Benefits of power flow control

Advanced Research Projects Agency for Energy, U.S. Department of Energy

Benefits of Power Flow Control Hardware and Software Technologies

Lotte Schlegel, Chris Babcock and Josh Gould 9/27/2013

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Contents Purpose and Scope ...................................................................................................................................... 3

Characteristics, Capabilities and Technologies of a Flexible Grid ................................................................ 3

Power Flow Control Technology Defined ................................................................................................ 4

Hardware ............................................................................................................................................. 5

High Voltage Direct Current ............................................................................................................... 5

HVAC Power Transmission Controllers (PTC) ..................................................................................... 7

Software ............................................................................................................................................. 9

Topology Control Algorithms (TCAs) .................................................................................................. 9

Value Analysis of Power Flow Control ....................................................................................................... 11

Identification of Value Propositions ...................................................................................................... 12

Asset Management ............................................................................................................................ 12

Reliability and Security ....................................................................................................................... 13

Congestion Relief ............................................................................................................................... 14

Integration of renewable energy ..................................................................................................... 14

Economic Efficiency ......................................................................................................................... 15

Summary of Power Flow Control Technology Value .............................................................................. 18

Stakeholders in the Transmission Grid Influence Technology Investment Decisions ............................ 18

Conclusion/Next steps ............................................................................................................................... 21

References ................................................................................................................................................. 24

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Purpose and Scope Electricity is dynamic – supply must meet demand that changes by the second in the electric grid. While electricity markets have evolved to price supply dynamically and demand response systems have developed to manage demand on a dynamic basis, the transmission grid is inflexible. When the flow of electrons is disrupted by a storm, an accident, or congestion choking the lines like cars on an interstate, it affects the wallets of people and businesses. The electric transmission grid costs consumers billions in congestion costs, is difficult and expensive to upgrade, and does not respond quickly to contingency events – costing $79 billion annually in power interruptions (Hamachi LaCommare, 2004). Transmission infrastructure in the U.S. is aging -‐ as of 2008, 70% of transmission lines and transformers are 25 years or older and 60% of circuit breakers are 30 years or older (DOE, 2008).

The electric grid of the future will need to be sufficiently flexible, responsive, and reliable to support variable generation resources, reduce areas of transmission congestion, and respond quickly to system disruptions due to severe weather events. The impending upgrades to infrastructure present an opportunity to include technologies to improve resiliency of the grid. Increasing the flexibility of the electric transmission grid can be the cornerstone to addressing all of these challenges.

ARPA-‐E’s Green Electricity Network Integration “GENI” program envisions a future in which the transmission grid can be controlled to optimize the use of cost-‐effective, clean generation resources while providing high-‐quality, reliable power1. To that end, ARPA-‐E is funding research into transformative hardware and software technologies that could significantly change the ability to control the flow of electricity in the power grid.

This analysis is intended to add to the conversation about the benefits of a flexible transmission system achieved through power flow control technologies. Specifically, it will describe and categorize the benefits of power flow control technologies and define the impact of technologies used for power flow control. It will also make recommendations for further studies and analyses on power flow control.

Characteristics, Capabilities and Technologies of a Flexible Grid Historically, the electric grid was designed to be a passive, one-‐directional system. To improve the grid’s reliability and turn intermittent power sources into major contributors in the U.S. energy mix, the grid needs to be designed and operated to be smarter and more flexible. Power flow control is one way to increase the flexibility and resiliency of the electric grid. Power flow is determined by the impedance of a transmission line and the difference in voltage at each end (M.I.T., 2011)2. Power flow control is the

1 More at http://arpa-‐e.energy.gov/?q=arpa-‐e-‐programs/geni 2 From MIT’s Future of the Electric Grid. “Two factors determine power flow: the impedance of a line and the difference in the instantaneous voltages at its two ends. Impedance is the combination of resistance and reactance. Resistance accounts for energy that is lost as heat in the line. It is analogous to the physical resistance exerted by water on a swimmer or wind on a cyclist. Energy lost in this way can never be recovered. Reactance accounts for energy associated with the electric and magnetic fields around the line. This energy is analogous to

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ability to change the way that power flows through the transmission grid using hardware and software to maximize system value. These technologies can change the effective impedance of the network or the sending and receiving voltages to influence the path of electrons flowing through the transmission grid. This enables the ability to hold power on a transmission line at a certain level or direction. Electrons follow the path of least resistance (or lowest impedance), and the result of changing the pathways of the grid is to change the way that power flows through the transmission system. Specifically, power flow control can be used to remove congestion, respond to contingency events (e.g. loss of a generator or transmission line), and mitigate power quality issues.

Power flow control includes the faculties to control the voltage or impedance on given major transmission lines, switch lines on and off, direct power from one line to another to increase the capacity of a transmission route, provide voltage support, transport power efficiently over long distances, and quickly reverse the direction of power flow from one area to another in response to contingencies. A system planner can optimize power flow on a system by choosing among technologies to enable each of these capabilities as appropriate. In order to fully integrate power flow control at the system level, information systems, hardware technology, and human operators at ISOs/RTOs, generators, and transmission and distribution companies coordinate to match system supply and demand at every moment. For instance, information (such as forecasting of weather, supply and demand), sensors, communication devices, and control technology work together to enable physical changes to the transmission grid. As power flow control hardware technologies are added to the system, coordinated control of the transmission grid will maximize the efficacy of power flow control and ensure reliability across the system. Changes are likely required to optimize the coordination of the grid with the addition of power flow control technologies -‐ for instance, as variables and options are added to the system, either a central operator with sufficient computational power to respond to dynamic grid conditions or coordinated distributed control will be necessary to ensure system optimization.

Power flow control can increase reliability and resiliency, optimize transmission asset efficiency and help prioritize new transmission construction by increasing the capacity of the transmission grid, reduce cost to electric consumers, facilitate grid-‐interconnection of generation, storage, demand response, and detect and minimize the impact of unforeseen disruption events such as extreme weather. The following sections will describe the technologies that enable power flow control and the value that power flow control capabilities afford to different stakeholders in the electric grid.

