The ongoing adoption of clean energy sources is evident across the world. Every country aspires to be powered with renewable energy as soon as possible and as such, establishment of renewable energy parks, investment in the sector, incentives on offer – everything is gaining face. But transitioning completely to a renewable society would take more than just creating the necessary infrastructure. What’s also necessary is to adapt the current infrastructure according to this change – even minute aspects like Electrical Protection System.
The current protection system works on the principle of sensing the ‘state of change’. It involves devising a mechanism which senses a “UNIQUE BEHAVIOUR” whenever there is an occurrence of fault (typically P-P or P-E). Our existing electrical energy sources rely on such protection system. These are the generators driven by steam, gas, nuclear or hydro turbines. Being prime movers with very high inertia, these turbines generate power directly into AC (Alternating Current).
These sources keep feeding the current into the grid even in case of a fault – and the fault current can be up to 300 to 400% of their steady current rate, along with a dip in the voltage. The ‘unique behaviour’ sensed by the Electric Protection System is this state of high current. The maximum electrical protection is based on the over-current such as IDMT (Inverse Definite Minimum Time), DMT (Definite Minimum Time), Instantaneous over-current and earth-fault protection. The figure is attached for reference for pictorial understanding.
While this holds for the conventional energy sources, renewable energy (RE) electrical sources such as Solar PV or Wind are slightly different because these are connected to grid either through a converter-inverter combo or just the inverters. These are purely power electronics based devices. Further, the generators in these sources – solar PV modules for solar energy and Wind Turbine Generators for wind energy – have nil or negligible inertia (nil in solar PV due to static nature and negligible in WTG due to small size).
Additionally, as an inherent property, the Solar Cell Short circuit current (Isc) is only 25% more than the Max Power current (Imp). For their own safety, the power electronic devices almost immediately limit the current and do not allow them to overshoot their rated current. Therefore, the fault current contribution from renewable sources is negligible whenever there is a fault in the grid or distribution system. Consequently, over-current based protection need to rely only on the reverse fault current from conventional sources. The figure is attached for reference for pictorial understanding.
Now, as renewables continue to increase their presence, this is bound to change. Because for a grid powered by renewable energy, it is possible that the current doesn’t shoot up even when the voltage dips in case of a fault. As a result, there will not be any ‘unique behaviour’ for the system to detect, making troubleshooting & fault location the major issue.
To tackle this, a new regime of protection devices/system will be required. This will be a “Unit” type protection such as “special current differential protection” and will need to be sensitive enough to acknowledge the current reversal and operate. Even, “sensitive impedance protection” will be required all over.
These new protection devices will be required everywhere and with over-lapping zones (line current differential with Back-up impedance protection) even in the Medium Voltage and Low Voltage system. This will be a much costlier system than conventional over-current based protection, and even a smart grid with AI might be required to diagnose a fault.
The high cost is due to the fact that while conventional electrical energy sources are a source of power as well as fault, renewable energy-based electrical energy sources are the only source of power and not of fault. Detection of a fault is thus more difficult in a grid powered by renewables.
A clean energy infrastructure, if unable to support our needs, isn’t any good either. While the transition is a must, it needs to be perfect as well. Therefore, focusing on an effective electrical protection system is essential and must be taken up shortly. Because using renewables to power grids susceptible to faults can result in an inconvenient, lengthy breakdown. We need to be better safe than sorry – be prepared than just be aware.
Author: Sudhir Pathak, Head – Engineering (Solar Grid, Rooftop & International), HFE