What Conflicts Exist Between FA and UFLS in Power Systems and How to Solve Them?
Feeder Automation (FA) and Under-Frequency Load Shedding (UFLS) are two critical protection and control mechanisms in power systems. While both aim to ensure safe and stable system operation, they carry potential conflicts in logic and timing that require careful coordination.
Feeder Automation (FA): Primarily addresses local feeder faults (e.g., short circuits, ground faults) in distribution networks. Its objective is to quickly locate and isolate faulted sections and restore power to non-faulted areas via network reconfiguration using switches. FA emphasizes rapid local power restoration.
Under-Frequency Load Shedding (UFLS): Responds to severe frequency drops in the interconnected grid (e.g., due to generator tripping, sudden load increase, or tie-line disconnection causing power deficit). It systematically sheds pre-designated non-critical loads to prevent frequency collapse, restore power balance, and stabilize system frequency. UFLS prioritizes overall system frequency security.
Under-Voltage Load Shedding (UVLS): Monitors system voltage in real time. When voltage drops below a preset threshold, the UVLS scheme determines whether to act based on predefined logic. If conditions are met, it sequentially sheds loads to reduce reactive power demand or enhance reactive support, thereby restoring voltage to normal levels.
Conflict Case Examples
Case 1: In 2019, in North America, FA-induced power restoration triggered a secondary frequency collapse.
Case 2: In 2020, in East China, FA operation following a short-circuit fault caused erroneous UFLS activation.
Case 3: In 2021, wind farm disconnection triggered overlapping actions between UFLS and FA.
Case 4: In 2022, during a typhoon in South China, FA network reconfiguration led to excessive load shedding.
Event Description
In 2022, 110kV Line A and a power plant’s grid-connected Line B were operating on Bus Section I of a 110kV substation. A fault on Line A caused Switch A to trip. However, since the plant’s Line B switch remained closed, power continued to be supplied to the substation. As a result, the voltage on Bus Section I did not fall below the undervoltage threshold, preventing the 110kV automatic transfer switch (ATS) from initiating. Similarly, the plant supplied power via Transformer No. 1 to the 10kV Buses I and IV, whose voltages also remained above threshold, so the 10kV ATS did not activate.
As the plant continued to supply load, system frequency gradually declined. 5.3 seconds after Switch A tripped, frequency dropped to 48.2 Hz. The plant’s under-voltage and under-frequency separation device, set at 47 Hz and 0.5 s, did not operate. However, the substation’s UFLS relay, set at 48.25 Hz and 0.3 s, detected the frequency of 48.12 Hz and operated correctly, shedding several 10kV feeders (Lines C, D, E, F, G). All secondary equipment operated as expected.
On-site Review
The 110kV substation’s Switch A tripped correctly due to protection action, and UFLS operated, disconnecting Lines C, D, E, F, and G. The substation switches issued trip signals, triggering FA activation. The fault was identified between the substation switch and the first line switch. FA initiated on all five lines, locating the fault between the substation outlet and the first switch. However, no fault was found during on-site inspection, confirming a false FA operation.
Solution
Enhance synchronization of load shedding information. For lines with UFLS/UVLS protection, support blocking of automatic load transfer functions.
Implement robust load transfer blocking: in fully automatic centralized FA schemes, upon receiving a load shedding signal, immediately block the FA execution function for the affected lines.
