Power Swing which is basically caused by the large disturbances in the power system which if not blocked could cause wrong operation of the distance relay and can generates wrong or undesired tripping of the transmission line circuit breaker.
Power swings can cause the change in load impedance which
under steady state conditions, whereas within the relay’s operating
characteristic, to induce unwanted relay operations at different network
locations. These undesirable measurements may aggravate the power-system
disturbance and cause major power outages, or even power blackout.
Particularly, distance relays should not trip unexpectedly during dynamic
system conditions such as stable or unstable power swings, and allow the power
system to return to a stable operating condition. Thereby, a Power Swing Block (PSB) function
is adopted in modern relays to prevent unwanted distance relay element
operation during power swing . The main purpose of the PSB function is to
differentiate between power faults and power swings, and block distance or
other relay elements from operations during a power swing.
Out-of-Step (OOS) phenomena, which is same as an unstable
power swing . Uncontrolled tripping of
circuit breakers during an OOS condition could cause equipment damage, pose a
safety concern for operating personnel, and further contribute to cascading
outage and shutdown of larger areas of the power system. So, the main purpose of
the Out-of-Step Trip (OST) function should be taken into account to accomplish
differentiation stable from unstable power swings, and separation to system
areas at the predetermined network locations and at the appropriate
source-voltage phase-angle difference between systems, in order to maintain
power system stability and service continuity.
The power system disturbances cause big oscillations in
active and reactive power, low voltage, voltage instability and phase or
angular instability between the generated and consumed power which results in
loss of generation and load which effected both the power generation and the
end customers. During the steady state
condition, power systems operate on the nominal frequency (50Hz or 60Hz). The
complete synchronism of nominal frequency and voltage at the sending and
receiving ends cause complete balance of active and reactive power between
generated and consumed active and reactive powers. In steady state operating
condition Frequency= Nominal frequency (50 or 60 Hz) +/– 0.02 Hz and
Voltage=Nominal voltage +/– 5% [1].
Power system faults, line switching, generator
disconnection, and the loss or application of large blocks of load result in
sudden changes to electrical power.
Whereas the mechanical power input to generators remains
relatively constant.
The electrical power, Pg transferred
from the generator, an electric machine, to the load is given by the equation:
where:
Eg = Internal voltage
and is proportional to the excitation current
El = Load Voltage
X = Reactance between the
generator and the load
Angle that the
internal voltage leads the load voltage
Pm = Mechanical Turbine
Power of the generating unit
Pg = Electromagnetic
Power output of the generating unit
Pa = Accelerating Power
The mechanical power, Pm,
is provided by the turbine and the average mechanical power must be equal to
the average electrical power. When a system disturbance occurs there is a
change in one of the parameters of the electrical power equation. For faults, typically the reactance between
the generator and the load (X), the load voltage (El), or some combination of
these two parameters causes the electrical power to change. For example, for a
short circuit the load voltage is reduced, for a breaker opening the reactance
increases. When a generation unit trips, the required electrical power from the
remaining generators increases. In this case, the instantaneous mechanical
power provided by the turbine is no longer equal to the instantaneous
electrical power delivered or required by the load. When the load on a unit is
suddenly increased, the energy furnished by the rotor results in a decrease in
the rotor angular velocity . And this decrease in rotor velocity will cause
oscillations in rotor angle and can result in severe power flow swings.
Generator disconnection due to
fault
Suppose we have two generators G1&G2 in parallel, and
both the generators are sharing load. On the sudden disconnection of G2, there
will be an increase in load on G1 and due to this there will be the
oscillations in the rotor angle of G1, which is represented in Fig.
In Fig, d is the steady state rotor angle and d’ is the change in rotor angle due
to oscillations which will result in
the oscillation of nominal voltage, and this oscillation
in the nominal voltage causes loss of synchronism between the generators in
parallel or between the generation and load.
Depending on the severity of the disturbance and the actions of power system controls, the system may remain stable and return to a new equilibrium state experiencing what is
referred to as a stable power swing. Severe system disturbances, on
the other hand, could cause large separation of generator rotor angles, large swings of power flows, large fluctuations of voltages and currents, and eventual loss of synchronism
between
groups of generators or between neighboring utility
systems. Stable Power Swing: Small disturbances which can be control by the
action of Power System and the system remain in its steady state condition.
Unstable Power Swing: Severe disturbances can produce a large separation of
System Generator Rotor angles, large swings of power flow, large fluctuations
of voltages and currents, and eventually lead to lose synchronism.
Power Swing Effect on the Distance Relay
Power swings can cause the load impedance, which under steady state conditions is not within the relay’s
operating characteristic,to enter into the relay’s operating characteristic.
Operation of these relays during a power swing may cause undesired tripping of
transmission lines or other power system elements, thereby weakening the system
and possibly leading to cascading outages and the shutdown of major portions of
the power system.
Distance or other relays should not trip during such as
stable or unstable power swings, and allow the power system to return to a
stable operating condition. Distance relay elements prone to operate during
stable or transient power swings should be temporarily inhibited from operating
to prevent system separation from occurring at random or in other than
pre-selected locations. A Power Swing Block (PSB) function is available in
modern relays to prevent unwanted distance relay element operation during power
swings. The main purpose of the PSB function is to differentiate between faults
and power swings and block distance or other relay elements from operating
during a power swing. However, faults that occur during a power swing must be
detected and cleared with a high degree of selectivity and dependability.
Severe system disturbances could cause large separation of the rotor angles
between groups of generators and eventual loss of synchronism between groups of
generators or between neighboring utility systems. When two areas of a power
system, or two interconnected systems, lose synchronism, the areas must be
separated from each other quickly and automatically to avoid equipment damage
and power blackouts. Ideally, the systems should be separated in predetermined
locations to maintain a load-generation balance in each of the separated areas.
System separation may not always achieve the desired load-generation balance.
In cases where the separated area load is in excess of local generation, some
form of load shedding is necessary to avoid a complete blackout of the area.
Uncontrolled tripping of circuit breakers during an Out-of- Step (OOS)
condition could cause equipment damage, pose a safety concern for utility
personnel, and further contribute to cascading outages and the shutdown of
larger areas of the power system.
Therefore, controlled tripping of certain power system
elements is necessary to prevent equipment damage and widespread power outages
and to minimize the effects of the disturbance.
The Out-of-Step Trip (OST) function accomplishes this separation. The
main purpose of the OST function is to differentiate stable from unstable power
swings and initiate system area separation at the predetermined network
locations and at the appropriate source-voltage phase-angle difference between
systems, in order to maintain power system stability and service continuity.