IEC 60255 Curves
Equation:
Where,
TMS denoted time multiplier setting
Is
denoted the current setting.
I
denoted the actual current.
k
and α denoted the curve type constants.
See table below.
|
Curve type |
k |
α |
|
Standard
inverse |
0.140 |
0.020 |
|
Very inverse |
13.5 |
1 |
|
Extremely
inverse |
80 |
2 |
|
Long time
standard inverse |
120 |
1 |
The objective of protective device coordination study
is to effect a time-current coordination among the devices that achieve the
expected system protection and electrical service continuity goals.
As presented on
the attached, these graphs not only plot the time-current curves (TCC) of
Relays, Circuit Breakers and Fuses on the log-log paper but also damage curve
of equipment and current landmarks. In addition, there is an associated one
line diagram located at right side of graph to show the settings and ratings.
According to
these curves and landmarks on the graphs, it can tell their coordination situations
among protective devices easily. Any dis-coordination includes cross or
overlaps even disorder of curves shall be fixed and repaired to get properly
interval. The results and remarks of PD coordination refer to the attached.
The motor starting curves on the time-current curve graph
is also modeled in the SKM.
Besides short circuit study, the protective devices
coordination also play a vital role to enable Arc Flash Study could be
continuous.
In USA, the National Occupational Safety and Health Administration (OSHA) requires that in non-residential locations, employers should conduct arc hazard assessment and analysis, confirm the calculation results of arc flashover hazard distance and incident energy, and select appropriate personal protective equipment for staff. Print the evaluation result as a label and stick it on the body of the electrical switch.
Once accident happened, if failing to comply with the legal requirements, the employer will be required to pay high penalty for compensation.
In case of severe event, the employer may be required to stop all working case for investigation until finish all necessary processes. The loss will be quite heavy. In general, all employers are willing to accept and comply with the specified regulations.
In the contrast, in Taiwan, domestic regulations are less and few specified in this respect, only like stipulated by the Ministry of Labor states that "employers shall have responsibilities to supply safety and health equipment and method to prevent harm caused by electricity.
Obviously, Taiwan needs to improve arc flash risk environment to prevent electrical personnel from dangerous.
Use softwares, saving tedious calculation time and avoid manual calculation errors may occur, reliability is greatly improved, therefore, the use of analysis software has generally been accepted by engineer, is a trend.
Power system
analysis software include CYME, EasyPower, EDSA, ETAP, PSS / E and SKM etc., if
you want to learn how to operate one by one that need to take a lot of time. Up
to now, I actually used two of them frequently, ETAP and SKM. PSS/E is not often.
The main
difference between them is that ETAP focus on the output of the report, SKM is
good at express on one line diagram.
In Taiwan,
Taipower requested renewable energy company use PSS/E at Grid Impact Study in
UHV cases. PSS/E often running in Power companies globally.
In brief, power system in Taiwan is almost equal to the Taipower system, main three (3) portions include power generation systems, transmission systems and distribution systems.
There are several voltage categories in Taiwan, from high to low, from transmission system to distribution system, 345kV, 161kV, 69kV, 22.8kV, 11.4kV and low voltages 110V and 220V etc. respectively.
In Taiwan recently, Renewable Energy includes Solar and Wind power is very hot, several big scale projects are under construction. They supplied very positive contribution.
The reference power system diagram is excerpted from the Taipower Company website as below.
