In this part of the tutorial, we shall be able to produce a Normal Inverse time-current curve. Given the values below, we will be plotting the values on an XY Scatter graph with data points connected without markers.
I (A) | t (s) |
---|---|
600 | 2.75 |
800 | 1.60 |
1200 | 1.01 |
1600 | 0.80 |
1600 | 0.80 |
1600 | 0.14 |
4800 | 0.14 |
4800 | 0.10 |
Try plotting the values and I presume that you will be getting a graph similar to Figure 1.
Figure 1
As electrical engineers, doing protective devices coordination is always a part of our tasks. You may say that I could always use ETAP or EDSA or SKM Power Tools to do the job.
If your company has the money to buy all these software, then you are right. The most important thing however is, do you know how to use the sophisticated power system analysis software.
At the end of this tutorial, the electrical engineer will be able to create coordination graphs for any particular application. This tutorial also aims to provide tips during the process.
The requirements of this tutorial are the following
This is a sample of coordination curve we shall be doing on the next part of the tutorial.
The Common specifications for high-voltage switchgear and controlgear standards ( IEC 60694:2002 ) tables 1 and 2 provide the rated insulation levels of switchgears and controlgears. In these tables, the withstand voltage applies at the standardized reference atmosphere (temperature, pressure and humidity) specified in IEC 60071-1.
The rated withstand voltage values for lightning impulse voltage (Up), switching impulse voltage (Us) (when applicable), and power-frequency voltage (Ud) shall be selected without crossing the horizontal marked lines. The rated insulation level is specified by the rated lightning impulse withstand voltage phase to earth.
In the initial part of this tutorial, we were able to calculate the size of the power factor correction capacitor bank required to improve the power factor of a plant from 0.75 lagging to 0.95 lagging. In this part of the tutorial, we shall be able to calculate the savings generated from the improved power factor.
Depending on the power utility company, the following values may vary.
Example : Utility Company A has the following charges.
Average Cost /khw : $0.09 (Other costs such as demand charges are already integrated in this value)
Power Factor Charges:
Below 0.80 lagging : +5.00 % x Total Energy Consumption
Above 0.90 lagging : -3.75 % x Total Energy Consumption
Note: This is the first part of the PF Correction tutorial series.
In industrial plants, induction motors comprises majority of the load. Without power factor correction, the nominal industrial plant overall power factor ranges from about 0.70 to 0.80 lagging.
A low power factor means that the I2R losses is high. Some electrical utility companies also penalize plant with low power factors as this leads to a high system loss. On the hand, plant with high power factor are given incentive discounts on the billing which will make any investment on a power factor correction equipment viable.
Type test (design test) is defined as the series tests which are made upon the completion of the development of a new equipment design to establish representative performance and to demonstrate compliance with the relevant standard. Once made, these tests need not be repeated unless the design is changed so as to modify its performance. In such a case only the relevant tests need be repeated.Type testing is normally done by a testing authority independent from the equipment manufacturer. The test authority will be issuing Type test certification for the particular equipment.Switchgears and controlgears could be classified as type-test assembly (TTA) or partially type test assembly (PTTA).
Selecting the right equipment to perform the right task during design is a tedious job. A very good example is the variable speed drive (VSD) versus Soft Starter(SS).
On low capacity drives (<25kW), there is no significant cost difference between the two equipment. In high capacity drive application (rating above 1MW) and moving into the medium voltage (6.6kV - 11kV), there will be significant difference in the total cost of ownership between the two equipment.
Lightning is a high-voltage discharge (usually negative) within clouds (intra-), to each other (inter-), or to the earth. The cloud-to-ground (CG) flash is the one we are usually concerned with lightning protection. The charged cells in clouds normally attract charges of opposite polarity on the earth's surface or on high objects located directly below them. When the charge reaches a critical level causing the insulation between cloud and earth breaks down, it develops a stepped ionized path, frequently to the earth, resulting in a high current discharge (stroke) that neutralizes, for the moment, these cloud and earth charges. The discharge current increases from zero to a maximum of 1 us to 10 us, and then declines to half the peak value of 20 us to 1000 us. Lightning flashes usually consist of a sequence of individual return strokes that transfer significant electrical charge usually from the cloud to earth.
The best things in life are free. This is what I felt when I am doing this exercise, comparing results from InterPSS with ETAP.
To verify on how an open source software match up to an expensive commercial software like ETAP, I tried comparing the results side by side. I used ETAP 5.5.6 and InterPSS 1.4.04 in my comparison.
In any project, doing a simple short circuit verification and load flow analysis will save the electrical engineer headaches during testing, commissioning and system reliability testing of the project.
I know some electrical engineers who don't even bother doing a manual calculation because they assume that what they have experienced in their past projects will be the same as their current. This is totally wrong. Every project, despite having similarities have different electrical parameters. Making assumptions and not verifying it mathematically is totally unacceptable.
Short circuit calculation plays a very important role in determining the ratings of electrical equipment. Using software has the advantages, however, to ensure that the software results are correct, hand calculation could be done as a check. Depending on the complexity of the system, hand calculation may be very tedious and impractical.
To demonstrate a simple hand calculation, see the following tutorial at download section.
As the final article on voltage drop and cable selection series, I am now providing a complete worksheet for cable selection based on the IEC standards 60228 and 60364-5-54. I created the worksheet in such a way that it could be printed with your company name, logo, your project, your name, cable number, load description and many more.
Electrical design requires selecting the cable with the correct temperature rating. Designers tend to select the cable with the highest temperature rating, this is not acceptable as the first cost of the project will be affected considerably.
A cable temperature rating is based on the type of material used for its insulation. In this article, we shall be discussing how to correctly select the cable temperature rating for certain installation requirements based on the environmental condition.