Power Flow Control Technology Defined Both hardware and software technologies have power flow control applications. This analysis will focus on two types of hardware technologies – High Voltage Direct Current (HVDC) transmission cables and

the potential energy stored when riding a bicycle up a hill. It is recovered (in the ideal case) when going down the other side. In an AC line in the U.S., this energy is stored and recovered 120 times per second, and thus is quite different from the behavior of energy stored in devices such as batteries. The resistance of a line is determined by the material properties, length, and cross-‐section of the conductor, while reactance is determined by geometric properties (the position of conductors relative to each other and ground). In practical transmission lines, resistance is small compared to reactance, and thus reactance has more influence on power flow than resistance.”

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substation equipment; and High Voltage Alternating Current (HVAC) power transmission controllers that use power electronics to augment the existing AC grid. In addition, the capabilities enabled by software control algorithms such as topology control are discussed.

Hardware

Hardware can efficiently direct the flow of power on the grid, help stem energy losses, and enable the grid to be more responsive and resilient. Advances in materials and engineering are decreasing the costs of power flow hardware; many of the concepts of which have been around for a long time. The descriptions below include power flow control technologies that already exist and are in wide spread use in the grid today, as well as emerging technologies not yet in use that show tremendous promise for power flow control applications.

High Voltage Direct Current transmission systems are composed of one or more DC transmission lines or cables between a converter (combined rectifier and inverter), which converts AC to DC or vice versa. The DC lines/cables in concert with the most recent voltage source converter (VSC) technology enable rapid control of the direction of power flow. Both voltage and current source converters can invert DC to a matching AC frequency of an interconnected AC grid, which affords HVDC the ability to connect two asynchronous AC systems. DC poses fewer technical challenges compared to AC because it is not necessary to match frequency, phase or voltage. DC can be configured as a monopolar (one cable) or bipolar (two cable) system which offers cost savings over tripolar AC designs which require one cable for each of the three phases. Because of this and the lower line losses (30-‐50% lower as compared to AC), HVDC transmission lines are the least expensive option for transmitting power over long distances (Reed, 2012). HVDC transformers have been more expensive relative to HVAC. The distance at which a given HVDC line becomes more cost effective than HVAC at a given voltage is the difference between line and terminal costs including the difference between losses (see Figure 1). Also, when connected with an AC grid, HVDC can mitigate power factor issues (current lagging/leading voltage) by providing reactive power support, and can provide black start capabilities3 with VSCs.

HVDC transmission systems are used to transport power over long distances and sub-‐sea. HVDC lines with VSCs allow for bi-‐directional control of power flow and can be directly scheduled and dispatched. Bi-‐directionality allows for the export of energy from control area A to control area B under certain conditions, and re-‐dispatch for import of energy from area B to area A in other scenarios. One example of bi-‐directional flow is the HVDC cross channel 2,000 MW link that imports electricity to Britain from France during much of the year, but exports power to France during the summer when demand is high or to meet load during scheduled outages. The Cross Sound Cable between Connecticut and Long Island is also bi-‐directional, although power flows from Connecticut to Long Island for most hours of the year.

3 Black start is the process of restoring power to a power plant, normally without relying on the power of the transmission grid. Typically in the case of a wider grid outage, black start is provided in a sequence: a portable generator is used to start one power plant, the proximal transmission lines are energized and the power used to start the next base load generator, and so on. Voltage Source Converters can be used for black start as they can synthesize a balanced set of three phase voltages.

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HVDC technology has gained popularity since the mid-‐20th century. Historically, the limitations to its practical use have been the high cost of the power electronics required for the converter. Recent technical breakthroughs have reduced the cost of power electronics and increased their application. HVDC is now being deployed globally, with dozens of projects in the global pipeline, and is of particular importance to integrating distant, renewable energy generators such as offshore wind farms. When considering new transmission corridors, HVDC is more favorable to HVAC because of the smaller footprint of the transmission towers. HVDC proponents envision a future in which DC cables are embedded within the existing AC grid and multi-‐terminal HVDC allows for a superimposed HVDC network that will help to integrate remote resources, improve system stability and reliability via AC-‐DC interties, and increase control of power flows through the system. HVDC technologies are being developed by numerous vendors, including General Electric with funding from the ARPA-‐E GENI program.

GE Global Research is developing two ARPA-‐E funded projects to improve HVDC technology – multi-‐terminal HVDC and improved cable insulation.

The multi-‐terminal HVDC Networks with High-‐Voltage High-‐Frequency Electronics project is developing multi-‐terminal HVDC-‐compatible converters to improve the ability to network HVDC and integrate renewable energy into the grid. Nanoclay Reinforced Ethylene-‐Propylene-‐Rubber for Low-‐Cost HVDC Cabling project is developing low-‐cost insulation for HVDC transmission cables. Cables will be less expensive and suppress excess charge accumulation, which will protect the insulation.

Fig. 1. Breakeven distance for HVDC transmission lines HVDC becomes cost competitive with HVAC over a distance at which line losses at a given voltage are lower than a comparable HVAC line.

Source: Pike Research, 2012

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HVAC Power Transmission Controllers (PTC) can control impedance, voltage and phase and hold power at a desired level and direction of flow. PTC devices use a combination of solid state power electronics and other static equipment to modulate the state of a given AC transmission line by injecting and removing voltage and impedance. These coordinated actions result in controllable voltage/current phase shift to manage real and reactive power flows, controllable line impedance to increase or decrease current, and the ability to balance the current phase between the three phases of an AC transmission system.

Historically, these capabilities were accomplished by Flexible Alternating Current Transmission Systems (FACTS), which employed similar power electronic devices in substations and were typically large and capital intensive. Today, advances in technology are decreasing the cost and footprint, and increasing the reliability and operability of these devices, making HVAC PTC viable solutions for power flow control applications. Such devices are being developed by several ARPA-‐E GENI teams, including Smart Wire Grid, Varentec, Oak Ridge National Laboratory, and Michigan State University. Phase Shifting Transformers

Phase shifting transformers change the voltage phase angle between primary and secondary windings, changing the input and output voltages of a line and thereby controlling the active power that can flow in the line. Effectively, they inject a voltage in series with the line. This enables control of power flow between two power systems, balances loading, and improves system stability.

