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ELECTRIC SPRING (ES) -a promising smart grid technology Ms. PRATHIBHA ASST.PROFESSOR, DEE Most PV systems are set up to disconnect from the grid whenever they detect a significant fault. If a single home’s PV system trips off-line, it’s only a headache for the owner. But if hundreds or thousands of t h e m d o s o simultaneously, it could upset the network’s delicate balance, t u r n i n g a n otherwise small disturbance into a n o u t a g e blacking out an entire city or county. Throughout much of the developed world, electric utilities are facing an unprecedented challenge. Growing numbers of customers are installing solar PV systems on their homes or businesses. The power they’re injecting into distribution lines is causing voltage- and frequency-control problems that threaten to destabilise the grid. While this is not yet a major problem, it could become one as distributed solar systems proliferate. The cause of the problem is the inverter, an electronic system that converts the d i r e c t c u r r e n t ( D C ) supplied by the PV panels i n t o t h e a l t e r n a t i n g current (AC) that flows o n t h e p o w e r g r i d . Although they supply AC at the right voltage and frequency to sync with the distribution grid, they are otherwise passive. They can’t sense what is happening on the grid and adjust themselves accordingly. But newer “smart” inverters can prevent a PV system from going off-line when it doesn’t have to. By doing so, they can actually make the grid more stable, by preventing the sudden deterioration of voltage and frequency that would otherwise occur when hundreds or thousands of PV panels are suddenly taken off-line. Smart inverters are poised to fill a big need in the fast-evolving electricutility industry. As more and more homeowners put PV panels on their roofs, the power they are supplying is reducing the need for big, centralised generating plants. The upshot is that increasing n u m b e r s o f t h e s e traditional power plants are getting retired, and g r i d o p e r a t o r s a r e scrambling for ways to keep their networks running with the same high level of reliability that their customers have long taken for granted. The combination of smart inverters and new control methods will be essential for helping utilities transition to the grid of the future, in which vast amounts of wind- and solar-generated electricity will be the norm. A smart i n v e r t e r c a n “ r i d e through” voltage or frequency dips and other s h o r t - t e r m g r i d disturbances and if these inverters have communications capabilities, they can let grid operators monitor
capabilities, they can let grid operators monitor and control them in response to changing conditions. paired, or coupled, their motions will naturally align and picture several weights hanging from springs that are connected to a rigid and fixed board. g r i d f r e q u e n c y w i l l s i m i l a r l y c a u s e t h e inverter to adjust its p o w e r o u t p u t t o compensate. The use of “Electric Springs” is a voltage instability and frequency deviations. In recent years, Electric Springs (ESs) have been developed as a new means of addressing these issues. Besides regulating a c v o l t a g e s i n t h e distribution network, ESs can also regulate the load power flow to response the variable renewable sources. Therefore, it brings a new control paradigm, i.e., the load d e m a n d f o l l o w i n g v a r i a b l e p o w e r generation. The power grids of the future may need advanced inverters that don’t simply follow what the grid is doing but actually help f o r m t h e g r i d , b y r e s p o n d i n g i n s t a n t a n e o u s l y t o disturbances and working in concert to help keep the g r i d s t a b l e . T h i s approach, known as Virtual Oscillator Control (VOC), was developed by an NREL team led by Brian Johnson working with Sairaj Dhople of the University of Minnesota; Francesco Bullo at the University of California, S a n t a B a r b a r a ; a n d Florian Dörfler at ETH Zurich. The basic idea behind VOC is to leverage the properties of oscillators— that is, anything that moves in a periodic f a s h i o n , s u c h a s a mechanical spring, a metronome, or a motor. When two oscillators are As the weights are given a pull to set the springs bouncing, the springs will tend to bounce in an uncoordinated fashion. However, if the board itself is suspended from the ceiling on springs, it becomes a feedback mechanism that responds inversely to the pushes a n d p u l l s f r o m t h e bouncing weights. Within a short period of time, the weights will all start bouncing at the same frequency, all in lockstep. For virtual oscillator control, the trick is to m a k e e a c h i n v e r t e r respond to the grid much like a spring: If the grid voltage drops, the inverter adjusts its output such that it “pushes” against the voltage change. Likewise, a surge in grid voltage will induce an inverter to “pull” the voltage back to the n o m i n a l r a n g e . A n increase or decrease in novel way of distributed voltage control while simultaneously achieving effective Demand-Side M a n a g e m e n t ( D S M ) through modulation of noncritical loads in r e s p o n s e t o t h e f l u c t u a t i o n s i n intermittent renewable energy sources like Wind, solar etc. The role of demand side management is going to be critical once t h e p e n e t r a t i o n o f renewable energy sources becomes significant. This would bring about a paradigm shift from the traditional centralised control of hundreds of power plants to match the demand to decentralised control of millions of loads to balance the supply (generation). In a distribution network, uncertainty and variability associated with renewable energy sources, such as solar power and wind power, can easily cause p o w e r f l u c t u a t i o n s , An ES connected in series with the noncritical load (e.g., a water heater) that has high voltage variation tolerance forms the socalled smart load. The critical load that requires a well-regulated mains voltage is parallel with the smart load branch. By operating under inductive or capacitive mode, the E S c a n i n j e c t a controllable voltage and accordingly change the uncritical load voltage so as to maintain the voltage across the smart load at a c o n s t a n t l e v e l . I n addition, it also enables uncritical load to vary its power consumption and thus contributes to the d e m a n d r e s p o n s e . Overall, the smart load c a n b e i n t e n d e d t o provide reactive power compensation to the system by adjusting the output voltage of ES and ensure a tightly regulated voltage across the critical load.
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