Medium-Voltage Interconnection Engineering

Studies, substation protection & control, collector routing, and SCADA integration — from the point of interconnection to the point of demarcation.

Abstract



Connecting new generation or large load to an existing medium-voltage (MV) network demands disciplined interconnection engineering across four linked domains: system studies, substation protection-and-control design, collector and feeder routing, and supervisory control integration. This paper sets out Keentel Engineering’s approach to each, with particular attention to interconnections onto electrically active buses — where existing harmonic filters or reactive-compensation equipment make harmonic and reactive coordination decisive to a safe, code-compliant energization.

1.  Introduction

The expansion of utility-scale solar, energy storage, and large industrial loads has increased both the frequency and the complexity of medium-voltage (MV) interconnections. Most occur in the MV class commonly spanning roughly 13.8 kV to 34.5 kV, where a new facility must be joined to an existing network that was not originally designed to accommodate it. Whether the new facility exports to the utility grid or operates behind the fence to offset on-site load, the engineering at the point of interconnection (POI) determines whether energization is safe, code-compliant, and operationally sound.



Interconnection engineering is frequently underestimated because the new generation or load equipment — inverters, transformers, switchgear — is well understood in isolation. The difficulty lies at the seam: the interaction between the new facility and an existing, often brownfield, electrical system whose as-found condition, protection philosophy, and electrical loading must be respected. This paper describes Keentel Engineering’s integrated approach across the four domains that define a complete MV interconnection package — system studies, substation protection-and-control (P&C) design, collector and feeder route engineering, and balance-of-plant (BOP) supervisory control — and the engineering judgment required to deliver them as one coordinated design.

2.  Interconnection Topologies and the Point of Interconnection

An MV interconnection begins with a clear definition of the POI and the route from it to the new facility’s demarcation. The POI is commonly established by adding a new MV circuit breaker — vacuum or SF₆, with a disconnect on each side — into an existing switchgear lineup or onto an existing bus. The choice between a dedicated breaker bay, a line tap, or a bus extension is driven by the available fault duty, the host’s protection scheme, physical space, and the operational flexibility the owner requires.

The path from the POI to the facility is rarely a single construction type. In practice it combines segments: insulated MV cable in existing and new cable tray, overhead conductor on pole structures, and underground cable in direct-buried or duct-bank construction. These segments are joined by transition structures — cable-to-overhead risers and overhead-to-underground riser poles — that frequently also house gang-operated air-break switches and revenue or production metering. Each transition is an engineered interface with its own insulation-coordination, grounding, and mechanical requirements. Defining the metering and ownership boundary early, and carrying it consistently through every drawing, prevents the scope ambiguity that often delays interconnection approvals.

3.  System Studies

System studies establish the technical basis for a safe energization and for the protection settings that will govern the interconnection in service. The applicable studies, their purpose, and the consensus standards that govern them are summarized below, followed by commentary on the two that most often prove decisive.

3.1  Harmonics and Resonance on Electrically Active Buses

Inverter-based resources inject harmonic currents that must be evaluated against the existing network at the POI. The evaluation becomes critical when the interconnecting bus already hosts power-factor or harmonic-filter banks, or static VAR compensation with a thyristor-controlled reactor. Tuned filters and compensation equipment reshape the system’s frequency response, and the addition of inverter capacitance and control dynamics can shift resonant points toward characteristic harmonics. A frequency-scan and harmonic load-flow study — screening for resonance and confirming compliance with IEEE 519 voltage- and current-distortion limits at the POI — is therefore not a formality but a gating analysis. Where interactions are identified, mitigation may include filter retuning, detuning reactors, or inverter control adjustments, each of which is best resolved on paper before energization rather than in the field.

