Question

can you calculating this question and explain why? thanks

A distribution network shown in Figure 3 consists of a 40-MVA and 33/22-kV step-down transformer with a reactance of 10% and two distribution lines each having a reactance of 5 Q2. The load connected to each bus is also shown in the figure. The CO-8 overcurrent relays are used to protect the system. The time-current curve of the CO-8 relay is shown in Figure 4 on page 5. Assume that the coordination time interval between two adjacent relays is 04 4. and that the fault level at 33 kV is infinite. Bus 3 L1 Bus 4 Bus 2 Bus 1 10 MVA L2 T1 CB5 CB3 CB2 CB1 10 MVA 10 MVA 33 kV CB4 22 kV Figure 3 (a) Select the ratios for all CTs. (5 Marks) (b) Select the current plug settings and time dial settings for the relays at Bus 2, Bus 3 and Bus 4 (12 Marks) (e) Calculate the operating times of the relays at Bus 2 and Bus 3 ifa three- phase fault occurs in the middle of L2. (4 Marks) (d) What is the weakness of the existing overcurrent protection scheme? Typical time curves type CO-8 over current relay 50-60 hertz Current tap setting A 6 0.5 0.6 08 1.0 1,5 2.0 2.5 11 time dial setting 10 3.5 6 3 2 7 10 12 0.5 3 4 5 6 7 8 10 12 1416 20 2 1 Relay input current in multiples of current tap setting Figure 4 Relay operating time (seconds)

Answer 1

Weakness of the existing overcurrent protection schemes

Existing protection schemes are self-contained entities that use independent local measurement chains to deliver their functionality. However, the increasing complexity of power systems has given rise to System Integrity Protection Schemes (SIPS), which use wide area measurements to deliver more complex functionality.

The measurements used by each of protection systems will vary significantly in terms of the type of measurement, the acceptable delay, the required reporting rate, the required resolution and the required accuracy.

SIPS are designed to protect the system from this specific set of contingencies using a set of pre-determined actions that are designed based on offline system studies. These actions will be executed when a specific set of input conditions are satisfied . For a scheme to be classed as a SIPS the actions implemented must go beyond simply isolating the faulted elements.

The conditions required to trigger a SIPS and cause it to operate can include events (e.g. the loss of a line), the system response (e.g. the measured frequency being below a threshold), or a combination thereof. Furthermore, most SIPS are armed by one condition and then triggered by another condition. The use of SIPS is now a worldwide practice [21] and an ever increasing number of these schemes are being designed and implemented