When and how to mitigate arc flash hazards in medium-voltage electrical systems
Mitigating arc flash hazards in medium-voltage systems could be performed in synergy to achieve optimal system reliability and safety
- Understand how a typical protective relay upgrade process aligns with arc flash hazard mitigation
- Learn how hierarchy of risk control from NFPA 70E: Standard for Electrical Safety in the Workplace could be incorporated in mitigating arc flash hazards.
- Follow the various steps in upgrading relay process and review engineering software simulation and case studies.
Medium-voltage electrical system insights
- Arc flash hazard mitigation goals can be achieved, without compromising power system reliability, by harnessing the advancement in protective relaying technology.
- Understanding when and how to upgrade protective relays is key to ensuring electrical equipment does not fail or cause a problem.
Protective relays are the brain and intelligence behind a medium-voltage protective device. They are critical infrastructure for proper medium-voltage system operation and protection and increase system reliability.
With electromechanical and solid-state protective relays being obsolete and nearing their end of life, upgrading them with their microprocessor counterparts is paramount for reliability of power systems. Microprocessor relays provide advanced communication, monitoring and automation capabilities along with all basic protection and control platforms.
The advancement in technology in the power industry, coupled with increase in awareness about arc flash hazards, has brought a wave of arc flash hazard mitigation processes and procedures. If the protective relay upgrade project is planned, the arc flash hazard mitigation could become an amazing complementary result along with the relay upgrade. The key to this process is: adequate planning and selecting appropriate relays to achieve both goals.
Protective relays are an integral part of the medium-voltage power system because they ensure protection for equipment during fault and abnormal operating conditions. If these protective devices are installed, engineered, programmed and maintained properly, they keep the power system safe and reliable.
It is critical to upgrade them to the current technology and standards. Many electromechanical and solid-state relays are becoming obsolete, which means upgrading them is inevitable — either through unplanned failures or during planned outages. As older protective relays near their end life, they cannot be relied on for the equipment protection they provide. Instead of providing system protection and reliability, these relays now become a safety hazard and liability.
One of the critical factors affecting arc flash hazard is the protective device fault clearing time. The faster the fault is cleared, the lesser the incident energy resulting in lower arc flash hazards. One of the best methods of mitigating arc flash hazard is reducing the fault clearing time and this is where the advanced microprocessor relays come into the picture.
Most advanced relays have capabilities of detecting the fault faster and interrupting them a lot quicker than their electromechanical counterparts. Another feature that microprocessor relays provide is their advanced communication and automation abilities. This means the overcurrent device could be operated from a safe distance, which increases the working distance and reduces the arc flash hazard.
A well-planned relay upgrade project could easily be complemented with the arc flash hazard mitigation one. Upgrading protective relays and mitigating arc flash hazard have a common goal of maintaining power system reliability and promoting worker and equipment safety. With meticulous planning, well-thought equipment (protective relay) selection along with smart engineering practices can help achieve both processes and their common goal (see Figure 1).
Typical protective relay upgrade process aligning with arc flash hazard mitigation
Project planning and raising user awareness: The initial and the most important step in upgrading relays has been planning the entire project with keeping arc flash mitigation as one of the goals or end result. This includes understanding customer’s expectations and goals and providing consultation and coaching. Many customers are not aware that upgrading relays could aid them in mitigating arc flash hazards and in some cases may not be aware of the arc flash hazards at all. It is vital to educate the user about potential arc flash hazards and how upgrading the relays could help mitigating them.
Gathering data and procuring protective relays: The next step in this process is collecting accurate data such as existing device settings, drawings, short circuit, coordination and arc flash studies, operating sequence and other original equipment manufacturer documents. Having well-established and updated documentation helps in maintaining existing equipment operation procedure and provides room to improve functionality on existing system.
Procuring relays follows the data gathering process. The key here is being judicial while selecting the proposed relay. For instance, selecting an overcurrent relay with capabilities of having arc flash sensor would be an excellent choice for both upgrading the relay and mitigating the arc flash hazard. This selection of relay aligns with the common goal of this paper and could be a cost-effective solution for arc flash hazard mitigation. Procuring relays as soon as project scope has been outlined helps in reducing the original equipment manufacturer lead times as well.
