Power Substation Design Calculations – A Checklist of 18 Studies for Engineers to Consider

Power Substation Design Calculations

What does it take to design a masterpiece of a substation? Quite a bit, honestly. In this article, I will share 18 substation design calculations or studies that will set you up to create a beautiful substation. Here’s the list.


System Planning

Short-circuit study

Short circuit study - substation design calculations
Figure 1: Short circuit study – oneline depicting the flow of fault current. Arrows indicate the direction of the flow. Image courtesy: Electrocon
Why conduct short-circuit study?
Hundreds, if not thousands, of generators are tied to the power grid. Rotating loads like induction motors are integrated as well.
When a short-circuit occurs
– generators pump current into the fault.
– motors (which store energy in the magnetic field) backfeed into the fault.
Under-rated equipment subjected to this sudden current in-rush may catastrophically fail.
Outcome of short-circuit study
The study furnishes momentary and interrupting fault current magnitudes along with impedance to the fault location (either in symmetrical components X1-X0 or X/R ratio). More information here.

You should use the results to
– Procure substation equipment rated to withstand, and when designed for, interrupt fault current
– Determine trip setting for relays
– Perform other studies such as bus calculations and ground grid study

Load flow study

Substation Design Calculations - Load Flow Study
Figure 2: Load flow study – oneline showing power flow. Orange arrows indicate overloaded line. GIF source: CREDC.
Why conduct load flow study?
The grid operates in equilibrium. When load increases, the equilibrium shifts to a new high, with generators ramping up to keep up. This increase in power (consequently the continuous current) overheats underrated equipment and causes the insulation to deteriorate or fail.
Furthermore, some amount of power is lost as heat on the transmission lines. This results in a voltage drop at the receiving end of the line.
Outcome of load flow study
The study furnishes voltage drop, continuous current, and power factor at different nodes in the grid.

You should use the results to
– Procure substation equipment rated to withstand continuous current
– Determine trip MVA for transmission lines
– Determine pickup settings for overcurrent relays
– Determine transformer LTC settings
– Correct power factor using capacitors

Note: Minimum ratings are also standardized by the Regional Transmission Operator (RTO). For example, this design criteria document for transmission owners (under PJM) mandates continuous current for line terminal equipment. If the load flow study prescribes a lower rating, you should discard the results in favor of RTO requirements.

Insulation coordination study

Insulation coordination study - Substation design calculations
Figure 3: Insulation in the power grid is ubiquitous. It isolates energized conductor or bus from touching the grounded structure.
Why conduct insulation coordination study?
The power grid is subjected to lightning strikes and temporary over-voltages. A lightning strike occasionally carries greater than 1 million-volts, 100 kilo-amps, and 20 giga-joules of energy.

How “beefy” should the insulation be to survive this? You could spend a lot and over insulate or, when the budget is a concern (which is typically the case), build a station with reduced insulation.
Outcome of insulation coordination study
Choose standardized BIL rating (Basic lightning impulse Insulation Level) for the substation from IEEE 1313.1 standard. If you are choosing lower insulation level, pay close attention to the ratings of surge arrester and its install location.

Results are incorporated in the following drawings:
– General arrangement plan drawing (because BIL rating dictates phase-phase and phase-ground clearances)
– General arrangement section view drawing
– Equipment installation detail drawing

Note: There are no formulas to calculate BIL. It has been established based on experience and lab tests. At 345kV and above voltage, a switching surge has higher voltage magnitude than lightning. Choose BSL levels from the same standard for this voltage class.

Electrical – Substation Design Calculations

Protection and control analysis

Relays inside a control building
Figure 4: Protective relays inside the control building. Some are capable of tripping the high voltage circuit breaker in the yard.
Factors affecting relay choice for substation protection - substation design calculations
Figure 5: Several factors are considered when picking a relay. Significant ones are listed here.
Why conduct protection and control analysis?
Power substations contain expensive pieces of equipment. Some form of protection is required to prevent them from going up in smoke. Protection of modern substations is implemented using microprocessor relays.

Relays are required to:
– Trip and isolate only the faulted zone. In other words, minimize widespread outage.
– Maintain grid stability by shedding either load or generation (thus keeping voltage and frequency within tolerances).
Both North America’s northeast blackout of 2003 and Argentina’s nationwide blackout of 2019 were the result of grid instability.
Outcome of protection and control analysis
– Specify protection and control logic for the substation equipment.
Specify SCADA and communication system for automation, annunciation, and remote control purposes.
– Create relay settings that coordinate with other relays (in the station and at remote-end).
– Create relay settings that generate high-speed tripping to disconnect generators or loads (during abnormal conditions) to maintain grid stability.

