Part 3: Electrical Engineering Queries Asked by Our Valuable Students
Q1.What is a Simo code and what is its function?
SIMOCODE is the flexible, modular motor management system for low-voltage motors. It can easily and directly be connected to automation systems via PROFBUS or PROFINET. It combines in just one compact system all required protection, monitoring, safety, and control functions. The motor management system thus increases plant availability and allows significant savings to be made for installation, commissioning, operation, and maintenance of a system.
Q2. What are some effects of open phase on the primary side of the transformer?
Well, the answer to this question has many implications. To understand, the open phase condition in transformer is also called “single phasing”. When it occurs the currents and the voltages on the primary and the secondary side of the transformer highly depends on the following:
- Transformer Winding (Delta, Wye , Delta-Grounded etc.)
- Transformer Core construction
- Transformer Loading
Usually, the detection of the open-phase or single-phase condition is difficult to detect. In fact, the ground overcurrent or negative-sequence over current protection may not even detect the open phase condition due to the symmetry and balance of voltages and currents. To do that a detailed analysis of the open phase condition is done, and certain methodologies are implemented to identify and understand the effect of single phasing in a transformer.
Q3. How to calculate Arc Current and Incident Energy below 600V?
To provide safety to the people in the work environment Arc Flash studies are done. Appropriate procedures and personal protective equipment are then employed to minimize potential injuries.
Calculating the Arc Flash currents, and Incident Energy Levels have a step-by-step procedure. The following link will give you an in-depth look into the manual Arc Flash calculation according to the IEEE (Institute of Electrical and Electronics Engineers) Std. 1584 2002:
https://myelectrical.com/notes/entryid/130/arc-flash-calculations-arc-flash-safety
Q.4 What do you mean turn on maintenance switch?
A Maintenance Switch is a toggle switch that is connected to the upstream protective device (relay or trip units) having the “Maintenance Mode” option. The maintenance mode makes use of a different trip setting that reduces the interruption time as compared to the standard instantaneous protection. When the switch is manually operated, the maintenance switch turns the upstream breaker function to “Maintenance Mode”. Under this setting, the protective device can improve the operating safety conditions by reducing the fault clearing time thus lowering the incident energy levels.
In summary, the maintenance mode or switch changes the settings, so it is sensitive to the change in current. Typically, the protective device (relay or breaker) operates on the lowest set instantaneous setting. If a fault were to occur, the protective device would operate fast without worrying about coordination. The speed of device operation reduces the incident energy levels significantly… and it enhanced safety… When personnel is working on a gear, the maintenance mode/switch would be “turned on” until the work is finished.
Q5. When we consider arc flash is a hazard…. What level of hazard concerning arc flash to different voltage rating of power source? which is most fatal to deal with arc flash form D.C. or A.C.?
To answer the first part of your question there are several factors and parameters considered when we can call Arc flash Hazard. For example, arc flash and incident energy levels produced by it differ according to the equipment, voltage at the bus it is type and equipment type. Similarly, when we talk about the power sources, we are referring to the voltage levels and current which are first use in calculating the Short-Circuit-Current and this value of the current is used to calculate the Arc Flash Hazards. For more details you can refer to the article on
IEEE Xplore: “IEEE Guide for the Specification of Scope and Deliverable Requirements for an Arc-Flash Hazard Calculation Study in Accordance with IEEE Std 1584(TM)”
Secondly, although our understanding of the Arc Flash has expanded in the recent years but most of the work is done on AC arc flash. Moreover, 2002 IEEE 1584 standard includes only AC arc flash hazards and no standards for the calculation of DC Arc flash. However, NFPA (National Fire Protection Association) 70E Arc Flash standards 2012 and 2015 extracted significant information from the paper: “Arc Flash Calculations for Exposures to DC Systems” and “DC-Arc Models and Incident-Energy Calculations”. It conclusively mentioned that the maximum possible power in a DC arc flash occurs when the arcing voltage is 50 percent of the system voltage. Also, since we must know the Arc Flash Energy levels which matters most, whether they occur into the AC or DC power system, but their method of calculation is surely different.
Q6. How are relay settings calculated in real System like radial distribution system?
Radial distribution and other distribution schemes are not usually considered in calculating the relay settings. Once the system distribution scheme is set up and relays are placed, relay settings are then configured according to the voltages, fault currents, loads, sources, and equipment protection. These settings have certain features which ensures desired selectivity, sensitivity, speed, and safety of the system.
Q7. What do you know about protection relays, what are the main roles in a circuit or protection system?
Relays, in general, are switches which can make or break the circuit to serve the crucial purpose of protecting power systems. They can be electronically or electromechanically operated. Advancement in relays have enable them to perform several tasks which include:
- sending trip signals to circuit breakers when required (e.g., in case of fault current)
- acting as the brains of overcurrent, differential, earth fault, distance various other protection schemes
- acts as an interface between high power loads and control circuitry
- control and operate switches and other electromechanical devices remotely at large distances
- programmable/ numerical relays incorporate microcontrollers which can be programmed to achieve the desired objective and level of protection
Q8. Is single derived from its three-phase system directly or per phase model of that three-phase system? Which one is right?
Single Line Diagram is per phase representation of a three-phase system. It is a simplified notation of a three-phase system, that is why it is also known as One-Line Diagram.
Q9. What is pre fault voltage in distance relay?
Pre-fault voltage is the voltage of the system (particularly at the location of fault) when normal load current is flowing through it. The system voltage will be at the pre-fault level until any fault current starts to flow through it. The term ‘pre-fault voltage’ has no meaning in the context of distance relays. The general concept applies everywhere in the power systems.
Q10. Scenario: 6x 11kV 6MVA generators each feeding a 11/33kV 6MVA transformers. Then paralleled up on a common bus. Question: can we lump generator and transformer values together in the form of MVA to simplify the circuit. i.e., a single generator and single transformer?
Yes, we can lump the generators and transformers for circuit representation, but it also depends on the type of circuit or say its application.
