Eltec International

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Load Flow Studies

Load flow calculations provide active and reactive power flows and bus voltage magnitude and their phase angle at all the buses for a specified power system and operating condition.

Load Flow Analysis helps to ensure that cables, transformers, transmission lines are sized appropriately to carry required load. To make sure that the transformer taps are set appropriately to obtain satisfactory voltage profile within the area of study.

Required when

 There is a significant plant expansion

 New local generation is or is proposed to be added

 New utility feed has been installed

 New large motors have been added to the system

 New transformers have been installed

 Addition of significant loads

Short-Circuit Studies

Short-circuit studies help to determine fault levels at various buses. These studies are very useful in determining post fault bus voltages in the entire system, post fault network currents in the entire system as well as negative and zero sequence currents in various electrical network elements. These calculations provide the bulk of the information needed for protection system design, protection setting calculations and relay coordination. The studies are to be carried out for various operating scenarios of the network with existing earthing system and the performance of the system shall be reviewed and remedies were suggested.

Short circuit analysis is done ensure that the existing and newly installed breakers will not be overstressed or fail to protect under short circuit conditions.

Required if:

  • There is a significant plant expansion
  • New local generation has been added
  • New utility feed has been installed
  • New large motors have been added to the system
  • New larger transformers have been installed
  • Transformers have been replaced with lower impedance units

The study is made assuming a three phase fault condition, since this is the type of fault which will produce the maximum current. Maximum current occurs because there is less impedance between the source of power and the point of the fault than there is in any other type of fault (i.e., phase to phase, 2 phases to ground, 1 phase to ground). Impedance between the power source and the point of fault is the deterrent to current flow; the more impedance, the less current flow and vice versa.

Current-limiting or impedance sources in a power system are transformers, reactors, cables, switches, breakers, and connections. Current sources are generators and, at the time of faults, capacitors and motors. In the Short Circuit Study, the points at which faults are assumed are located on the source side of the device which will be called upon to interrupt the power system when an actual fault occurs. This is done to determine whether or not the device will be able to interrupt the current available at the time of the fault.

Many times the source side of the interrupting device is an electrical bus in a piece of switchgear. When current flows through the bus bars, forces are exerted between the different phases of the bus. When a fault occurs, the current magnitude is greater than normal current flow, and the forces between the bus bars are increased by the square of the current increase. Thus if the fault current is four times normal, the forces between the phases are sixteen times as great.

The Short Circuit Study gives us the information necessary to determine if our breakers and fuses are capable of interrupting a fault, as well as the means of determining whether or not the bus sections of the switchgear are supported adequately to withstand the forces generated from the fault currents.

In addition, this type of study tells us the maximum current available to operate an electrical protective device (i.e., Molded Case Circuit Breaker, Low Voltage Power Circuit Breaker, Relays, Fuses, etc.).

Protection & Coordination Studies

Protection & Coordination Studies involve preparing coordination time-current characteristic curves to determine the required settings/sizes of the protective devices to maximize selectivity. It is necessary to achieve proper fault identification and fault clearance sequence. The relays must be able to distinguish between the normal operating currents including short time over currents that may appear due to certain equipment normal operation (example: Motor starting currents, Transformer inrush currents) and sustained overcurrent due to fault conditions. During fault conditions, these relays were made to operate quickly, isolating the faulted section of the network and allowing for continued operation of the healthy circuits. In the event of failure of primary relays meant for isolating the fault within its primary zone of protection, backup relays were tested to operate after providing sufficient time discrimination for the operation of primary relays. Hence, the operation of backup relays be coordinated with those of the operation of the primary relays. The flexible settings of the relays (namely plug or tap setting, the time dial setting and possibly selection of suitable time-current operating characteristics), will be set to achieve the desired objectives.

Protection Device Coordination is done to ensure that the breakers operate in a desired sequence. Breaker closest to the fault should be the one isolating the fault. Other upstream breakers should operate only if the closest breaker fails to open.

Required if:

  • There is a significant plant expansion
  • New local generation has been added

Protective device coordination means that downstream devices (breakers/fuses) should activate before upstream devices. This minimizes the portion of the system effected by a fault or other disturbance. At the substation level, feeder breakers should trip before the main. Likewise, downstream panel breakers should trip before the substation feeder supplying the panel. 

If you've experienced nuisance tripping, or have adjustable trip breakers/relays without documentation supporting the existing settings, a coordination study is a great investment. If your facility uses adjustable trip circuit breakers and/or relays, do you know what the existing device settings are, and why? In far too many cases, no-one knows what the settings are, where they came from, or whether they make any sense at all.

Don't wait for an outage to find out there's a problem

This study examines the size and settings of protective devices in system. National Electric Code requirements and ANSI/IEEE standards are considered to determine if existing devices are adequately applied. Recommended sizes and/or settings are provided in a written report aimed towards improving system protection and coordination. 

The Protection / Coordination Study provides the following benefits:

* Helps reduce unnecessary downtime. 
* Provides recommended settings for adjustable trip circuit breakers and relays. 
* Helps increase coordination (selectivity) between devices. 
* Identifies deficiencies in system protection. 
* Provides recommended solutions to help correct problem areas. 
* Reviews and discusses the use of system devices with respect to National Electric Code requirements, and appropriate ANSI/IEEE standards.

Harmonic Studies

The wide and ever increasing applications of power electronic devices and other electronic and digital controllers, such as variable speed drives, uninterruptible power supplies (UPS), static power converters, rectifiers, Static Var Compensator (SVC), etc., power system voltage and current quality has been severely affected in some areas. In these areas components other than that of fundamental frequency can be found to exist in the distorted voltage and current waveforms. These components usually are the integer multipliers of the fundamental frequency, called harmonics. In addition to electronic devices, some other nonlinear loads, or devices including saturated transformers, arc furnaces, fluorescent lights, and cyclo-converters are also responsible for the deterioration in power system quality.

Using computer simulation, the phenomena of power system harmonics can be modelled and analysed. The Software Module provides you with the best tool to accurately model various power system components and devices to include their frequency dependency, nonlinearity, and other characteristics under the presence of harmonic sources. This module employs two analytical methods, Harmonic Load Flow Method and Harmonic Frequency Scan Method. Both methods are the most popularand powerful approaches for power system harmonic analysis. By using those two methods in combination, different harmonic indices are computed and compared with the industrial standard limitations; existing and potential power quality problems, along with security problems associated with harmonics can be easily revealed. Causes to those problems can be identified and different mitigation and corrective schemes can be tested and finally verified.

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