Substation Design: Values to report to IESO

In the province of Ontario, prior to the commissioning of any new transmission station, the range of import and export of active and reactive power are key values that are to be communicated with Independent Electricity System Operator (IESO).

For example, in case of a new 115 KV transmission station consisting of a single 20/ 26.6/ 33.2 MVA OA/FA/FA transformer as a load center with the possibility to export  power, the following "Equivalent Engineering Value Range" shall be communicated with IESO:

Active Power: -20 to +40 MW

Reactive Power: -15 to +24 MVAR

The calculation method for active power is as follows:

Transformer capacity is 20/26.7/33.3 MVA. Considering a power factor of unity and 20% overload, then:
Imported Active Power = 33.3*1.20*1 = 40 MW.
The reverse power flow through the transformer is normally considered to be 60% of its capacity. With a unity power factor we will have:
Exported Active Power = 0.6 *33.3*1= 20 MW.

The calculation method for reactive power is as follows:

Transformer capacity is 20/26.7/33.3 MVA. With 20% overload it will reach to 40 MVA. Considering a power factor of 0.8, then:

Imported Reactive Power = 40* SQRT(1- 0.8^2) = 24 MVAR.

On the other hand, the reverse power flow through the transformer is normally considered to be 60% of its capacity. Therefore, 0.6 *33.3 MVA = 20 MVA. The power factor of the 115 kV line is not less than 0.9. Therefore, it shall provide 0.436*20 =8.7 MVA.

The other logic to determine the maximum reactive power flow injected by the facility is when the power flow is towards the facility however, power factor is on the capacitance side. Considering a power factor of 0.9 to 0.95 with 40 MVA can result a reactive power flow of about 15 MVAR. This approach dictates more flow towards the grid. Therefore - 15 MVAR is selected as minimum.

October 2012