DCIAB APPLICATION

Voltage Regulating Distribution Transformers (VRDT)

Traditionally, voltage regulation can be done by using an off circuit tap changer, which allows for transformer windings to be added or substracted in distinct steps only when the tap changer is de-energized. Voltage regulating distribution transformers (VRDT) belong to a new class of distribution transformers that are equipped with an on-load tapchanger to enable a stable voltage output under load. 

Standard grid planning follows ANSI C84, requiring distribution system voltages to be within ±5% of nominal voltage with respect to 10-minute average voltage [1]. Voltage regulation from primary feeders is distributed throughout the entire distribution lines, creating a voltage limit constraint within the voltage bandwidth. At the primary substation feeders, voltage usually remains stable and independent of the feed in load. However, the delivered voltage experiences a larger spread further down the distribution line. In conventional distribution transformers, voltage on the primary side is linked to voltage on the secondary side. The effect of this mechanism means high
voltage on the primary side equates to high voltage on the secondary side. With the implementation of a voltage regulating distribution transformer, a decoupling effect takes place between the medium and low voltage grid [2]. From the distribution voltage profile, voltage starting from the substation to the line voltage regulator may be above nominal voltage so that there is room for voltage drop over the distribution transformer to the connection points of the last meter in the line. This is true for both peak load and low load scenario.

The VRDT decouples MV from LV grid such that any voltage that comes from the primary side, it does not affect the voltage on the secondary side. Connected customer meters continue to receive stable secondary voltage despite the condition of the voltage from the primary side.

Figure 1.
Voltage Regulating Distribution Transformer

Table 1. First Gen vs. Next Gen DCIAB Comparison

OLTC Specifications

A standard distribution transformer integrated with an on-load tapchanger becomes a voltage regulating distribution transformer (VRDT). Manufactured by Maschinenfabrik Reinhausen, the ECOTAP® VPD® is a compact load tap changer that enables service transformers to respond to changes in voltage under load. Having the ability to respond to changes in voltage under load and the decoupling of medium voltage from low voltage grids, the VRDT opens up various potential applications for utilities, industries and distributed generation. Based on a high speed, resistor-type vacuum technology, this OLTC can operate either in 9 or 17 positions at 1.25 or 2.5 percent per step in common configurations. Designed for single phase and three phase, the equipment is rated at 30 A or 100 A, with the highest equipment voltage at 40.5 kV. The ECOTAP® VPD® OLTC exemplifies high reliability by enabling 500, 000 operations exceeding the typical lifetime of the transformer. The ECOTAP® VPD® comes with motor drive, the load tap changer and a control unit. 2. DCIAB as an application of VRDTs 2.1 ComEd Overview Commonwealth Edison Company, also known as ComEd, is the largest electric utility in the state of Illinois, serving 4 million customers throughout northern Illinois and 70% of the state’s population. ComEd provides electric services within an 11,400-square-mile territory and manages over 90,000 miles of power lines [3]. In 2012, as part of the Illinois Energy Infrastructure Modernization Act, ComEd stepped up significant electric grid improvements throughout its service area through a 10-year grid modernization plan [4]. The utility recognizes the needs for a strong and modern electric grid as the basis for its modernization plan. As a result, grid infrastructure upgrades are part of the core initiatives at ComEd. 

Figure 2.
A traditional substation that requires easement, fencing and wildlife protection

First Gen Distribution Center-in-a-Box

The fisrt generation DCIAB project started in 2005 as part of the initiative within ComEd’s Innovation Team. Driven by efforts to be more cost effective, the team came together with equipment manufacturers to design the existing equipment, a substation design project called Substation-in-a-Box. This equipment is a line voltage regulator in the form of a 34.4 kV substation with 3 single-phase voltage regulators and a switch. The transformer is equipped with a 600-A recloser that is connected inside the LV air compartment. The recloser controls are mounted on the outside of the cable compartment for accessibility. This design made a big leap, transforming a typical distribution center substation into a more compact solution and therefore enabled ComEd to add distribution services directly at the location where the growing loads requirements exist.

The 2005 DCIAB project serves to increase reliability and reduce substation design and construction costs. Unlike the traditional distribution center substation layout, which requires security fencing for overhead incoming and outgoing wires, to and from the exposed transformer bushings, the pad-mounted DCIAB style allows both the 34.5 kV and the 13.2 kV lines to run underground. This new concept, with
its reduced footprint, would be unfenced, with less overhead lines that leads to better visual aesthetic to the community.

Figure 3.
First Gen DCIAB

Figure 4.
Next Gen DCIAB

Next Gen Distribution-Center-in-a-Box (DCIAB)

Located 100 miles southwest of Chicago, the rural town of Tonica is a village in LaSalle County, Illinois. Economic growth in Tonica fueled the need for additional substation capacity. In 2019, a ComEd team went to the drawing board and came up with a re-design of the existing First Gen DCIAB. The next generation DCIAB features
a novel design with several new characteristics. The 3 individual single phase voltage regulators are now replaced with an on-load tap-changer (LTC), the ECOTAP® VPD® from Machinenfabrik Reinhausen, further reducing the footprint of the substation significantly. It is installed on the high side voltage with a regulating range of ±10%. The 40.5 kV, 100. A three-phase LTC provides the same capability as the previous 3 individual single-phase voltage regulators, enabling voltage regulation under load to ensure Tonica residents receive voltage within the prescribed limit. As part of the project’s priorities based on safety and reliability, the DCIAB retained its dead front design to enhance safety for operating personnel, a cornerstone feature in the previous version of the equipment. In addition, a de-energized tap changer is also added for safety and convenience while maintaining equipment.  

Figure 5.
Cable connections are dead-front inside the pad-mounted transformer

Cost-Benefit Analysis of Next Gen DCIAB

Compared to the first gen DCIAB, the next gen DCIAB features a significantly smaller footprint than its predecessor. This allows for a single 34kV to 13.2kV transformer to be installed “right-of-way” without building a substation or the needs for additional fencing and wildlife protection. The total cost estimate for the current DCIAB transformer equipment is approximately half of its original model, as illustrated in the table below. ComEd commissioned the DCIAB project in October of 2020 and subsequently energized the project in March 2021.

Conclusion

A new class of distribution transformers, the voltage regulating distribution transformer, is fast becoming an important and economical solution for grid operators to solve challenges for the distribution grids. These challenges stem from the increased loads due to electrification movement, distributed energy resources, and regulatory pressure to reduce capital expenditures as a benefit to ratepayers. The DCIAB project from Commonwealth Edison is a clear example of its viability in the grid
operators’ toolkit. For ComEd, it is now the new standard for DCIAB designs. For other US utilities, new applications continue to be assessed where the VRDT may serve as a beneficial solution for the distribution grids.

References

[1] ANSI Standard C84.1-2011 Electric Power Systems and Equipment – Voltage Ratings (60 Hz).

[2] Sojer, Manuel “Voltage Regulating Distribution Transformers As New Grid Asset.” 4th International Colloquium "Transformer Research and Asset Management, Pula, Croatia, May 10 – 12, 2017.

[3] ComEd Company Information. https://www.comed.com/AboutUs/ Pages/CompanyInformation.aspx.
Accessed 10 December 2021. 

[4] Significant Electric Grid Modernization Work Coming to ComEd Service Area. https://www.comed.com/News/Documents/newsroomreleases_03082012.pdf Accessed 10 December 2021.

[5] Amburgey, Glynn and Thomas Bruhl. “Substation in a Box 2.0.” TD World, March 2012, https://www.tdworld.com/grid-innovations/distribution/article/20960832/substation-in-abox-20. Accessed 10 Dec. 2021.