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Protection Question No.32 - Directional overcurrent
Author : atiqun nabi - From: England
 
#1
Sat, March 28, 2009 - 11:46
1- Directional-overcurrent fundamental

The traditional directional-overcurrent relays are combinations of directional and overcurrent relay units in the same enclosing case. Any combination of directional relay, inverse-time overcurrent relay, and instantaneous overcurrent relay is available for phase- or ground-fault protection.
Directional control is a design feature that is highly desirable for this type of relay. With this feature, an overcurrent unit is inoperative, no matter how large the current may be, unless the contacts of the directional unit are closed. This is accomplished by connecting the directional-unit contacts in series with the shading-coil circuit or with one of the two flux-producing circuits of the overcurrent unit. When this circuit is open, no operating torque is developed in the overcurrent unit. The contacts of the overcurrent unit alone are in the trip circuit. Without directional control, the contacts of the directional and overcurrent units would merely be connected in series, and there would be a possibility of incorrect tripping under certain circumstances. For example, consider the situation when a very large current, flowing to a short circuit in the non-tripping direction, causes the overcurrent unit to pick up. Then, suppose that the tripping of some circuit breaker causes the direction of current flow to reverse. The directional unit would immediately pick up and undesired tripping would result; even if the overcurrent unit should have a tendency to reset, there would be a race between the closing of the directional-unit contacts and the opening of the overcurrent-unit contacts.
Separate directional and overcurrent units are generally preferred because they are easier to apply than directional relays with inherent time characteristics and adjustable pickup.
The operating time with separate units is simply a function of the current in the overcurrent unit; the pickup and time delay of the directional unit are so small that they can be neglected. But the operating time of the directional relay is a function of the product of its actuating and polarizing quantities and of the phase angle between them.
However, the relay composed of separate directional and overcurrent units is somewhat larger, and it imposes somewhat more burden on its current-transformer source.
The electromechanical overcurrent protection systems and directional overcurrent systems have been used extensively for protecting distribution networks and subtransmission networks. These protection systems are also applied for backup protection of the transmission systems. However, the current trend of using static protection systems and numerical protection systems is on the increase. This is due to the availability of high-speed processors at low cost at present, and the improved performance offered by numerical protection systems in terms of accuracy, consistency and flexibility.
Nowadays, some algorithms for implementing numerical directional overcurrent protection systems have previously been developed and reported in the literatures. In general, the algorithms comprise two separate parts. The first is that which uses the current magnitude for implementing the inverse time response characteristic specified. This is the numerical implementation of the operating principle of the electromechanical system. The second part is that for determining the fault direction. The algorithms published use active- power calculations for the purpose of fault direction detection. However, the scheme for fault direction detection based on power has a number of disadvantages. In fault operating conditions, the magnitude of the active-power can be very low, which will affect the required discrimination, depending on the threshold setting. Furthermore, correct operation of the scheme is not achieved in close-up faults where the voltages at the protection location collapse to zero or very small values.
Some references proposed directional relays based on the use of compensated impedances for forming signals which are compared for fault direction detection. However, the choice or setting of the compensated impedance, which is system-dependent, will influence the relay performance.
Also some methods draw on digital signal processing and the use of symmetrical-phase-sequence components of voltages and currents together with memory voltages in pre-fault condition. The phase-variable voltages and currents at the protection location in the phasor form, which are obtained by discrete Fourier transform (DFT), are transformed to those in the symmetrical-phase sequences. In the first stage, the fault type is identified using the sequence components of voltages / currents. For asymmetrical faults, the phase angles of the voltage and current in the negative-phase-sequence are then used for fault direction detection, on the basis of the system impedance characteristic in the negative-phase-sequence. Close-up faults impose no difficulty in the operation of this detection scheme, as the negative-phase-sequence voltage remains non-zero even when the voltages in one or two phases are reduced to zero. The use of negative-phase-sequence components offers high sensitivity which is independent of load currents, and avoids the problems encountered in close-up faults in detecting the direction of phase-to-phase faults or earth faults involving one or two phases.
For three-phase balanced faults where both negative- and zero-phase-sequence components are negligible, phase voltages and currents are applied to achieve the required directional property. The use of memory voltages in the case of three-phase close-up faults. With numerical protection, this is achieved without the difficulty encountered in the analogue protection systems. The fault direction detection method proposed can be combined in a straightforward manner with the algorithm which implements the inverse time response characteristic of overcurrent protection.


2- TEST

The first electrical test made on over current relays should be a pickup test. Pickup is defined as that value of current or voltage which will just close the relay contacts from the 0.5 time-dial position. Allowing for such things asmeter differences and interpretations of readings, this value should be within ± 5 percent of previous data. One or two points on the time-current curve are generally sufficient for maintenance purposes. Reset the relay to the original time-dial setting and check at two points such as 3 and 5 times pickup. Always use the same points for comparison with previous tests. The instantaneous unit should be checked for pickup using gradually applied current. Whenever possible, current should be applied only to the instantaneous unit to avoid overheating the time unit. The target seal-in unit should also be tested using gradually applied current. The main unit contacts must be blocked closed for this test.
In addition to tests recommended for the overcurrent relay, the directional unit of the directional overcurrent relay should be tested for minimum pickup, angle of maximum torque, contact gap, and clutch pressure. A test should also be made to check that the overcurrent unit operates only when the directional unit contacts are closed.


 
Author : Hamid - From: IRAN
 
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