A Coordinated Electric System Interconnection Review—the utility’s deep-dive on technical and cost impacts of your project.

Challenge: Frequent false tripping using conventional electromechanical relays
Solution: SEL-487E integration with multi-terminal differential protection and dynamic inrush restraint
Result: 90% reduction in false trips, saving over $250,000 in downtime

The three operating regions you have to design to

Device	Output vs voltage	Response	Best suited to	Main limitations
Mechanically switched capacitor or reactor	Proportional to voltage squared	Seconds; discrete steps; limited switching operations per day	Steady-state reactive supply, voltage profile, loss reduction	No dynamic capability; step voltage change on switching; capability collapses when most needed
Static var compensator	Capacitive branches proportional to voltage squared	A few cycles; continuously controllable	Continuous control where cost matters and deep voltage support is not the driver	Square-law capability loss; harmonic filters are part of the plant and interact with the network
STATCOM	Approximately proportional to voltage — constant current capability	One to two cycles closed loop; converter response faster still	Voltage stability margin, weak interconnections, fast disturbance recovery, flicker and unbalance compensation	Higher capital cost; converter losses; adds a converter and its control dynamics to the network
Synchronous condenser	Governed by machine capability and excitation	Excitation response in the hundreds of milliseconds; inherent inertial response instantaneous	System strength and inertia, short-circuit contribution, black start support	Rotating plant with maintenance and losses; slower controlled response than a converter
STATCOM with energy storage	Reactive as a STATCOM, plus real power within the storage rating	As STATCOM for reactive; real power limited by storage	Where a real power deficiency is part of the problem	Cost and complexity of the storage; different failure and maintenance profile

Stage	Keentel scope	Outcome
Screening	Series-compensation proximity, radial contingency review, UIF and short-circuit ratio screening, frequency scans	Early identification of which SSO types apply
Detailed studies	PSCAD/EMTDC EMT studies with vendor real-code models, impedance-based and frequency-scan analysis	Clear evidence of stability margins or risk
Model quality	EMT model review, benchmarking against positive-sequence models, model acceptance support	Models that system operators accept
Mitigation	Control retuning recommendations with vendors, SSDC and bypass logic requirements, protection and monitoring specifications	Practical, coordinated solutions
Interconnection support	SSR and SSO study reports for ISO and utility requirements, response to reviewer comments	Fewer delays in the interconnection process

Resilience: designing CDUs to the facility Tier objective

Liquid cooling makes heat removal more critical, not less. Cold plates hold only seconds of thermal buffer. If TCS flow stops, processors throttle or shut down very quickly. Resilience design must therefore cover:

Pump redundancy: N+1 pumps at minimum, with automatic changeover.
CDU redundancy: N+1 CDUs on a common TCS header, or 2N CDUs serving A and B manifolds for fault-tolerant designs.
Power to CDUs: dual-corded CDUs fed from A and B sources, with pumps and controls on UPS power so coolant keeps flowing through a utility failure and generator start. This is the liquid-cooling equivalent of continuous cooling.
Facility water continuity: chilled water or condenser water must also continue, through thermal storage, UPS-backed pumps or a high enough FWS temperature that the TCS can coast on stored thermal mass for the transfer period.
Concurrent maintainability: isolation valves so any CDU, pump, filter or manifold section can be serviced without stopping the others.
Leak management: leak detection, drip trays, automatic isolation valves and procedures for fluid spills.

Electrical engineering considerations

CDUs and liquid-cooling plants are electrical loads with their own design requirements:

Pump motors and VFDs. Large CDUs use variable-frequency drives on their pump motors. These introduce harmonic current that must be included in the facility's harmonic study and IEEE 519 evaluation.
Critical mechanical load on UPS. Putting CDU pumps on UPS increases UPS and battery sizing. The mechanical UPS load must be included in redundancy and ride-through calculations.
Feeder and protection design. Dual feeds, automatic transfer, selective coordination and arc-flash labelling apply to CDU power distribution just as they do to IT power.
Generator transient response. CDU pumps restarting together after a transfer create motor inrush on the generators. Staged restarts and soft acceleration reduce the step load.
Load dynamics. AI training loads can swing rapidly between high and low power. Cooling controls must track those swings without temperature overshoot, and electrical systems must handle both the IT and the cooling load changes.

