In This Issue
- Protective System Maintenance Program
- Transmission Line Loadability
- Generator Loadability
- Relay Performance during Stable Power Swings
- Coordination for Performance during Fault
Applications of ASPEN Tools for PRC Compliance
North American Electric Reliability Corp. (NERC) started issuing recommendations after the Northeast blackout of 2003, when the causes of the blackout became known. These recommendations soon turned into standards with names that begin with the letters "PRC". Currently there are a number of PRC standards that directly govern how protective relays are set and maintained. The following is a description of ASPEN tools designed for complying with these standards.
PRC-005 - Protective System Maintenance Program
This standard requires the utilities to have an ongoing program to keep the protection systems in good working order. Among other things, this regulation requires the relays and associated equipment (communication system, battery backup, trip coils, etc.) to be tested and calibrated from time to time. In the ASPEN Relay Database C/S, we have implemented an "Activity" feature (in version 10, available since 2012) to help users schedule and keep records of the tasks required under this standard.
PRC-023 - Transmission Line Loadability
Features for PRC-023 compliance were implemented in OneLiner v10, in 2005. Figure 1 shows the load region as a shaded cone in the distance-relays window. The relay passes the test if this cone does not intersect the relay characteristics. In the overcurrent-relay window, the loadability limit is shown graphically with an annotation with a callout (Figure 2). The relay passes the test if the relay's pickup lies to the right of the callout's vertical bar. In OneLiner v10, we also added a Check | Relay Loadability command to verify the compliance of all the phase distance relays and phase overcurrent relays system wide. A utility can use both the graphical displays and the reports from the check command as evidence of compliance.
In version 14, OneLiner will have the ability to check the loadability of transformers, as required by PRC-023-3.
Fig. 1: Distance relay characteristics with a load region.
Fig. 2: Overcurrent relay curve with an annotation.
PRC-025 - Generator Loadability
In the events leading to the 2003 blackout, a number of the generators tripped when the voltage in the transmission network dropped to 0.85 per-unit. A detailed analysis revealed that, prior to the trip, the generators were generating near their MW limit, while supplying VARs as high as 1.5 times the numerical value of the rated MW because of a controlled response call "field-forcing." This condition apparently confused some phase relays to conclude that the generators were feeding into a fault. PRC-025 provides guidelines for setting relays at and near generators to prevent a recurrence.
OneLiner v14 (to be release near the end of 2015) has tools to comply with PRC-025-1 for distance and overcurrent relays:
- At the generator side of a step-up transformer (GSU) that connects the generating unit(s) to the network.
- At the plant side of a GSU that connects a wind or solar plant to the network.
- On a line that is used exclusively to connect the GSU to the network.
For a distance relay, the PRC-025 limit is shown as a slanted dotted line (Figure 3). The relay complies if its characteristics do not interest the dotted line. For an overcurrent phase relay, the limit is shown using an annotation similar to that of Figure 2. Checking logic is also available to overcurrent relays that are voltage-restrained or -controlled. In all cases, the calculations, including all the intermediate steps, are shown in text in the TTY Window.
Fig. 3: Distance relay characteristics with a generator loadability limit.
PRC-026 - Relay Performance during Stable Power Swings
This standard seeks to improve reliability by preventing relays from tripping unnecessarily due to stable power swings.
For a distance phase relay, the calculation for the unstable power-swing region is quite involved. Among other things, the entire network has to be reduced to a 2-bus equivalent that comprises of the end buses of the relay branch in question. Then the region is formed as the intersection of a lower loss-of-synchronism circle, an upper loss-of-synchronism circle, and a lens that connects the end points of the total system impedance. The end result is a pair of wing-shaped curves, shown dotted in Figure 4. A relay complies with PRC-026 must have its characteristics lie within the area embraced by these two dotted curves. The calculations and the coordinates of the dotted curves are documented in text in the TTY Window. For an overcurrent phase relay, the limit is shown using an annotation similar to that of Figure 2.
Fig. 4: Relay characteristics and the unstable power swing region.
PRC-027 - Coordination for Performance during Fault
The stated purpose of this standard is "to maintain the coordination of Protection Systems installed for the purpose of detecting faults on Bulk-Electric-System Element and isolate those faults, such that the Protection Systems operate in the intended sequence during fault."
We will provide two different tools in OneLiner v14 to assist users to comply with this standard.
First, in OneLiner v14, there will be an updated Fault All Buses command to help users identify buses where the 3-phase or single-line-to-ground fault current has changed 15% or more since the last established baseline. It works as follows. The user first runs the Bus Fault Summary command as usual for the entire system or for a selected list of buses. The text output of this command serves as the baseline. At a later time (no more than 6 years, according to the standard), the user can run the same Bus Fault Summary command, but this time with a new option to fault "buses in an existing bus fault summary report". The output will show the fault currents for the same buses as the baseline. In addition, it will list all the buses whose fault currents have change 15% or more.
Second, the standard requires the user to perform a protection system coordination study at these buses. OneLiner already has a number of tools for coordination checks, including the Check | OC Minimum Pick, Check | OC Instantaneous Setting, and Check | Primary/Backup Coordination. We intend to add a new checking command to OneLiner v14 (available in a maintenance release in the first half of 2016) that utilizes stepped-event simulations as the basis for checking relay coordination.