Performing engineering studies, creating relay setting and updating existing drawings: Performing accurate and up-to-date engineering studies consisting of at least short circuit, coordination and arc flash analysis, with minimal assumptions, is another critical step of the relay upgrade process. It is not recommended to convert the existing settings into the upgraded relays. Electromechanical and solid-state relays have very limited functionality and capabilities, whereas microprocessor relays have elaborate functions.
For instance, an overcurrent (ANSI device 50/51) electromechanical relay only has time overcurrent and instantaneous overcurrent settings, whereas a feeder protection microprocessor relay has both of these functions with multiple group settings, directional element, reverse power and so on.
Updating existing drawings to replace existing equipment is good and highly recommended practice while upgrading relays. This process helps in the field wiring of new relays and provides customers with updated protection schemes and documents that could be used for troubleshooting in future. Updating existing drawings also provides design engineers with an opportunity of incorporating remote operation of overcurrent devices. Remote close and trip of circuit breakers could easily be incorporated in the upgraded system. This is one of the vital components of engineering controls for arc flash hazard mitigation.
Off-site wiring, uploading settings and testing relays: It is recommended wiring relays and their associated test switches to the relay doors or insert panels off-site. Another good practice is to upload relay settings, test relay elements and logic off-site. This reduces the outage duration in the field and helps accomplish milestones in the upgrade process.
Reducing outages is one of the important factors in the relay upgrade process. Not being able to schedule an outage is one of the hindering factors on relay upgrade projects. Relays are a huge part of the power system automation and protection. Thus, any facility can only afford to have minimal downtime involving them.
Figure 2: These images are the before (left) and after pictures of the medium-voltage switchgear after relay upgrade work has been performed. Courtesy: National Field Services
Medium-voltage systems will be without any protection and automation features if relays are safely taken out of the service. This could compromise the reliability and safety of the power system. Many relay upgrade projects are postponed or suspended because of the lack of switchgear shutdown or inability of any facility to schedule the outage.
On-site demo, install, test and commission: The final and most important step in relay upgrade project is bringing everything together in the form of a field installation. It is always recommended documenting all existing wires and maintaining integrity and aesthetic property of wires in compartment while wiring.
Another important final step is commissioning the entire system. This includes at minimum verifying polarities, current transformer (CT) ratios, phase rotation and direct current trip checks to list a few. The goal is to improve the system reliability and safety without any nuisance power interruptions. Those calculations should be redone and documented at this time. It is imperative to have detailed plans and procedures for existing system uninstallation, new relay installation, testing and commissioning, contingency plans for unexpected surprises during this entire upgrade process (see Figure 2).
Arc flash hazard mitigation methods
Arc flash hazards can be calculated by calculating the incident energy. This can be accomplished by using any commercial software. The main components of incident energy calculations are the amount of available fault current, the overcurrent protective device fault clearing time and working distance. This article dives into the second two components: fault clearing time and working distance. The methods and goals of relay upgrade aligns with NFPA 70E: Standard for Electrical Safety in the Workplace hierarchy of controls for hazard mitigation. Some of the arc flash mitigation engineering methods are listed below:
Eliminating the hazard is at the top of the hierarchy. This is the most effective method of arc flash hazard mitigation because this method eliminates the hazard. The following engineering methods are useful in eliminating the arc flash hazard from the medium-voltage systems. These methods are based on reducing the fault clearing time.
Method 1: Using reduced instantaneous setting with alternate group/maintenance mode setting
This is one of the simplest and most effective methods of reducing incident energy and arc flash hazards. Most microprocessor relays have provision for multiple group settings. The groups could be transferred over automatically using relay programming logic or manually using the selector switches that could be mounted anywhere in the switchgear.
For instance, Group 1 would have the normal system protection settings. Short circuit, coordination and arc flash study needs to be performed to design the reduced instantaneous settings. This reduced instantaneous value would then be programmed into Group 2 settings.
The reason for creating separate groups is to maintain the system reliability and functionality. The user would have to switch settings into Group 2 before performing any switching or interacting with the medium-voltage equipment. Once this electrical work has been performed, it is imperative to return the maintenance switch back to its original position or change the relay logic back to enabling Group 1 protective device settings.