Commissioning a liquid-cooled data hall

Flush and clean the TCS piping and manifolds to the specified cleanliness, using high-purity flush water.
Fill and vent with the specified coolant, verifying glycol concentration and chemistry.
Pressure test for leaks at design and test pressures, including quick-disconnect fittings.
Verify sensors and controls: flow meters, temperature sensors, pressure transmitters, leak detection and BMS points.
Load test with heat load banks connected through the cold-plate interfaces or rack manifolds, across the operating range.
Prove the heat balance on both loops at several load steps, and the approach temperature against the design.
Test failure scenarios: pump failure, CDU failure, power transfer to generator, valve failure and loss of facility water, confirming temperatures stay within the IT vendor's limits.

How Keentel Engineering helps

Keentel Engineering supports data center owners, developers and EPCs with the electrical engineering that liquid-cooled facilities depend on, and coordinates with the mechanical engineer of record and cooling vendors on the integrated design.

The short answer: a large load that drops off the grid all at once can hurt the grid as much as a power plant that trips.

For decades, utilities expected customers' equipment to trip during a disturbance. A few homes or a factory dropping off for a second did not matter. That changed with the arrival of loads of hundreds of megawatts — the size of a mid-size power plant — often clustered in the same area.

When a fault happens on a transmission line (for example, lightning or a tree contact), voltage across a wide area dips for a fraction of a second until protection clears it. Data center electronics sense that dip and, if set to protect themselves aggressively, switch to backup power or shut down. If many megawatts disappear at once:

Frequency jumps up, because generators are suddenly producing more than is being used.
Voltage can swing high, because lightly loaded lines push voltage up.
Other equipment may then trip, which is how a small event turns into a large one.

SPP's draft ride-through document states this directly: there are no industry standards that require loads to ride through faults, yet uncontrolled tripping of large loads "may cause or exacerbate cascading outages." So SPP is creating load requirements modeled on what generators already have to meet.

A HILL is defined in the SPP Tariff; the ride-through document points to the Tariff rather than repeating the size threshold. In practice, the label covers very large, fast-growing loads such as data centers, crypto mining and large industrial facilities. Check the current SPP Tariff for the exact definition that applies to your project.

The study process sits under SPP Tariff Attachments AQ, AX, BA and BB and SPP's business practice BP 7850. The draft ride-through rules apply:

To new requests: HILLs that submit through SPP's transmission service process, or that are supported by a High Impact Large Load Generating Facility, on or after the effective date of SPP Revision Request RR 696 (the date is still a placeholder in the draft).

To earlier requests, more softly: HILLs that applied to their Transmission Owner before that date must make "reasonable efforts" to maximize ride-through within their equipment's capability and document those efforts if asked.

To generation on the same site: solar, batteries, gas generators or synchronous condensers that share a Point of Interconnection (POI) with a HILL have their own ride-through rules (Section 6 below).

One important nuance: the rules can be limited to the power-electronic part of a site. SPP's example is a 50 MW facility with 45 MW of conventional industrial load and 5 MW of power-electronic load; the requirements would apply only to the 5 MW, if SPP's study shows that exemption does not harm reliability.

Voltage at the POI (per unit of normal)	What it looks like	Must stay connected for at least
Above 1.20	Severe overvoltage	Not required (may ride through or trip)
1.10 to 1.20	High voltage	0.5 seconds
0.90 to 1.10	Normal range	Continuously — no tripping
0.80 to 0.90	Mild sag	2.0 seconds
0.50 to 0.80	Moderate sag	0.5 seconds
Below 0.50	Deep sag, usually a nearby fault	0.15 seconds (about 9 cycles)

"Per unit" simply means a fraction of normal voltage: 0.80 per unit on a 345 kV line is about 276 kV.

