EFFECTS OF LOAD ON TRANSMISSION LINE RELAY APPLICATIONS AND SETTINGS


One of the principal influences on protective relay settings is load; hence, maximum load current level, in turn, may influence fault detection sensitivity.

Phase overcurrent relays
Phase overcurrent relays must be set to avoid operation on all of those “normal” conditions to which they may be subjected, such as transformer inrush, motor starting current, maximum emergency load conditions, and maximum recoverable swing conditions.

This usually entails a time overcurrent pickup setting above a maximum load current level and/or a coordinated instantaneous pickup setting to ensure security of the relays against misoperation. The sensitivity achievable is, therefore, somewhat coarse, but many applications in which they are used do not require extreme sensitivity.

Ground overcurrent relays
Ground overcurrent relays have the advantage of utilizing a current source that supplies little or no normal current to the relays. The sensitivity achievable is substantially better than that afforded by phase overcurrent relays. Only unbalanced load current and normal system unbalance affect the setting of these devices.

Directional overcurrent relays
Directional overcurrent relays have the same restrictions as phase and ground overcurrent relays for load flow in their tripping direction. Properly selected directional elements block tripping for load flow and faults in the nontripping direction.

Phase distance relays
Phase distance relays have a relatively fixed reach; they operate most sensitively when fault currents are present and less sensitively when only load current exists. Fault currents typically lag voltage by 60° or greater. Load current typically leads or lags voltage by 30° or less. Although less sensitive in the load angle region, phase distance relays may require a setting for adequate fault coverage that may limit line loading.

Ground distance relays
Ground distance relays may also be susceptible to the error associated with ground fault resistance and out of-phase sources. Further, they may have overreach and underreach characteristics for the “leading” and “lagging” phases in responding to phase-to-phase-to-ground faults, unless provision is included to compensate for these factors.

Many ground distance relays operate on phase current and voltage inputs, making them susceptible to operate under heavy load conditions. For this reason, ground distance relays are usually supervised by ground overcurrent elements, which must be set to avoid operation for heavy unbalanced loads. Load current will also influence the “reach” of these devices where fault resistance is involved (Giuliante, McConnell, and Turner).

Pilot systems—two terminal
The influence of load on pilot systems is highly dependent on the nature of the protective relaying scheme. Those systems using overreaching distance measurement, such as directional comparison blocking, permissive overreaching transfer trip (POTT), and directional comparison unblocking, have the advantage of limited load angle sensitivity and have the absence of a critical reach due to the nature of the relaying system.

These relays have very little influence from load except in very long line applications, and this is often accommodated by blinders that prevent operation of the protective relaying system under balanced, three-phase load conditions. For direct underreaching transfer trip (DUTT) schemes, the pilot distance relays are set short of the remote line terminal.

This setting makes the scheme less susceptible to tripping under heavy loading conditions. However, the reach variation of the distance relay as a result of prefault load current is much more critical than for the overreaching schemes.

It is imperative that load or fault current, or any combination of the two, never be able to cause operation of the Zone 1 relay for any condition other than a fault on the protected line. Phase comparison and current differential schemes are not normally susceptible to operation under load condition because of their inherent nature of comparing current into the line at one terminal with current out of the line at the other.

However, load does influence the setting of fault detectors in phase comparison blocking schemes and in current differential schemes when operation following channel failure is allowed. Current differential schemes may be sensitive to tapped loads, and settings should be chosen accordingly.

Also, high levels of through load current may reduce the fault detection sensitivity of both phase comparison and current differential schemes. When transmission cables are used, special considerations may be required for the fault detector settings because of the capacitance of the cables.

Pilot systems—three terminal
Load in a three-terminal line application may represent an outfeed condition for an internal fault. Depending on the particular type of relaying system, this may produce an undesired blocking effect.

Three-terminal applications generally have at least one weak source and, consequently, care must be exercised to ensure that either the contribution to an internal fault exceeds this load current outfeed, or the relaying system bases its response on the total internal fault current.

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