Distributed Series Reactor The Distributed Series Reactor (DSR) is a technology being developed by Smart Wire Grid, a startup based in Oakland, California. DSRs are small, single-‐turn transformers that inject inductance onto a transmission line. The level of inductance is tunable to alter the overall line impedance and thus the flow of current. DSRs are distributed along transmission lines, in all 3 phases, and can communicate with each other to form a variable impedance system. They can also operate autonomously to alter flows at a specific point on the line. As such, the technology can help to reduce congestion and balance power flow within a system.

Magnetic Amplifier for Power Flow Control Oak Ridge National Laboratory is developing an electromagnet-‐based amplifier-‐like device that will allow for complete control over the flow of power. The prototype device is a low cost iron-‐based magnetic amplifier.

Dynamic Power Flow Controller Varentec is developing low cost transmission controllers to dynamically control voltage and power flow with ARPA-‐E funding. The technology would enable early detection and fail-‐safe protection of the transmission grid to maintain its operating state.

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Shunt Compensators Shunt devices are used to control transmission voltage, reduce reactive losses, dampen power oscillations and are connected in shunt to a transmission line. A Static Synchronous Compensator (STATCOM) is a VSC usually connected to the grid through a shunt transformer. STATCOMs do not require the bulk capacitors and inductors that are used in the thyristor-‐based Static Var Compensators (SVCs) which are still in widespread use today. Instead, the STATCOM generates reactive power entirely electronically and can act as either a source or sink of reactive power. The STATCOM can also exchange real power between the grid and an energy storage device connected at its DC terminals. VSCs based on Insulated-‐gate bipolar transistor (IGBT) technology4 have much faster switching times than other compensator technologies, which makes them particularly useful for dynamic voltage support and power factor correction. A STATCOM does not affect power flow on a transmission line directly. However, by using local shunt reactive power injection to change the voltage profile of a transmission line (e.g. support voltage at the midpoint of a long line), it can enable a line to be loaded more heavily (e.g. to thermal limits) without exceeding steady state stability margins or voltage drop limits. In contracts, a power flow controller is connected in series with a transmission line and has the ability to force a change in power flow on the line, essentially by introducing a controllable voltage in series with the line.

Series Compensators A Static Series Synchronous Compensator (SSSC) is a VSC connected in series with a transmission line. It has the ability to raise, lower, or even reverse the power flow on the line by injecting a relatively small voltage in series. For a wide range of power flow control, only reactive power output from the VSC is needed. However, additional control capabilities such as independent control of real and reactive power flow, can be obtained if a source/sink of real power is connected to the DC terminals of the VSC. Currently, there are no examples of SSSC installations in transmission grids except those installed as part of the three Unified Power Flow Controller demonstration projects. A stand-‐alone SSSC is a more versatile (and potentially lower-‐cost) power flow controller than a Thyristor-‐Controlled Phase Angle Regulating Transformer with a similar MVA rating, which is the closest comparable device. At present, back-‐to-‐back HVDC is being considered in some places to solve loop flows and other transmission problems, but requires two converters rated for full transmitted power. In most cases the problem could be solved with a single fractionally rated SSSC.

4 IGBT technology is a power semiconductor device that forms an electronic switch. They are high efficiency, fast switching and can handle high voltages and current when many devices are stacked in parallel.

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Thyristor Controlled Series Capacitors (TCSC) are a family of equipment that provides a controllable capacitance (or in some cases, an inductance) connected in series with a transmission line to reduce (or increase) the total reactance of the line.

Unified Power Flow Controllers (UPFCs) UPFCs provide the functionality of both shunt and series compensators. They control real and reactive power flow and provide voltage support for the connecting bus5. Historically, UPFCs have taken up significant space, been very expensive, and required the construction of large transformers. There are only three operational UPFCs in the world, each of which was tailored to meet a particular utility’s problem. However, grid operators are largely uncomfortable with the series compensation capabilities of UPFCs, and as a result these operating modes are rarely used, leaving the UPFCs to operate largely as a STATCOM (for more, see Marcy UPFC case study in this document). Moreover, the company that built the UPFCs – Westinghouse – was acquired by Siemens, which no longer sells or supports the devices. An ARPA-‐E team from Michigan State University is building a transformer-‐less UPFC which addresses these issues and can control power flows from intermittent resources including wind and solar resources.

Software

Advancements in computing and data communications can optimize grid operations, match power delivery to real-‐time demand, and find effective ways to manage sporadically available renewable power sources and grid-‐level power storage.

Topology Control Algorithms (TCAs) are a network solution to optimally activate (close) and deactivate (open) transmission lines to decrease the cost of the transmission system. This is based on the counter-‐intuitive, but demonstrated, phenomenon that closing a congested pathway improves the overall system flow6. TCAs are integrated into software that controls the grid’s hardware infrastructure,

5 Real power is power delivered to the end user to do work (measured in watts). Reactive power is current energizing the system components (measured in volt-‐amperes reactive-‐ VAR). 6 Closing a congested pathway can open the electric flow at the system level. This has been demonstrated by ISOs and researchers, including the Brattle Group, Argonne National Labs, and a team from Texas A&M. To illustrate this concept a team from Texas A&M showed that when a 50MW line was dropped in a 3-‐line, 3-‐generator system, the feasible cost to serve load dropped. This concept is demonstrated in the diagrams below:

Transformer-‐Less Unified Power Flow Controller Michigan State University is developing a power flow controller to improve the routing of electricity from renewable sources through existing power lines. The UPFC will eliminate the need for a transformer and construction of new transmission lines. It will optimize energy transmission and help reduce transmission congestion.

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and change the shape of the grid by actuating line switching hardware or by controlling the HVAC PTC devices listed above. The net effect of changing the shape of the grid is to change the way that power flows through the transmission system.

TCAs are not a new concept; they have been employed by operators of wireless ad-‐hoc networks for radios (since 1970’s) and computers (since 1990’s) by optimizing the transmission power of each node to improve signal flow in the network. For the electric power industry, recent advances such as phasor measurement units (PMUs), low-‐latency communication systems, and the reduced cost and improved speed of computer processors allow for TCAs to be an effective solution for power flow in the transmission grid. TCAs are being developed through the ARPA-‐E GENI program by Texas A&M and Boston University.

