Adding a BESS to a Data Center Campus

What Battery Storage Actually Does to Power Quality at the Point of Interconnection, Which Studies Have to Prove It, and Why the Inverter — Not the Battery — Decides the Outcome

1. Executive Summary

Battery energy storage has moved from the periphery of data center design to its centre. Storage is now being deployed on data center campuses to bridge outages, shave peaks, defer utility infrastructure, provide the load flexibility that increasingly determines how quickly a large load can be interconnected at all, and in some markets to participate in wholesale services. The commercial case is well understood. The electrical consequences are frequently not.

The framing that causes the most trouble is treating storage as a container of energy. A BESS is a power-electronic converter with a battery attached. Everything the grid experiences — harmonic current injection, reactive support, ramp behaviour, fault current contribution, ride-through, stability under weak-grid conditions — is determined by the power conversion system and its control tuning, not by the cells. The energy capacity sets how long it can do something. The inverter sets what it does.

On a data center campus the analysis is harder than on a standalone storage site, for one reason: the site is already an unusually distorted electrical environment before the BESS arrives. A large data center is a concentration of rectifier front ends — UPS systems, IT power supplies, variable-speed drives on the mechanical plant — sitting behind a long medium-voltage cable system with substantial distributed capacitance. Adding a converter of comparable rating to that environment is not a superposition problem that can be waved through; harmonic currents from different sources can cancel or reinforce depending on phase, and the network the BESS is injecting into may already have a parallel resonance sitting near a harmonic order the converter produces.

This paper works through what actually changes when a BESS is added at a data center campus: the correct harmonic compliance framework and why the metric is total demand distortion at the point of common coupling rather than voltage total harmonic distortion at a panel, how network resonance is created and excited, what governs voltage fluctuation and flicker, why grid strength and control interaction dominate the analysis on weak interconnections, what protection and overvoltage behaviour changes, and the complete study set an owner should expect. It closes with a mitigation toolkit, a compliance measurement protocol, and a twenty-question FAQ.

The sentence the source graphic gets right

The battery may be the asset, but the question that determines whether the project is buildable is how the grid — and the campus’s own power quality — respond once it is connected.

That question is answered by studies performed during design, using validated manufacturer models of the specific converter being purchased. It is not answered by a datasheet line stating that total harmonic distortion is below five percent.

2. Why Storage Is Appearing on Data Center Campuses

The design implications differ by use case, and the use case must be settled before the electrical design begins, because it determines the converter rating, the duty cycle, the point of connection, and the interconnection pathway.



• Bridging and ride-through. Lithium storage replacing or augmenting conventional UPS energy storage, extending autonomy and reducing the frequency of generator starts. Connection is typically at low voltage within the critical power path.
• Peak shaving and demand management. Reducing billing demand and deferring utility service upgrades. Connection is typically at the campus medium-voltage bus.
• Load flexibility as interconnection currency. This is the fastest-growing driver. Where utilities and system operators are constrained on large-load interconnection, the ability to curtail or shift campus demand on request — without dropping IT load — is increasingly what determines the energisation date. Storage is the mechanism that makes a firm IT load look flexible from the utility’s side of the meter.
• Generator reduction. Displacing some engine-generator capacity or runtime, with consequences for air permitting, fuel storage, and the resilience case that has to be demonstrated during commissioning.
• Wholesale market participation. Frequency response, regulation, or capacity, which brings the facility into a different regulatory and performance regime and imposes telemetry, model, and ride-through obligations.

Each of these places the BESS at a different point in the single line and subjects it to a different interconnection framework. Storage inside the critical power path at low voltage is a facility design problem. Storage on the medium-voltage campus bus operating in parallel with the utility is an interconnection problem, and the study obligations, model requirements, and protection requirements change accordingly.

3. The Inverter Is the Asset

The power conversion system determines grid behaviour through a small number of design and tuning choices, and the electrical engineer needs the manufacturer’s specifics on each of them before any study can be run.