Results are incorporated in the following drawings:
– Oneline, AC schemes, DC protection schemes, relay panel wiring drawings, SCADA and communication drawings
– Control building layout
– Relay panel front views

DC system – battery calculations

DC system in a substation - Substation design calculations
Figure 6: Substation DC power system. During normal operation, the batteries are trickle charged by the charger and remain on stand-by. The charger also feeds the DC loads (from AC system via a rectifier).
Why conduct battery calculations?
– Motors which operate high voltage switch and ones that charge the spring inside the circuit breaker
– Microprocessor relays inside the building and inside the power equipment

all work using DC power.

A battery that not only packs enough energy but also provides the discharge characteristics to operate substation equipment is needed.
Outcome of battery calculations
Specify batteries with enough amp-hour capacity to support the continuous load for 8 hours and momentary load (such as breaker and switch operation) for a minute or more. The popular battery chemistry in the industry is lead-sulphuric acid.

Specify battery charger that is capable of charging the battery.

Results are incorporated in the following drawings:
– Station DC power oneline

AC system – auxiliary power transformer calculations

A typical auxiliary AC system in a substation.
Figure 7: A typical auxiliary AC system in a substation. For a new substation, the load voltage rating must be determined early on. This is because a transformer can be bought in any one of the following configurations 120VAC 1-ph, 240VAC 1-ph, 208V 3-ph wye, 240V 3-ph delta, etc.
Why conduct auxiliary power transformer calculations?
Not all loads in the station rely on DC power.

The HVAC system, transformer fans, lights, cabinet heaters, lift-stations/sump pumps, battery charger, etc require AC power.
Outcome of auxiliary power transformer calculations
Specify auxiliary transformer capable of supplying the demand.

Results are incorporated in the following drawings:
– Station auxiliary AC power oneline

Ground grid study

Ground grid study - substation design calculations
Figure 8: Ground potential rises when a lightning strike or a short circuit current is injected into earth. This is depicted by the red peaks. Image courtesy CDEGS. The goal for the study is to shave the peaks i.e. create equipotential surface even when an impulse is injected.
Substation ground grid - substation design calculations
Figure 9: A ground mesh designed to mitigate the ground potential rise. Image credit: Triden on electricialtalk.
Why conduct ground grid study?
A lightning strike on tall power transmission structures is inevitable. When this surge is buried into the earth, it needs a path to dissipate. If this path is unavailable (due to high resistivity soil, for instance) the ground potential rises at the point of contact.

This is a hazardous situation. Anyone walking in this area is subjected to an electric shock because of the potential difference developed between feet (with reference to Figure 8, one foot on red peak and the other on blue valley). This is called step potential. The same concept applies to touch potential.
Outcome of ground grid study
Install mesh grounding system as shown in Figure 9 to create an equipotential surface. Drive ground rods into the earth (10′ or 20′ or 40′ as determined by the study) and tie the mesh to the rod – allowing the mesh to access low resistivity soil.

Because earthing is impacted by soil resistivity, in certain cases, the native soil needs to be replaced to get the desired results.

Results are incorporated in the following drawings:
– Ground grid plan drawing
– Ground grid installation details drawing

Lightning protection calculations

lightning study - rolling sphere method - substation design calculations
Figure 10: Rolling sphere method shown. Area below the sphere is protected. Thus the larger the sphere the more area covered. The sphere is rolled over lightning masts and shield wires only. Substation image credit: WAPA. Image marked-up for illustration purpose only; it does not represent actual lightning protection condition.
lightning study - rolling sphere method - substation design calculations
Figure 11: Result from the rolling sphere study. Equipment left unprotected shown as well. Image credit: Joe Young.
Lightning study - fixed angle method - substation design calculations
Figure 12: Another method of determining lightning protection is the fixed angle method. A mast tall enough to protect critical equipment is specified. The area inside the cone is protected. This study is ideal for small substations only. Image credit: Biren Patel.
Why conduct lightning protection calculations?
Substations need a shield to protect itself from lightning strikes.
Outcome of lightning protection calculations
Install a combination of lightning masts and shield wires that provides adequate lightning strike coverage.

