Protective relays are the brain and intelligence behind a medium-voltage distribution system (Figure 1). They serve as critical infrastructure for proper medium-voltage system operation, protection, and system reliability. Upgrading electromechanical to microprocessor relays and mitigating arc flash hazards in a medium-voltage system can be performed in synergy to achieve optimal system reliability and safety.
KEEPING UP WITH TECHNOLOGY
With electromechanical and solid-state protective relays becoming obsolete and nearing their end of life, upgrading with microprocessor counterparts is paramount for power system reliability. Microprocessor relays provide advanced communication, monitoring, and automation capabilities, along with all basic protection and control platforms. This advancement in technology, coupled with increased awareness about arc flash hazards, has brought a wave of new arc flash hazard mitigation processes and procedures. If a protective relay upgrade project is well-planned, arc flash hazard mitigation can yield a complementary result. The keys to this process are adequate planning and selecting the appropriate relays to achieve both goals.
This article provides a snapshot of these concurrent topics and shows how the communication and automation capabilities of microprocessor relays can be leveraged to mitigate arc flash hazards. It dives into various methods and procedures for arc flash mitigation including maintenance mode settings, programming a breaker open/close delay, remotely opening and closing the device, installing a virtual main, and expanding the differential zone of protection supplemented by SKM software simulation results and case studies from field projects. Arc flash hazard mitigation goals can be achieved without compromising power system reliability by harnessing the advancements in protective relaying technology.
Protective relays are an integral part of a medium-voltage power system because they ensure protection for equipment during fault conditions. Engineering, programming, installing, and maintaining these devices can keep our power systems safe and reliable, and 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 through unplanned failures or during planned outages. Older-style protective relays near their end of life cannot be relied upon for the equipment protection they provide. Instead, these relays have now become a safety hazard and liability.
One critical factor affecting arc flash risk is overcurrent protective device (OCPD) fault-clearing time, and this is where advanced microprocessor relays come into the picture. The faster the fault is cleared, the lower the incident energy. Most advanced relays are capable of detecting and interrupting a fault faster than their electromechanical counterparts. They also provide advanced communication and automation abilities. This means the overcurrent device can be operated from a safe distance, which increases the working distance and reduces the arc flash hazard.
A well-planned relay upgrade project can easily be complemented with an arc flash hazard mitigation project. Upgrading protective relays and mitigating arc flash hazards can satisfy the common goals of maintaining power system reliability and promoting worker and equipment safety. Meticulous planning and well-thought-out protective relay selection along with smart engineering practices can help achieve both processes and their common goal.
PROTECTIVE RELAY UPGRADE PROCESS
Many customers are not fully aware of potential arc flash hazards or that upgrading relays can help mitigate them, so educating them throughout the process is imperative.
Project Planning and Raising User Awareness
The initial step in upgrading relays is planning the entire project while keeping arc flash mitigation as one of the goals. This includes understanding the customer’s expectations and goals and providing consultation and coaching.
Gathering Data and Procuring Protective Relays
The next step is to collect accurate data such as existing device settings, drawings, short circuits, coordination and arc flash studies, operating sequences, and other OEM documents. Well-established and updated documentation helps to maintain existing equipment operation procedures and provides room to improve the functionality of the existing system.
Procuring relays follows the data-gathering process. The key is to be judicious when selecting the proposed relay. For instance, selecting an overcurrent relay that includes an arc flash sensor would be an excellent choice for upgrading the relay and mitigating the arc flash hazard. Selecting this type of relay aligns with the common goal and can be a cost-effective solution for arc flash hazard mitigation. Procuring relays as soon as the project scope has been outlined helps reduce OEM lead times.
