Protection-Class Fault Indicators Improve Response, System Visibility and Equipment Life
By Steve Watt, Joanna Hofer and Shankar Achanta, Schweitzer Engineering Laboratories Inc.
Eighty percent of power distribution overhead faults are momentary. These momentary faults are often detected and cleared by reclosers that are positioned along distribution lines. Reclosers divide the distribution lines into sections, and they open or close these line sections independently of the main line, clearing faults without disrupting service to a whole line or an entire distribution system. Even though this recloser-aided precision fault clearing keeps the power on for customers in regions that are unaffected by the fault, a typical distribution system can have areas that present challenges when faults occur. A new generation of specialized sensors, however, are helping advance protection to a new level.
for Single-Minded Protection Schemes
Because most faults clear themselves, reclosers minimize the impact of faults by interrupting the fault current, allowing momentary faults to clear. This allows breakers at the substation or along the distribution lines to automatically restore power to the line after a momentary fault.
While this method is effective, distribution protection engineers continue to face fault-clearing challenges. One reason for this is that each recloser control is typically programmed to operate according to one protection scheme. For line sections that include both underground and overhead lines or those that have fuse-protected laterals with varying ranges of criticality and accessibility, employing a single protection scheme is not always ideal.
For example, some areas of the same section might require delays for fuse blowing (trip saving) while other areas would benefit from faster tripping (fuse saving). Applying a single scheme to these varied sections results in missed opportunities to optimize power system metrics, such as System Average Interruption Duration Index (SAIDI), Momentary Average Interruption Frequency Index (MAIFI) and Customer Average Interruption Duration Index (CAIDI). These metrics provide insights that help operators reduce future outages to better serve their customers.
Protection-Class Fault Indicator Systems Optimize Line Protection
A new, economical protection approach is to improve the performance of the reclosers already on a system. Distribution protection engineers can do this by installing a protection-class fault indicator system. This new type of fault indicator system sends fault detection signals to the recloser control or relay so that protection devices make better decisions. Because the system quickly communicates that there is a fault–typically within 6 milliseconds of a fault occurrence–the recloser control or relay has the information it needs to apply the best protection scheme for the situation while the fault is active on the line.
Utilities applied faulted circuit indicators (FCIs) for decades to locate faults, but the idea of using them to improve protection is new. To enhance protection, the FCI system must provide information to recloser controls and feeder relays faster by several orders of magnitude. These high-speed sensor systems are optimal for the following medium-voltage applications:
“- Hybrid fuse-saving and fuse-blowing protection schemes that use the same recloser or relay
“- Precise protection selectivity at underground-to-overheard transitions
“- Coordination delay bypassing to accelerate protection and reduce the duration of damaging fault current
Protection-class fault indicator systems consist of fault transmitters and receivers. Fault transmitters contain a current transformer and clamp onto an overhead feeder line. Fault transmitters harvest energy from the line to which they’re attached, allowing them to operate without batteries. When a preset trip threshold is exceeded, the radio in the fault transmitter sends high-speed wireless signals to the fault receiver. Each transmitter also sends a periodic radio link status to the receiver to indicate that the transmitter is operational.
In a protection-class fault indicator system, the fault receivers can simultaneously receive fault information from multiple fault transmitters, which greatly improves the wireless communications performance because each transmitter can communicate without delay. The receiver communicates to a protective relay using a high-speed protocol.
The protective relay or recloser control uses the fault information from the transmitter and receiver system to optimize protection schemes during a fault. If radio communications from the transmitters are lost, the relay or recloser control continues to operate with standard protection schemes.
Mixed Protection Schemes
For a given distribution feeder segment, a protection-class fault indicator system optimizes recloser control operation to reduce outages (SAIDI) or momentary faults (MAIFI). Adding a transmitter and receiver system to a recloser control or relay allows distribution protection engineers to mix protection schemes and create hybrid schemes that balance SAIDI and MAIFI metrics.
Fuse Blowing as Primary and Fuse Saving as Secondary
Fuse-blowing schemes are commonly used in urban areas where feeders have numerous taps. In this scheme, a fault on a tap causes a fuse at the beginning of the tap to blow. The outage is confined to the tap, and the recloser control prevents reclosing attempts on the main line, which improves MAIFI metrics. The downside of this approach is that temporary faults can cause a fuse to blow and create a permanent outage for the tap, impacting SAIDI metrics. To optimize both SAIDI and MAIFI metrics, distribution protection engineers can employ a hybrid fuse-blowing and fuse-saving scheme that uses the protection class fault indication system. They can apply fault transmitter sensors on taps where fuse saving is preferable. These taps might have a critical load or experience frequent momentary faults. When the relay or recloser identifies faults on these taps, protection can be instantly switched to fuse saving, tripping before the fuse blows.
