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TNB system is interconnected to Singapore PUB system via 230 kV tie Hines. The load and generation at Singapore PUB is assumed to be balance at 4000MW. Since the frequency decay rate is the function of percentage overload in the interconnected system, having the Singapore PUB connected would reduce the percentage overload and hence reduce the frequency decay rate. Fig. 2 shows the effect of tripping and not tripping the tie line to Singapore PUB in 6-stage scheme, The rate of frequency decay increase from 1.2 Hz/s to 2.3 Hz/s when the linc is tripped.

Figure 2 TNB system is interconnected to Singapore PUB system via 230 kV tie Hines. The load and generation at Singapore PUB is assumed to be balance at 4000MW. Since the frequency decay rate is the function of percentage overload in the interconnected system, having the Singapore PUB connected would reduce the percentage overload and hence reduce the frequency decay rate. Fig. 2 shows the effect of tripping and not tripping the tie line to Singapore PUB in 6-stage scheme, The rate of frequency decay increase from 1.2 Hz/s to 2.3 Hz/s when the linc is tripped.

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After the system collapse of 29" September 1992, TNB implemented a 4-stage load shedding scheme of IS7TOMW in July 1993 based on a credible generation loss of 1500MW [11]. The 4-stage scheme was revised again immediately after the August 3 1996 system collapse when considerably more than 1500MW of generation was lost, to a 6-stage 2700MW scheme (Table I}.
After the system collapse of 29" September 1992, TNB implemented a 4-stage load shedding scheme of IS7TOMW in July 1993 based on a credible generation loss of 1500MW [11]. The 4-stage scheme was revised again immediately after the August 3 1996 system collapse when considerably more than 1500MW of generation was lost, to a 6-stage 2700MW scheme (Table I}.
It was decided to have an independent review of this 6- stage scheme to determine whether any improvement could be discovered. The static review of the 6-stage scheme was done using the formula given by Western Council Coordinating Council (WSCC) [12]. As a mule of thumb to minimize over shedding, it is always better to have more stages with smaller load at each stage. Moreover, tripping a big block of load at one time will give a large impact to an already weakened system. A new 1|1-stage scheme was recommended as shown in Table fl. The new 1t!-stage scheme was designed to minimize the changes to the 6-stage scheme. It shed the same amount of load in the same frequency range as in the 6-stage scheme. The effect of minimizing over-shedding is illustrated in Fig. 1 where the percentages of over-shedding for both schemes are compared. It shows the results of static analysis for the 25 scenarios. In the first scenario, a 300MW generation deficiency is modeled. Further 1OOMW generation loss is added for the subsequent scenario. The over-shed percentage is defined as the ratio of actual amount of !oad shed to the amount of generation loss. Out of the 25 scenarios, the 11- stage scheme shed less load in 14 scenarios.
It was decided to have an independent review of this 6- stage scheme to determine whether any improvement could be discovered. The static review of the 6-stage scheme was done using the formula given by Western Council Coordinating Council (WSCC) [12]. As a mule of thumb to minimize over shedding, it is always better to have more stages with smaller load at each stage. Moreover, tripping a big block of load at one time will give a large impact to an already weakened system. A new 1|1-stage scheme was recommended as shown in Table fl. The new 1t!-stage scheme was designed to minimize the changes to the 6-stage scheme. It shed the same amount of load in the same frequency range as in the 6-stage scheme. The effect of minimizing over-shedding is illustrated in Fig. 1 where the percentages of over-shedding for both schemes are compared. It shows the results of static analysis for the 25 scenarios. In the first scenario, a 300MW generation deficiency is modeled. Further 1OOMW generation loss is added for the subsequent scenario. The over-shed percentage is defined as the ratio of actual amount of !oad shed to the amount of generation loss. Out of the 25 scenarios, the 11- stage scheme shed less load in 14 scenarios.
Keeping the tie line between utilities during system disturbances is a controversial subject. On one hand, the system is exposed to more disturbances because of larger geographic area. On the other hand, the system frequency variation due to disturbance is reduced because of the bigger equivalent system inertia. Neglecting the higher frequency of disturbances, the benefit of interconnection between utilities can be easily demonstrated by static analysis.
Keeping the tie line between utilities during system disturbances is a controversial subject. On one hand, the system is exposed to more disturbances because of larger geographic area. On the other hand, the system frequency variation due to disturbance is reduced because of the bigger equivalent system inertia. Neglecting the higher frequency of disturbances, the benefit of interconnection between utilities can be easily demonstrated by static analysis.
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