Module 7 : Out of Step Protection

Lecture 26 : Power Swing Detection, Blocking and Out-of-Step Relays

 

Introduction (contd..)

  Guide lines for Setting Out-of-Step Tripping Relay
  The primary question in setting the out of step relay discussed in previous section is to freeze the location of the line segments L1and L2. i.e. decide the perpendicular distance MX and . In general, this setting should be such that the swing locus will remain between for at least a preset interval usually 0.005sec. This time is the operating time of the auxiliary relays which evaluates the sequence of events and determines a loss of synchronism condition. Usually, this represents what is achieved in actual system and hence does not pose any series rejection. Also, the characteristics L1and L2 should not be set so far apart to pick up on load conditions.

In other words, the angle subtended by X on R and S should be larger than and as maximum loading condition will always have angle below . The maximum rate of slip can be estimated by carrying out transient stability simulations.
  Consider the system shown in fig 26.5a. In the case of out of step condition, optimum location to break the system into multiple islands is at bus-B.
  If during out of step condition, the electrical center also appears in the line segment BC, then the required separation is achieved naturally. However, as we know the location of electrical center is not fixed and it depends upon system conditions like ES, ER, number of lines in service, Thevenin's impedances ZS1 and ZS2 etc.

Now, if due to system conditions, the electrical center location shifts to section CD, then uncontrolled system separation will take place at cut - 2, where generation load balance is not obtained.

Hence, it is desirable to block distance relay operation on power swings and install an out of step tripping relay at Bus-C. The respective power swings are shown in fig 26.5b. Now, for this scenario, the loss of synchronism requires a transfer trip signal to be generated to breakers at bus B. In general, the point of best separation is not fixed and it depends upon loading and generating conditions.
  Thus, supervising control with system operator intervention may be required to decide the islanding location. Now WAM (Wide Area Measurement) technology has opened up new options for system protection.
 
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