When distributed generation causes net reverse power flow on a distribution feeder, several voltage-related issues can occur. According to studies published by the National Renewable Energy Laboratory (NREL) and Sandia National Laboratories, utility voltage violations constitute the most commonly occurring constraint that limits solar generation capacity on a distribution feeder.
Some common causes of voltage-related impacts include:
■ Excessive voltages (above 105% nominal). Significant reverse power flow on a feeder, typically greater than 1 MW, can cause excessive voltage rise and result in voltages greater than 105% of nominal voltage.
■ Over-operation of load tap changers (more than 30 daily taps). Solar and wind are variable generation sources. For solar generation, partly cloudy days, especially windy ones, can result in up to 100 daily cloud pass events. When combined with the smaller variations of existing feeder loading, this can result in substantially overtaxed tap changers.
■ Poor power quality and rapid voltage changes (low voltages below 95% and sudden voltage variations greater than 3%). Variability due to solar and wind can cause a sudden loss of generation. This is known to result in voltages below normal operating limits and poor power factors, as well as aggravating voltage variations, such as customers experiencing persistent appliance noise.
1. This graph shows voltage on a distribution feeder with variable solar generation causing reverse power flow. Courtesy: AMSC
Figure 1 shows actual field voltage recordings from a feeder experiencing problematic reverse power flow due to variable solar generation. Conventional line regulators and switched capacitor banks are far too slow to compensate for these negative impacts, leaving utilities and project developers with limited choices for resolving these issues. Traditional utility methods of reconductoring a feeder or constructing a dedicated feeder for a generating plant can incur costs of up to $800,000 per MW of plant rating. Such traditional wire-based approaches can easily render an otherwise attractive investment economically infeasible.
Land of Solar and Wind Opportunity
Many attractive sites for distributed solar and wind plants are located in semi-rural or rural areas where land is readily available and well-suited for solar plant development. Unfortunately, the best sites for land are often the worst when it comes to available grid infrastructure. Specifically, with respect to voltage, downline feeder locations corresponding to more rural areas, such as locations more than 5 miles from the nearest utility substation, are the ones most sensitive to the effects of reverse power flow and variable generation explained above.
Agricultural communities in particular can benefit from leasing land for solar and wind plants to create an attractive and non-seasonal income supplement to farming. However, agricultural communities are commonly served by longer distribution lines that confound the connection of generating plants. This lack of a strong grid infrastructure can be a serious barrier to making such a local economy-boosting option feasible.
Recent progress in power electronics is enabling the industry to cost-effectively address these distributed generation challenges. An example of one such solution is a power electronics device known as a STATCOM (STATic synchronous COMpensator).
STATCOMs are shunt-connected grid inverters that are optimally designed to compensate variable generation like that created by solar or wind. For more than 30 years, STATCOMs have been used extensively in transmission systems to address a variety of applications, including utility voltage control and grid code compliance for large renewable generator plants.
2. This graph shows the performance of a distribution-class STATCOM regulating voltage on a circuit with variable solar generation. Compared to the performance obtained with traditional utility regulation equipment shown in Figure 1, the D-VAR VVO utilized in this case significantly improves system operation. Courtesy: AMSC
AMSC’s D-VAR VVO is a distribution-class STATCOM that meets both the high-performance and low-cost requirements of distribution applications. Figure 2 shows a more consistent daily voltage profile with a single distribution-class STATCOM installed on the feeder.
AMSC’s distribution-class D-VAR VVO solution was designed from the ground up for installation and operation within distribution circuits, and features the following design characteristics:
■ No routine maintenance.
■ Excellent safety, without batteries.
■ Extremely low operating losses (less than 1% at full output).
■ Excellent reliability due to no moving parts and a completely sealed design.
■ Compact feeder-ready design enables installation within existing utility right-of-way boundaries.
■ Compliance with stringent distribution equipment standards, including dielectric integrity, short-circuit withstand rating, and enclosure integrity.