3.2  Protection Coordination in a Brownfield Context

A new feeder must coordinate selectively with the host’s existing bus and transformer protection so that a fault on the new circuit is cleared by the nearest device without tripping unrelated load. This requires an accurate as-found protection single-line, current short-circuit data at the interconnecting bus, and a settings package developed and checked against the IEEE C37 family of standards. Behind-the-fence interconnections add a further dimension: the protection must also address islanding, reverse power, and the host facility’s operating and ride-through requirements, which differ materially from a pure grid-export scheme.

4.  Substation Interconnection and Protection-and-Control Design

The substation package translates the studies into a constructible interconnection. For a new MV bay added to an existing substation, the physical scope typically includes the general arrangement, elevation views, and structural and foundation design for the new equipment, together with the substation bill of materials. The protection-and-control scope includes the switching diagram, protection single-line, three-line diagrams, AC and DC schematics, control-building and relay-panel layouts, grounding plans and details, conduit plans and schedules, cable and cable-tray details, and wiring diagrams.



On brownfield sites, much of this work is executed by updating owner-furnished CAD or native-PDF drawings so the new bay integrates cleanly into the facility’s existing documentation set. The quality of the as-found record — an accurate single-line, verified fault data, and reliable equipment ratings — is the single largest determinant of design efficiency. Keentel’s practice is to reconcile the as-found drawings against field reality early, so that downstream P&C deliverables rest on a verified basis rather than an inherited assumption.

5.  Collector and Feeder Route Engineering

The route from the substation to the facility’s demarcation is engineered as a complete civil-electrical system. It begins with design criteria and a project summary, and develops through cover sheets and plan-and-profile drawings that capture the mixed cable-tray, overhead, and underground construction described in Section 2. Specifications for geotechnical investigation and topographic survey define the field data the design depends on, while an existing-infrastructure location and avoidance plan is essential on congested industrial corridors where the route shares space with roadways, rail, and live utilities.

The electrical core of the route work is conductor sizing: ampacity, loss, derating, and sizing calculations performed in accordance with IEEE 835, the Neher-McGrath method, and ICEA practice, accounting for installation method, mutual heating in tray and duct bank, and soil thermal resistivity. Around this sit the duct-bank design, directional-bore specifications, MV termination specifications, and the grounding, surge-protection, and cathodic-protection details that protect the cable system over its life. Fiber and splice details, phasing drawings, scope-delineation drawings, and construction specifications complete a package that contractors can build without interpretation.

Several recurring conditions separate a routine interconnection from a demanding one, and they are worth naming explicitly because they shape both scope and risk:

Interconnection onto active buses

Where existing filters or reactive compensation are present, harmonic resonance and reactive coordination must be resolved analytically before energization, as discussed in Section 3.

Reactive coordination with the host

The new facility’s reactive capability and its plant controller must coordinate with the host’s voltage schedule and any existing compensation, rather than operate in isolation.

Behind-the-fence versus grid export

The interconnection mode changes which studies, protection functions, and metering requirements apply; treating the two interchangeably is a common and costly error.

Brownfield congestion and outage coordination

Routing through an operating industrial site requires existing-infrastructure avoidance, outage windows, and verified as-builts, each of which affects schedule as much as design.

Verified as-found data

Accurate single-lines and current fault data are prerequisites; design built on stale records propagates error through every downstream deliverable.

6.  Balance-of-Plant SCADA and Communications

Bringing the facility under coordinated control is the final domain. The physical design defines the communication data-flow and architecture, the network and fiber paths, patch panels and splicing, communication-rack and control-enclosure layouts, power wiring, I/O schematics, and field-panel design. The programming scope produces the SCADA point list, the power-plant controller logic, the human-machine interface (HMI), the control narrative and system description, the network plan, the historian configuration, and reporting.



Two principles govern good BOP SCADA design. First, the point list and control narrative are written before the screens, so that the interface reflects a deliberate control philosophy rather than an accumulation of available signals. Second, integration and commissioning are planned from the outset — on-site integration and off-site commissioning support are scoped as engineering tasks, not afterthoughts — because the value of a control system is realized only when it is verified end to end against the as-built plant.

