In a recent study, we explored the challenges and solutions for integrating renewable energy sources, particularly solar PV power plants (SPPs), with traditional power Generator methods in a weak regional power system.
The study aimed to address grid instability issues observed in a regional power network, where a newly energized 10 MW capacity SPP faced frequent disconnections. Our focus was on recommending cost-effective and timely mitigation strategies to enhance the SPP's energy evacuation capabilities.
Methodology:
The study utilized the latest power flow models, incorporating updated infrastructure, load, and generation data. This approach allowed for a detailed analysis of the system's current operating conditions, particularly concerning voltage and frequency stability.
Key Findings:
System Operation at Stability Limits: The regional power system was found to be operating at the edge of its steady-state stability limit, indicated by depressed voltages and prolonged frequency oscillations.
A PV analysis on the Base Case, at the Interconnecting 20 kV bus (IP), showed that too much power is already being transferred into that bus (0 MW additional power into the bus puts the bus voltage at 0.848 p.u. – far below the lower acceptable limit of 0.95 p.u (Fig 1)
Figure 1: Base Case PV Figure 2: Compensated Case
If 15 MVAR of shunt capacitors are installed at the IP bus, an additional ~11.5 MW can be transferred into that bus from the 110 kV bus before the Low Voltage Transfer Limit is reached (Fig 2)
Conversely, reducing the load transfer into the IP bus to a maximum of @ 0.85 pf achieves the Low Voltage Transfer Limit for 0.95 p.u. bus voltage as an alternative to installing voltage support RPC. (fig 3)
Voltage Collapse Risk: Low operating voltages, averaging about 85% of nominal, were observed, signifying a high risk of voltage collapse.
The PV analysis concludes that the present peak load transfer of 15.2 MW @ 0.85 pf, at noon peak output of 11.3 MW, unity pf, already exceeds the recommended load transfer.
QV Analysis:
This analysis helps in determining the amount of reactive power that can be added to a location without compromising voltage stability. A higher reactive power margin indicates better voltage stability under normal conditions. The study presents two scenarios: one where there is a positive reactive power margin, allowing for more reactive power to be added without risking voltage collapse, and another where there is no reactive power margin, indicating the system is at or near voltage collapse due to insufficient reactive power.
In this case study, the power bus was found to have a reactive power deficit of approximately 3.6 MVAR. This implies that no further reactive power can be transmitted through certain transformers in the system without risking voltage stability. However, if 15 MVAR of shunt capacitors were installed at this bus, the reactive power margin would become positive, allowing for additional reactive power transmission without stability concerns.
Figure 3: Reduced Load + Uncompensated Figure 4: Base case QV curve
The final findings of the study reveal that the current peak reactive load transfer in the system, combined with certain solar photovoltaic power plant outputs, exceeds the advisable reactive power transfer limit by about 3.6 MVAR, based on the modeled system configuration, loads, and generation capacities.
Weak Grid-Generation Link: The high impedance of transmission lines and the current configuration significantly contributed to the grid's instability.
Lessons Learned:
Load Management: Ensure voltage levels at all buses remain within 95-105% of nominal, potentially requiring a reduction in peak system load.
Frequency Deviation Control: Implement smaller load switching blocks, not exceeding 2-2.5 MW, to minimize frequency deviations.
Reactive Power Compensation: Installation of stepped, switched shunt capacitor banks at key substations to improve voltage support.
Data Recording and Analysis: Maximize the use of event recording capabilities in protective relays to aid in understanding and mitigating trip events.
Conclusion:
This study highlights the need for strategic planning and implementation of infrastructure improvements to ensure stable and efficient integration of renewable energy sources into existing power systems. By adopting these recommendations, the regional power network can enhance its resilience and sustainability, paving the way for a more robust and green energy future.
