In our previous article, “The Need for Distributed, Dynamic, and Decoupled Power Flow Control in Distribution,” we discussed a diverse set of trends that are challenging distribution utilities in their core mission, delivering reliable and affordable power to end customers. As a result of these trends, the requirement for multi-function power regulation that is distributed in control, to pinpoint specific feeder locations, dynamic in response, to address increased renewable generation variability and intermittency, and decoupled in nature, to provide predictability and enhance operational simplicity, has created the need for a new class of active grid infrastructure. This new infrastructure can only be made reliable, efficient and effective if enabled by power electronics and managed under a common control framework.
The Rise of Utility-Scale Power Electronics
The ability to effectively manage today’s electric grid relies on detailed knowledge of the system and what actions must be taken to reliably and economically deliver high quality power to all customers at all times. Historically, utilities have applied design rules and assumptions when building or optimizing distribution feeders to ensure that voltage is delivered within ANSI C84.1 limits during steady-state and worst case peak conditions. These rules and assumptions allow distribution engineers to calculate the length and gauge of conductors as well as size and location of conventional electromechanical devices such as tap changers, voltage regulators and capacitor banks to maintain a desired primary feeder voltage profile. So long as the underlying assumptions hold true, the primary feeder voltage profile has historically been a reliable proxy for voltage delivered to end customers, albeit often at the cost of requisite system margin. As a result of the various time-varying distribution grid trends highlighted in Part 1, however, many utilities, particularly those on the forefront of integrating high penetration levels of DG or implementing CVR and peak reduction programs, have already begun to witness the following: 1) increasing disparity between field telemetry and expected feeder power quality metrics, and 2) existing electromechanical devices reaching their operational limits and failing more frequently. By leveraging advancements in power electronics, a new class of utility-owned and utility-operated power regulation systems, or Agile Grid Infrastructure, is now available to augment the design of the past and transition the distribution grid into the modern era of real-time power control.
Power electronics, or the application of semiconductor switching devices to the conversion and control of electric power, provide the necessary foundation for Agile Grid Infrastructure. While power electronics has been widely adopted by a variety of industries ranging from renewable generation to electric transportation, it has seen limited adoption in utility distribution systems due to the inherent challenges of developing utility-scale and utility-grade equipment. Early attempts to productize power electronics in recent years did not meet the fundamental business requirements of the distribution utility – cost competitiveness with existing alternatives, high system efficiency, lifetimes comparable with existing distribution equipment, ease of deployment, and maintenance-free operation. The notion of Agile Grid Infrastructure, however, encompasses these fundamental requirements while leveraging other benefits enabled by power electronics to provide all of the following characteristics:
- More cost-effective than existing alternatives – traditional grid reinforcement (e.g. circuit cutover or reconductoring) is cost prohibitive
- High system efficiency – low losses and low operational cost
- Long lifetime – rugged design for long life in harsh outdoor environments
- Maintenance-free operation – passive cooling with no fans or pumps that fail
- Multi-function capability – voltage regulation, reactive power (VAR) compensation, harmonic cancelation, phase balancing, sensing and more
- Fast response time – sub-cycle response time to power fluctuations
- Continuous operation – solid-state design results in no wear-and-tear
- Operational simplicity – ability to operate autonomously or integrated with SCADA/DMS
- Deployment flexibility – small volumetric footprint and easy to install
- Common control – management under a single control framework for maximum flexibility and interoperability
Making the Right Choice to Solve Multiple Challenges
When considering all of the possible challenges facing the distribution grid today and in the near future – CVR and peak demand reduction, reliable integration of rooftop and utility-scale PV, power quality and voltage assurance for sensitive loads, reliable support for EV charging, improved feeder capacity utilization, etc. – it is imperative for distribution engineers to consider the right suite of products that will meet the needs of their entire distribution system, one that is multi-functional and coordinated under a common control framework.
To illustrate this point, consider the following simplified views of CVR and integration of rooftop solar, two distribution grid challenges that can benefit from distributed, dynamic and decoupled power management along the secondary. CVR programs are typically limited by the most severe secondary voltage drops along a feeder. As a result, they benefit from boosting voltage along the secondary and reducing the amount of voltage drop from the distribution transformer to the customer. On the other hand, rooftop solar benefits from bucking secondary voltages to counteract the voltage rise seen in the middle of the day when solar output is high and load is low. When viewed as individual applications, a distribution engineer might optimize for one over the other, a solution that only boosts voltage for CVR or one that only bucks voltage for PV. When viewed together, however, it is clear that the right solution, and the one that will result in the lowest systemic costs to the distribution utility and its ratepayers, must be able to boost and buck voltage. More importantly, since the effectiveness of CVR depends entirely on delivering lower voltage levels to the end loads on a 24/7 basis, it becomes imperative to regulate voltage directly and precisely, especially in the presence of PV, which pushes local voltages up and erodes the intended CVR savings.
This simple example is helpful in illustrating the need to consider a suite of products that is capable of providing a broad range (boost and buck) of distributed, dynamic and precise voltage regulation, and knowing when to do so. However, there are other functions that utilities also need, namely power factor correction and voltage and current harmonic cancelation, to improve grid efficiency and overall power quality. Accordingly, Agile Grid Infrastructure based on a suite of products that can provide multiple functions at the right times and under the right conditions is the most effective way to technically and economically address the wide range of challenges utilities face today.
Although the timely advances in power electronics signal the beginning of Agile Grid Infrastructure, not all solutions are created equal. Multi-function solutions that address several important applications and enable subsidy-free business cases based upon competitive up-front capital costs, recurring operational savings, and ease of deployment, will bring the most value to utilities. Fortunately, viable solutions now exist that meet both these multi-faceted technical challenges and stringent utility business case requirements. The next article, the final in this three-part series, will focus on the quantifiable business case for Agile Grid Infrastructure based on this suite of multi-functional power electronics products.