Power Swing




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.