Arc flash results from the rapid release of energy due to an arcing fault between phases, either phase-to-neutral or a phase-to-ground. Ionized air creates electrically conductive superheated plasma that can exceed temperatures of 5,000°F. It often results in serious personnel injury, equipment damage and production losses.
The arc flash phenomenon has been present since electricity was discovered. According to data from the Mine Safety and Health Administration (MSHA), there were 381 cases of non-contact electrical burn injuries between 1996-2005. Most people would wrongly assume that this only affects electricians, but it also affects mechanics, prep plants workers, etc., really anyone present is in danger during an arc flash event.
During an arc flash event, an explosion occurs and as current begins to pass through ionized air large volumes of ionized gases are expelled. Metal from the vaporized conductors are expelled. In less the 0.2 seconds, miners will encounter blinding light, intense heat, thermo‐acoustic effects, molten metal, toxic gas and shrapnel. They may also inadvertently come in contact with energized components. Anybody in this environment at this time would be severely injured.
The causes of arc flash include: accidental contact with energized parts (dropping tools), inadequate short circuit protection, tracking across insulation surfaces, insulation failure, wiring errors, contamination (coal dust on electrical surfaces), corrosion and improper work procedures.
The working section in an underground coal mine is constantly changing, which means the electrical supply (cables and load centers) is subjected to increased movement compared with static counterparts on the surface. This increased movement results in increased wear on trailing cables, connectors and other components.
The design of electrical protection systems has to take several items into consideration, such as physical working conditions (water and mineral contamination). What about the existing system capacities/design? Does it have available fault current, short circuit protection, higher system impedance and insulated bus bars? As far as containment and protection, certain personal protective equipment (PPE) is required and arc-resistant enclosures are available that channel arc pressure and gas to safe areas.
Arc flash hazards are broken out into five risk categories. Each category has clothing requirements for personal protection, which is determined by the calories per square centimeter.
Arc resistant enclosures have been designed to direct the arc safely away from areas where workers would be present. Some designs place a series of low-voltage circuit breaker panels on the end. Each panel has a section where the breaker is located. If an event occurs, arc pressure builds and opens a flap and the arc flash energy enters an arc chute. The arc chute forms a mechanical maze lengthening the flame path causing it to go through flame arresting vents. Also the volume of each compartment is decreased by one-third, further mitigating the arc pressure.
The mining industry presents a number of special problems and unique requirements when it comes to electrical protection. In addition to the previously mentioned mobility issues, methane can be present, so flameproof and intrinsically safe equipment is generally used.
Existing protection consists of individual screened/shielded cores in the trailing cable; sensitive earth leakage (ground fault) intrinsically safe electrical lockouts (a breakdown in insulation or failure prevents the motor from starting); and short circuit/overload protection. Overload protection has developed quite well over the years. Many relays have a way to model the thermal characteristics of the motor very well with many settings and adjustments. These, however, tend to be disabled during motor start-up leaving systems unprotected.
Short circuit protection has limitations as well. A high trip threshold is required to avoid operation during motor starts. It is typically eight to 10 times full load motor-running current. Short circuits take time to develop.
Many existing systems are not suitable for long cable runs, where cable impedance may limit fault current and short circuit operation. In this case the current is not high enough or a depression in voltage does not activate the short circuit protection.
Arc flash protection systems currently in use today use a combination of optical light sensors attached to cable joints, bus bars and other electro mechanical connections. Optical systems are used to detect serious explosive releases of energy, those typically seen during short circuits and electrical arcing. The sensors are trying to detect a flash of light from the arc. That coupled with a current monitoring device to try to pick the arc up during failure. This is a damage limitation exercise because the arc has already happened. Simply put, can it be switched off before major damage is sustained.
The previously mentioned containment devices can be difficult to install in smaller cubicles and in high density low voltage panels. They are also difficult to retrofit to existing equipment. These difficulties are exacerbated where flameproof equipment working in a gaseous atmosphere is involved.