3. A D-VAR VVO distribution-class STATCOM installed in a 15-kV class feeder. Courtesy: AMSC
Figure 3 shows a typical D-VAR VVO installation on an existing right-of-way.
It’s All About Voltage
Since the inception of modern power delivery systems, distribution utilities have taken responsibility for controlling and reliably managing voltage by owning and operating the voltage regulating assets on their distribution circuits. As increased variable distributed generation causes greater load dynamics in the distribution grid, utilities must invest in a more dynamic voltage regulation system.
Distribution solar plants incorporate devices called inverters between the solar panels and the utility grid to extract DC power from the panels and convert it to the AC power required by the distribution grid. Inverters are owned by the plant operator and physically located within the power plant, where they are connected to the low-voltage AC bus that is also owned by the plant operator.
Incorporating utility-owned D-VAR VVO STATCOMs as the primary dynamic voltage regulation solution and implementing commonsense configuration guidelines for solar and wind inverters can provide plant developers with a reliable and repeatable approach for connecting their distribution power plants to the utility grid.
4. An existing distribution feeder with a D-VAR VVO allowing connection of a new distribution solar plant. The D-VAR VVO protects the utility and its existing customers from experiencing power quality issues. Courtesy: AMSC
Figure 4 shows an example of a utility circuit with a D-VAR VVO and a new distribution solar plant. The utility-owned D-VAR VVO ensures that existing customers served by this circuit do not experience power quality problems due to the new solar plant, while also protecting the utility’s mechanical tap changers from mis-operation and excessive wear and tear.
Financing Options for Utility-Owned STATCOMs
For any generation project, a utility-owned voltage regulation approach that incorporates the D-VAR VVO distribution-class STATCOM will allow the utility to concentrate on what they do best—voltage control and management—while simultaneously enabling the developer to focus on what they do best—rapid and reliable plant development. Utility-owned dynamic voltage regulation will permit both the utility and the plant developer to maximize system operational reliability for the service lifetime of a plant. With an average installed capital cost in the range of 2% to 3% of the cost of the generating plant, the D-VAR VVO distribution-class STATCOM offers utilities a broadly feasible low-cost upgrade.
Two financing models can be employed to accommodate different environments. In the first model, the developer pays, and the utility owns and operates. This model is similar to traditional upgrades offered by utilities, such as protection upgrades or a dedicated feeder upgrade. Cost recovery details are defined in an interconnect agreement signed by the developer and the utility. Successful approaches can range from the developer paying up front for the upgrades, to a monthly fee structure for the plant, or some combination of both.
In the second model, the utility pays for the upgrade, with utility rate-base recovery. This model is no different from the way utilities recover costs for other voltage-regulating assets necessary to ensure system reliability. When connecting a new generating plant to an existing distribution feeder, the D-VAR VVO can be employed as a primary voltage regulator on the circuit. This model can be most appropriate for utilities experiencing broader adoption of distributed generation on several utility circuits, as well as utilities that are pursuing renewable targets and need to accelerate and streamline interconnections of renewable generation.
Meeting Today’s Challenging Distribution Needs
Demand for connecting generating plants to the distribution grid will only continue to grow. Distribution-connected generation offers utilities and developers a unique option where system issues such as congestion and complex transmission system interconnection requirements can be avoided. In the U.S. alone, there are thousands of MW-class distribution power plants currently in the planning phase. Many of these projects will require cost-effective solutions to address voltage issues if they are to be funded and constructed.
STATCOM technology has been applied successfully in the transmission grid for more than 30 years, establishing itself as a trusted tool for ensuring system reliability. Derived from STATCOM’s proven performance and engineered for today’s distribution systems, the D-VAR VVO provides utilities with a powerful, cost-effective, and easily implemented solution for meeting today’s changing and challenging distribution requirements.
—This article was contributed by AMSC, a system provider of megawatt-scale power resiliency solutions.