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Starting impedance relay protection

Fiber and communications infrastructure knowledge brief Published 06 Jun 2024

Prepared for preliminary research by the ROGER FIBRE knowledge desk. Confirm optical interfaces, port or core capacity, electrical input and output, enclosure protection, thermal design and installation limits against current standards and product documentation.

Starting impedance in relay protection refers to the threshold impedance at which a protective relay detects abnormal conditions, such as motor starting or faults, and initiates tripping.

Definition and Principle

In protective relaying, impedance relays (also called distance relays) operate based on the measured electrical impedance between the relay location and the fault or motor terminals. The relay compares the voltage-to-current ratio, and if the measured impedance falls below a predetermined threshold, it triggers a trip to isolate the fault or protect the equipment. For motor starting, this threshold is often referred to as the starting impedance.

Application in Motor Starting

For large synchronous motors, especially those driving heavy machinery like compressors or extruders, the starting current can be 350–450% of rated current, and the terminal voltage may drop to 70–80% of nominal during direct-on-line starting. An under-impedance relay (ANSI code 21) can be used to supervise the starting process by monitoring the impedance. If the motor experiences a blocked rotor or critical starting condition, the relay trips to prevent overheating or damage.

However, studies have shown that using an under-impedance relay for starting supervision has limitations, because these relays are primarily designed for normal running fault protection. Alternative solutions, such as a supervision switch relay (ANSI code 14) based on rotor speed, may provide safer and more reliable protection during starting.

Setting Considerations

The starting impedance must be set above the minimum expected impedance during normal starting to avoid nuisance tripping. It should account for motor inertia, load torque, and voltage dips during startup. In distribution networks, relays are often set to operate when the current exceeds 1.

3 times the starting current for inverse or very inverse time characteristics, ensuring selective and fast protection. Proper coordination with other protective devices is essential to maintain system stability and prevent unnecessary outages.

Summary

Starting impedance in relay protection is a critical parameter that ensures motors and electrical systems are protected during high inrush currents or abnormal starting conditions. While under-impedance relays can be used, careful setting and consideration of motor characteristics, load, and network conditions are necessary. In some cases, rotor-speed-based supervision may offer a more reliable solution for critical motor starting applications.

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Starting impedance relay protection

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Such failure to start can occur even when the relay appears to be correctly set. To prevent it the relay should be disabled at a time after rotation begins and before the relay can operate.

Impedance Relay

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Protection Basics

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Distribution Automation Handbook

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doi: 10.1007/978-3-319-20919-7_3

Impedance relays are used whenever overcurrent relays do not provide adequate protection. This section pro-vides exercises about how to use impedance (distance) relays to protect a power network.

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Perform power system simulations of selected faults and observe how a given protection principle (overcurrent, impedance, and differential) works. Set the relays for a given power system. Verify by

Principles and Characteristics of Distance Protection

Distance protection, in its basic form, is a non-unit system of protection offering considerable economic and technical advantages. Unlike

Distribution Automation Handbook

The relay operates if the measured impedance falls below the set start value. The set start value determines the so-called reach of the relay, which defines at which distance faults seen from the

What is Impedance Types Distance Relay?

The relay whose working depends on the distance between the impedance of the faulty section and the position on which relay installed is known as the

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Technical note

This reference is intended for preliminary fiber and communications infrastructure research. Optical interface, fiber type, port or core capacity, electrical input and output, enclosure protection, cooling, installation limits and applicable standards must be verified for the specific project.

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