It should be noted, creating 100% coverage is impossible. Therefore a probability study is conducted to determine the likelihood of a lightning strike on the unprotected equipment. If the risk is acceptable then the coverage is reduced or not installed at all.

Results are incorporated in the following drawings:
– General arrangement plan drawing
– General arrangement elevation or section view drawing

Lighting calculations

Lighting study - substation design calculations
Figure 13: Substation lighting determined by lighting study.
foot-candles due to lighting - substation design calculations
Figure 14: Lighting study shown. The numbers indicate foot-candles; typically 3 feet from the ground. The requirement varies from utility to utility. The goal is to create a bright, well-lit area near major equipment. Image source.
Why conduct lighting calculations?
Substation security and safety of personnel is important. A well lit area serves this purpose.
Outcome of lighting calculations
The height and angle of LED head that provides the required foot-candles of light intensity are calculated.

Results are incorporated in the following drawings:
– Substation lighting plan drawing

Voltage drop calculations

voltage drop calculations - substation design calculations
Figure 15: Voltage drop study. Substations are vast. A piece of wire connecting a battery to motor load can span hundred’s of feet. Because the copper or aluminum wire has resistance, some power is lost as heat resulting in voltage drop along the wire. Due to this drop, the voltage at the receiving end may not be adequate to start the motor. Substation image credit: WAPA.
motor operator voltage drop calculations - substation design calculations
Figure 16: Although the DC system is designed for minimal voltage drop, various equipment are designed to operate under worse conditions. For instance, the 125Volts DC motor operator in this image can work with voltage as low as 90Volts DC. That is 28% voltage drop.
Why conduct voltage drop calculations?
Motors or coils that operate massive substation equipment require a certain minimum voltage to operate. Failure to do so renders it inoperable.
Outcome of voltage drop calculations
Determine wire size (1/0AWG or #2 gauge or #6 gauge etc.) such that voltage developed at the receiving end is within equipment working limits.

Results are incorporated in the following drawings:
– Cable schedules
– Wiring drawings

Conduit fill calculations

Conduit fill study - substation design calculations
Figure 17: Conduits for pulling electrical cables. Conduit fill study determines the quantity of cables that can be pulled through each. Image credit: MTA
Why conduct conduit fill calculations
This is fairly straightforward. Pulling more wires than what is possible will break below grade PVC conduits especially at the bends.
Outcome of conduit fill calculations
Specify the quantity of wires that can be pulled.

Install a combination of handholes or manholes or cable troughs to make cable pulling easy.

Results are incorporated in the following drawings:
– Conduit plan drawing
– Conduit installation details drawing

Civil / Structural – Substation Design Calculations

Land survey and site grading analysis

Land topography - substation design calculations
Figure 18: Drawing created based on a land survey. The lines on the contour indicate areas of the same altitude. Picture credit: Landmarksurveyors
Power Substation Design Calculations - A Checklist of 18 Studies for Engineers to Consider 1
Figure 19: Substations where site grading played a significant role. Left image credit FE Penn power hoytdale substation. Right image credit: FE Lorain county substation.
Why conduct land survey and site grading analysis?
How much money is spent on building a new substation is largely influenced by the location. Cutting and filling the earth to create a flat property (in a mountainous area), creating the right slope to channel water away from substation, stormwater management, etc is an expensive affair.
Outcome of land survey and site grading analysis
Determine if the land is feasible for construction.

Create a grading plan. Design drainage facilities like culverts, ditches, detention ponds, lift-stations etc, to handle stormwater.

Create compensatory storage when building in a floodplain.

Results are incorporated in the following drawings:
– Site grading plan drawing
– Site grading detail drawing
– Property plan drawing

Geotechnical investigation and foundation calculations

Soil boring - PEguru.com
Figure 20: Soil sample collected for analysis. Portions of sample have been scooped out on the right. Picture credit: Nauman khan.
Power Substation Design Calculations - A Checklist of 18 Studies for Engineers to Consider 2
Figure 21: Transmission tower being lowered onto anchor-bolts. The design of this foundation factors-in soil condition (among others). Picture credit WAPA ED5 substation..
Why conduct geotechnical investigation and foundation calculations?
Soil conditions play a significant role in the design of foundations.
Outcome of geotechnical investigation and foundation calculations
Geotechnical investigation reveals information on soil conditions, underground obstructions or hazards, seismic conditions, topography, etc. Learn more here.