Automated Grid Disruption Response System Texas A&M is developing a Robust Adaptive Topology Control (RATC) system designed to detect, classify, and respond to grid disturbances by reconfiguring the grid to maintain economically efficient, reliable operations. The system would help to prevent outages and minimize the time it takes for the grid to respond to interruptions, and make it easier to integrate renewable resources into the grid.

Transmission Topology Control for Infrastructure Resilience to the Integration of Renewable Generation Boston University is developing a technology that helps grid operators manage power flows and integrates renewable resources by optimizing the transmission system. The system would have the capability of turning power lines on and off to manage transmission congestion, increase use of renewable resources, and improve system reliability. The fast optimization algorithms would enable near real-‐time change in the grid.

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Value Analysis of Power Flow Control Power flow control benefits the entire transmission system as well as transmission owners, generators, operators, planners, regulators, and consumers. Transmission benefits can be numerous and diverse, including:

• Reduce energy transmission losses • Mitigate transmission outages • Defer and prioritize transmission investments • Increase transfer capability from one part of the system to another • Reduce cycling of base load generators to increase asset efficiency • Increase wheeling of power in and out • Reduce loop flows • Meet public policy goals

Any one of the technologies described above can help to achieve these benefits. However, to maximize the benefits of power flow control and to maintain system reliability, some system coordination is required in order to understand the system-‐level effect of the installation of power flow control technologies, to plan for future asset mix, and to optimize operations of the physical grid and electricity markets. Power flow control is achieved when software technologies in concert with well-‐placed hardware work together to optimize the transmission system. Ultimately, planners, operators and regulators may need to consider several additional factors to realize the full potential and system benefits of power flow control technologies, including:

• Market/regulatory structure for wide area control – to make sure that market structure and technical capabilities are aligned to properly value the benefits of power flow control technologies

• Software – synchronous access to cloud resources for optimized coordinated control • Sensors – accurate, real-‐time, dispersed estimation sensors to measure and communicate the

conditions of the electric grid in real time and ensure

This analysis does not consider the many complementary technologies that would help to maximize flexibility and control including PMUs, advanced metering infrastructure or distribution-‐level technologies, or incentives and market structures that could enable power flow control. The analysis is solely focused on the high-‐voltage transmission technologies and software applications described above.

One can think of the value of power flow control technologies in terms of the total costs and benefits of a transmission grid with power flow control capabilities as compared to the total cost and benefits of the system without these capabilities. However, one of the difficulties in quantifying the value of power flow control capabilities is that system optimization requires that there be short-‐term beneficiaries of a change in power flow, and corresponding entities that might see a drop in revenue in the short-‐term, as any change to the physical constraints of the electric grid can affect the price that generators or transmission owners are paid for electricity. This analysis explores five distinct value streams of power

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flow control, defines the associated benefits and costs, and identifies the stakeholders and how they might be affected at a system level.

Identification of Value Propositions

Asset Management Transmission infrastructure in the United States is built to meet peak demand, which leads to sub-‐optimal utilization outcomes at a system level during non-‐peak periods. Reliability standards and favorable FERC-‐established rates of return provide incentives for transmission investment. At the same time, much of the existing transmission infrastructure is reaching the end of its useful life, and new transmission is difficult, expensive, time-‐consuming, and highly litigious to build. Transmission owners are also faced with competing calls for capital to meet reliability and environmental priorities. Research from the Edison Electric Institute shows that its shareholder-‐owned utility members increased their investment in transmission infrastructure, investing $11.1 billion in 2011 and planning to spend $54.6 billion through 2015 (Edison Electric Institute). Several power flow control technologies could increase the capacity of existing transmission lines and defer new investment in construction or help prioritize construction of new lines to optimize the use of the transmission grid. While increasing the capacity of transmission lines would produce system-‐level benefits, ultimately some transmission owners and electricity generators would see lower revenues in cases where they currently benefit from congestion.

HVDC

In some scenarios, power flow control technologies could decrease transmission losses and increase transmission utilization. Most notably, HVDC lines have lower losses in transporting power over long distances, and technological advances in insulation could increase this benefit further. For instance, GE Global Research is developing a nanoclay reinforced ethylene-‐propylene-‐rubber for low-‐cost HVDC cabling that could bring down the cost of HVDC cable by as much as 80%. Such a decrease in the cost of HVDC would lower the distance at which HVDC is cost competitive with HVAC, and increase its affordability as an option for integration into the AC grid.

HVDC requires smaller transmission right of ways, so new construction or reconductoring of transmission lines can be easier to achieve. This is particularly important in heavily populated areas, which often suffer from transmission congestion. In these cases, transmission planners may consider using existing transmission right of ways to install buried HVDC cable to increase transmission capacity without permitting a completely new transmission pathway.

Power Transmission Controllers and Topology Control Algorithms

HVAC PTCs such as DSRs and STATCOMs can increase the capacity of AC transmission infrastructure and reduce the need for a new transmission line, to optimize the existing AC transmission. Because repowering existing assets could be less costly, a transmission owner could prioritize capital expenditures and deploy resources for new transmission lines in parts of the system where it would

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make the most difference. In addition, they can increase the flexibility and adaptability for grid operators to use existing AC lines.

Topology control allows for line switching to optimize economic efficiency and minimize congestion. In some cases, employing topology control alone would increase the utilization of transmission lines and defer the need for new transmission construction. One common concern about topology control is that it might increase circuit breaker operations and maintenance expenses. Under a scenario with topology control, circuits will be switched more frequently, but in non-‐fault conditions with much less current. Circuit breakers have a robust design to deal with fault conditions are expected to operate will in a topology control case. However, equipment manufacturers will need to validate and support this new use case. Circuit breakers that are old and past warranty may be of greater concern in than newer devices. While it is thus possible that line switching could increase the need for maintenance on breakers that are used more frequently in switching than static scenarios, the system-‐level benefits should outweigh the costs.

Reliability and Security Where power systems are designed to meet one or two contingency extreme events, power flow control capabilities could help to mitigate the impact of one or two outages by providing alternate power flow paths to continue to serve load. The economic impact of the infamous northeastern August 2003 blackout was estimated to be $4 to $10 billion in the United States, highlighting the importance of the electric grid in today’s economy (U.S.-‐Canada Task Force, 2004). Reliability is top of mind for system operators, regulators, policy makers, and businesses in the U.S. today, as reflected in the regional implementation of North American Electric Reliability Corporation (NERC) standards. Power flow control technology could increase the flexibility and responsiveness of the grid.

