Improper relay logic threatens your electrical assets and investments

05/06/2014


While event recorders are powerful tools, their output is only as good as their input. Integration engineers need to identify all elements that should be recorded in the event of a fault. However, it’s important to remember that event recorders have limited data capacity and will begin dropping old data from the file once that capacity is reached. Therefore, some elements should be left out of the event recorder depending upon how often they toggle. Best practices call for the engineer to structure the high-speed data capture for trips, abnormal voltage, or currents to aid in event analysis during future trip investigations.

3. Arc flash mitigation capabilities

Many microprocessor-based relays include an optional protection feature for arc flash detection. This feature can immediately respond to arc flash incidents by detecting a combination of excess light and current. The feature can help workers avoid devastating injury and prevent costly equipment damage.

To determine if this relay feature is needed in a specific environment, protection engineers can complete an arc flash hazard analysis to identify the potential for and location of possible arc flash hazards. Such an analysis may include recommendations to utilize microprocessor-based relays for additional arc flash protection. In these instances, the arc flash mitigation capabilities should be programmed into the protective relay, using logic to instruct the relay to immediately interrupt the circuit if developing arc flash conditions are detected.

4. Motor/generator protection features

Most industrial facilities, wastewater treatment plants, manufacturing facilities, and refineries operate a large number of motors, some of which cost tens of thousands of dollars. Large motors are expensive to repair or replace and lead times are long. Each motor can be effectively protected by one microprocessor-based relay. However, the features within the relay must be programmed correctly, per the motor manufacturer’s specifications and mechanical load, taking into consideration dynamic response of the facility’s power system.

Some motor protection relays allow for limiting the maximum starting time or maximum starts/hour to allow for adequate cooling of the motor during high-current, low-speed operations. Often, this function is underutilized or misunderstood and is either disabled or set to a value that does not adequately protect the motor. To take advantage of this function, protection engineers perform motor starting and load flow studies to determine the maximum motor starting times that can be expected under the most restrictive conditions. Relays are then programmed to prevent damage but still allow for adequate start times without nuisance tripping.

The installation of microprocessor-based relays isn’t “plug and play.” Relying on default logic can provide a false sense of security that may not be revealed until after severe equipment damage or unplanned outages have occurred.

Utilities and industrial facilities frequently make a critical mistake when upgrading to microprocessor-based relays by failing to customize the relays’ built-in programmable logic, thus forfeiting the relays’ ability to properly protect the electrical system. With the help of an experienced protection and/or integration engineer, utilities and facility owners can identify the features and capabilities within each relay that should be leveraged. By investing in proper relay logic customization and programming, facilities ensure optimum protection for their electrical systems and simultaneously realize the full value of their upgrade investment.

Steve Nollette is a supervising engineer for Emerson Network Power’s Electrical Reliability Services.

Key Points:

  • Changes in equipment and in electrical systems require expert installation of protective relays systems, but this often does not take place.
  • Simplifying the control system control system is a benefit, but can only be realized if custom logic has been programmed and tested.
  • An arc flash hazard analysis can identify the potential for and location of possible arc flash hazards. If found, that also can be programmed into the relay.
  • The installation of microprocessor-based relays isn’t “plug and play.” Relying on default logic can provide a false sense of security that may not be revealed until after severe equipment damage or unplanned outages have occurred. 

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