The conventional means of arc protection is typically very expensive to install and does not lend itself well to retrofit applications of existing switchgear and power centers. There is, however, an alternative approach.
Alternative Approach: Phase Angle Measurement
It was realized that improvements to short circuit would be very difficult to achieve by reducing the detection current. After all, the motors have to start. Sensitivity could be improved during starting, but this required the introduction of a time delay. Of course, this is the last thing required for short circuit protection. It is, however, possible to use phase angle as a method of separating the motor running current from the fault current. Looking at a typical motor load vs phase angle shows this is practical.
The y-axis on the graph represents the motor full load current. The x-axis is the power factor or phase angle. On the right, a typical DOL starting characteristic curve can be seen. The dotted line on the top of the graph is the standard short circuit protection. It is set at eight times the full load current. The existing short circuit protection needs to get to that level before it will activate.
By using the power factor, the fault current can be separated and a system can be developed with very sensitive tripping characteristics. Under normal operating conditions, the trip threshold would never be breached because the power factor would remain constant or remain within the region indicated on the graph. Arcs and restive series arcing would show itself as a restive fault, improving the power factor, which would push across the trip threshold. That’s what the system identifies: a change in power factor as well as a significant change in current.
This graph also shows the trip characteristic as a straight line. That doesn’t necessarily have to be the case. It can be changed. In a situation with multiple motors connected to one power system, the characteristics can be changed to suit. It might create a slight shift in the motor characteristics depending on the number of motors running at once. The system would have a slight desensitization to take multiple motors into account, but the same principle would apply.
If we take a look at the motor load conditions and the percentage of fault current needed to activate the trip threshold, in all of the cases it is equal or less than the motor full load current. This has resulted in a very important benefit and valuable new protection. A trip will be initiated if the fault arc resistance are now in series, that is a single break in a single core. Resistance arcing, maybe a loose connection or a single break in a single core, can now be detected, which is a big advantage. This technique can detect very low levels of arcing within the system before it escalates into a catastrophic failure.
For the protection to be realized, three voltage reference signals are required, one for each phase. Each phase also has three current signals (both phase and magnitude) derived from the motor CTs. Using a very high-powered digital signal processor, the detection system monitors each of the voltage and current waveforms independently. At each zero crossover point the power angle is measured, and compared against the three phase current magnitude.
From the tripping characteristic it can be seen that the power angle shifts due to the increase of the resistive current as a result of the applied fault. This measurement technique provides the ability to detect and discriminate between either short circuit or series arc based faults.
The device detects fault levels at less than full load and ignores the motor inrush current during its starting period.
Using an algorithm, the system discriminates between the two types of faults. It looks at the difference between the current and voltage on the phase angle diagram. It checks whether it’s inside or outside of the known operating characteristics. Has the current increased in any of the three phases? If all three phases have increased, it’s likely to be a short circuit to ground. If two phases have increased and one has decreased, it may likely be a loose connection, a series arc type fault.
It should be noted that opening and closing the contactors will cause an arc as well. That has to be taken into account. That can be done in a number of ways: it can be limited in the software or it can be linked into the start/stop controls on the existing system.
The algorithm has several benefits:
- Early detection of loose or deteriorating connections in cable junctions and conductors
- Improved short circuit protection allowing longer cable runs from the supplying power center—no longer inhibited by the impedance of the cable;
- Short circuit protection sensitivity down to 0.25xFull Load Current;
- Protection active even during motor starts; and
- Retrofittable to existing switchgear with a few modifications and minimal disruption.
In the future, the system may be applied to variable speed drives. They present a few more difficulties due to the wave shape and how the sinusoidal wave is built. The panacea would be to have this method as bus bar protection, where one device looks after all of the equipment. Theoretically, it may be possible, but not at the present.
This article was adapted from a presentation made by John Simpson and Kurt Carlson at Longwall USA 2011 (www.longwallusa.com), which took place in Pittsburgh during June 2011. Simpson is the development manager, P&B Protection Relays for PBSI Group Ltd. Carlson is the power distribution manager for SMC Electrical Products.