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What is the purpose of the Equipment Evaluation Study module?

Background

- To compare distribution equipment ratings to the results of the Demand Load Study module.
- To compare distribution equipment ratings to the results of the Load Flow Study module.
- To compare distribution equipment ratings to the results of the Short Circuit Study module. As long as available fault duty X/R ratios are at or below rated equipment X/R ratios the Short Circuit Study module results will match Equipment Evaluation Study module results.
- To convert fault duties to the equipment X/R ratio base if the available fault duty X/R ratio is greater than the equipment rated X/R ratio. In this case the Short Circuit Study module results will not match the Equipment Evaluation Study module results.

What is the purpose of a Demand Load Study?
- To calculate the connected, demand and design loads throughout the distribution system.
- The results can be used to determine or validate feeder and transformer ratings.

Are electrical engineers required to perform a demand load study?
- No, however it is good engineering practice.

What is the purpose of a Load Flow Study?
- To calculate bus voltage levels to compare to equipment ratings.
- To calculate branch current flows to compare to equipment ratings.

Are electrical engineers required to perform a load flow study?
- Yes, per NEC Articles 210.19 for Branch Circuits and 215.2 for Feeders. Both articles specify voltage drop limits.
- NRC requires load flow studies to verify system performance.

What is the purpose of a Short Circuit Study?
- To calculate maximum available symmetrical fault duties to compare to LV equipment short circuit ratings, and MV/HV equipment interrupting ratings.
- To calculate maximum available peak fault duties to compare to LV equipment unpublished peak ratings, and MV/HV equipment closing and latching ratings.

Are electrical engineers required to perform a short circuit study?
- Yes, per NEC Article 110.9: “Equipment intended to interrupt current at faults levels shall have an interrupting rating sufficient for the nominal circuit voltage and current that is available at the line terminals of the equipment.”

What are good references for engineers performing load flow and short circuit studies?
- A Practical Guide to Short-Circuit Calculations, Conrad St. Pierre, to order go to www.epc-website.com.
- IEEE Std. 141-1993, IEEE Recommended Practice for Electric Power Distribution for Industrial Plants (IEEE Red Book).
- IEEE Std. 399-1997, IEEE Recommended Practice for Industrial and Commercial Power Systems Analysis (IEEE Brown Book).

What do I mean be by manually compare?
In the old days I’ve had to pull the data from the output reports or data blocks in order to develop summary tables in my report. That was a time consuming task, prone to errors! The Equipment Evaluation Module automates this task. If we understand this module we can greatly improve our efficiency and analysis skills.
If this little exercise does not convince you to use the Equipment Evaluation module, nothing will!

Example 1
Create a new project in PTW consisting of the existing distribution system shown in figure 1.




Select the LV Switchboard ratings required for the application.

- Voltage – Standard LV switchboards are designed to be installed on distribution systems rated up to 600V. However, this is not the rated voltage of the switchboard. The rating is set at the nominal voltage of the distribution system it is installed on.

V = 480V

Select the LV Switchboard ratings required for the application.

- Voltage – Standard LV switchboards are designed to be installed on distribution systems rated up to 600V. However, this is not the rated voltage of the switchboard. The rating is set at the nominal voltage of the distribution system it is installed on.

V = 480V

Current – The continuous current rating of the switchboard is selected by the design engineer. There is no standard sizing practice used throughout the industry. In this case we will base our switchboard ampacity rating on the maximum current rating of the source transformer.

I = 2300kVA / (√3 0.48kV) = 2766 A

Ratings in this range include 2500 and 3000 amps. Select 3000A.

Short Circuit Rating – The short circuit rating must be greater than the available fault duty. The available fault duty at 480V consists of contribution from the utility through the transformer

SC utility = MVA transformer / ( MVA transformer / MVA utility + Z transformer ) SC utility = 2 MVA / ( 2 MVA / 500 MVA + 0.0575 ) = 32.52 MVA

and the contribution from the motors.

Z new = Z old (V old / V new) 2 = 0.17 (460 V / 480 V) 2 = 0.156 Ω p.u.
MVA motor = 0.746 HP / ( pf * h * 1000 )
MVA motor = 0.746 1000 HP / ( 0.85 * 0.92 * 1000 ) = 0.954 MVA
SC motor = MVA motor / Z new = 0.954 MVA / 0.156 Ω p.u. = 6.12 MVA

The total short circuit capacity available is

SC total = SC utility + SC utility = 32.52 + 6.12 = 38.64 MVA
SC total = 38.64 MVA / (√3 * 0.48 kV ) = 46.48 kA

Ratings in this range include 42 kA and 50 kA. Select 50 kA.

Since this is a new project and we do not know the manufacture select a UL rated LV Switchboard.



Next run an Equipment Evaluation



Then click on the Run Study button, check the appropriate boxes, and click OK.



After the calculations are completed close the Study Messages window, click on the non-protective devices button and view the Calc kA field.



Distribution Equipment X/R ratios

The ANSI, IEEE and UL standards provide manufacturers with basic design and performance requirements. One aspect is the ability to withstand and interrupt fault current. Table 1 lists applicable standards and short circuit test parameters for selected types of electrical distribution equipment.

Table 1 – Distribution Equipment Short Circuit Test Data