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Voltage control of distribution systems: the modern grid

This author no longer works for DNV GL.

Hawaii’s House and Senate are close to reaching an agreement on House Bill 623, which would set the state’s Renewable Portfolio Standard (RPS), or supply mix goal, to 100% by 2045.  The current target is at 40% by 2030, which already classifies the Aloha State as the most ambitious kid in the class.  Most of the states in the contiguous United States have adopted an RPS of 10-30% within the next 20 years, other than California, which is set to reach 33% by 2020, and most other states range between 10-30%.

Renewable energy sources, such as solar and wind, are intermittent and mostly unpredictable relative to their conventional counterparts.  Consequently, the integration of large amounts of renewable energy translates into increased complexity of system operations and security of supply.  Hawaii’s ambitious goal is further exacerbated by the fact it is an isolated island system, which means no neighboring entity can come to the rescue if they need to purchase power when their supply is inadequate.

The rapid variation in power output (e.g., fast moving, scattered clouds shading solar PV panels) causes flicker issues which can cause issues ranging from being annoying to damaging fine electronics.  The lack of accurate forecasting techniques (e.g., wind speed, cloud coverage, air humidity) means that the security of supply is highly dependent on the whims of Mother Nature.

The studies we’ve conducted for various utilities across the world suggest that a renewable energy penetration level of roughly 25-30% of the total capacity is the threshold before the risk of power quality issues become too significant.  This holds true for conventional distribution systems with minimal or no mitigation solutions, such as energy storage, Demand Response (DR) programs, or advanced Voltage/VAR Optimization (VVO) control strategies.

In a system where power is purely produced by renewable energy sources, system operators loose significant control over generator dispatch, which means losing a key resource in maintaining a generation to load balance.  This amplifies the role of VVO control strategies and intelligent distribution management.  Distribution system assets will have to be harmonized and able to dynamically adapt to the energy source fluctuations through remote operations.

Beyond conventional approaches such as control of substation LTCs, switched capacitors, and voltage regulators, innovative concepts are being considered.  One such technological advance being developed is the deployment of large amounts of small, variable reactive power devices on secondary networks, which would allow finer muting of voltage spikes and drops.  Another solution being discussed is compensating customers for the use of their smart inverters to provide voltage/VAR support, which would make the prospect of integrating large amounts of distributed energy resources less daunting.  Additionally, the full integration of conventional voltage and VAR control assets with DR programs and distributed energy resources, which could be controlled through algorithms programmed in Advanced Distribution Management Systems (ADMS), would allow for system operators to optimize systems operations by leveraging considerable computational power.  The concept of harmonizing DR programs with conventional assets is presented in the following diagram.

Figure 1: Harmonizing DR programs with conventional assets

Figure 1: Harmonizing DR programs with conventional assets

In general, VVO can be thought of as a Tempur-Pedic mattress that alleviates the prickles from the bed of nails that are renewable energy intermittencies and corrects the bad posture that is the uncertainty of supply.

Will the Lower 48 States adopt Hawaii’s Green Vision if their implementation proves successful?  Unless Climate Change concerns supersede economics, it would be very unlikely.  Hawaii power mix consists of a significant amount of oil-fuelled generators, and oil costs roughly three times the national average since it has to be shipped to the middle of the Pacific Ocean.  This is not the case for the continental United States, however a lot can change in 30 years, at which point Hawaii will be well ahead of the curve with regards to systems operation optimized to handle highly renewable energy sources.

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