Electrical – Substation Design Calculations

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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 equipmentall 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.
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 certainminimum 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