit limitations (minimum load for stability, rate of loading constraints,
peculiar unit faults, etc.)
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