• Switching frequency and modulation strategy, which set where converter-produced current distortion appears in the frequency spectrum. Modern converters push the dominant switching content into the kilohertz range, which is a different problem from the low-order harmonics produced by the six-pulse rectifiers that shaped the classic distortion literature.
• Output filter topology — typically an inductor-capacitor-inductor arrangement — whose capacitance is itself a network element that participates in resonance, and whose damping determines behaviour at the filter’s own resonant frequency.
• Current control bandwidth and the phase-locked loop tuning, which govern stability margin against network impedance and are the mechanism behind converter-network control interactions on weak systems.
• Reactive capability and the reactive priority setting, which determine what voltage support is available and whether the converter gives up real power to provide it.
• Ramp rate limits and their configurability, which govern the voltage fluctuation the site imposes on the network.
• Fault current contribution and duration, typically limited to a small multiple of rated current, which is the single most consequential difference between an inverter-based resource and a synchronous machine for protection design.
• Ride-through configuration and the priority between ride-through obligations and equipment self-protection, which is where a converter that meets its datasheet can still trip in a way the interconnecting entity considers non-compliant.

The model is the deliverable



Studies require validated converter models — a positive-sequence model for load flow and stability work, and an electromagnetic transient model for control interaction, weak-grid, and harmonic behaviour.

Generic library models produce answers that are wrong in the specific ways that matter, because the behaviour of interest is a property of the manufacturer’s control code. Model availability, quality, and the right to use the model should be procurement requirements written into the converter specification, not requests made after the order is placed.

4. Harmonics: The Right Metric at the Right Point

Harmonic compliance conversations go wrong in three predictable ways: the wrong metric, the wrong measurement point, and the wrong assumption about who owns the limit.

4.1 Current Distortion Is Measured as Total Demand Distortion

The governing North American framework expresses current distortion limits as total demand distortion, referenced to the maximum demand load current at the point of common coupling, rather than as total harmonic distortion referenced to the instantaneous fundamental. The distinction is not academic. A converter operating at ten percent of rating can show an alarming current total harmonic distortion figure while contributing a trivial amount of actual distorting current. Judging a project by that number produces both false alarms and false comfort.

The allowable total demand distortion is not a single value. It scales with the ratio of available short-circuit current at the point of common coupling to the maximum demand load current — a stiffer connection relative to the load is permitted more distortion, because the resulting voltage distortion is smaller. Individual harmonic orders carry their own limits, tightening as order increases, with even-order harmonics held to a fraction of the odd-order limits.

4.2 The Point of Common Coupling Is a Specific Place

Limits apply at the point of common coupling — the point where other customers are or could be served — not at a panel inside the facility, not at the converter terminals, and not at the low-voltage side of the campus transformer. Internal distortion is a facility engineering matter governed by equipment withstand and transformer heating, not by the interconnection limits. Conflating the two produces expensive filter specifications aimed at a problem the utility never had.

4.3 Voltage Distortion Limits Belong to the Utility

The framework assigns current limits to the customer and voltage limits to the system operator, with the voltage limits tightening as system voltage rises. In practice this becomes a shared responsibility: the customer’s current injection acting on the utility’s system impedance produces the voltage distortion the utility must hold. This is why background distortion measurement before energisation matters. A site that arrives at a point of common coupling already near the voltage distortion limit has far less headroom than the current-limit table alone suggests, and discovering that after commissioning is a mitigation project rather than a design decision.



4.4 Above the Classic Spectrum

Converter switching content in the range above the traditional harmonic measurement band — the supraharmonic region — is not addressed by the classic limit tables and is increasingly the source of real problems: interference with power line communication and metering, audible noise in magnetics, and interaction between converters from different manufacturers sharing a bus. A data center campus with a large storage converter, dozens of UPS modules, and hundreds of drives is exactly the environment where this appears. It should be measured, not assumed away.