HVDC

HVDC technology provides several reliability benefits. Specifically, a DC circuit breaker with instantaneous response time will allow for quick fault detection and response, which, in conjunction with other power flow control technologies, can prevent a system-‐level problem and re-‐route power to enable continual, uninterrupted service. Similarly, directional switching of power flow enables routing options post-‐contingency. The ability to reverse power flow in response to a contingency can decrease generation capacity requirements for ancillary services.

In the case of an HVDC intertie between two asynchronous grids, VSCs can provide black start service from one grid to another, significantly decreasing response time without the need for reserve installations that would otherwise be idle much of the year.


DSRs, STATCOMs, and TCAs each provide reliability benefits. DSRs can control potential transmission overload and bypass congested lines, increasing transmission utilization, decreasing congestion, and thereby increasing dispatch options. The built in device-‐to-‐device communication system in DSRs

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enables dynamic, autonomous response and eliminates risks associated with other central-‐control communications devices. The AC regulation function of STATCOMs can automatically control transmission contingency conditions and prevent problems or decrease recovery time. TCAs will optimize transmission line switching under normal and contingency conditions – bypassing congested lines and finding the optimal path to serve load.

In order to quantify the specific benefits of power flow control technologies on a particular system, it would be necessary to model the grid response under contingency conditions using reliability software, and then again with power flow control technologies built in and estimating the economic value of the reduction in load loss (Budhraja, Mobasheri, Ballance, Dyer, Silverstein, & Eto, 2009).

Congestion Relief Transmission congestion happens whenever preferable or low cost generation is unable to serve electric load due to a physical limit on the transmission system. Market efficiency is based on optimal economic operation of the grid by dispatching the lowest-‐cost generation. Congestion disrupts this process and leads to dispatch of higher cost generation to meet demand in the importing location, and exerts downward pressure on prices in exporting areas. Reducing congestion on the transmission grid will reduce congestion pricing for energy and ancillary services and allow for economic dispatch of generation while balancing transmission lines. At a system level, the cost of constructing new transmission lines or adding power flow control technologies must be weighed against the benefits of doing so. Congestion is often a problem in or around densely populated areas, where permitting new transmission lines can be particularly difficult. In these cases, there may be a clear system-‐level benefit to power flow control technologies. Congestion relief brings multiple benefits in terms of integration of renewable energy and economic efficiency of energy markets.

Integration of renewable energy Multiple renewable integration studies have validated the substantial system level and societal benefits of increased renewable energy penetration. Wind and solar energy generators reduce the system operating costs by displacing fuel expenses and deferring upgrades to existing conventional generators; in addition to lowering generation fleet carbon emissions. In the Western Wind and Solar Interconnection Study (WWSIS), it was found that by tapping the large solar and wind resource in the Western Connection, up to 35% of the required energy could be delivered by renewables (GE Energy, 2010). This results in a 40% reduction in the annual system OPEX. In the Eastern Wind Integration and Transmission Study (EWITS), a 10% reduction in annual system OPEX was achieved by incorporating 30% of the energy requirement from wind in the Eastern Connection (EnerNex Corporation, 2011). EWITS also calculated an 18% reduction in CO2 emissions.

The challenge to incorporating variable, uncertain renewable energy is that the current system infrastructure and operational practices were designed for dispatch-‐able and predictable generation supplies. However, renewable energy generators, such as wind and solar, are variable and uncertain (non-‐perfectly predictable) due to the nature of wind and cloud coverage. This variability and

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uncertainty has the potential to exacerbate transmission congestion as the penetration of renewable generation increases. Conversely, there might be an under supply of energy or system frequency disruption if the renewable generators slow or stop production (due to ramping).

To mitigate these challenges, system operators can require additional reserve capacity to supplement renewables and come online quickly to stabilize system frequency in the event of ramping of the energy resource. Other generators must perform load following to match their output to any changes in the energy supply-‐demand balance. Furthermore, local generators are called upon in instances when congestion prevents renewable energy from serving the load. In this case, current practice empowers grid operators to curtail renewable generators if their supply cannot be reliably transmitted due to congestion elsewhere in the system. In all these cases, the operation of reserve generators is generally higher cost than the renewable generators. Some system operators have begun to utilize forecasts of renewable energy regions to aid in more economic reserve scheduling and transmission system operation. However, the accuracy of these forecasts at present is marginally better than assuming persistence. Poor information leads to inefficient dispatching and un-‐necessary cycling of conventional generators which is a less efficient operational method that outputs greater emissions and more wear and tear on the asset.

These additional operational requirements of renewables are manageable, but lessen the total achievable system benefits due to the increased demand for real-‐time reserves and inefficiencies in the near-‐term asset scheduling and curtailment practices. For example, the integration of wind energy in ERCOT is estimated to cost an additional $0.66/MWh due to deployment/operation of reserves, the cost of base load cycling, and transmission congestion (Ahlstrom, 2013). In terms of capital outlay for reserve capacity, it is estimated that PJM spends $3 per each additional MW of wind power capacity (The Brattle Group, 2013).

The renewable integration studies have found that these practices and associated costs can be largely avoided if the grid were flexible to compensate for the variable, uncertain supply. Power flow control technologies can achieve sufficient transmission system flexibility to lower renewable integration costs, reduce congestion, and allow for even further economic utilization of renewable energy by minimizing curtailment. In addition, a more interconnected and controllable transmission system will facilitate the network benefits of geographic averaging of renewable resources and more accurate wind and solar forecasts.

Economic Efficiency Power flow control technologies can increase the economic efficiency of the electric grid through lower losses and by enabling economic dispatch of transmission and generation assets. HVDC devices, DSRs and TCAs can be installed on the existing transmission grid to allow for the necessary flexibility to lower integration costs through the mitigation of curtailment-‐causing system bottlenecks and congestion.

