GE UR8LH

Description

Product Description

The GE UR8LH is a specialized, single-function protective relay from the GE Multilin Universal Relay (UR) series, designed exclusively for underfrequency load shedding (UFLS) applications in electrical power systems. This relay’s core purpose is to detect a dangerous decline in system frequency—a condition that occurs when generation is insufficient to meet load demand—and automatically disconnect predefined blocks of non-critical load to prevent a total system collapse or blackout. The GE UR8LH operates as a critical component of a utility’s or industrial facility’s system integrity protection scheme (SIPS). By implementing a coordinated scheme of multiple UR8LH relays, system operators can automate a staged, graduated response to frequency decay, shedding load in steps to allow generators to rebalance and stabilize the grid. This device is essential for maintaining the stability and security of isolated grids, industrial plant power systems, and utility distribution networks.

Technical Parameters and Features

• Device Type: Dedicated Underfrequency Load Shedding Relay.
• Protection Function: Single-function device for underfrequency (81) protection with multiple stages. It does not include overcurrent, voltage, or other common protection elements found in multifunction relays.
• Frequency Measurement: High-accuracy frequency measurement (typically ±0.002 Hz or better) with selectable measuring modes (phase-phase, phase-neutral).
• Programmable Stages: Typically features 8 independent frequency stages (as indicated by the “8” in UR8LH). Each stage can be set with:
  • Frequency Setpoint: The threshold (e.g., 59.3 Hz, 58.8 Hz).
  • Time Delay: A definite-time delay (e.g., 0.1 to 10 seconds) to ride through transient disturbances.
  • Output Relay Assignment: Each stage is assigned to a specific output relay to trip a specific breaker or load group.
• Rate-of-Change (df/dt) Element: Often includes a rate-of-change of frequency element (81R) for faster, more sensitive detection of severe generation loss, which can initiate load shedding even before a fixed frequency threshold is crossed.
• Voltage Restraint: Includes voltage supervision (27/59) to block operation during conditions where frequency measurement may be unreliable, such as during a loss of potential or severe voltage distortion.
• Inputs: Voltage inputs (typically 2 or 3 phase-to-phase or phase-to-neutral) for frequency measurement.
• Outputs: Multiple Form-C output relays (often 4 or more) to execute the shed commands.
• Communication: Basic communication ports (RS485) for integration via Modbus RTU or DNP3 for setting changes and event retrieval, though its primary operation is standalone and logic-based.

Advantages and Key Features

• Simplicity and Reliability for a Critical Task: As a dedicated device, the GE UR8LH is optimized for one job. This reduces complexity, minimizes the chance of misoperation due to incorrect settings for other functions, and provides a highly reliable, deterministic response to frequency events.
• High-Speed, Accurate Frequency Measurement: Its measurement system is designed specifically for precise frequency tracking, which is critical for making correct shedding decisions. The inclusion of a df/dt element allows it to react more quickly to severe events than a relay using only fixed frequency thresholds.
• Flexible and Configurable Staging: The 8 fully configurable stages allow engineers to design a tailored load-shedding scheme that matches the specific generation and load profile of their system, shedding the right amount of load at the right time.
• Secure with Voltage Supervision: The voltage restraint logic ensures the UR8LH will not operate incorrectly during PT fuse failures or other abnormal voltage conditions, a critical security feature to prevent unnecessary load loss.
• Proven Performance in Demanding Applications: Part of the widely used and trusted GE UR family, it has a strong track record in utility and industrial power systems worldwide for a mission-critical protection function.

Application Case Studies in Various Fields

• Industrial Plant Cogeneration Facility: An islanded plant with its own generators uses multiple GE UR8LH relays. If a large generator trips, causing a rapid frequency drop, the relays shed less critical production lines in stages to keep the primary process and essential services online.
• Municipal Utility Distribution Substation: Installed on distribution feeders to implement a city-wide UFLS scheme. If the regional grid experiences a major generation loss, the UR8LH relays shed predetermined residential and commercial load blocks to prevent a total blackout.
• Offshore Oil & Gas Platform: Protects the isolated power system on the platform. The loss of a large gas turbine compressor motor would cause a severe frequency dip; the relay sheds HVAC and water injection loads to preserve power for safety and critical production systems.
• Renewable Energy Integration Point: At the substation connecting a large wind farm, a UR8LH relay can be part of a scheme to shed non-firm load if the wind generation suddenly drops off, stabilizing the local grid.

Comparison with Competing Products

Compared to using a multifunction feeder protection relay (like a GE F60) or a programmable logic controller (PLC) for underfrequency load shedding, the GE UR8LH offers a focused alternative:

1. Dedicated vs. Multifunction: A multifunction relay can perform UFLS, but its firmware and processing cycles are shared among many functions. The UR8LH is a dedicated hardware device where all resources are focused on fast, secure frequency measurement and trip output, which can be advantageous for the most critical, high-speed schemes.
2. Simplicity and Commissioning: The settings and logic for the UR8LH are straightforward and purpose-built. Configuring complex UFLS logic in a PLC or a multifunction relay’s flexible logic requires more programming, increasing the risk of errors during commissioning and testing.
3. Cost-Effectiveness for the Application: For an application requiring only UFLS, the UR8LH can be a more cost-effective solution than allocating a costly multifunction relay for this single task. It is a “right-sized” tool for the job.

Selection Advice and Important Notes

• Requires a Coordinated System Study: The settings for each stage of the GE UR8LH (frequency, time delay, df/dt) must be derived from a detailed system stability study. Incorrect settings can cause over-shedding (instability) or under-shedding (collapse).
• Coordinate with Other Relays: The operation of the UR8LH must be coordinated with other system protection (generator underfrequency, islanding schemes) to ensure a proper sequence of events during a disturbance.
• Ensure High-Quality Voltage Signal: The accuracy of the relay depends on a clean, stable voltage signal. Use dedicated potential transformers (PTs) and ensure wiring is properly shielded. Never share PT circuits with high-power auxiliary loads.
• Regular Testing is Mandatory: Given its critical role, the UR8LH scheme should be tested regularly using a secondary injection test set to verify the frequency setpoints, timers, and output relay operation. Test the voltage restraint feature as well.
• Document the Load Shedding Scheme: Maintain clear documentation of which breaker/load is assigned to each output relay and stage. This is vital for system operators and for troubleshooting after an event.

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