7.  Engineering Challenges and Special Considerations

8.  Milestone-Based Delivery and Quality Assurance

Keentel structures interconnection work around progressive design milestones — commonly 10%, 30%, 60%, 90%, Issued-for-Construction (IFC), and As-Built — so that the owner, the host, and any reviewing authority can evaluate the design at defined gates and fund it accordingly. Studies typically progress on a parallel track keyed to 30%, 60%, 90%, IFC, and post-IFC closeout. All deliverables are produced under the oversight and quality assurance of a licensed Professional Engineer, with the engineering seal applied in the jurisdiction the project requires.

9.  Conclusion

A medium-voltage interconnection succeeds or fails at the seam between new equipment and an existing system. The studies prove the energization is safe; the substation P&C design makes it constructible; the route engineering carries power reliably to the demarcation; and the SCADA design brings the facility under coordinated control. Treating these as one integrated package — anchored in verified as-found conditions and delivered through disciplined design gates under professional-engineer oversight — is what keeps interconnection projects predictable. Keentel Engineering provides this full scope, and welcomes the opportunity to discuss how it applies to a specific interconnection.

10.  Standards & References

The engineering described in this paper is governed by, and developed in accordance with, the following consensus standards, among others:



• IEEE 519 — Harmonic control in electric power systems
• IEEE 1313 / IEC 60071 — Insulation coordination
• IEEE 1584 / NFPA 70E — Arc-flash hazard analysis and electrical safety
• IEEE 80 / IEEE 81 — Substation grounding and ground-system measurement
• IEEE 835 / Neher-McGrath / ICEA — Cable ampacity and derating
• IEEE C37 series / IEC 60909 — Protection, switchgear, and short-circuit calculation
• NESC and NEC — Overhead/underground construction and electrical installation
  

Representative Case Studies (Anonymized)

The following are anonymized to protect client confidentiality; identifying details, locations, and figures have been generalized.

1 — Behind-the-Fence Solar Tie-In onto an Active Industrial MV Bus

Sector: Heavy industrial host facility, on-site solar. Challenge: The proposed interconnection tied into an existing MV bus that already carried tuned harmonic filters and static VAR compensation. Adding inverter-based generation raised a real risk of harmonic resonance and IEEE 519 non-compliance at the POI. What Keentel did: Built a frequency-scan and harmonic load-flow model of the inverter plant against the existing filters and compensation, screened for resonance, and coordinated reactive behavior with the host's voltage schedule — then carried the result into the protection single-line and relay settings. Outcome: Resonance risk was identified and mitigated analytically before energization, and the interconnection design proceeded to a sealed, constructible package without field surprises.

2 — Utility-Scale Solar Collector & Substation Interconnection Bay

Sector: Utility-scale solar. Challenge: Routing a medium-voltage feeder from a new substation bay to the collection system through a congested corridor sharing space with roadways, rail, and live utilities, using mixed overhead and underground construction. What Keentel did: Delivered the new bay's general arrangement, structural/foundation, and full P&C set, plus the route engineering — plan-and-profile, ampacity/derating calculations, duct-bank and bore specifications, and an existing-infrastructure avoidance plan. Outcome: A coordinated IFC package the contractor could build without interpretation, with the route laid out to avoid existing infrastructure and minimize crossings.

3 — Large Computational Load Transmission/MV Interconnection Support

Sector: Large industrial / data-center class load. Challenge: A high-capacity load seeking interconnection needed a conceptual single-line covering multiple phased load levels and a modeling package consistent with utility/ISO requirements. What Keentel did: Developed a phased conceptual single-line and a power-system modeling package (power flow, dynamics, and short-circuit), supported the interconnection application, and responded to utility/ISO modeling review comments — all under licensed PE oversight. Outcome: A technically sound application basis that let the owner evaluate phased load levels from a single, consistent engineering set during the study process.

Technical FAQ — MV Interconnection Engineering