HVDC

Long-‐distance HVDC installations improve market access to remote resources. When congestion is appropriately managed, HVDC facilitates lower energy prices. Lower line losses of HVDC can further

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reduce the overall cost to serve remote load by 30-‐50%. The most economic generation, including renewable generation resources, are often not located in close proximity to major load centers. To tap these resources, a transmission system must be developed. For long distance connection, HVDC conductors offer the most value because of 5-‐10% less line loss than similar capacity AC conductors (Bahrman, 2009). Along with the advantages of smaller transmission towers and no need for intermediate substations, lower line loss equates to lower overall system cost. For a 1000 mile system rated for 6000MW an 800 kV HVDC system is $670/MW-‐mi less expensive than a 765 kV AC system (Bahrman, 2009).

HVDC can be used to route power around a congested area of the AC grid, bringing less expensive power or renewable generation situated at a distance to market in an area of higher demand. For example, the Trans Bay Cable delivers power from Pittsburg, California to San Francisco, providing an alternate route for generation to serve 40% of the city’s peak energy needs. Similarly, the Neptune HVDC cable running from New Jersey to Long Island enables power flow directly to Long Island, skirting areas of transmission congestion in New Jersey and New York and serving 30% of electric needs of Long Island.

The bi-‐directional flow capabilities of many HVDC installations could allow for the change of flow to address particular points of congestion where congestion stress points shift with changing supply and load patterns. For example, the Cross Sound Cable, a merchant transmission line between CT and Long Island, largely sends power from CT to Long Island but on occasion sends power the other way in response to changing conditions.

Back-‐to-‐back HVDC –AC intertie capabilities enable ties between asynchronous grids and can thereby increase transfer capacity, allowing for access to supply from a contiguous grid system and decreasing the cost of reliability services. HVDC that is multi-‐terminal or bi-‐polar with bi-‐driectional capabilities will increase the interconnection further and allow for economic dispatch in multiple directions. For example, the Cross Sound Cable can send power from Connecticut to Long Island or from Long Island to New York depending on system conditions.

With greater HVDC connectivity of disparate renewable generators and loads, the negative system effects of renewable intermittency are largely displaced. Using multi-‐terminal HVDC transmission systems with VSCs that allow bi-‐directional power flow, system operators can take advantage of varying geographical resource profiles. For example, the proposed Clean Line Energy transmission projects leverage periods of excess wind energy in the SPP to deliver power to MISO or PJM (Galli, 2012). When SPP is not producing wind energy, MISO might be, or likewise PJM might be producing solar energy. By connecting large geographical areas, the average amount of energy available to serve loads is higher and more predictable than an individual resource area alone; and HVDC systems are the most cost-‐efficient manner to create the connection. The geographical averaging effect improves energy forecasts (as forecast error is smaller for larger geographies), reduces the system impact of ramp events, and thus reduces base load cycling and the use of/need for reserve capacity. Additionally, a more interconnected transmission system allows for reserve capacity sharing between balancing areas, which reduces the total reserves required below that which any single balancing area would need to carry to meet load and frequency regulation requirements.

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HVDC collection systems enable a new design paradigm for renewable energy generation stations. With AC collection systems, solar PV electricity is collected as DC at each panel and then converted to synchronous AC electricity. For wind, generators produce asynchronous AC electricity, which is converted to DC and then to synchronous AC electricity. If renewable generators were designed to connect to an HVDC collection system, PV panels would not need an inverter and wind turbine-‐side converters would be reduced in complexity to output DC. This not only reduces the costs of developing renewable generator stations -‐ by 7% for solar (Goodrich, 2012) -‐ it also lowers the collection losses when there are long feeder lines connecting the generators to the transmission system. Vestas estimates a 30% improvement in reducing energy losses for wind farms developed for HVDC collection instead of AC (Manjrekar).


DSRs and topology control algorithms could increase the flexibility of the transmission grid and thereby increase the economic efficiency of generation dispatch. DSRs allow operators to bypass congested lines by increasing capacity and distributing power flow among portions of the AC grid, thereby increasing transmission utilization, decreasing congestion, and allowing for economic dispatch of generation. For instance, variable impedance devices such as Smart Wire Grid’s DSRs can increase AC transmission system utilization. A Smart Wire Grid simulation of 3,000 modules on six transmission lines in an eastern RTO reduced the average bus marginal cost by over 6% in a summer peak scenario (Smart Wire Grid, 2013). DSRs balance the load being transmitted across each phase and allow for the increase in transmission capacity.

Power flow control technologies designed to alleviate congestion can have a great advantage to easing the integration of renewables. Smart Wire Grid’s DSRs have been demonstrated to create a variable impedance transmission network that allows power flow to bypass congested lines. A simulated study in the Pacific North West found that with an investment of $58 million (~3000 devices), the variable impedance system created was able to unlock and additional 2.8GW of wind energy by reducing congestion (Smart Wire Grid, 2013). This benefit would be achieved without adding any additional transmission lines, and thus deferred significant investment for the transmission owners.

Likewise, this same effect can be accomplished by optimally switching transmission lines to change the impedance characteristics of the transmission system. TCAs can be deployed by system operators to optimize their switching decisions based on real-‐time events on the grid. TCA simulations in power flow modeling software has shown a reduction in wind curtailment instances from 33% to 14% by switching lines (Qiu, 2013). A simulation of the impact of TCA using historical PJM data demonstrated over $100M in annual savings from congestion relief (The Brattle Group, 2013). Again, these benefits were gained with very little capital investment which allows transmission owners to invest elsewhere in their system.

Other HVAC PTC devices that can provide voltage and frequency support, such as STATCOMs and phase-‐shifting transformers, have been used to improve the integration of wind and solar generators. These devices, which also allow for power flow control, provide dynamic response to fluctuations in the power quality of renewable generators.

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Summary of Power Flow Control Technology Value One power flow control technology can have multiple benefits depending on its application in the grid. To understand the possibility of various power flow control technologies at a glance, see Table 1. Technical capabilities alone are not sufficient to achieve economic efficiency of the system with the deployment of a power flow control technology. Market and regulatory barriers can prevent use of the technical capabilities even when it would be economic, highlighting the need for clear understanding among transmission owners, system operators, and regulators of both technical capabilities and benefits of technology at the system level.

