Power System and Actual Operational Experiences

Most of the indigenous load is supplied from the bulk 500 kV-power system via the parallel 275 kV tie transmission lines. This metropolitan system consists entirely of 275 kV, 154 kV and 66 kV underground cables. If the power supply from the bulk 500 kV- power system is interrupted by a fault on the tie transmission line, the metropolitan power system will be separated as a heavily overload system with large shunt capacitance.

We have therefore installed an Islanding Protection System with active and reactive power balance control functionality in order to protect the most important loads in the metropolitan area from serious blackouts.

On 22 November in 1999, the power system in area (1) in Figure 2 was separated from the main power system by an accident involving the 275 kV overhead tie transmission line. An Air Self-Defense Force jet training plane severed the tie transmission lines. During the course of this incident, the successful operation of the Islanding Protection System ensured that the most important customers in the metropolitan area were not affected by the power failure.

Metropolitan Power System Features

Figure 4 provides detail of the power system in (1).  The peak demand is approximately 3600 MW in summer and the local power system generating capacity is 700 MW.

The total charging capacity of the 275 kV, 154 kV and 66 kV underground cables is 700 MVar. If the metropolitan power system is separated from the bulk 500 kV power system, a voltage drop will be experienced and the frequency will also drop rapidly due to severe excessive overload. As the shortage of generating capacity is very large, the system may collapse as a result of generator under frequency tripping unless high speed load shedding is initiated.

However, the integrity of the separated system cannot be assured solely by load shedding. If many loads are shed for active power balance control, the reactive (inductive) power of the loads and reactive power loss of the transformers will also be lost, so a system possessing a large shunt capacitance will suffer from the effects of serious overvoltage caused by severe reactive power unbalance. Therefore, it will not be possible to recover the system frequency because the power consumption will not be reduced as a consequence of the overvoltage. [load voltage characteristics such as P = P0 (V/V0)2] Finally, the power system may collapse.