Where the numbers come from. The table blends two familiar references. The first is the ITIC (formerly CBEMA) curve, the long-standing chart of how much voltage disturbance IT power supplies should tolerate. The second is ERCOT's July 2025 proposal for large electronic loads. SPP's appendix explains the logic:

Below 0.5 pu for about 9 cycles (150 ms): loads close to a fault see deep sags, but most faults are cleared in 4 to 7 cycles, so loads should ride through normally cleared faults.
0.5 to 0.8 pu for 0.5 seconds: loads farther from the fault see shallower sags that can last up to 30 cycles when a fault is cleared by backup protection.

What the load should do during a moderate sag (0.5 to 0.8 pu). It should keep drawing power from the grid, reduced roughly in proportion to how far voltage has dropped. Briefly using less power actually helps the grid recover, which is why SPP allows it, but the load must stay connected.

This is the requirement most likely to catch data center projects out. When voltage falls below about 0.5 pu, a server power supply's rectifier can no longer pull useful power from the grid. SPP accepts that, and gives the load two options:

Transfer to UPS. The uninterruptible power supply carries the servers from its batteries while the grid is disturbed.
Trip. The load may disconnect.
Either way, the load must come back to at least 90% of its pre- disturbance grid consumption within 1 second after voltage recovers to 0.9 pu or higher.

Why this matters: many UPS systems are set up to protect the computers, not the grid. After a sag they may stay on battery for several seconds, or ramp back slowly, to be sure the grid is stable. To the grid, a 250 MW data center that stays on battery for 10 or 15 seconds after every nearby fault looks the same as a 245 MW plant tripping offline. Under SPP's draft, that response would not meet the 1-second rule.

SPP allows one exception. In a weak grid, a fast snap-back can cause oscillations, so SPP and the Transmission Owner may agree to a longer recovery time. That is a negotiated exception, not a default.

What to do about it in practice:

Ask your UPS manufacturer for the return-to-grid (re-transfer) time and ramp rate after a sag, and whether firmware settings can achieve 1 second.
Make sure the dynamic model you give SPP uses those exact settings. A model that is more conservative than the real equipment can fail a study the equipment would pass.
If the site is on a weak, radial connection, raise the recovery-time question with SPP and the Transmission Owner early.

Instantaneous voltage (pu, after surge arresters)	Must ride through for at least
Above 1.80	Not required
Above 1.70	0.2 milliseconds
Above 1.60	1.0 millisecond
Above 1.40	3.0 milliseconds
Above 1.20	15.0 milliseconds

Frequency	Must stay connected for at least
61.8 Hz or higher	Not required
61.2 Hz or higher	299 seconds
58.8 to 61.2 Hz	Continuously
Below 58.8 Hz	299 seconds
Below 57.0 Hz	Not required

In plain terms:

Across the normal and moderately disturbed range, the load must keep running for about five minutes, long enough for operators and automatic controls to restore balance. SPP says HILLs should not trip within these limits unless internal protection requires it.Reclosing — riding through more than one hit.

Transmission lines often use automatic reclosing:

After a fault the breaker opens, waits, and closes again to test whether the fault has cleared. If it has not, the grid sees a second voltage dip. SPP's rule: HILLs should not use protection that trips on these repeated dips while POI voltage stays within the ride-through limits. If equipment protection truly requires such logic, the HILL must still ride through at least six fault-clearing attempts within 90 seconds. A study case with a fault, then a second fault a few hundred milliseconds later, is testing exactly this.

Constant current, not constant power. Power equals voltage times current. A load in constant-power mode tries to keep its power the same when voltage drops, so it pulls more current, which drags voltage down further — the opposite of what the grid needs. A load in constant-current mode lets its power fall in step with voltage. SPP requires HILLs that are mainly power electronics or variable-speed drives to:

Use constant-current control during disturbances, with a reasonable current limit (the limit is still "TBD" in the draft).
Avoid constant-power control.
Avoid protection that trips on repeated reclosing dips (Section 6).

