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

More Commonly Used ANSI Relays

More Commonly Used ANSI Relays are as follows:

No.  Description
--   -----------

11 Multifunction device - contains other numbers as appropriate (new in 1996 edition).
21 Distance relay
25 Synchronizing or synch-check relay
27 Undervoltage relay
32 Directional power relay
37 Undercurrent or underpower relay
43 Manual selector
46 Reverse-phase, phase-balance, or negative-sequence current relay
47 Phase-sequence voltage relay
49 Thermal relay
50 Instantaneous overcurrent relay
51 Time-overcurrent relay
52 Power circuit breaker
55 Power factor relay
59 Overvoltage relay
62 Time delay stopping relay
67 Directional overcurrent relay
68 Blocking relay
78 Phase angle or out-of-step relay
79 Reclosing relay
81 Frequency relay
85 Carrier or pilot relay
86 Lockout relay
87 Current differential relay
89 Power disconnect switch
94 Auxiliary tripping relay
101 Breaker control switch

Add suffix letters and numbers - e.g.: 67N is neutral (ground) directional overcurrent relay 21P-1 is zone 1 phase distance relay.

Gas Trapped in Distribution Transformers

Distribution transformers are generally sealed tank with gas space these days as opposed to the previous generation of conservator type transformers.

The oil filling of tanks of these transformers is made in open air, usually with a simple hose in the transformer. This type of oil filling results in trapping air in the oil, the windings and the core assembly and structure. Trapped air consists of Nitrogen, Oxygen and a small quantity of Carbon Dioxide.

This does not happen in high voltage power transformers since they are filled under vacuum and they go through an extensive degassing process.

The trapped air in distribution transformers may vary greatly from one transformer to the other and with time. The trapped air will migrate at different speed to the gas space.

During the transformer test, discharges might happen between the winding and the tank due to poor workmanship or insulation failure. In these cases different hydro-carbon gases are generated and added to the gas content due to the spark in the oil.

After the required retrofit action on the failed insulation, the gas content of the oil is measured and analyzed by experts to verify the safe operation of the transformer under the tests and normal operation of the transformer.

Based on the above-mentioned oil filling procedure, one should not be concerned by Nitrogen and Oxygen in analysis of dissolved gas.

In the test result of the gas content of these transformers, five gases that are more of concern are: Hydrogen, Methane, Ethane,Ethylene and Acetylene. Also, the ratios that shall be taken into account more seriously are: CH4/H2, C2H2/C2H4, C2H2/CH4, C2H6/C2H2 and C2H4/C2H6.

MV Cable Insulation and System-grounding

There are three levels of conductor insulation for medium-voltage cables: 100, 133,and 173% levels.

The solidly grounded system permits the use of 100% insulation level. When the fault on the other system will raise the system voltage above normal during the time of thefault,133% insulation level should be speciÞed if the fault is cleared within one hour.

When the fault will remain on the system for an indefinite time, 173% voltage level insulation should be used

Ref: IEEE Std 141-1993