2.5 Q UANTITIES I NFLUENCING D ISTANCE M EASUREMENTS .1 Multiterminal and Tapped Lines
2.5.2 Parallel Lines
2.5.2.1 Zero-Sequence Mutual Compensation Methods
5 Z0M
Line 1
Line 2
S R
Load
Relay
6
SLG Fault
Figure 2.114 Mutually coupled parallel lines with a single zero-sequence source
To overcome the overreaching and underreaching problems mentioned above, protection engineers can choose between different methods:
1. In the ground distance relay of the protected line, introduce the residual current from the offending parallel line, and apply a compensation algorithm in the apparent impedance calculation.
2. Assign and adapt individual settings groups to different operating conditions (parallel line in service, out of service, or out of service and grounded) by considering the effective mutual coupling of the different operating conditions.
2.5.2.1 Zero-Sequence Mutual Compensation Methods
It is theoretically possible to compensate a ground distance relay such that the effect of the zero-sequence mutual coupling on its reach is minimal. This is typically accomplished by bringing the current from the offending parallel line, IRM, into the ground distance relay of the protected line and applying a zero-sequence mutual compensation factor, k0M, to offset the increase or decrease of the zero-sequence voltage induced in the protected line.
The different mutual compensation methods to consider are discussed in more details in [38]. Some of these methods often lead to misoperation of relays connected to the parallel nonfaulted line unless the user is extremely careful with the ground distance relay settings. As the system complexity grows with an increasing number of lines, the user must take extreme care in using ground distance mutual
evaluations and extensive review of the power system under faulted conditions. The application of zero-sequence mutual compensation may be undesirable for a number of reasons listed below, and many relay manufacturers discourage its use or do not provide such compensation in their ground distance relays:
• It is not possible to obtain the zero-sequence current from the parallel line when the lines run in parallel for only a portion of their total length and do not terminate at the same substation at one or both ends.
• It is not possible to obtain the zero-sequence current from the parallel line when the line is out of service for maintenance and is grounded at both line ends, because the application of grounding points are away from the CT locations and the line is isolated from the line breakers with line disconnect switches.
• It is not possible to use mutual compensation when several lines share the same right of way, or more than two circuits share the same transmission tower.
• Application of zero-sequence mutual compensation may cause the ground distance relay of the unfaulted line to loose directionality for a close-in reverse line-to-ground fault on the parallel line, because the zero-sequence compensation current may overpower the actual line currents and allow the ground distance relay to operate.
• Protection engineers do not prefer to mix currents from different line terminals into one relay panel because of the possibility of incorrect installation, for safety considerations, and to avoid testing mistakes.
The residual current from the offending parallel line, IRM, can be routed to the relay shown in Figure 2.115 in order to compensate for the zero-sequence mutual coupling. The voltage measured at the relay location is represented by the following expression (ignoring fault resistance):
)
Z1L = positive-sequence line impedance IA = A-phase current measured by the relay
3I0 = residual current measured by the relay (3I0 = IA + IB + IC)
Z0M = zero-sequence mutual coupling impedance IRM = residual current from offending line
Rearranging (2.52) in the form of an apparent impedance measured by the relay in p.u.:
) ZAPP = apparent impedance measured by the relay
The term k0M • IRM in the denominator of (2.53) is the error term if we do not make any corrections using a zero-sequence mutual compensation method.
3
1
4
2
5 Z0M
Line 1
Line 2
S R
Relay
Figure 2.115 Mutual compensation using the zero-sequence current from Line 1 into the ground distance relay in Line 2 Method 1
Measure IRM and use
L 1
M 0 RM
Z
• 3
Z
•
I in the denominator of ground fault location calculations.
This method is fraught with difficulties for the sound (or nonfaulted) line relaying system. The residual current, IRM, of the offending line can be much larger than the residual current measured on the healthy line when a close-in fault occurs on the offending line, and a single zero-sequence source is located behind the relaying terminal. IRM equals 3I0 for both relaying terminals at the opposite end. The resulting apparent impedance is greatly reduced and positive at the terminal where IRM is less than 3I0 (assuming 3I0 and IRM are in phase) because IRM is used in the denominator. If IRM and 3I0 are in antiphase, the sign of the apparent impedance measured is correctly negative but errant in magnitude.
Method 2
Measure IRM and use a k0 factor, which is dependent upon the ratio of IRM / 3I0. This k0 factor is then used in the ground fault distance algorithms. This method simply uses the ratio of IRM / 3I0 to determine which k0 factor is appropriate. Switch the compensation factor, k0, used in the ground distance calculations depending on the magnitude of IRM. If IRM / 3I0 ≤ 1 (± margin), the ground distance calculations use one k0 factor. If IRM / 3I0 > 1, the ground distance element uses an alternate k0 factor. IRM is not used in the ground distance calculations but is used as a simple switching indicator for the k0 factor.
A weakness of this method is that it does not take into consideration the direction of IRM. Therefore, undesirable overcompensation occurs when the parallel faulted line experiences sequential tripping.
Method 3
Measure IRM and use a k0 factor, which is dependent upon the ratio of IRM / 3I0 and the direction of IRM
relative to 3I0. This k0 factor is then used in the ground fault distance algorithms. This method requires that the relay perform a directional decision independent of the apparent impedance calculation.
This method is the most secure and can work for some applications if implemented appropriately. The difficulties with this method come with changing switching arrangements or the isolation of zero-sequence sources.
Method 4
Ignore the residual current from the offending line. Compensate for the known under- and overreach with appropriate Zone 1 and Zone 2 distance relay reach settings or with different zero-sequence current compensation factors for Zone 1 and overreaching zones.
There are actually few problems associated with this method. Later in this section, we make simple
zero-sequence current compensation factors for Zone 1 and overreaching zones. Alternate settings groups are necessary for different system switching configurations.
Table 2.15 shows the different switching states of the parallel line and the measured impedances in each case for a fault at the end of the parallel line, as shown in Figure 2.114, with the following assumptions:
• The phase current and zero-sequence current of the protected line are equal (IA = 3I0).
• The offending line residual current, IRM, is equal to the residual current of the protected line.
Table 2.15 Calculated impedances for different switching arrangements depending on the state of the parallel line
State of Parallel Line Calculated Impedance
In service
Out of service and grounded at one point only or not grounded ZApp=Z1L
Out of service and grounded at both line ends
)
The state of the offending parallel line can change dynamically from in service to switched off and not grounded because of a circuit breaker opening at one or both line ends. Ground distance relay settings of the protected line may be too slow to adapt in real time, and for this reason, a user needs to find a common settings parameter group that would serve both situations (e.g., offending parallel line in service or out of service and ungrounded). We refer to this as parameter Settings Group A in this section of the report.
The third condition, during which the offending parallel line is out of service and grounded at both line terminals, is a typical maintenance scenario and can use a different relay settings group that is activated during line maintenance conditions. We refer to this as parameter Settings Group B in this section of the report. Note that some electric utilities apply single-point grounding methods during line maintenance conditions. In such cases, the user may want to introduce an additional Settings Group C that is not compromised due to mutual coupling considerations.
Consequently, this kind of compensation of the mutual coupling effect requires at least two independent parameter sets (A and B) in the relay. In addition to that, we need two different zero-sequence current compensation factors, one for the underreaching zone (Zone 1) and one for the overreaching zones (Zone 2, etc.). Alternatively, different settings can be applied for the phase-to-ground and phase-to-phase fault reach of the overreaching zones.