Smart Electrical enables you to calculate bus load totals on a specific power distribution board, bus, or on all the buses in the plant. This calculation facilitates the sizing of electrical equipment such as generators, distribution transformers and their feeder cables, buses, UPSs and so forth. After completing the calculation, Smart Electrical stores the results in the database. You can calculate the bus load totals for all the existing buses in the plant or just for those PDBs or buses that you select in the Electrical Index or the Electrical Engineer. After the software completes the calculation, you can generate a load summary report for the calculated loads based on the calculated and stored properties.
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The software takes into account any existing equipment that is connected in parallel.
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When making calculations, Smart Electrical considers the worst-case scenario in its calculation results and does not take into account any load flow that would be taken into account by a power analysis application such as ETAP. That is, for calculation purposes, Smart Electrical always assumes that the total connected load is supplied from one source only. If there are multiple sources that supply the same connected load, Smart Electrical assumes that each source has the capability to supply the connected load. Smart Electrical, unlike any power analysis application, does not account for multiple sources that share a load. Consequently, in Smart Electrical, the total load for a distribution network can appear artificially high unless the Circuit Mode property is set to either Connected or Disconnected as required on specific circuits.
Calculation Process: Drill-Down and Roll-Up
The calculation process consists of two parts, drill-down and roll-up.
Drill-Down
The calculation starts from the highest level of a particular power flow that you select in the Electrical Engineer. Then the calculation process continues down the power flow all the way to the electrical consumers at the lowest level of the Electrical Engineer hierarchy. The software performs this process automatically. You can tweak the calculation process by switching the specific circuits on or off as you require. Also, you can define converting equipment PDBs as consumers. In these cases, the drill-down process does not continue beyond the pertinent converting equipment or PDBs. Also, in the Properties window, you can change the Is Load property of an electrical item to False. Note that the software does not include in the drill-down process any items below the circuits that have been switched off. Nor does it include any converting equipment items that have not been defined as consumers.
Roll-Up
During this calculation process, the software collects the information from the lowest level of a particular power flow that you select in the Electrical Engineer and goes up the flow through the wiring, converting equipment, and feeder circuits all the way to the buses connected by couplers or risers and up to their incomer circuits. Then, the software gathers the values from the buses on the lower level of the electrical hierarchy and adds them to the values of the buses on a higher level all the way to the top of the selected electrical hierarchy. Note that the software stores the incomer totals in the calculation properties of the incomer circuits in the Properties window of the incomers.
Electrical Load Data
When performing a load summary calculation, the software uses the values of the following load properties:
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Rated active (kW at rated/nominal/100% load conditions) — Prated
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Rated reactive (kVAR at rated/nominal/100% load conditions) — Qrated
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Consumed active (kW at operating point/consumed load conditions) — Pconsumed
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Consumed reactive (kVAR at operating point /consumed load conditions) — Qconsumed
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Operating mode (Continuous, Intermittent, Standby or Spare)
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Coincidence factors for each and every mode of operation (x,y,z,zz)
Motor Power Calculation Mechanism
For most types of load, Smart Electrical calculates the electrical reactive and apparent power values as a vector sum based on the mechanical power and the power factor (cos φ). This is the common electrical formula derived from the power triangle shown below. The software displays the calculated values in the Active power, Reactive power, and Apparent power fields. You can enter values in the Rated power, Break power, Demand factor, and Power factor fields only.
The software performs active, reactive, and apparent load calculations twice — for rated and consumed (brake) power. For motors (dynamic loads), the power factor and efficiency values that are used for consumed power calculation are derived from a three-point graph which is interpolated from the given power factor and efficiency values at 100%, 75% and 50%.
Example:
Let's assume that a three-phase motor working as an intermittent load has the following values:
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Rated power = 100 kW (the nameplate mechanical power that is derived by a particular motor)
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Brake power (also called shaft power) = 80 kW (80% of the rated power)
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Y = 0.5
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Power factor at full load = 0.85
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Efficiency at full load = 0.9
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Power factor at break point = 0.826 (at 80 % of the rated active power)
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Efficiency at break point = 0.884 (at 80 % of the rated active power). These calculated values are the result of calculating the power factor at 75% = 0.82 and efficiency at 75% = 0.88.
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For motors, these values are interpolated from three given points. For static loads the values are provided only once.