5. The Data Center Complication

Standalone storage projects are analysed against a network. A data center campus adds a large, non-linear, dynamically varying load between the converter and the point of common coupling, and that changes the analysis in four specific ways.

5.1 Superposition Does Not Hold Naively

Harmonic currents from the storage converter and from the facility’s rectifier loads combine as phasors, not as magnitudes. Depending on the phase relationship at each order, they can partially cancel or reinforce. A study that adds distortion budgets arithmetically will be wrong in both directions, and the direction it is wrong in is not predictable without modelling the actual sources with their phase information.

5.2 The Facility Load Varies and So Does the Answer

IT load, mechanical load, and storage dispatch all vary independently. Compliance is not a single operating point. The harmonic study must evaluate the credible combinations — minimum facility load with the converter at full charge, full facility load with the converter idle, converter at full discharge during a demand response event — because the worst case for total demand distortion is often minimum demand rather than maximum, and the worst case for resonance is a specific capacitance and load damping combination that occurs at neither extreme.

5.3 Transformer Heating Is a Separate Calculation

Harmonic current increases transformer losses disproportionately, because eddy current loss rises with the square of harmonic order. The relevant assessment is a harmonic loss factor calculation against the transformer’s capability, or specification of a transformer rated for the harmonic duty. Data center campus transformers see this loading continuously, not intermittently, and a transformer selected on kVA alone in a high-distortion environment is a transformer with an unstated derate.

5.4 The Critical Load Is Sensitive to the Same Things

Voltage distortion and fluctuation at the campus bus propagate to the UPS input. UPS systems evaluate their source and will transfer to battery if input quality falls outside their acceptance window. A storage converter whose behaviour degrades campus voltage quality can therefore cause the facility’s own critical power systems to abandon the utility source — the same class of failure discussed in the commissioning literature as a generator-to-UPS interaction, arriving through a different door.

6. Resonance: The Network Creates It, the BESS Excites It

Resonance is the most misunderstood item on every BESS power quality summary. A converter does not create resonance. Resonance is a property of the network — the interaction between system and transformer inductance and the capacitance of cables, filters, and any power factor correction equipment. What the converter does is inject current at frequencies that may coincide with a resonant condition that already exists, at which point a modest injected current produces a large voltage.

6.1 Where the Capacitance Comes From on a Data Center Campus

• Medium-voltage cable. Campus distribution is cable, not overhead line, and often a great deal of it. Distributed cable capacitance is frequently the dominant capacitive element on a large campus and it grows with every phase of build-out.
• Converter output filter capacitance, from both the storage converter and any other inverter-based resource on site.
• Power factor correction and harmonic filter banks, where fitted.
• Surge capacitors and instrument transformer burden, minor individually and occasionally relevant in aggregate.

6.2 The Consequence of Getting It Wrong

A parallel resonance near a harmonic order the converter produces amplifies that order, producing voltage distortion far above what the injected current would suggest, overheating capacitors and transformers, and in the worst case causing protective device operation on a system with no fault. Because campus cable capacitance grows as phases are built, the resonant frequency migrates across the build-out. A study performed on Phase 1 does not describe the campus at Phase 3.

The study that answers this is a frequency scan

A harmonic impedance scan across the frequency range of interest, performed at the point of connection for each credible network configuration — each phase of build-out, each transformer and feeder switching state, minimum and maximum load damping — identifies where the resonant peaks sit relative to the orders the converter produces.

It is inexpensive during design and it is the only way to know whether a filter is needed, what it should be tuned to, and whether a proposed filter will move a resonance onto a worse order rather than away from one.

7. Voltage Fluctuation, Flicker, and Ramp Rate

Voltage fluctuation from a storage system is a power-change problem, not a harmonic problem, and it is straightforward to estimate. A change in the real and reactive power exchanged at a bus produces a voltage change governed by the change in apparent power relative to the short-circuit strength at that bus and the resistance-to-reactance ratio of the source impedance. Two consequences follow directly.

