Every time a fault or a topological change affects the power balance in the system, the instantaneous power imbalance creates oscillations between the machines. Stable oscillations lead to transition from one (pre-fault) to another (post-fault) equilibrium point, whereas unstable ones allow machines to oscillate beyond the acceptable range. If the oscillations are large, the stations’ auxiliary supplies may undergo severe voltage fluctuations, and eventually trip. Should that happen, the subsequent resynchronization of the machines might take a long time. It is, therefore, desirable to trip the machine(s) exposed to transient unstable oscillations while the plant auxiliaries remain energized.
The frequency of the transient oscillations is usually between 0.5 and 2 Hz. Since the fault imposes almost instantaneous changes on the system, the slow speed of the transient disturbances can be used to distinguish between the two. For the sake of illustration, let us assume that a power system consists of two machines, A and B, connected by a transmission line. Figure 9.34 represents the trajectories of the stable and unstable swings between the machines, as well as a characteristic of the mho relay covering the line between them, shown in the impedance plane. The stable swing moves from the distant stable operating point towards the trip zone of the relay, and may even encroach on it, then leave again. The unstable trajectory may pass through the entire trip zone of the relay. The relaying tasks are to detect, and then trip (or block) the relay, depending on the situation. Detection is accomplished by out-of-step relays, which have multiple characteristics. When the trajectory of the impedance seen by the relays enters the outer zone (a circle with a larger radius), the timer is activated, and depending on the speed at which the impedance trajectory moves into the inner zone (a circle with a smaller radius), or leaves the outer zone, a tripping (or blocking) decision can be made. The relay characteristic may be chosen to be straight lines, known as “blinders,” which prevent the heavy load from being misrepresented as a fault or instability. Another piece of information that can be used in detection of transient swings is that they are symmetrical, and do not create any zero or negative sequence currents.
In the case when power system separation is imminent, out-of-step protection should take place along boundaries that will form islands with matching load and generation. Distance relays are often used to provide an out-of-step protection function, whereby they are called upon to provide blocking or tripping signals upon detecting an out-of-step condition. The most common predictive scheme to combat loss of synchronism is the Equal-Area Criterion and its variations. This method assumes that the power system behaves like a two-machine model where one area oscillates against the rest of the system. Whenever the underlying assumption holds true, the method has potential for fast detection.
The frequency of the transient oscillations is usually between 0.5 and 2 Hz. Since the fault imposes almost instantaneous changes on the system, the slow speed of the transient disturbances can be used to distinguish between the two. For the sake of illustration, let us assume that a power system consists of two machines, A and B, connected by a transmission line. Figure 9.34 represents the trajectories of the stable and unstable swings between the machines, as well as a characteristic of the mho relay covering the line between them, shown in the impedance plane. The stable swing moves from the distant stable operating point towards the trip zone of the relay, and may even encroach on it, then leave again. The unstable trajectory may pass through the entire trip zone of the relay. The relaying tasks are to detect, and then trip (or block) the relay, depending on the situation. Detection is accomplished by out-of-step relays, which have multiple characteristics. When the trajectory of the impedance seen by the relays enters the outer zone (a circle with a larger radius), the timer is activated, and depending on the speed at which the impedance trajectory moves into the inner zone (a circle with a smaller radius), or leaves the outer zone, a tripping (or blocking) decision can be made. The relay characteristic may be chosen to be straight lines, known as “blinders,” which prevent the heavy load from being misrepresented as a fault or instability. Another piece of information that can be used in detection of transient swings is that they are symmetrical, and do not create any zero or negative sequence currents.
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| FIGURE 9.34 Trajectories of stable and unstable swings in the impedance plane. |