Performing Engineering Studies, Creating Relay Settings, and Updating Existing Drawings
Performing accurate and up-to-date engineering studies consisting of at least a short circuit, coordination, and arc flash analysis with minimal assumptions is another critical step in the relay upgrade process. It is not recommended to convert existing settings into upgraded relays. Electromechanical and solid-state relays have limited functionality and capabilities, whereas microprocessor relays have elaborate functions. For instance, an overcurrent (ANSI device 50/51) electromechanical relay has only time overcurrent and instantaneous overcurrent settings, while a feeder protection microprocessor relay provides these functions as well as multiple group settings, directional element, reverse power, and more.
Updating existing drawings while replacing or upgrading equipment is a highly recommended practice. This process helps with wiring new relays in the field, provides the customer with updated protection schemes and documents that can be used for future troubleshooting, and provides design engineers with an opportunity to incorporate remote operation of overcurrent devices. Remote circuit breaker close and trip, which is one of the vital engineering controls for arc flash hazard mitigation, can be incorporated into the upgraded system.
Offsite Wiring, Uploading Settings, and Testing Relays
Wiring relays and associated test switches to relay doors or insert panels offsite is recommended. Another good practice is to upload relay settings and test relay elements and logic offsite. Doing the work offsite reduces the outage duration in the field and helps accomplish milestones in the upgrade process.
Reducing outages is an important factor in the relay upgrade process, and being unable to schedule an outage can hinder a relay upgrade project. Relays are a huge part of the power system’s automation and protection, and taking relays out of service leaves it without protection and automation features, compromising system reliability and safety. Many relay upgrade projects are postponed or suspended because the facility outage cannot be scheduled.
Onsite Demo, Installation, Testing, and Commissioning
The final step in a relay upgrade project is bringing everything together for field installation (Figure 2). It is always recommended to document all existing wiring and maintain the integrity and aesthetic properties of wires in each compartment.
Another important step is to commission the entire system. At a minimum, this includes verifying polarities, CT ratios, phase rotation, and DC trip checks. The goal is to improve system reliability and safety without causing nuisance power interruptions. It is imperative to have detailed plans and procedures for removing the existing system, installing new relays, and testing and commissioning the system while having contingency plans for any surprises during the upgrade process.
MITIGATING ARC FLASH HAZARDS with NFPA 70E—2024
Arc flash hazards can be estimated by calculating the incident energy 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 the working distance. The methods and goals of a relay upgrade must align with the NFPA 70E hierarchy of controls for hazard mitigation.
Elimination
Eliminating the hazard is the most effective method of arc flash hazard mitigation since it removes the hazard. Two engineering methods are useful in eliminating the arc flash hazard from a medium-voltage system. These methods are based on reducing fault clearing time.
- Using reduced instantaneous (50) settings with the alternate group/maintenance mode setting. This is one of the simplest and most effective ways to reduce incident energy and arc flash hazards. Most microprocessor relays provide for multiple group settings.
The groups can be transferred over automatically using relay programming logic or manually using selector switches mounted anywhere in the switchgear. For instance, Group 1 would use the normal system protection settings. Short circuit, coordination, and arc flash studies must 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 system reliability and functionality. The user must switch settings into Group 2 before performing any switching or interacting with the medium-voltage equipment. Once this electrical work has been completed, it is imperative to return the maintenance switch to its original position or change the relay logic back to enabling Group 1 protective device settings.
Figure 3 shows the SKM PTW time current curve (TCC) along with the calculated fault current on how this reduced instantaneous setting could be calculated. As long as this reduced instantaneous setting value is set below the fault current line (dotted line), the reduced instantaneous setting can be calculated.
- Using the virtual main concept. This method, which involves installing additional hardware and relays, is an effective method for reducing arc flash hazards in existing systems. In the past, most power systems were designed without a physical main overcurrent protective device such as a circuit breaker or fused disconnect. Thus, whenever a medium-voltage switchgear powered up a medium- to low-voltage transformer, the incident energy on the secondary side of the transformer was very high. Without a main interrupting device, there were few options to reduce the incident energy.
The concept of a virtual main fills the void on a switchgear that is not designed with a main circuit breaker. In this method, current transformers (CTs) are installed on the secondary side of the transformers and/or the switchgear. Instantaneous overcurrent devices located upstream of this transformer are designed to operate if any fault current events occur on this downstream switchgear.