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Active power (electrical):
P = P mechanical /h (h — efficiency at working point))
Prated = 100 kW/0.9 = 111.1kW
Pconsumed = 80 kW/0.884 = 90.5kW
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Reactive power:
Q = P * tan (arcos (cos j))
Qrated = 111.1 * tan (arcos 0.85) = 68.9 kVAR
Qconsumed = 90.5 * tan (arcos 0.826) = 61.8 kVAR
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Apparent power:
S = √(P2 + Q2)
Srated Ö(111.12 + 68.92) = 130.7 kVA
Sconsumed = √(90.52 + 61.82) = 109.6 kVA
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Full load current (FLC) for a three-phase load (with 380 VAC line voltage):
FLC = Srated / (U * √3)
FLC = 131000 / (380 * √3) = 198.6 A
Static Load Power Calculation Mechanism
For static loads, the power calculation is rather simple. No interpolation is performed by the software to find the power factor and efficiency values at work point. The software calculates the rated power kW, kVAR, and kVA values using efficiency and power factor values once at 100% of a full load and once at demand (working) work point. However, you have to enter the power factor and efficiency values yourself. The rest of the calculation is the same as for motors.
Special Cases (Junction Boxes and Cabinets)
For some item types, such as junction boxes and other panels, the software does not perform the triangular power calculation even if the Is Load property has been set to True. For these items, you need to enter the active and reactive power values manually. Smart Electrical does not calculate apparent power for these items.
Smart Electrical does not calculate compensated values for loads. These calculations are performed
for circuits and buses.
Compensated and Uncompensated Electrical Load Values
As opposed to individual loads, for which only uncompensated values are calculated and retained, for circuit and buses, the software operates with two sets of load data values:
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Uncompensated values — rated and consumed electrical load kW, kVAR, and kVA values without applying coincidence factors. The software rolls up these values from the loads connected to specific circuits or buses.
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Compensated values — rated and consumed electrical load kW, kVAR, and kVA values multiplied by coincidence factors. It is possible to instruct the software to use one of the following coincidence factors as shown on the Reports tab of the Preferences dialog:
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Use each load coincidence factor — coincidence factor values of individual loads
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Use Bus PDB coincidence factor — bus coincidence factor values
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Calculate according to largest non-continuous load — Instructs the software to calculate the total normal running and peak loading according to the largest non-continuous load connected to the current bus including all the rolled-up loads that are connected to the downstream buses. For details, see Calculate Bus Loading According to the Largest Non-Continuous Load.
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Example
Let's assume that an intermittent load has the following values:
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Rated active power = 11.1 kW
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Rated reactive power (calculated or entered manually) = 6.9 kVAR
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Consumed active power = 8.5 kW
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Consumed reactive power = 5.9 kVAR
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Operating mode = intermittent
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Y = 0.5 (intermittent coincidence factor)
The uncompensated values would be as follows:
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Uncompensated rated active power = 11.1 kW
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Uncompensated rated reactive power = 6.9 kVAR
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Uncompensated consumed active power = 8.5 kW
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Uncompensated consumed reactive power = 5.9 kVAR
The compensated power values would be as follows (uncompensated values multiplied by the coincidence factor):
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Compensated rated active power = 5.55 kW
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Compensated rated reactive power = 3.45 kVAR
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Compensated consumed active power = 4.25 kW
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Compensated consumed reactive power = 2.95 kVAR
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Bus and circuit totals depend on the settings of your preferences. Therefore, run the bus load calculation to update the total values in the bus, circuit, and load properties.
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Electrical items are included in calculations if they are defined as consumers. In addition to loads, you can define converting equipment items, cabinets, local panels, and junction boxes as consumers. To define an electrical item as a consumer, you have to set the Is Load property in the Properties window for that item to True or define the electrical item as a Consumer in the relevant Common Properties dialog.
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If you define a converting equipment item, local panel, or junction box as a consumer and perform a drill-down calculation, the software uses the rated and consumed electrical load values of the equipment as the load values, the software ignores the entire branch that is connected downstream from the equipment. Note that you enter the pertinent rated and consumed electrical load values on the Common Properties dialog of the equipment item.
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It is also possible to instruct the software not to perform a drill-down and roll-up of a bus. You can do this by setting the Drill Down Enable Flag property to False in the Properties window. In this case, the software will take into account only the manually entered consumed / rated active or reactive values of that particular bus.
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Loads associated with PDBs are taken into account whether or not they are connected via circuits.
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Circuit connection mode (Connected or Disconnected) affects the calculation results. Disconnecting a circuit excludes the downstream connected load from the calculation.
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The circuit properties store the total value of the load that is connected to it as follows:
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The properties of feeder circuits that feed direct loads store the load data.
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The properties of feeder circuits that feed downstream buses whether directly or through converting equipment store the downstream bus load total values. Practically, these values will be the same as for the buses that are fed by these circuits.
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The properties of coupler circuits store the total values of the bus is connected to the corresponding riser provided that your preferences have been set do so. If your preferences are set to exclude the couplers, the calculation results for the coupler circuits will be null.
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The properties of riser circuits store the total values of the bus that is connected to the feeding coupler. If your preferences are set to exclude risers, the calculation results for riser circuits will be null.
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If your preferences are set to take into account converting equipment losses, the circuit total calculation results will include the kVAR and kW losses as continuous values.
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It is also possible to calculate a bus load summary for a freestanding bus.
For more information, see the following topics: