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Overview of Distributed GenerationWhat is DG?

Distributed Generation (also called Distributed Energy Resources, or DER) is electricity generated within the distribution grid, i.e., close to or directly at the consumer level. It is energy produced and used in contrast to the typical system of electricity generated in far-off central station power plants, sent through transmission lines, and finally through a distribution system to the end users. Types of DG include biomass, wind turbines and solar (photovoltaic, PV) panels.

Nations around the world confirm policy changes toward renewable energy targets as high as 20% by 2030. DG specifically is estimated to rise globally to 165 gigawatts of power by 2023, with the predominate increase coming from solar (PV). The growth of DG is due to several factors, including political movements towards

renewable energy, economic incentives, and reliability and security concerns related to the centralized model. DG’s continued penetration revolves around the lowering cost of pertinent technologies, e.g., solar panels. This cost curve implies that DG (particularly PV) will become utilized more and more by consumers as technology improves.

What Does DG Mean for the Distribution Grid?

The power distribution grid was designed to deliver consistent energy to loads in industrial, residential and commercial areas – a one-way street. However, energy production within the distribution network, rather than at a power plant, is a game changer. For instance, photovoltaic (PV) and wind energy generate inherently irregular and unpredictable output; day-to-day weather patterns and cloud pass vary generation output wildly across even one distribution area. Because a large

percentage of DG is from wind and PV, grid operation in turn becomes laden with voltage flicker, voltage swells, poor power factor, harmonics and other power quality issues for which established equipment simply cannot respond quickly, effectively or often enough.

Maintaining voltage within a desirable bandwidth to consumers requires a comprehensive equipment strategy. Largely, they are devices that manipulate the two elements of AC current; “real” power (typically expressed as Watts or Megawatts), which is the component that actually does wok. Real power is realized when voltage and current are in phase with each other. Conversely, “reactive” power (expressed in VARs or MVARs) manifests when voltage and current are not in phase. Reactive power is unable to provide actual work, but by either injecting or absorbing it, VARs are a potent and proven method of regulating voltage. This is called VAR compensation.

Distributed Generation Revolution: Dynamic Solutions for Grid Adaptation

A white paper by AMSC

Executive SummaryThe driving elements of the power grid are shifting due to technological innovations. For instance, steady cost reductions of solar panels, coupled with their increased efficiency, have turned rooftop solar installation from a trend to a veritable energy staple. Growing economic benefits are transforming former energy consumers into energy generators. However, consumer-based generation is not what the grid was built for.

Originally designed in the early 20th Century, the grid brought electricity by means of load-distant generation, through long transmission lines, and finally to end users via a distribution network. The 21st Century introduction of energy generation within the distribution grid itself, called Distributed Generation (DG) or Distributed Energy Resources (DER), has required a system built for one-way delivery to be a bi-directional one. This white paper will explore these dynamic changes to the grid, leading to considerations for the future of distributed generation, and provide strategic solutions, including the application of dynamic volt/VAR compensation units, as manufactured by AMSC, purposefully designed for increasing DG grid adaptation.

As DG penetration continues to rise, managing reliability and power quality have become defining issues for the utilities industry. Of particular concern is the intermittent nature of wind and solar DG output. Changes in sunlight strength due to cloud pass events, and

lags and bursts of wind vary voltage generation for some sites, while other sites continue normal operation within the same distribution area. Conventional equipment (capacitor banks, voltage regulators) will be unable to compensate for increased DG penetration that is already demanding a more responsive grid infrastructure. As part of solar installations, smart inverters provide some localized grid support functions; however, utilities do not typically own them, leaving these devices outside of a utility-coordinated, tactical control scheme.

AMSC’s innovative dynamic volt ampere reactive (VAR) technology has been applied successfully transmission systems for the past 15 years. These systems manage voltage irregularities, mitigating capacitor bank switches, thus extending grid equipment lifespans. With the growing adoption of DG, utilities now need effective solutions to upgrade distribution grid infrastructure in a way that doesn’t impact service costs to customers. Appointing small, utility-owned volt/VAR compensation devices directly to the low voltage DG site (typically 120V to 480V) would be inefficient, costly and difficult to maintain. Application of such technology on distribution feeders, however, could be used to manage voltage flicker, voltage swells and other power quality concerns for neighborhoods, business parks, and communities with a single installation.

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Historically, the compensation for fluctuations caused by demand/supply shifts or other events have been managed by devices that either coarsely add or remove VARs (capacitor banks), or voltage regulators that step-adjust to keep voltage within an acceptable range. Being mechanical in nature, all of these methods are relatively slow to act. Step voltage regulators (VR) are suitable to compensate load deviations of greater than 10 minutes in duration, while switched caps are suitable for compensating for daily load fluctuations. In practice, VR and cap banks typically operate no more than 30 and 4 times per day, respectively.

Even with such technological impairments, capacitor banks and VRs have been highly effective in the past. Utilities have been able to predict large changes in loads due to day/night cycles, making the slow operation of these devices relatively unproblematic. They are also inexpensive to install. Nonetheless, the dynamic response limitations of these power quality management systems are incompatible with minute-to-minute voltage fluctuations caused by renewable energy generation. If the infrastructure doesn’t adapt, the chances of poor power quality are widened and the equipment will wear at a rate much faster than anticipated.

Smart Inverters Help, But Aren’t Utility-Controlled

Placed at solar panels, smart inverters are a component of every PV installation to convert the DC current produced by sunlight into grid-ready AC current. As DG-related grid complications can be reduced at the point of interconnection, smart inverters possess software that provides grid support functions locally, including limiting power ramp rate and fixing power factor for a single site. Configuring these functions at installation time can be a cost-effective tool for utilities to leverage. However, they are not typically owned by the utility but rather by the PV-generating customer themselves, which makes them difficult tools to strategically utilize.

In terms of VAR capacity, smart inverters have significant limitations. The ratings vary from manufacturer to manufacturer, as well as differing due to installation site and control settings (the sizing and rating of a smart inverter is dependent on the PV panel being used). As they are not utility-operated, this leaves a smart inverter’s capabilities, however helpful, out of the utility’s control scheme. Since the utility understands the full-picture demands of distribution network, i.e.,

where certain power quality concerns are most pressing, it is imperative that they have dynamic equipment that they not only own and control, but is also flexible enough to respond to the unique power quality issues of DG.

Utility-owned, Secondary Power Electronics Are Uneconomical at Scale

In order to have optimal control, it may seem prudent to apply utility-owned volt/VAR compensation devices right at the DG site on the secondary circuits. This would manage instability before it affects the grid, and give utilities administrative control as well. However, installing so many units (potentially as many sites as there are consumers, in the case of rooftop solar) is unrealistic in scale, time and cost. Additionally, having so many units would entail heavy maintenance; the more devices within the network, the greater the probability of failure due to malfunction or environmental damage, resulting in increased maintenance costs.

A highly effective solution at scale would be powerful and flexible enough to control voltage and power factor from multiple, diverse sources, rather than for each DG installation. This optimal device would be installable upstream from DG sources on 15kV-class feeders, the most widely utilized feeders in distribution system. This location is ideal because it would act upon the

principal DG sites that cause power quality concerns along the distribution grid, e.g., small industry, residential, agricultural and offices, while requiring the least amount of installations.

The Ideal, Utility-Controlled SolutionD-VAR VVO

AMSC’s dynamic volt ampere reactive technology solution, called D-VAR, has effectively been used to manage voltage profiles and power factor in wind farms, in solar plants, and utility grids globally. Using D-VAR systems, utility scale renewable generation has been able to meet POI voltage/PF regulating requirements, assist in high and low voltage ride-through capabilities, solve harmonic problems, and improve power quality related grid interconnection. Additionally, grid planners have benefited from D-VAR’s dynamic voltage regulation capability system and advanced controls to reduce voltage stability problems and optimize T&D networks.

In response to increasing distribution grid complexity, AMSC offers a specialized edition of its D-VAR line called “D-VAR VVO.” D-VAR VVO possesses the patented volt/VAR compensating capabilities of its parent technology such as moderating step-voltage changes smoothly and rapidly,

Fig. 1 The D-VAR VVO installed in an above ground distribution feeder.

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and mitigating the stress on current infrastructure. Whereas capacitor banks are binary, and only able to add or remove fixed VAR quantities, AMSC’s dynamic Volt/VAR compensation technology gives D-VAR VVO the ability to add or absorb VARs in an adjustable, “dial-like” fashion.

D-VAR VVO is a unique, purpose-built device that is designed to integrate effortlessly into the distribution system with the appropriate capabilities, the right size, and at the optimal location for installation. Rather than attempting to manage DG interconnection at numerous sites with a device at each, D-VAR VVO is installed on 15kV-class feeders to regulate voltage for a whole region of the distribution circuit, i.e., the power quality for hundreds of customers can be managed with each installation. This means that there are less devices needed to optimize the distribution grid, minimizing cost while providing precise voltage and power factor control.

Due to an innovative design with no moving parts (e.g., no fans or pumps, naturally cooled), D-VAR VVO requires no routine maintenance and operates without any perceptible acoustic noise. It also needs no outside communication or controls to perform its dynamic functionality, unless desired as part of a utility’s control strategy. AMSC’s strategic innovations sum up to a cost-effective, practical solution to the inevitable impediments of DG.

