Response of Grid Interconnected Solar PV inverters to Transmission System Faults

Transcription

1 Response of Grid Interconnected Solar PV inverters to Transmission System Faults David Piper, P.E. Operations Planning & Analysis SCE Grid Control Center July 18,

2 OVERVIEW 1. Overview of the event that raised awareness of fault induced loss of PV generation 2. Overview of Subsequent Events 3. Identified Causes: 1. Frequency Miscalculation 2. Momentary Cessation 4. Review of PV Models 5. Simulation 6. Key Findings 2

3 FAULT INDUCED LOSS OF PV GENERATION In August 2016 the Blue Cut Fire caused twelve faults on the South of Lugo 500 kv corridor Four of these faults resulted in an unexpected reduction in PV generation throughout the SCE system The most severe fault resulted in the temporary loss of approximately 1,178 MW of transmission connected PV NERC/WECC created a joint task force to determine the root cause

4 OVERVIEW Since August 2016, a total of twelve transmission faults have resulted in a temporary loss of generation from PV resources 1400 Reduction in PV Output Per Event PV Resource Loss (MW) Event # All faults cleared normally, within 4 cycles. Inverters typically resumed normal operation within 5-7 minutes. 4

5 MEASURED FREQUENCY DURING 1178 MW EVENT SCE s PMU system indicated that frequency remained above Hz Hz Hz 5

6 Total Solar Output (MW) IDENTIFIED CAUSE #1: INVERTER MISCALCULATION OF FREQUENCY The task force discovered that inverters from one manufacturer can miscalculate frequency during transmission faults. Many of these inverters calculated frequency below 57 Hz or above 61.7 Hz These inverters tripped due to over/under frequency protection and remained offline for 5-7 minutes 8/16/ :45 Event Time (HH:MM) As of July, 2017 these inverters make up 34% of PV inverters connected to SCE s transmission system 6

7 MITIGATION OF FREQUENCY TRIPPING The manufacturer developed revised inverter protection settings. These revisions include a definite time delay to the frequency trip set points. Low frequency trip setting: seconds High frequency trip setting: seconds Reduce the total restoration time to 150 seconds if a trip does occur. 68 NERC High Frequency Limit 66 NERC Low Frequency Limit 64 Manufacturer High Frequency Setting Frequency (Hz) No Trip Zone Manufacturer Low Frequency Setting Trip Time (s) 7

8 IDENTIFIED CAUSE #2 MOMENTARY CESSATION DUE TO VOLTAGE SAG Most commercial inverters are designed to momentarily cease the production of electric power if voltage exceeds the normal operating voltage range of 0.9 pu to 1.1 pu. After voltage returns within the operating range, these inverters will start increasing output in a ramped manner. The inverter s ramp rate limiter is set during commissioning and is usually configurable. Typical values range from 134% per second to 10% per minute. SCADA systems can mask this behavior because the full recovery (or some portion of it) can occur faster than one sampling interval 8

9 MOMENTARY CESSATION EXAMPLE Diagram shows the real and reactive power output of the plant, as measured by the digital fault recorder at the point of interconnection. Most inverters shut down and remained offline for approximately 750 ms The inverters resumed normal output approximately 1.5 seconds after the fault cleared

10 REVIEW OF PV DYNAMIC MODELS

11 REGC_A MODEL REVIEW lvpl1: LVPL* breakpoint Maximum pu output at LVPL breakpoint e.g pu 0.9 pu voltage (brkpt) brkpt: LVPL characteristic breakpoint voltage Voltage point that relates to lvpl1 Defines start of blocking characteristic zerox: LVPL characteristic zero crossing Voltage at which point inverters are completely blocked lvplsw: LVPL switch If lvplsw=1, then LVPL is enabled If lvplsw=0, then LVPL is disabled rrpwr: LVPL rate rate limit (pu) Defines ramp rate of return from momentary cessation *LVPL=Low Voltage Power Logic 11

12 REEC_A MODEL REVIEW Several flags determine the plant reactive power control mode 12

13 WT4G WIND TYPE 4 (FULL CONVERTER) Legacy model Should not be used for future interconnections Existing models should be replaced by 2 nd generation models 13

14 ANALYSIS OF MODELS IN WECC BASECASE Many PV plants are still modeled as WT4G Many PV plants have LVRT logic disabled Many PV plants are using default blocking voltages (zerox = 0.4 pu) Many PV plants are using default ramp rate limit values (rrpwr=10pu) 14

15 SIMULATION

16 SIMULATION Simulations were performed to evaluate the impact of momentary cessation. GE s PSLF software was used to simulate the worst case fault 3 phase, normally cleared 500 kv bus fault. Assumptions Load: Light spring Generation Mix: Maximum 2017 level of PV in the southwest United States California synchronous generation sources were minimized. Various ramp rate limit settings were evaluated to determine the impact that the recovery ramp rate has on system frequency.

17 Total Power (MW) SIMULATION RESULTS s 2s 4s 6s 8s 10s Frequency (Hz) Time (s) The worst-case fault caused the temporary shutdown of 5563 MW of PV generation. The ramp rate limiter has a significant impact on system frequency. 1s 2s 4s 6s 8s 10s

18 Simulation: PV Models: KEY FINDINGS Transmission system faults may cause widespread temporary shutdown and/or tripping of PV inverters. Some inverters have been identified to miscalculate frequency, a condition that is currently being fixed. Most other inverters have been identified to momentarily cease the production of electric power when voltage exceeds the normal operating voltage A high ramp rate limit after momentary cessation can help to reduce the impact of this lost generation on the system. DFR and PMU records can allow Power System Planners to compare models to real events Models can be evaluated to determine if reactive power control strategy and blocking characteristics accurately reflect reality. 18

19 Questions? 19