Exemptions, all conditional on the study. SPP's footnotes offer several exemptions, each only if the HILL Delivery Point Study shows reliability is not harmed:

Exemption	What it relieves
Non-power-electronic load	The whole requirement set may apply only to the power-electronic part of the site
Variable-speed drives (VSDs)	The 0.15-second deep-sag minimum, constant-current mode, the constant-power ban and transient overvoltage limits — but VSDs still must restore 90% of consumption within 1 second
Weak grid	A longer recovery time than 1 second, agreed with SPP and the Transmission Owner

This creates a real design trade-off. Moving chillers onto UPS can improve how cleanly the site rides through a fault, but it also turns that load into power-electronic load that the rules fully apply to. Running chillers on VSDs can bring them under the VSD exemptions instead. Design and modeling teams should decide this together, early.

On-site generation behind the same POI. If the site has its own generation sharing the POI, those units must coordinate their ride-through with the load:

Inverter-based resources (solar, batteries): IEEE 2800-2022 Clause 7, except voltage protection (7.2.1) and mandatory frequency tripping (7.3.1). Reactive current gets priority during low voltage, with active power cut as little as possible. The Transmission Owner sets the nominal voltage, and SPP may specify the positive- and negative-sequence reactive current response through the HILL generator interconnection agreement (HILLGIA).
Synchronous generators, Type 1 and 2 wind, synchronous condensers: NERC PRC-024-4.

SPP judges a HILL almost entirely through computer models, so the model is the project during the study. Two kinds of model are involved.

RMS (positive-sequence) models in PSS®E. These are what SPP runs in its big regional studies. They represent the grid in averaged, cycle-by-cycle terms and are fast enough to simulate the whole SPP system. A data center RMS model usually combines:

A power-electronic load model for the UPS-fed IT load. The PERC family of data center load models has been developed through the NERC Load Modeling Working Group; they are user-written models (delivered as compiled DLL files) until they are added to the standard PSS®E library.
A composite load model (CMLD) for cooling and other mixed load. It splits load into motor types (A, B, C, D), power-electronic and static parts, each with its own protection, such as motor undervoltage trips.

EMT models in PSCAD. These represent every switching device and waveform in detail, microsecond by microsecond. They are slow, so SPP uses them only near the project and only when the grid is weak (Section 3).

Pitfall	What goes wrong	How to avoid it
Software version mismatch	A user-model DLL compiled for one PSS®E version will not load in another, and the study stops	Ask SPP which PSS®E version the study uses; test the DLL in that exact version before submitting
Wrong point of interconnection	Model connects at the facility substation rather than the TO's POI bus; SPP cannot drop it into its case	Model from the POI bus the TO gives you, including any new POI transformer and line
Raw case only	SPP needs an .idv script that adds the project to its own case	Deliver an .idv (plus .raw), following the TO's template and bus numbering
Reactive power that does not match the load mix	The composite load's initial compensation is set from the power-flow Q; if Q is too low, motors trip after a fault and Q swings negative	Set power-flow Q consistent with the motor composition; test a POI fault before submitting
UPS settings that are too protective	Long time on battery or slow ramp-back fails the 1-second recovery rule	Use the vendor's real re-transfer settings and confirm they meet SPP's limits
Constant-power control	Higher current during sags, worse voltage recovery	Use constant-current settings in both equipment and model
RMS and EMT models that disagree	SPP's model verification step flags inconsistent responses	Build both from one data set and compare the same events side by side

Design and equipment

UPS vendor has confirmed, in writing, how the UPS behaves below 0.5 pu and how fast it returns to the grid (target: 90% within 1 second of 0.9 pu)
UPS and drives are set to constant-current behavior during disturbances
No protection trips the site on repeated reclosing dips; if any must, it tolerates six attempts in 90 seconds
Frequency protection does not trip between 58.8 and 61.2 Hz, and allows 299 seconds outside that band
Cooling strategy decided: chillers on UPS, on VSDs, or direct-on-line — and which exemptions that relies on
On-site generation (if any) meets IEEE 2800-2022 Clause 7 or PRC-024-4 and is coordinated with the load

