Utility feeds

Utility feeds represent the external power source from the electric utility company. This is where grid power enters your facility's electrical system.

What utility feeds represent

The utility feed models the infinite bus - an idealized power source with:

• Constant voltage magnitude
• Unlimited short-circuit capacity
• Zero impedance (ideally)

In reality, utilities have finite short-circuit capacity and source impedance. Utility feed parameters model these real-world characteristics for accurate power flow and fault analysis.

Creating a utility feed

1. Drag Utility from the component toolbar.
2. Drop on canvas (typically at top of diagram).
3. Connect bottom handle to service entrance equipment or main bus.
4. Configure source voltage and short-circuit capacity.

Utility feeds typically appear at the top of the diagram following industry SLD conventions (power flows from top to bottom).

Source voltage configuration

Specify the utility delivery voltage:

Distribution voltages (common in North America):

Transmission and subtransmission:

Voltage must match utility service agreement and incoming transformer configuration.

If the voltage is not in the list, choose Custom and type it in kV in the box below. The same choice is on bus, transformer and generator voltages. A value above 1,200 kV is refused, which catches a voltage typed in volts.

Operating voltage

Power flow analysis holds the utility's connection at its Operating Voltage, a percent of the nominal voltage. Set it on the utility's Study tab; the default is 100 and the allowed range is 80 to 120. Enter the voltage the utility actually delivers, for example 104 for a feeder that runs 4% high. Every bus the utility feeds moves with it.

A scenario can change the operating voltage, so you can compare a high-voltage and a normal case of the same design. Short-circuit and arc flash studies do not use it; they keep their own pre-fault voltage basis.

Short-circuit MVA input

Short-circuit capacity (also called fault level) determines maximum available fault current at the utility connection point:

Typical utility short-circuit MVA:

Get actual short-circuit MVA from utility company. This information may be on your utility service agreement or available by request.

Calculating fault current from MVA:

Fault Current (kA) = SC MVA / (√3 × Voltage kV)

Example: 500 MVA at 13.8kV

I_fault = 500 / (1.732 × 13.8) = 20.9 kA

X/R ratio

The X/R ratio (reactance-to-resistance ratio) of utility source impedance:

Typical X/R ratios:

X/R ratio affects:

• Power factor of fault current
• DC offset during asymmetric fault
• Circuit breaker duties

Higher X/R (more inductive) = longer time to zero-crossing = harder to interrupt.

Use 15 as default if unknown. Get actual value from utility for critical applications.

Input methods

On the utility's Study tab, choose how the utility's fault level is entered:

Switching methods converts the values you entered to the new method, for every fault case. Studies read only the selected method's values.

Fault cases

Utilities often give more than one fault case, such as a strong case for equipment duty and a weak case for arc flash, where slower protection can make the lower current worse. You can enter up to four cases on one utility.

A new utility has one case, and the Study tab shows its values as above. To add another:

1. Select the utility and open the Study tab.
2. Click Add case. A new column appears, filled with a copy of the first case's values and named "Case 2".
3. Type a name for each case in the Case row, for example Max and Min. Names must differ on one utility.
4. Enter each case's values in its column.
5. In the Studies row, choose the studies each case is included in: Short circuit, Arc flash, or both.

Changes save as you type. All cases use the utility's input method; Base MVA and the single-phase fault current are shared by every case.

How the studies use the cases:

• Short circuit evaluates every case included in it and reports, at each location, the highest fault current.
• Arc flash evaluates every case included in it and reports, at each bus, the highest incident energy. If any included case gives no result at a bus, for example because its current is outside the IEEE 1584 range or below the protective device's curve, that bus has no result and names the case.
• Load flow does not use the fault level.

Each study needs at least one case of each utility. If you remove every case from a study, that study does not run and names the utility to change. The Studies row stays visible on a one-case utility whose case is not in both studies, so you can see why.

To delete a case, open the menu beside its name and choose Delete case. A case that holds values asks you to confirm, because the deletion cannot be undone. Deleting the first case makes the next case the first.

> Note: A study evaluates every combination of the cases each utility includes in it, up to 16 calculations. A project with one utility uses one calculation per case.

A scenario can change a utility's cases. The added cases are overridden together: changing one value of an added case stores the scenario's copy of all of them, and the reset button beside them returns all of them to the base design.

A scenario can also switch the input method. Every case is converted, as in the base design, and the base design keeps its own method and values. The Reset input method to base value button beside the method tabs returns the method and every case to the base design.

Slack bus relationship

The utility feed connects to the slack bus in power flow analysis:

• Slack bus balances system real and reactive power
• Provides voltage angle reference (0 degrees)
• Typically the bus immediately downstream of utility feed

Designate the bus connected to your utility feed as the slack bus. See Buses for slack bus configuration.)

Grid contribution to faults

During short-circuit analysis, the utility feed contributes fault current:

Symmetrical fault current:

I_fault = SC MVA / (√3 × V_kV)

Asymmetrical (peak) fault current:

I_peak = I_fault × √2 × √(1 + 2e^(-2π/X/R))

Higher short-circuit MVA = higher fault current = higher interrupting capacity required for downstream breakers.

Service entrance configuration

Typical utility service entrance components:

1. Utility feed (modeled component)
2. Service entrance cable - From utility to main disconnect
3. Main service disconnect - First overcurrent device
4. Service entrance equipment - Main switchboard or panelboard
5. Main bus - Distribution point for building feeders

Connect utility feed to service entrance components in this order.

Multiple utility feeds

Some facilities have redundant utility services:

Dual utility feeds:

• Two separate utility connections
• Automatic or manual transfer switch
• High reliability for critical facilities
• Model as two separate utility feed components

Single utility feed with generator backup:

• Utility feed as normal source
• Generator as emergency source
• Automatic transfer switch between sources
• Model utility feed and generator separately

Metering point

Utility revenue metering typically occurs at or near the utility feed connection:

• Service entrance location for <600V
• Primary side of service transformer for medium voltage
• Billing demand calculated from metering data

The utility feed component represents the point after the meter (on the customer side).

Related topics

• Buses - Connecting utility feed to main bus (slack bus
• Short-circuit analysis - Utility contribution to fault currents
• Power flow analysis - Slack bus and power balance