First, the magnitude of the fluctuation is set as much by the strength of the interconnection as by the size of the converter. The same converter produces a small voltage step on a strong system and a visible one on a weak system. Second, because the converter’s reactive capability is fast and controllable, much of the fluctuation is correctable by the converter itself if the control mode and the reactive priority are configured to do so. A storage system is more often the remedy for voltage fluctuation on a campus than the cause of it.



Flicker — the perceptibility of rapid voltage variation — is assessed by a standardised measurement method producing short-term and long-term severity indices, evaluated against planning levels the system operator sets. The relevant question for a data center campus is rarely the storage converter operating on a smooth dispatch signal; it is fast events. Step changes during demand response dispatch, transitions between charge and discharge, and coincidence with mechanical plant starting are the cases that need evaluation.

The design controls are ramp rate limiting configured in the converter, reactive support enabled with an appropriate priority, and, where the interconnection is genuinely weak, dynamic reactive equipment sized from the study rather than from a rule of thumb.

8. Grid Strength and Control Interaction

The ratio of available short-circuit strength at the point of interconnection to the rating of the inverter-based resource is the single most predictive number for whether a storage project will behave. A high ratio means the network holds voltage and the converter’s controls have ample stability margin. As the ratio falls, the converter’s own current injection begins to move the voltage it is measuring, the phase-locked loop and current controllers are operating against a network impedance comparable to their own control impedance, and stability margin erodes.

For a campus with a large storage converter, other on-site inverter-based generation, and a long radial supply, the effective strength seen by the converter can be considerably lower than a headline short-circuit figure at the utility substation suggests. Where multiple inverter-based resources share an interconnection, the aggregate matters, and system operators in the most affected regions have developed weighted and site-specific formulations precisely because the simple ratio overstates strength when converters interact.



The failure modes at low grid strength are not gradual degradation. They are oscillatory instability at frequencies that positive-sequence simulation cannot represent, unexpected tripping during and after nearby faults, and sustained interactions between converters from different manufacturers that neither exhibits alone. Sub-synchronous control interaction with series-compensated transmission is the best documented example and is not the only one.

This is why electromagnetic transient modelling is a requirement rather than a refinement at weak interconnections. Positive-sequence stability tools cannot represent converter control dynamics at the timescales where these phenomena live, and a study set that stops at load flow and short circuit has not addressed the risk that actually threatens the project.

9. Grid-Following and Grid-Forming Behaviour

Most storage converters in service today are grid-following: they synchronise to a measured voltage and inject a controlled current. They require a stable external voltage reference and they contribute nothing to system inertia or to establishing voltage on a de-energised network.

Grid-forming control instead regulates voltage magnitude and angle behind an impedance, presenting a voltage source to the network. The practical consequences relevant to a data center campus are substantial: grid-forming converters can operate at very low grid strength where grid-following control becomes unstable, can contribute to frequency stability, and can establish voltage on an islanded network, which makes campus islanding and black start technically feasible in a way grid-following equipment does not.

The trade-offs are real. Grid-forming operation imposes different current headroom requirements during transients, the control mode interacts with the protection design, and interconnection requirements and model validation practice for grid-forming resources are still maturing. The decision should be driven by the grid strength at the interconnection and by whether campus islanding is an operational requirement, and it should be made early, because it affects converter procurement, protection design, and the study scope.

10. Protection, Fault Behaviour, and Overvoltage

10.1 Fault Current Contribution

An inverter-based resource contributes fault current limited by its control to a small multiple of rated current, for a controlled duration, with a waveform that is not the decaying symmetrical contribution a synchronous machine produces. Overcurrent protection philosophies built around the assumption of substantial fault current from every source do not translate. Where the storage converter is a significant portion of the source capacity at a campus bus, the coordination study must model the converter’s actual current-limited contribution rather than an equivalent machine, and directional, differential, or communication-assisted schemes may be required where overcurrent alone cannot discriminate.

