Low Voltage Ride through Hybrid Energy System
Researches in 2017 showed that, by 2020, around 20% of the total energy production worldwide will be generated from renewable energy. The share of renewables in global electricity generation jumped to nearly 28% in Q1 2020 from 26% in Q1 2019. The increase in renewables came mainly at the cost of coal and gas, though those two sources still represent close to 60% of global electricity supply. But the major problem with the standalone system is that the sources are not continuous.
This intermittent nature of the sources can tamper the power system stability.
Hence we can combine two or more such renewable energy sources to form a hybrid renewable energy system. Such hybrid systems are more promising and effective in generating power, especially in remote areas, as compared to individual systems.
They could bring out the advantages of each renewable source being combined and also complement the demands of conventional power systems. There are so many renewable energy sources available, but Wind and solar power projects are widely getting implemented that they are of free access and environment – friendly.
But for the connection of new generation systems into our existing grids, the transmission system operators define minimum requirements that should be met, which is called Grid Code.
The major challenge with a grid connected hybrid system is that they must contribute with the power quality and power system stability. During a fault at the grid side, it’s the voltage at the point of common coupling (PCC) which drops suddenly. This will adversely affects the entire hybrid system as the drop in voltage abruptly increases the rotor speed of the wind energy conversion system (WECS) generator and also affects the normal operation of the PV system.
Thus, in order to protect the renewable systems, it was customary practice to disconnect the renewable systems upon faulty grid conditions.
But, nowadays, due to higher penetration of renewable systems into the grid, disconnection of such a large number of renewable systems instantly from the grid during fault can aggravate the power system stability issues.
Because removal of such large scale hybrid generation during voltage dip will further cause the voltage to go down, which in turn results in the disconnection of more generation units, leading to a cascading failure.
Figure 1 – Result of a voltage drop test at a PV system. In this diagram the voltage drops to about 20% of the nominal voltage for a time of approx. 550ms. The PV inverter recognizes the voltage drop and feeds a reactive current of approx. 100% of the nominal voltage into the system for the duration of the fault in order to support the grid. After fault clearance the active power output is increased to the value prior to the occurrence of the fault within 160ms. (credit: J. Dirksen; DEWI GmbH, Wilhelmshaven)
IEEE 1547a
Therefore, came a renewed grid code, IEEE 1547a standard in 2014, which is referred to as Amendment 1 to IEEE 1547 standard in 2003, demanding that these new large scale generation plants should possess Low Voltage Ride Through (LVRT) capability.
As per this Amendment 1, there will be coordination between the grid operators and those of the renewable systems, on how these renewable systems can regulate the voltage by changes to real and reactive power.
It aims at maintaining sustainable power delivery during the faulty conditions and instead of getting disconnected from the grid, the generation units should ride through the low voltage conditions and support the grid. This will provide a much robust grid, as the Distributed Energy Resource (DER) is clearly allowed to provide low voltage ride through.
By providing appropriate control to the DER systems, we could make the system ride through the voltage dip condition by injecting reactive current at the point of common coupling PCC, and thereby improving the voltage that has fallen due to fault. But this method involves inclusion of some complex control circuitry.
Hence, among the various solutions to accomplish LVRT capability, the better choice is to employ FACTS (Flexible AC Transmission Systems) devices.
FACTS technologies provide advanced solutions as, Unified Power Flow Controller (UPFC) – like FACTS devices can provide independent and simultaneous control of both real and reactive power flow. Due to instantaneous reactive power injection, steady state is reached faster. During the time of large transients, the DC link storage of the FACTS device may not be sufficient as it is limited to a definite value.
So, a backup energy storage system (ESS) like Super Capacitor can be coupled to the device’s DC link to improve the dynamic performance of power systems. Integrating an ESS into a FACTS device can lead to improved controller flexibility by providing dynamic decentralized active power capabilities.
Thus, the enhanced performance of the combined FACTS/ESS will have greater appeal to transmission service providers, when it comes to the case of achieving LVRT capability in Grid connected hybrid systems.
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