Substitution
Eliminating the hazard is not feasible in all practical applications. If the hazard cannot be eliminated, substitution is the next method on the hierarchy of risk control. This method involves designing engineering processes to replace something that poses a high risk with a less hazardous option. The following innovative methods to substitute arc flash hazards in a medium-voltage system are based on increasing the working distance.
- Designing relay logic for remote switchgear operations. This method is simple and effective to implement in most power systems equipped with automation. Most modern medium-voltage systems are equipped with remote operation. The design engineer can simply incorporate remote bits into the circuit breaker operation logic to achieve this. Since the protective relays would open or close the circuit breaker, the operator does not have to be physically present during the switching process. This method increases the working distance from a few inches to several feet, thus keeping the operator out of harm’s way. The only caveat for this method is that the switchgear must have automated switching capabilities, which could be lacking in some older switchgear.
- Using a delay timer in relay push buttons and control switches. This method is simple and effective for older and newer switchgear systems, and the switchgear does not have to be automated. Using this method, the relay is programmed with logic for circuit breaker open and close operations, and minimal changes are made to wiring during the upgrade process.
A push button on the front panel of most microprocessor relays (Figure 4) initiates a small delay of 10 or 20 seconds, providing ample time for an operator to push the button and safely move out of the switchgear room. For relays that do not have a push button, the control switch that operates the circuit breaker can be programmed in a similar fashion. This process is cost-effective and can be achieved with simple engineering design modifications.
ENGINEERING PROJECT: CASE STUDY
SKM PTW software was used to prepare and analyze this typical engineering project incorporating several of these methods to reduce arc flash hazards. The protective relay was programmed with reduced instantaneous settings that reduced the incident energy on a medium-voltage switchgear bus from 22.8 cal/cm2 to 1.14 cal/cm2. The protective relay pushbuttons were programmed with a delay of 20 seconds for additional arc flash hazard mitigation.
The virtual main concept was implemented to reduce the incident energy on a low-voltage switchgear bus. Current transformers (CTs) were installed on the secondary of the 2,000 kVA transformer. These CTs were connected to the existing protective relay. Short circuit, coordination, and arc flash engineering studies were performed on this new system, and instantaneous settings were designed for the 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. The single-line diagram created for this project is provided in Figure 5.
CONCLUSION
Protective relays are the backbone of protection, automation, and control for medium-voltage systems. They are critical to power system performance and reliability and utmost importance should be given to maintaining and upgrading these relays. Mitigating arc flash hazards is a vital part of electrical safety that helps protect lives and maintain the efficiency and reliability of the power system. The arc flash reduction methods in this article can be carried out along with the relay upgrade process as a comprehensive project. Using engineering brainpower and harnessing advancements in technology, we can keep our electrical system safe and reliable.
Bibek Karki, PE, is an Engineering Manager at IPS PowerServe in Lewisville, Texas. He has 10-plus years of experience in power system studies, electrical testing, and maintenance. Karki’s responsibilities at IPS include performing power system studies including short circuit, load flow, equipment evaluation, coordination studies, motor starting, harmonic wave, and arc flash hazard analysis using SKM and ETAP. He has performed studies at commercial and industrial sites, electrical distribution substations, water treatment plants, oil pumps, data centers, and wind farms. He also performs preventative maintenance, installation, modification, testing, and repairs of electrical switchgear, circuit breakers, transformers, protective relays, and associated power distribution equipment. Karki was recognized as a 2022 40 under 40 winner by Consulting-Specifying Engineer. He is a NETA Level 4 Certified Senior Technician, a NICET Level III Technician, and a registered Professional Engineer in 26 states. He has been an IEEE member since the start of his student career and is now an IEEE Senior Member and part of the IEEE Power & Energy Society. Karki earned an MS in electrical engineering from Southern Methodist University.