10.2 Ground Fault Overvoltage and Effective Grounding

When a converter and its interconnection transformer remain energised on a portion of the system that has been separated from the utility source by an upstream device operating for a ground fault, the grounding arrangement of the interconnection transformer determines whether the unfaulted phases rise to a level the system’s equipment insulation and surge arresters can tolerate. This is a design decision — winding configuration, grounding transformer, neutral treatment — that must be settled against the utility’s grounding practice, and it is a common cause of late-stage interconnection redesign when it is deferred.

10.3 Load Rejection and Transient Overvoltage

A converter charging at full rate that suddenly loses its source, or discharging into a load that disappears, produces a transient the design must bound. Combined with transformer energisation inrush, sympathetic inrush between adjacent transformers, and the interaction with surge arrester duty, this belongs in the transient study scope rather than in the assumption that converter controls will handle it.



10.4 Islanding and Interconnection Compliance

Unintentional islanding detection, ride-through obligations during voltage and frequency excursions, and disturbance monitoring and reporting obligations attach to the resource according to its size, voltage level, and the interconnection framework it falls under. The requirements differ materially between a distribution-connected resource and a transmission-connected one, and between market regions. Establishing which framework applies is a first-week question, not a permitting-phase question, because it drives model requirements, telemetry, protection, and testing scope.

11. The Study Set

The following is the study scope an owner should expect for a storage system interconnected at a data center campus. Scope varies with size, voltage, and jurisdiction, but a proposal that omits several of these is a proposal that has deferred risk rather than priced it.

12. The Mitigation Toolkit

Mitigation should follow from the studies, in order of cost and intrusiveness. Specifying a filter before running a frequency scan is the most common and most expensive sequencing error in this work.

13. Measurement and Compliance Verification

A compliance claim that rests on simulation alone is incomplete. The verification sequence that holds up is straightforward and is frequently skipped.

1. Measure background power quality at the point of common coupling before the storage system is energised, using instrumentation of the appropriate measurement class, over a period long enough to capture the site’s weekly load cycle. Without this baseline, no post-energisation measurement can distinguish the storage system’s contribution from what was already there.
2. Record the facility’s own distortion contribution before energisation as well. On a data center campus this is substantial and it is the owner’s, not the utility’s.
3. After energisation, measure across the operating envelope — charge, discharge, idle, and transition — at the load conditions the study identified as governing, not merely at a convenient one.
4. Evaluate against the limits using the statistical basis the standard specifies rather than instantaneous peaks. Compliance frameworks are written around percentile values of aggregated measurement intervals, and a single high sample is not a violation any more than a single low sample is a pass.
5. Repeat the assessment at each phase of campus build-out. Cable capacitance, load, and short-circuit strength all change, and the resonance condition moves with them.
6. Retain the measurements. When a power quality dispute arises with the utility or with an equipment vendor, the party holding a defensible baseline and a documented commissioning survey is in a completely different position from the party holding a simulation report.
   

14. Reading the Graphic Correctly

The four-panel summaries of BESS power quality effects that circulate widely are directionally useful and technically loose in ways that matter. Five clarifications.

Clarification 1 — The metric is total demand distortion, not THD

Stating that a storage system "increases THD levels" points at the wrong number. Interconnection compliance for current is assessed as total demand distortion at the point of common coupling, referenced to maximum demand load current. Voltage total harmonic distortion is a separate limit belonging to the system operator. Using the terms interchangeably produces both unnecessary filter purchases and missed non-compliance.

Clarification 2 — The converter does not create resonance

Resonance is created by network inductance and capacitance. The converter excites a condition that already exists. This matters practically because it locates the fix: the answer is a frequency scan and, if required, a change to the network — filter, reactance, or point of connection — not a search for a better-behaved converter.