How D-VAR VVO Works

D-VAR VVO is a dynamic reactive power compensator based on power electronics technology. The D-VAR VVO manages variability caused by DG/DER and other threats to distribution voltage control by injecting or by absorbing VARs as needed with sub-cycle response time (compared to 60 seconds of typical VR and switched caps), with limitless daily operations (compared to <30 for VR and <4 for switched caps). Because the power electronics operate continuously, there are no time intervals between volt/VAR compensation tasks (compared to 10 minutes typical for switched caps), which is ideal for the increasingly dynamic distribution grid.

D-VAR VVO is a shunt device, making installation directly on H-pole 15kV-class feeders seamless. With this arrangement, it adds no series impedance as voltage regulators do, and minimizes BoS costs (i.e., no transformers, substations, fences, etc.) Since 15kV feeders are an optimal installation location where power quality can be managed for hundreds of DG sites simultaneously, D-VAR VVO is purposely designed with 15kV standards in mind, including protection gear.

While 15kV feeder application has clear economical advantage, the varied DG engagements occurring along many feeder

miles necessitates a customizable device; on one lateral of the distribution grid, there may be thirty rooftop solar DG, and at another there may be only ten. Being a “one-stop” DG solution, D-VAR VVO has the flexibility to be installed in either single phase or three phase formats. These systems can compensate at the sub-cycle in the range of 333kVAR, +/-500kVARs up to +/-2MVARs, respectively, at a single location.

Events triggering voltage flicker, voltage swells, harmonic delinquencies and poor power factor can be autonomously stabilized through D-VAR VVO’s dynamic control modes. D-VAR VVO can be configured to operate in whichever mode best suits the application (volt/VAR, grid PF, or grid VAR), and also allows for scheduled sequencing between modes. While no external control is required, D-VAR VVO provides utilities with secure access to its control functions. This means it can be a standalone, autonomous unit or can be installed within a fleet of devices operating towards a centralized control scheme. D-VAR VVO incorporates AMSC’s proven secure DnP3.0 network interface, which can be readily configured to communicate via preferred SCADA, DMS, and distribution automation protocols.

DG(PV)

DG(PV)

DG(PV)

D-VARVVO

Step VR

15kV Feeder

Substation

Loads(customers)

Loads(customers)

D-VAR VVO Switched caps

Partly Cloudy Days:“Megawatts a

Minute”

Corrects for loadchanges (time)

Daily operationslimit (type)

Reverse powerconditions

> 10 mins. 3-4 cycles

No limit

Robust

> 60 minutes

< 4

Can’t reducevoltage

< 30

“Sensitive”

Fig. 2 The performance capability of the power-electronics based D-VAR VVO.

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Summary Century-old techniques of managing utility load – in one direction – are unfit for the new age of distributed generation. The distribution system requires new innovations that are not only cost-effective and dynamic to adapt it to the ever-increasing penetration of DG, but also utility-controlled towards the greatest efficacy. Employing volt/VAR compensation equipment on-site could reduce voltage variations; however, this would require numerous (and growing) devices, which in turn would escalate installation, maintenance and operating costs. The best solution would have the flexibility and power to serve expansively – with one to a few installations required in a feeder. At the seamless meeting of utility industry needs and responsive solutions, AMSC’s D-VAR VVO provides essential stability to the increasingly dynamic distribution grid, offering voltage and power factor control for DG outputs at the ideal location on 15kV feeders.

D-VAR VVO Advantages for DG Grid Adaptation Include:

• DG & DER voltage instability concerns are mitigated

• Ability to manage voltage violations from multiple, diverse DG sources

• Shunt device, easily integrated, optimal location on 15kV feeders

• Solves resonance issues associated with cap banks

• No routine maintenance

• Unique, dynamic device, acting in milliseconds

About AMSC

AMSC (NASDAQ: AMSC) generates the ideas, technologies and solutions that meet the world’s demand for smarter, cleaner … better energy. Through its Windtec Solutions, AMSC enables manufacturers to launch best-in-class wind turbines quickly, effectively and profitably. Through its Gridtec Solutions, AMSC provides the engineering planning services and advanced grid systems that optimize network reliability, efficiency and performance. The company’s solutions are now powering gigawatts of renewable energy globally and enhancing the performance and reliability of power networks in more than a dozen countries. Founded in 1987, AMSC is headquartered near Boston, Massachusetts with operations in Asia, Australia, Europe and North America.

www.amsc.com© 2017 AMSC. AMSC, GRIDTEC, GRIDTEC SOLUTIONS, D-VAR, D-VAR VVO and SMARTER, CLEANER… BETTER ENERGY are trademarks or registered trademarks of American Superconductor Corporation or its subsidiaries.

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