TRANSMISSION LINE PROTECTION LENGTH CONSIDERATIONS BASICS

Short lines (SIRs > 4)
Typical protection schemes and communication channels used are as follows:
— Current differential (FO, PW, AT, MW)
— Phase comparison (FO, PW, AT, PLC, MW, R)
— POTT (FO, PW, AT, PLC, MW, R)
— Directional comparison blocking (FO, PW, AT, PLC, MW, R)

Short lines result in a small current magnitude difference and minimal voltage drop difference between close-in and remote faults. Non-pilot overcurrent relays usually cannot be set to discriminate between internal and external line faults. Non-pilot distance relays may be able to discriminate between internal and external line faults for lines with an SIR as high as 20.

However, it may not be possible to coordinate non-pilot distance relays on adjacent long lines with the short line distance relay zones. Therefore, pilot relay schemes utilizing communication channels are used. The most effective forms of pilot protection for short lines include current differential, phase comparison, POTT, and directional comparison blocking schemes.

None of these schemes require distance elements to be set for less than the line impedance. Schemes utilizing Zone 1 distance elements should provide the capability to handle arc resistance or fault impedance, which can be significant compared to the line impedance.

Current differential and phase comparison systems do not provide remote backup for adjacent system elements. However, non-pilot overcurrent and distance relay schemes can be applied as backup protection, provided time delays are adjusted to provide coordination.

Medium lines (SIRs < 4, but > 0.5)
Typical protection schemes used are as follows:
— Phase comparison (AT, PLC, MW)
— Directional comparison blocking (AT, PLC, MW)
— Permissive underreaching transfer trip (FO, MW)
— POTT or unblocking (FO, MW, PLC)
— Step distance
— Step or coordinated overcurrent
— Inverse time overcurrent
— Current differential

Lines with SIRs less than four allow more effective relay discrimination. Relay schemes utilizing under reaching elements can now be set. Zone 1 can be set to underreach the remote end and still protect 80 90% of the line.

The last 10–20% of the line can be protected by a Zone 2 overreaching element. Since selectivity and sensitivity can usually be met, the speed with which a fault needs to be cleared may require the application of pilot relaying. If slow fault clearing is acceptable, simple step distance or overcurrent relays can be applied.

Long lines (SIRs < 0.5)
Typical protection schemes used are as follows:
— Phase comparison (PLC, MW)
— Directional comparison blocking (PLC, MW)
— PUTT, POTT, or unblock (AT, PLC, MW)
— Step distance
— Step or coordinated overcurrent

Long lines have small SIRs, and most long lines are extra high voltage (EHV) or ultra high voltage (UHV) lines. EHV and UHV lines almost always require high-speed tripping of all terminals for stability purposes and to minimize damage caused by the fault.

Long lines can also have other system elements included in the line, such as series capacitors. This makes the total line impedance variable under certain conditions and introduces transient behavior that makes selectivity difficult. Phase and directional comparison protection schemes over PLC or MW are well suited to this type of application.d to provide coordination.

DISTANCE RELAY MINIMUM SETTINGS BASIC INFORMATION

The problem of low settings of line distance relays is that for lines that are very short or have a high SIR, available fault current and voltage at the relay location may not provide adequate operating margins. All distance relays have minimum settings criteria that must be met for proper operation.

The following concerns should be addressed:

a) Impedance characteristic.
Minimum fault operating currents must be know; usually, the shorter the reach, the greater the minimum current required to function. The apparent reach setting of the relay decreases with lower relay terminal voltages (Figure below), causing the relay to under reach.

Apparent relay reach vs. relay voltage

b) Directional action.
Minimum polarizing voltages must be known. Sensitivities in the range of 1% of rated voltage may be required; however, at this sensitivity, misoperations may occur for reverse faults due to the effect of arc drop.

c) Memory action.
The memory circuitry of the relay is used for low-voltage conditions by supplying a prefault voltage for polarizing. This circuit may have memory action that lasts only a few cycles.

d) Operating time.
Tripping time (Figure below) may vary with the distance to the fault, the basic minimum reach setting, the fault current magnitude, and the magnitude of relay voltage prior to the fault.

Location of Fault in Percent of Relay Setting

Variation of operating time with distance to the fault

Usually, the lower the ratio of Zapparent to Zsetting, the faster the relay operates except for low current conditions. Under conditions of low current, the relay operate time may actually increase.

e) Maximum torque angle.
Cable circuits may have a very small line impedance angle, especially for pipe-type cables. This may require a maximum torque angle that is not available on the relay and, thus, necessitate using a different range or impedance characteristic (i.e., quadrilateral) relay.

f) Continuous ampere rating.
The engineer must be aware of the continuous operating characteristics for a particular setting. It is possible for a setting to violate the continuous ampere rating, especially if the relay is an electromechanical style.

g) CT and VT errors.
Due to errors in the CTs and VTs, it is possible for a marginal setting to be unusable. On systems where the available voltages and currents are low, CT and VT errors may further reduce the available quantities to the relay.

h) Relay settings.
When the protected line is short, arc impedance must be incorporated into the settings for Zones 2 and 3. It may not be possible to increase the Zone 1 reach setting because of possible overreach of remote terminals, although some relaying principles automatically adapt to this need.

As each distance relay has its own characteristics, it may be necessary to perform fault studies under minimum conditions to confirm that the relay functions properly. If the relay does not meet the minimum requirements, then alternative relay schemes, such as current differential, phase comparison, or pilot-wire, should be considered.

DISTANCE RELAY – PROTECTION FOR LONG DISTANCE LINES

Distance relay is one of the most important protection schemes in the power system. Distance relaying is relatively complex in nature. This article however will try to give the readers a basic understanding of the concept and process.

There are basically three types of distance relays:

(1) an impedance type of relay measures the voltage-to-current ratio on a faulty line, which is the impedance to fault;

(2) a reactance type of relay measures a signal proportional to the imaginary component of the voltage-to-current ratio; and

(3) an admittance type of relay, also called a mho relay, measures current to voltage and is inherently directional.

The relays thus respond to impedance, reactance, or mho, rather than the fault current. Static distance relays may operate within 0.5 cycle of fault inception. External timers and the distance settings are used to provide selectively.

The characteristics of a distance relay are plotted on an R–X diagram. Thus, the characteristic of an impedance relay is a circle, and the relay will operate in either direction for an impedance setting within the circle.

The characteristic of a reactance relay is a straight line parallel to the R-axis and is nondirectional. A mho relay has a circular characteristic, which passes through the origin and hence is directional.

Variations in these basic characteristics are conventional. Consider that a distance relay is applied to a short-line, the arc fault resistance can alter the reach of the relay and result in a no-trip or nuisance trip.

Conversely, a reactance relay will remain unaffected by the arc fault resistance but may operate on load currents. It should be used in conjunction with other relays to restrict its reach along the R-axis and in a negative reactance direction.

For specific applications, the characteristics of a mho relay can be offset in the forward or reverse direction. Blinder relays have an angle impedance characteristic that can be set parallel to the impedance characteristics of a line.

Figure 1(a) shows characteristics of distance relays. The reverse offset mho characteristic is used for loss of excitation protection of a generator.

The concentric circles and blinder characteristics, shown in Fig. 1(b) are used for out-of-step protection.