Clarification 3 — Storage is usually the cure for voltage fluctuation, not the cause

A converter with fast reactive control and configurable ramp limits is one of the most effective voltage stabilisation devices available at a campus bus. Presenting voltage fluctuation solely as a risk of adding storage inverts the more common engineering reality, where the storage system is dispatched to correct fluctuation caused by the facility’s own load.

Clarification 4 — The dominant risks are missing from the four panels

Harmonics, voltage fluctuation, and resonance are the classic list. On modern projects the risks that actually stop or delay work are grid strength and converter control interaction, ground fault overvoltage and the interconnection transformer grounding arrangement, ride-through compliance and disturbance monitoring obligations, and model availability and validation. None of these appear on a four-panel summary and all of them are schedule-critical.

Clarification 5 — Compliance is a set of operating points



Every one of these effects varies with dispatch, facility load, network configuration, and build phase. A single-case study and a single commissioning measurement are not evidence of compliance; they are evidence of one condition. The study set and the measurement protocol both have to be built around the envelope.

15. Keentel Data Center and Storage Engineering Services

Keentel Engineering provides the interconnection engineering, power system studies, and facility electrical design that determine whether storage added to a data center campus behaves as intended and can be approved. Our practice in this area covers the following.

15.1 Interconnection and Point-of-Interconnection Engineering

• Large-load and generation interconnection support: load and resource characterisation, application-stage technical packages, coordination with the serving utility, transmission provider, and system operator, and management of study-phase technical questions.
• Point-of-interconnection design, substation design, and medium-voltage collection and campus distribution design for phased data center developments.
• Interconnection transformer configuration and effective grounding assessment, settled early rather than discovered during utility review.

15.2 Power System Studies

• Load flow, short circuit, protective coordination and selectivity, and arc-flash analysis, with inverter-based resources modelled as current-limited sources rather than as equivalent machines.
• Harmonic frequency scan and injection studies across build-out phases and operating combinations, with compliance assessed at the correct point using the correct metric.
• Flicker and voltage fluctuation assessment, grid strength evaluation, and transient stability analysis.
• Electromagnetic transient modelling for control interaction, weak-grid stability, ride-through verification, and switching and overvoltage transients, using validated manufacturer models.
• Transformer harmonic loading assessment, insulation coordination, and ground grid and step-and-touch analysis.

15.3 Facility Electrical Design and Owner’s Engineer Support

• Campus distribution topology, UPS and generator plant integration with storage, and white-space power distribution designed against the availability and maintainability requirements.
• Converter and equipment specification including model deliverable requirements, ride-through and control configuration, and factory test scope.
• Design review of EPC and vendor submittals, QA/QC of third-party study packages, commissioning specification and test script support, and power quality baseline and verification survey scoping.
• NERC compliance support for registered resources, including disturbance monitoring, ride-through, and model-related obligations.

Keentel Engineering holds a Florida Certificate of Authorization and maintains offices in Tampa, Austin, Sacramento, and Baltimore, supporting projects across the interconnections.

16. Frequently Asked Questions

Notice and Disclaimer

This document is original technical content prepared by Keentel Engineering LLC for general professional information. It is not project-specific engineering advice and does not constitute a design, a study, an interconnection application, or a compliance determination for any particular facility. Study scope, interconnection requirements, and power quality obligations vary by system operator, utility, voltage level, resource size, and jurisdiction, and the requirements applicable to a specific project govern that project.

Standards and regulatory references are provided by subject for orientation. The current published edition of each standard and the requirements adopted by the applicable authority, utility, or system operator govern. Equipment behaviour, ratings, and model characteristics must be taken from the specific manufacturer’s validated data for the equipment selected.

Keentel Engineering LLC is an independent engineering consultancy. Reference to any standard, code, market rule, industry organisation, or equipment category in this document does not imply affiliation with, endorsement by, or sponsorship from any such organisation or manufacturer.