A power distribution system comprises power source such as generator(s) and/or power feed(s) from the utility and a distribution system which typically include equipment such as Transformer(s), Switchgear(s), Motor Control Center(s), Distribution Panel(s), and loads such as motors, heaters, etc. All these equipment's are connected using cables. However, bus ducts or cable buses are typically used to connect transformer secondary's to switchgears.
The purpose of a power system study is to ensure that the electrical power distribution system e operates in a safe and reliable manner under both normal and fault conditions. Power system studies are typically conducted using specialized software tools. The commonly used ones are ETAP (Electrical Transient Analyzer Program) and SKM.
The following studies are most commonly carried out:
1. Load Flow
Load flow studies help determine voltages at various points, active and reactive powers, power factor correction, and optimize transformer tap settings.
2. Short Circuit
Short circuit studies help determine the current that will flow through the power distribution system in the event of a short circuit at a certain point. Short circuits can be caused by equipment malfunction or accidental damage.
Short circuit studies provide the necessary details to help size equipment in order to ensure that can carry short circuit current in the event of a fault.
3. Motor Starting
Motors draw a high current during starting and this is called as the inrush current. This is typically around 6 times the rated full load current. The high starting current can lead to excessive voltage drop in upstream buses and may cause tripping of circuit breakers; malfunction of other equipment due to low voltage.
Motor starting studies help determine impact of starting large motors on the power system. They can help ensure that transformers are adequately sized, taps are optimally selected, voltage drop during starting is within adequate limits, motor protection relays are set appropriately.
Motor starting studies are of two types static motor starting and dynamic motor starting.
Static motor starting studies are most commonly performed, and they take into consideration the locked rotor impedance during starting (acceleration) and this is the worst-case scenario. Dynamic motor starting on the other hand, is performed when details such as load -torque characteristics, load details are available. Dynamic motor starting studies are more complex given the number of parameters that needs to be considered and accuracy is dependent on the quality of input data.
4. Protective Relay CoordinationProtective relays help in isolating parts of the power distribution system in the event of an abnormal condition such as a short circuit. The intent is to minimize the impact caused by the abnormal condition. Protective relays sense abnormal conditions and can be set to alarm (at the control room or elsewhere) or trip circuit breakers to prevent power flow.
Protective relay coordination study relates to setting of protective relays such that they operate in a pre-determined sequence based on fault conditions. They are set such that they isolate the faulty area and minimize disruption to rest of the system.
5. Arc flash
About Arc Flash
Arc Flash is the unintended flow of electric current through air from one conductor to another or from one conductor to ground. Arc flash releases extremely high amounts of energy in the form of intense heat (above 35,000oF), light and sound (around 140 dB or more), creating arc blast wave (pressure upwards of 2,000 lbs/sq.ft), involving flying objects (molten metal and projectiles from the electrical equipment), fumes ( vaporized metal) and fire.
Arc Flash hazard is defined as a source of possible injury or damage to health associated with the release of energy caused by an electric arc. [NFPA 70E Art 100]
Injury caused by an arc flash is a function of the proximity of the workers to the source of hazard, the fault current and the duration of time for which the arc persists.
Typically arc flash incidents are caused by faulty installation, poorly maintained disconnect switches and circuit breakers, presence of dust, debris and foreign objects, corrosion, insulation failure, loose connections, condensation, carelessness such as dropping tools or tools left behind after installation/maintenance.
Arc flash studies are based on IEEE 1584 and NFPA 70E.
IEEE 1584 provides a mathematical model to determine the arc-flash hazard distance and the incident energy to which workers could be exposed during their work on or near energized electrical equipment.
NFPA 70E establishes safety processes that use policies, procedures, and program controls to reduce the risk associated with the use of electricity to an acceptable level. [NFPA 70E Fact Sheet]
How to protect against arc flash
Protection against arc flash can be achieved by one or more of the following methods:
Safety in Design
Power system / arc flash studies using industry standard software such as ETAP, SKM or equivalent.
Design power distribution networks to reduce the available fault current.
Use of protective relays to reduce the duration for which the fault persists.
Use of arc resistant switchgears and motor control centers.
Incorporating remote rack-in /rack-out, closing/opening capabilities.
Personal Protective Equipment (PPE)
Personal Protective Equipment (PPE) shall be selected in accordance with Article 130, Table 130.7(C)(15)(c) of NFPA 70E.
Administrative Controls
Use of warning signs and labels
Training and work practices
Pre-Job Briefing
Job Hazard Analysis
Lock out /Tag Out Procedures (LOTO)
Best Practices
Ensure that power system studies are performed by qualified personnel and the same is reviewed and updated when changes such as addition and deletion of loads are made to the power distribution system.
Perform periodic audits to ensure that arc flash labels affixed on equipment are current, and readable
Follow good maintenance practices
Pre-job briefings and Job hazard analysis
Follow a rigorous lock out tag out policy
It is recommended to review and update arc flash studies once in 5 years
As explained, arc flash is a dangerous phenomenon which can cause significant damage to people and property. The same can be mitigated by performing system studies and adopting safe work practices.
6. Grounding System Design
Purpose of a grounding system is to provide a low impedance path to ground. This helps prevent dangerous over voltages (potential difference) that can be fatal to people and can damage equipment.
Grounding grid is designed taking into consideration soil conditions, and fault levels. IEEE 80 and IEEE 665 are most commonly used standards followed for grounding system design. Commercially available software such as ETAP and SKM base their calculations on these standards.
The software helps in optimizing number of grounding grid conductors, grounding rods, and their locations such that the step and touch potentials are within acceptable limits.
Fauske and associates have the expertise, experience, and capabilities necessary to perform power system studies and audits. Please contact Venky Viswanathan, Principal Electrical Engineer at venky.viswanathan@westinghouse.com, for further details.