Table 1. Power Flow Control Technology Value Categories. Power flow control technologies can have different or multiple benefits depending on their position and application in the electric grid – asset management, renewable integration, congestion relief, economic efficiency, and reliability and security. Classes of technologies that are represented by one or more of ARPA-‐e’s GENI technology teams are represented in bold font.

Technology Value Categories

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Power Flow ControlTechnology

Asset Management

Renewable Integration

Congestion Relief

Economic Efficiency

Reliability&

Security

Value

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HVDC VSC X X X X X X X

HVDC LCC X X X X

TCSC X

UPFC X X X X X X X X X

Shunt -STATCOM X X X X X X X X

Series - SSSC X X X X X

Phase-Shifting Transformer

X X

DSR X X X X X X X

TCA X X X X X

GENINon-GENI

Stakeholders in the Transmission Grid Influence Technology Investment Decisions As previously discussed, quantifying the benefits of power flow control capabilities is particularly difficult due to the dynamic nature of the electric transmission grid and the differences in benefits to individual stakeholders as compared to the overall system benefits. At the same time, multiple

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stakeholders are often involved in technology investment decisions, and a level of agreement among them is necessary in order to optimize system efficiency.

Differences in regulatory structure among federal power authorities, investor owned utilities, merchant transmission owners, municipal utilities, and rural electric co-‐ops lead to substantial differences in the way certain groups assess power flow control technologies, even within similar stakeholder categories. As technology vendors consider the best value proposition and business model for their power flow control technologies, they should bear the regulatory environment and degree of restructuring of the electric market in mind. For an overview of influencers in the electric grid, see Figure 2.

Numerous influences on Transmission Owner’s investment and siting decisions

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Transmission Owner FERC

PUC

ISO/RTO

NERC/ coordinating

councils

Regulatory industry groups

$ Transmission CAPEX, OPEX, rate recovery

Tech

Decreasing influence

on investment

decisions Other regulators,

NGOs

Figure 2. Overview of influencers in the transmission grid. A utility or transmission owner investing in technology must be mindful and responsive to the interests of multiple stakeholders in the grid: the ISO/RTO that dispatches assets and determines set points for power flow control technologies, the regulators overseeing investment and siting decisions, the bodies responsible for overall reliability of the electric grid, and other interested parties who may intervene in a transmission case.

The benefits of power flow control technology to each stakeholder will vary by their business model and geographic and regulatory situation. To better understand the business models and motivations of various stakeholders in the electric transmission grid, see Table 2.

For the most part, power flow control will have positive economic outcomes, with the exception of those stakeholders who currently benefit from transmission congestion such as reserve generators and to a slightly lesser extent base load generators, renewable energy generators, and transmission owners.

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The beneficiaries of a change in power flow control will often be temporary and largely situation-‐dependent, as market conditions will remain dynamic in a world with power flow control. An overview of how each stakeholder’s situation might change as compared to current conditions is presented in Table 3.

Table 2. Motivations of stakeholders in the electric transmission grid. This table demonstrates the motivations of each stakeholder involved in the electric transmission grid, including their motivations, inherent conflicts and considerations, and a brief description of their revenue model. While every effort was made to provide a comprehensive overview, the differences in regulatory structure among federal power authorities, investor owned utilities, merchant transmission owners, municipal utilities, and rural electric coops should be considered when assessing the position of each stakeholder.

Conflicts of interest related to PFR investment

19ARPA-E Template

Stakeholder How do they make or recover $

Motivation Conflicts & Considerations

Transmission Owner

§ Rate of return (~13%) fortransmission investment

§ FERC technologyincentive rate

§ ~11.5% distributioninvestment

§ Projects that will be approved or financed –leadsto incremental build out of system (relativelyshort time horizon for utilities dependent onregulated rate of return)

§ Invest in what they know (wires) rather than newtechnology

§ Profit

§ Incentive towards construction to meetpeak - of new transmission lines ratherthan investment in technology toremove congestion etc.

§ Regulated: certainty of public benefitcase (to rate-base)

§ Merchant transmission need 20 year,low-risk opportunity

ISO/RTO Fees charged to:§ Generators§ Transmission owners (allocated to states and recovered in rate base)

§ Reliability (& compliance with standards)§ Reduced congestion§ Reserve margin§ Economic efficiency§ Known solutions

Split in priority/focus :§ Reliability§ Economic dispatch§ Capacity margins

Renewable Generator

§ Contracts (PPA, tariff) § Bankability§ Off-take certainty§ Reduced curtailment

Base Load Generator

§ Dispatch§ Regulated return (whereapplicable)

§ Bankability § Increase utilization § Compliance with regulations

§ Risk change schedule/dispatch§ No compensation for cycling & wear &

tear for slow ramping

Reserve /peakGenerator

§ Dispatch§ Ancillary services§ Regulated return (whereapplicable)

§ Increase utilization§ Ability to access ancillary services revenuestreams (where applicable)

§ Compliance with regulations

Risk lowering utilization by removingancillary service functions

FERC § Congressional approval§ Recovered from

regulated industries

§ Economic efficiency§ Reliability§ Policy implementation

TO needs to approach FERC with new technology to receive favorable return for new tech solution. Theoretically could change incentive for transmission technology investment over new wires.

PUC § Budget set at state level § Recovered from rate payers.

§ Customer rates § Economic efficiency§ Reliability§ Policy implementation

§Transmission investment on economicbenefits accruing to their state ratebase vs everyone else in market area

§ Public perception§ Re-election (where applicable)

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Table 3. Overview of beneficiaries as a result of power flow control improvements in the electric grid.