Interconnection data

POI bus, voltage and short-circuit / Thevenin data obtained from the Transmission Owner
Network from the POI to the load (POI transformer, line, facility transformers) taken from the TO's exhibit or design, with any assumed data flagged
Expected power factor and where it is measured agreed with the TO

Model package

PSS®E version used by SPP confirmed; all user DLLs load in that version
.idv script plus .raw, using the TO's bus numbers and template
.dyr records match the bus numbers in the .idv
Power-flow Q consistent with the dynamic load composition
Flat run, voltage sag, POI fault and fault-plus-re-fault cases run cleanly and results documented
PSCAD model built from the same data, with matching responses to the same events
Model report states every assumption, software version and setting source

Primary sources (SPP drafts, prepared for Revision Request RR 696):

SPP, High Impact Large Load Delivery Point Study (HDPS) Overview, V1.0, revision history dated September 23, 2025 (initial draft).
SPP, HILL Fault Ride Through Requirements, V2.0, revision history dated September 23, 2025 (initial draft).

Standards and references cited in those documents:

NERC TPL-001-5.1; NERC PRC-024-4; NERC PRC-029-1; IEEE 2800-2022; IEEE 1547; IEEE 1668-2017; ITIC (CBEMA) curve; ERCOT Large Electronic Load Voltage Ride-Through Performance Requirements proposal (LLWG, July 11, 2025); AESO Connection Requirements for Transmission-Connected Data Centers (August 22, 2025); CIGRE Technical Brochure 909; SPP Disturbance Performance Requirements; SPP Planning Criteria; SPP BP 7850.

Disclaimer:

This guide summarizes draft SPP documents in plain language for general information. The requirements are subject to stakeholder review, approval of RR 696 and future revision; several values (for example, the current limit) are still marked "TBD". It is not legal, regulatory or engineering advice for any specific project. Always confirm the current version of SPP's Tariff, business practices and requirements with SPP and your Transmission Owner before making design or filing decisions. PSS®E and PSCAD are trademarks of their respective owners.

In 1995, Sonny Patel earned his Electrical Engineering degree from the University of Illinois. But degrees don't build legacies — action does.

For three decades, he has worked the power industry from every side of the table: 16 years as a utility engineer at Exelon/Commonwealth Edison; generation leadership across hydroelectric, industrial steam turbine, and a 9 GW renewable fleet; NERC Regional Entity Senior Compliance Engineer and Audit Team Lead, auditing some of the nation's largest utilities; and testing and commissioning lead on equipment up to 765 kV — the very top of the North American grid.Utility. Generator. Regulator. Consultant. Few engineers have seen all four seats. Fewer still have sat in them.

His experience spans nuclear, hydro, conventional generation, renewables, oil and gas, mining — and today's data centers, where he is authoring a three-book series on data center design. He is a Licensed Professional Engineer in six states and a Licensed Electrical Contractor in Florida (Unlimited EC) — he doesn't just design the work; he's qualified to stand behind its execution.Today, as Founder and CEO of Keentel Engineering, Sonny leads a nationwide team of engineers delivering substation design, power system studies, NERC compliance, and commissioning — done right, coast to coast.Three decades. Every side of the table. One standard: accountable engineering.

In 1995, Sonny Patel earned his Electrical Engineering degree from the University of Illinois. But degrees don't build legacies — action does.

Utility. Generator. Regulator. Consultant. Few engineers have seen all four seats. Fewer still have sat in them.

Today, as Founder and CEO of Keentel Engineering, Sonny leads a nationwide team of engineers delivering substation design, power system studies, NERC compliance, and commissioning — done right, coast to coast.Three decades. Every side of the table. One standard: accountable engineering.