The PRC-25-1 Standard was created in response to a number of undesirable generator trips during the 2003 blackout when the voltage in the transmission network dropped to as low as 0.85 per-unit. A detailed analysis revealed that prior to tripping, these generators were generating near their MW limit, while supplying VARs as high as 1.5 times the numerical value of the their rated MW – due to a controlled response called “field-forcing”. This transient condition apparently confused some phase relays near the generators to conclude erroneously that the generators were feeding into a fault. PRC-025-1 provides guidelines for setting relays at and near generators to prevent a recurrence.
The Add | Generator Loadability Limit command in the OC Curves Window computes and draws the generator loadability limit in the form of a vertical line on the time-versus-current plot. The PRC-25-1 Standard requires the pickup of the phase overcurrent relay (in primary amps) to be higher than this value.
Note: The PRC-25-1 Standard is lengthy and complex. This command addresses only a subset of the Standard’s requirements. The user should consult the text of the Standard for details.
TO DISPLAY THE GENERATOR LOADABILITY LIMIT:
1. Select the Add | Generator Loadability Limit command.
Note: This command is dimmed and cannot be activated if (1) the first relay on the OC Curves Window is a ground relay, or (2) the horizontal axis is in MVA and not in amperes, or (3) fault solutions are being shown on the curves.
A dialog box will appear. The three input parameters vary depending on which of the three radio buttons is highlighted.
CASE 1: RELAY IS AT THE GENERATOR SIDE OF A 2-WINDING STEP-UP TRANSFORMER THAT CONNECTS THE GENERATING UNIT(S) TO THE NETWORK. THE RELAY CAN ALSO BE ON THE GENERATOR SIDE OF A 3-WINDING DUAL-SECONDARY GSU.
Note: This radio button is dimmed if the first relay on the OC Curves Window is not on the low side of a 2- or 3-winding GSU transformer.
Nameplate MVA rating: Nameplate MVA rating of the generating unit(s). In the case of a dual-secondary GSU, enter the rating of one of the two identical generators.
Rated power factor: Rated power factor of the generating unit(s)
Gross MW capacity: Gross MW capability of the generating units, as reported to the Transmission Planner. In the case of a dual-secondary GSU, enter the capacity of one of the two identical generators.
The method we use for this case is called “option 2b” in the Standard, which applies to phase relays (51) and voltage-restrained relays (51V-R) on the generator side of a GSU. The MW output of the unit(s) is assumed to be equal to the gross MW capacity. The MVAR output is assumed be equal to 1.5 times the numerical value derived from multiplying the nameplate MVA rating and the rated power factor.
The network-side voltage of the GSU is fixed at 0.85pu. The generator-side voltage is calculated from an iterative solution of the power-flow equation, which takes into account the turns ratio and impedance of the GSU.
The limiting current is calculated as 115% of the current based on the low-side voltage and the MW+jMVAR output of the unit(s). This limit is shown graphically as a vertical line near the bottom of the plot. The pickup of the relay (in primary amps) should be greater than this limit.
For a voltage-controlled relay (51V-C), the method of “option 3” in the Standard applies. The Standards requires only that the voltage-controlled setting be less than 75% of the calculated generator-bus voltage. The program shows the maximum voltage-controlled settting only in text in the TTY Window.
CASE 2: RELAY IS ON THE PLANT SIDE OF A GSU THAT CONNECTS A WIND/SOLAR PLANT TO THE NETWORK
Note: This radio button is dimmed if the first relay on the OC Curves Window is not on the plant side of a 2-winding GSU transformer and it is not on the wye-winding of a 3-winding transformer.
Aggregate nameplate MVA: The maximum aggregate nameplate MVA output from all the units within the plant.
Rated power factor: Rated power factor of the units.
MVAR from reactive power devices: Total static compensation in MVAR. Enter a positive value for capacitive MVARS, and negative for reactive MVARS.
The method we use for this case is called “option 11” and “option 12” in the Standard, which applies to phase relays (51) and directional phase relays (67 looking toward the network) on the plant side of a GSU that connects “asynchronous generators only (including inverter-based installation)” to the network. Most MW-size wind and solar plants fit this description. Again, the phase relays are on the plant-side of the GSU.
The MW+jMVAR output of the plant is computed based on the aggregate nameplate MVA and the rated power factor. The MVAR from reactive power devices is added to the MVAR output.
The network-side voltage of the GSU is fixed at 1.0 per-unit. The plant-side voltage is estimated using the transformer turns ratio alone.
The limiting current is calculated as 115% of the current based on the plant-side voltage and the net MW+jMVAR plant output. This limit is shown graphically as a vertical line near the bottom of the plot.
For a voltage-controlled relay (51V-C), the method of “option 6” in the Standard applies. The Standards requires only that the voltage-controlled setting be less than 75% of the calculated generator-bus voltage. The program shows the maximum voltage-controlled settting only in text in the TTY Window.
CASE 3: RELAY ON A BRANCH THAT IS USED EXCLUSIVELY TO CONNECT THE GSU TO THE NETWORK
Note: This radio button is dimmed if the first relay on the OC Curves Window is not on a line and it is not on a 2- or 3-winding transformer.
Nameplate MVA rating: Nameplate MVA rating of the generating unit(s).
Rated power factor: Rated power factor of the generating unit(s)
Gross MW capacity: Gross MW capability of the generating unit(s), as reported to the Transmission Planner.
The method we use for this case is called “option 15a” in the Standard, which is most commonly used for:
•Phase relays (51) on the high side of the GSU, looking “backwards” toward the network, or
•Phase relays (51) on a transmission line that is used exclusively to connect the GSU to the network. The Standard applies only to relays on the line that look toward the network, and NOT to the ones that look toward the GSU.
The MW output of the unit(s) is assumed to be the gross MW capacity. The MVAR output is assumed to be equal to 1.2 times the numerical value computed from multiplying the nameplate MVA rating and the rated power factor.
The voltage at the high side of the GSU is assumed to be 0.85 per-unit.
The limiting current is calculated as 115% of the current based on 0.85 per-unit voltage and the MW+jMVAR output of the unit(s). This limit is shown graphically as a vertical line near the bottom of the plot.
2. Press OK to show the PRC-025-1 limit.
The current limit is shown as a vertical line drawn near the bottom of the plot. (The exception is voltage-controlled phase relay of Case 1 and Case 2, for which the output is available in the TTY Window only.) The relay’s pickup (in primary amps) must be higher than this limit.
The calculations are documented in text in the TTY Window. A sample is shown below.
Curves Window
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