Fuse Saving as Primary and Fuse Blowing as Secondary
When fuse saving is the primary protection scheme, maximum effort is spent to clear all temporary faults on the feeder. This approach is commonly used in rural areas, large coverage territories, rugged terrain, areas with severe weather, and when personnel availability is constrained. The goal is to take only outages for permanent faults. Fuse saving improves SAIDI metrics and operations and maintenance costs. The downside of this scheme is that repeated reclosing results in poor MAIFI metrics. In a hybrid scheme, distribution protection engineers can use fault transmitter sensors on taps where fuse blowing is preferable. These might be taps with low, noncritical loads or locations where it is easy for service technicians to replace fuses. When the relay or recloser identifies faults on those taps, protection can be instantly switched to fuse blowing.
Protection at Underground-to-Overhead Feeder Transitions
Feeders with underground-to-overhead transitions present utilities with a protection challenge. Underground faults are usually permanent, while overhead faults are usually momentary. Utilities are often unwilling to reclose on faults near an underground-to-overhead transition because they do not want to reclose on underground faults, which most likely are permanent. Reclosing on permanent faults stresses the infrastructure and equipment and can damage cables and connectors. Therefore, knowing the precise location of a fault is critical to choosing the best protection scheme.
A protection-class fault indicator system enables fine-tuned protection at feeder underground-to-overhead transitions. Distribution protection engineers can place fault transmitters at the overhead-to-underground transition points. If a fault transmitter detects a fault, it sends a message to the receiver that is connected to the feeder relay. The relay can then change its protection scheme. This fine-tuned selectivity approach reduces outages and improves SAIDI metrics.
Bypass Coordination Delays
To clear faults from lines and apparatus along a distribution circuit, engineers preset a sequence of operations in overcurrent protection devices by specifying certain time-current characteristic curves and settings. This is known as coordination.
There is coordination between protective devices installed in a series configuration and also between protective devices and fuses. A protection-class fault indicator system provides a recloser or relay with the necessary information to reduce unnecessary coordination delays. For example, operators can use the system to determine when a fault is upstream from the next recloser, on unfused taps, or on unfused main line segments. The system can then trip faster in these cases.
Reducing the duration of faults not only improves reliability metrics but also has the following benefits:
“- Extended equipment life: Clearing faults before they can fully develop reduces stress on infrastructure
“- Improved system stability. Short fault-clearing times reduce voltage fluctuations and sags
“- Increased safety. Extended fault durations can lead to an increased chance of fire damage or electrocution
During fault conditions, voltages can fluctuate. Phases adjacent to the faulted phase can experience voltage sags. Shortening the duration of faults lessens the impact of voltage deviations, which is especially important for devices, such as consumer electronics, that have tight tolerances for power quality.
Conductors that encounter trees or other equipment during faults can create showers of sparks. The longer the fault stays on the line, the longer the shower of sparks continues, potentially starting catastrophic fires. Broken or fallen conductors also can be an electrocution hazard. Isolating faults faster reduces the chances of fires and other safety hazards.
The recloser control can retrieve the fault-detection information from the protection-class fault indicator system and send it back to a SCADA or outage management system. This provides line crews with more specific location and phase information for a fault, reducing patrol time.
Improved fault location information allows personnel to reduce patrol times and drive directly to the faulted tap or segment, improving CAIDI metrics. The ability to identify the faulted segment ahead of time is especially helpful when feeder lines are numerous, difficult to get to and far from major roads or when crews face poor weather conditions.
Protection-Class Fault Indicator Systems Can Improve Recloser Operations
Reclosers are valuable additions to any distribution system because they improve system reliability, but their operations can be greatly enhanced with sensors that can detect faults and transmit valuable location information quickly to a recloser control or relay. Adding protection-class fault indicator systems creates opportunities to optimize protection schemes in relays or recloser controls.
Distribution systems benefit when protection-class fault indicator systems are combined with advanced recloser controls and feeder relays. The key benefits include:
“- Improved SAIDI, CAIDI and MAIFI metrics from mixing fuse-saving and fuse-blowing schemes
“- More precise protection for underground-to-overhead transitions, which reduces permanent outages and the risk of reclosing on underground faults
“- Faster protection on radial feeders by eliminating unnecessary wait times for coordination
“- Increased equipment life
“- Improved system stability and power quality
“- Improved safety
“- Reduced patrol time for fault location
Advanced recloser controls and feeder relays have helped improve reliability metrics because they use more precise fault-detection methods, clear a greater number of temporary faults and enable effective fault isolation and restoration schemes. By adding protection-class fault indicator systems to existing protection systems, utilities can further improve reliability, realize additional distribution system benefits and optimize their power system. | PGI