Power flow control technology beneficiariesStakeholders Asset

ManagementRenewable Integration

Congestion Relief

Economic Efficiency

Reliability & Security

Value

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Transmission Owner

ISO/RTO

RenewableGenerator

Base Load Generator

Reserve Generator

FERC

PUC

Consumer

Likely benefit (financial or operational)

Benefit is dependent on situation

Revenue losses likely in current system

PFC generally produces beneficiaries…except in cases where stakeholders currently profit off of system inefficiencies

Conclusion/Next steps This document defined power flow control and identified and described technologies that enable power flow control. It identified the benefits of power flow control and how these benefits accrue to various stakeholders involved in the electric grid. It did not perform a detailed analysis of system-‐level benefits or provide case studies quantifying the impact of power flow control technologies. As power flow control technologies become more common on the electric grid, further analysis will be required to optimize their use at a system level. This should include:

Technology case studies and models

• Power flow control technology case studies and data sharing to document lessons learned For the existing cases where power flow control technologies are installed and operated, in-‐depth analyses will advance the understanding of the technical capabilities, costs, and benefits of the technology. Where possible, case studies should include quantitative analysis of the

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effects of the technology. Data sharing at a high-‐level will enable deeper understanding of the applications of power flow control technologies. Possible case studies include

o HVDC: Trans Bay Cable and its use and effects on transmission congestion Bi-‐polar HVDC applications such as Cross Sound Cable between Connecticut and

Long Island, Cross Chanel Cable between the UK and France o UPFC:

Marcy station UPFC in New York. What was the economic (market) response to its operating mode set points, before and after installation

o DSR: Case study on the Tennessee Valley Authority pilot installation

• Further describe and quantify the benefits of power flow control technology to a particular stakeholder Interested parties will seek more information on how the benefits of power flow control technologies change the economics of the system, particularly for cases where the benefits of a power flow control vary (e.g., those situation identified as “yellow” in Table 3– in what situations are these green and red?)

• Develop or identify a uniform model for analyzing transmission technologies Numerous stakeholders expressed interest in a uniform grid model of sufficient size to model system-‐level effects of combinations of technological installations.

• Add power flow control technology specifications to existing grid modeling software Recent modeling exercises may have been limited by the technical specifications available to modelers. To the extent that these set points can be added rather than programmed for each specific hypothetical or actual installation, decision makers would have more accurate models and understanding of the effects of technological installations.

System level technical and market analyses

• Technological analysis of what is required to enable power flow control at a system operator level Analysis may include modeling of optimal physical positioning of devices in the grid, reliability modeling, and economic modeling of the impact of increased transmission capabilities and the increased fluidity of changing grid topologies.

• Define level of coordination and control required within an RTO and among regions. In order to increase the flexibility of the transmission and distribution grid and meet goals around reliability, integration of renewable electricity at the utility and distributed scale, energy efficiency and demand response capabilities, we will need some centralized control and centrally coordinated distributed control. This will provide quick, responsive voltage support and meet the changing needs of the electric grid.

• Consideration of market design for a flexible transmission grid Changing grid topologies can change the economics of generator and transmission positioning

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and dispatch more fluidly than in the past. The community should ensure that market design is aligned to ensure flexibility and resiliency of the grid under a scenario with power flow control.

This work could be done by industry groups, academic organizations, or via coordinated public effort. Groups to engage for the purpose of research include

• Electric Power Research Institute (EPRI) • Edison Electric Institute (EEI) • Electricity advisory committee, DOE • Power Systems Engineering Research Center (PSERC) • CMU • MIT • Western Interconnection modeling stakeholders • Eastern Interconnection Planning Collaborative • ISOs/RTOs • Regulators

Groups to contribute experts for educational purposes include

• CIGRE Grid of the Future • IEEE • Georgia Tech • Washington State • Penn State • Texas A&M • Iowa State • Regulatory trade associations – NARUC etc • Institute of Public Utilities at Michigan State University

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References Ahlstrom, M. (2013). Questioning the Problem. EUCI Fast and Flexi-‐Ramp Resources Conference. Chicago.

Bahrman, M. (2009). HVDC Transmission: An Economical Complement to AC Transmission. WECC Transmission Planning Seminar.

Budhraja, V., Mobasheri, F., Ballance, J., Dyer, J., Silverstein, A., & Eto, J. H. (2009). Improving Electricity Resource-‐Planning Processes by Considering the Strategic Benefits of Transmission. The Electricity Journal .

DOE, D. o. (2008, January 9). Retrieved July 31, 2013, from National Energy Technology Laboratory: http://www.netl.doe.gov/smartgrid/referenceshelf/presentations/PSC%20Missouri_MGS_Utility%20Meeting_010908_Final_NETL%20Review_AP.pdf

Edison Electric Institute. (n.d.). Actual and Planned Transmission Investment by Shareholder Owned Utilties, 2006-‐2015. Retrieved August 20, 2013, from http://www.eei.org/issuesandpolicy/transmission/Documents/bar_Transmission_Investment.pdf

EnerNex Corporation. (2011). Eastern Wind Integration Study. Golden: National Renewable Energy Lab.

Force, U.-‐C. P. (2004). Final Report on the August 14, 2003 Blackout in the United States and Canada. Retrieved from FERC.gov website .

Galli, W. (2012). The Role of HVDC for Wind Integration in the Grid of the Future. CIGRE. Paris.

GE Energy. (2010). Western Wind and Solar Integration Study. Golden: National Renewable Energy Lab.

Goodrich, A. (2012). Residential, Commercial, and Utility-‐Scale Photovoltaic (PV) System Prices in the United States: Current Drivers and Cost-‐Reduction Opportunities. Golden: National Renewable Energy Lab.

Hamachi LaCommare, e. a. (2004). Understanding the Cost of Power Interruptions to U.S. Electricity Consumers. Berkeley, CA: Ernest Orlando Lawrence Berkeley National Laboratory, 2004.

M.I.T. (2011). Future of the Electric Grid. Cambridge: Massachusetts Institute of Technology.

Manjrekar, D. M. (n.d.). Wind: Challenges, Opportunities and PCS.

Qiu, F. (2013). A Study on Transmission Switching for Improving Wind Utilization. FERC Technical Conference on Increasing Market Efficiency through Improved Software. Washington, DC.

Reed, e. a. (2012). Medium Voltage DC Technology Developments, Applications and Trends. CIGRE U.S. National Committee 2012 Grid of the Future Symposium. University of Pittsburgh.

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Smart Wire Grid. (2013). Power Flow Control for the Grid. FERC Technical Conference: Increasing Real Time and Day Ahead Market Efficiency Through Improved Software (p. 14). Washington, DC: Smart Wire Grid.

The Brattle Group. (2013). Advances in Topology Control Algorithms (TCA). FERC Technical Conference on Increasing Market Efficiency through Improved Software. Washington, DC.

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