Figure 3 shows the time current curve (TCC) created using SKM Systems Analysis Inc. along with the calculated fault current on how this reduced instantaneous setting could be calculated. If this reduced instantaneous setting values is set below the fault current line (dotted line), the reduced instantaneous setting could be calculated.
Method 2: Using concept virtual main
This method involves installing additional hardware and relays in the existing system. However, this is an effective method in reducing arc flash hazards in existing systems. Most power systems in the past were designed without a physical main overcurrent protective device such as a circuit breaker or fused disconnect. Thus, whenever there is a medium-voltage switchgear powering up a medium- to low-voltage transformer, the incident energy is high on the secondary side of the transformers. Without an actual main interrupting device, there are not a lot of options of reducing the incident energy.
This concept of virtual main fills the void on the switchgear that is not designed to have an actual main circuit breaker. In this method, CTs are installed on the secondary side of the transformers and/or the switchgear. Two sets are typically included in this scheme. Then, instantaneous overcurrent devices located upstream of this transformer are designed to operate if there were any fault current events on this downstream switchgear.
Substitution in medium-voltage systems
Ideally, eliminating the hazard is the best way to reduce arc flash hazards. However, this might not be feasible in all practical applications. If the hazard could not be eliminated, then the next method on hierarchy of risk control is substitution. This method involves designing engineering processes to substitute the hazard.
The following two methods are innovative on substituting arc flash hazards in the medium-voltage system. These methods are based on increasing the working distance.
Method 1: Designing relay logic for remote switchgear operations
This method is simple to implement and effective in most power systems equipped with automation. Most modern medium-voltage systems are equipped with remote operation. The design engineer could simply incorporate remote bits in their circuit breaker operation logic to achieve this. Because the protective relays would open or close the circuit breaker, the operator does not have to be present physically in front of the switchgear during the switching process.
This method increases the working distance from a few inches to several feet, thus aids in keeping system operators out of the harm’s way. The only caveat for this method is that the switchgear must have automated switching capabilities, which could be an issue for older switchgear.
Method 2: Engineering controls
This method is simple and effective in both older and newer switchgear systems. The switchgear does not have to be automated, unlike the previous substitution method. This method could be attained by programming logic in the relay for circuit breaker open and close operation and performing minimal changes in wiring during the upgrade process.
Typically, the microprocessor relays have push-button in their front panel. Incorporating a small delay of 10 or 20 seconds provides ample time for an operator to push a button and safely move out of the switchgear room. For the relays that do not have the push buttons, the control switch that operates the circuit breaker could be programmed in similar fashion. This process is cost effective and could be achieved with simple engineering design modifications.
Medium-voltage electrical system safety
Protective relays are the backbone of the protection, automation and control for medium-voltage systems. They are critical to power system performance and reliability; thus, utmost importance should be given to the maintenance and upgrade of these relays.
Arc flash hazard mitigation is a critical part of electrical safety. This helps to protect lives and maintain the efficiency and reliability of the power system. All the arc flash reduction methods in this paper could be carried out with the relay upgrade process as a comprehensive project. Using engineering brainpower and harnessing the advancement in technology, we can keep our electrical system safe and reliable.
Case study: Reducing incident energy
SKM Systems Analysis Inc. software was used to prepare and analyze this typical engineering project that incorporates several methods of reducing arc flash hazards. The protective relay was programmed with the reduced instantaneous settings, which reduced the incident energy on medium-voltage switchgear bus from 22.8 cal/cm2 to 1.14 cal/cm2. This protective relay pushbuttons were programmed with the delay of 20 seconds for additional arc flash hazard mitigation.
The concept of virtual main was implemented in order to reduce the incident energy on low-voltage switchgear bus. Current transformers (CTs) were installed on the secondary of the 2,000 kilovolt amperes (kVA) transformer. These CTs were connected to the existing protective relay. Short circuit, coordination and arc flash engineering study was performed on this new system. Instantaneous settings were designed for this virtual main and programmed into the protective relay.
The incident energy on the low-voltage switchgear bus was reduced to 6.85 cal/cm2 from 137 cal/cm2. This is a significant decrease in incident energy.