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When frequency of the system falls below under frequency setting of a generator relay, the related generator is tripped. Tripping of this generator intensifies the imbalance between generation and consumption. Therefore, frequency decline will be aggravated and this may cause the under frequency relays of other generators to trip. Consequently a cascading outage of generators is started and eventually the whole system encounters a wide spread blackout. Conventional UFLS schemes are designed to prevent hazardous frequency decline following the disturbances which may cause considerable mismatches between generation and consumption. Excessive frequency decline may cause damage to the equipments of power system especially turbine blades in power plants. Therefore, every power plant is equipped with a type of under frequency relay which allows each level of frequency decline to exist just for a certain amount of time. After that time if the frequency is not recovered to the permitted level the generator will be tripped. A typical setting for under frequency relays of generators in 60 Hz system is listed in Table 17).

Table 17 When frequency of the system falls below under frequency setting of a generator relay, the related generator is tripped. Tripping of this generator intensifies the imbalance between generation and consumption. Therefore, frequency decline will be aggravated and this may cause the under frequency relays of other generators to trip. Consequently a cascading outage of generators is started and eventually the whole system encounters a wide spread blackout. Conventional UFLS schemes are designed to prevent hazardous frequency decline following the disturbances which may cause considerable mismatches between generation and consumption. Excessive frequency decline may cause damage to the equipments of power system especially turbine blades in power plants. Therefore, every power plant is equipped with a type of under frequency relay which allows each level of frequency decline to exist just for a certain amount of time. After that time if the frequency is not recovered to the permitted level the generator will be tripped. A typical setting for under frequency relays of generators in 60 Hz system is listed in Table 17).

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Fig.3: A typical LD-df/dt characteristic Tp-df/dt characteristics: The basic idea of this method is introduced in'*!, This method is similar to the first method; however instead of using f-df/dt characteristics Tp-df/dt characteristics are used. Tp is the intentional time delay for load shedding. Tp does not include the intrinsic delays such as operating time of relays and circuit breakers. In this method for large disturbances small time delays are used while for small disturbances conventional time delays are used. One type of Tp-df/dt characteristic is depicted in Fig. 2.
Fig.3: A typical LD-df/dt characteristic Tp-df/dt characteristics: The basic idea of this method is introduced in'*!, This method is similar to the first method; however instead of using f-df/dt characteristics Tp-df/dt characteristics are used. Tp is the intentional time delay for load shedding. Tp does not include the intrinsic delays such as operating time of relays and circuit breakers. In this method for large disturbances small time delays are used while for small disturbances conventional time delays are used. One type of Tp-df/dt characteristic is depicted in Fig. 2.
greater than 1.4 is designated large df/dt region; the region wherein Idf/dt| is between 0.9 and 1.4 is designated medium df/dt region and the region with Idf/dt| smaller than 0.9 is designated small df/dt region.
greater than 1.4 is designated large df/dt region; the region wherein Idf/dt| is between 0.9 and 1.4 is designated medium df/dt region and the region with Idf/dt| smaller than 0.9 is designated small df/dt region.
developed. The conventional lump and static system model is not able to provide accurate results and does not provide access to various system variables at different locations. The distributed and dynamic model of the network involves generators with governors and AVRs, transmission lines, power transformers, tap changers, shunt capacitors, shunt reactors, loads and under frequency relays. Fig. 4 depicts a schematic diagram of the network and Fig. 5 depicts the single line diagram of Khorasan network. Fig. 4: Schematic diagram of the Iranian network connected to the Khorasan network.
developed. The conventional lump and static system model is not able to provide accurate results and does not provide access to various system variables at different locations. The distributed and dynamic model of the network involves generators with governors and AVRs, transmission lines, power transformers, tap changers, shunt capacitors, shunt reactors, loads and under frequency relays. Fig. 4 depicts a schematic diagram of the network and Fig. 5 depicts the single line diagram of Khorasan network. Fig. 4: Schematic diagram of the Iranian network connected to the Khorasan network.
Fig.5: Single line diagram of the Khorasan network.
Fig.5: Single line diagram of the Khorasan network.
f-df/dt loci of the system: f-df/dt loci of the slack busbar, for all scenarios of stage 2.2 are obtained from the simulation results. For example the locus for a large disturbance with 720 MW generation outage is depicted in Fig. 6. Fig. 6: Locus of f-df/dt for 720 MW generation outage in Khorasan network.
f-df/dt loci of the system: f-df/dt loci of the slack busbar, for all scenarios of stage 2.2 are obtained from the simulation results. For example the locus for a large disturbance with 720 MW generation outage is depicted in Fig. 6. Fig. 6: Locus of f-df/dt for 720 MW generation outage in Khorasan network.
Fig.7: LD-df/dt characteristic
Fig.7: LD-df/dt characteristic
Fig. 8 shows the frequency of some busbars in Khorasan network for the disturbance of tie line outage in the case of using conventional UFLS. The amount of power transfer between areas before the tie line outage is 1100 MW. In this case, although all four steps of conventional UFLS scheme operate and 510 MW load is shed, the system does not remain stable and frequency does not recover.
Fig. 8 shows the frequency of some busbars in Khorasan network for the disturbance of tie line outage in the case of using conventional UFLS. The amount of power transfer between areas before the tie line outage is 1100 MW. In this case, although all four steps of conventional UFLS scheme operate and 510 MW load is shed, the system does not remain stable and frequency does not recover.
Fig.9: Frequency of the system in the stable case Fig.8: Frequency of the system in the unstable case
Fig.9: Frequency of the system in the stable case Fig.8: Frequency of the system in the unstable case
In order to further illustrate the performance of different UFLS schemes, several tie line outages are simulated on the system with different values of power transfer through tie line before the outage. For each value of generation outage, frequency of the slack busbar for different UFLS schemes is obtained by simulation. Figs. 10 to 12 depict the frequency of slack busbar for the tie line outages with 700, 900 and 1100 MW power transfer before the outage, respectively. Fig. 10: Frequency of the slack busbar for the tie line outage with 700 MW power transfer
In order to further illustrate the performance of different UFLS schemes, several tie line outages are simulated on the system with different values of power transfer through tie line before the outage. For each value of generation outage, frequency of the slack busbar for different UFLS schemes is obtained by simulation. Figs. 10 to 12 depict the frequency of slack busbar for the tie line outages with 700, 900 and 1100 MW power transfer before the outage, respectively. Fig. 10: Frequency of the slack busbar for the tie line outage with 700 MW power transfer
Fig. 11: Frequency of the slack busbar for the tie line outage with 900 MW power transfer Since the system will remain stable after the tie line outage, all UFLS shedding schemes designed in this work could be considered acceptable for this outage case.
Fig. 11: Frequency of the slack busbar for the tie line outage with 900 MW power transfer Since the system will remain stable after the tie line outage, all UFLS shedding schemes designed in this work could be considered acceptable for this outage case.
Fig. 12: Frequency of the slack busbar for the tie line outage with 1100 MW power transfer
Fig. 12: Frequency of the slack busbar for the tie line outage with 1100 MW power transfer
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