Based on this information:
– Remediate contamination or other hazards in the soil.
– Design foundations to mount buildings, steel structures, and substation equipment

Results are incorporated in the following drawings:
– Foundation plan drawing
– Foundation details drawing

Structural steel calculations

Power Substation Design Calculations - A Checklist of 18 Studies for Engineers to Consider 3
Figure 22: Steel structure analysis in RISA 3D software. Picture credit: Nauman Khan.
Why conduct structural steel reaction calculations?
Structural steel is subjected to horizontal loads (line tensions, wind load, etc) and vertical loads (dead weight of equipment mounted on it, ice load, etc). Any inadequacy in design may lead to structural failure.
Outcome of structural steel reaction calculations
Specify steel structure capable of handling the horizontal and vertical loads. The legs of tall structures are also susceptible to aeolian vibration. Install adequate cross-bracing or vibration dampers to prevent this condition.

Results are incorporated in the following drawings:
– Steel framing plan drawing
– Steel framing section view drawing

Bus calculations

Animated GIF - Find & Share on GIPHY
Figure 23: Video of what 40,000amps of fault current for 300 milliseconds does to a piece of substation bus.
Why conduct bus calculations?
As seen in the video, a piece of substation bus is subjected to extreme forces generated by fault current. A setup without adequate bus support and under-rated bus support insulators – will collapse.

The study also analyses the effects of wind loading, snow loading, bus material, etc.
Outcome of bus calculations
The goal would be to insert enough bus supports to stabilize the bus (considering all the forces acting on it). Also, specify extra high strength insulators where necessary.

Results are incorporated in the following drawings:
– General arrangement plan drawing
– General arrangement section view drawing

Miscellaneous – Substation Design Calculations

Transformer noise calculations

transformer noise - substation design calculations
Figure 24: The transformer core is made of laminated sheets of metal (to minimize eddy current loss). A downside to this is vibration induced by 60hertz alternating current. Noise is not only generated in core. The coil, mounting, housing, conduits, fans, pumps etc. all generate noise.
Why conduct transformer noise calculations
Power transformers generate audible noise, typically greater than 60db. This can be over-bearing.

If the substation is located in a city, the constant hum from a new transformer may be unacceptable by local ordinances (especially at night time.)
Outcome of transformer noise calculations
Specify standard sound transformer if the noise generated is acceptable. Otherwise, mitigate the noise using either sound walls or by specifying low-sound transformer.

Harmonic analysis

Power Substation Design Calculations - A Checklist of 18 Studies for Engineers to Consider 4
Figure 25: Substations feeding facilities containing VFDs or UPS systems and substations containing SVC’s or HVDC converter stations, require harmonic analysis. Picture credit: ABB drives US.
Why conduct harmonic analysis?
Non-linear loads (characterized by changing impedance as it draws power) injects harmonics into supply voltage and current. The resulting waveform is not smooth but one marred by spikes and dips. When these propagate into the high voltage system, they damage power transformers and affect the power quality of other customers.
Outcome of harmonic analysis
In substations where harmonics are present, specify equipment to filter them out. The methods and means of doing this are mentioned here.

Fire protection study

Power Substation Design Calculations - A Checklist of 18 Studies for Engineers to Consider 5
Figure 26: Firewall around oil filled transformer. Image credit: FortisBC Hollywood substation.
Animated GIF - Find & Share on GIPHY
Figure 27: Water deluge system for the transformer.
Why conduct fire protection study?
Short circuit at any location inside the substation generates an incredible amount of heat, as high as 40,000F – enough to cause the oil to catch fire, vaporize copper or aluminum material and burn any other material caught in its path.
Outcome of fire protection study
Install firewalls. Fireproof the control building, not just sidewalls but penetrations in the floor as well.

Passive fire protection system (like smoke detectors) are typically installed however, in urban areas, some form of fire suppression system is also installed. These active systems could be deluge system (figure 27) or clean agent suppression system.

That’s it, you have read every single study that’s conducted when building a new substation. Any thoughts? Drop them below.

Author credit: Sanity checks on the civil / structural section provided by Nauman Khan – Structural Engineer, PE.

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Matt

Arc flash analysis is missing.

Keith

Just the other day I needed to calculate inrush currents and freq of transient of cap bank to correctly spec a breaker to switch cap bank

ISMAIL

excellent work

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