APPLICATION OF CURRENT DIFFERENTIAL PROTECTION TO TAPPED TRANSMISSION LINES

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APPLICATION OF CURRENT DIFFERENTIAL PROTECTION TO
TAPPED TRANSMISSION LINES


1. Introduction

Microprocessor-based current differential relays offer superior protection for power transmission lines. The key advantages over distance relays include better sensitivity for high resistance faults, 100% line protection, and better performance in the single-pole-tripping mode, particularly during evolving and cross-country faults. Besides the well-known requirements of high-speed communication and sampling synchronization, a microprocessor-based differential relay requires Πas per principle of differential protection Πthat the currents are monitored on all the circuits connected to a protected line including tapped transformers and lines. This may create a problem as the tapped connections are meant to provide a cost-effective alternative to actual substations. The taps are made outside the main substations and may not be equipped with proper protection means such a Circuit Breakers (CBs) and Current Transformers (CTs). Also, high-speed communication from the tap site may be a problem. This either limits the application of line current differential relays or makes the tapped connections economically less attractive. This paper discusses application problems and solutions permitting the application of current differential protection to tapped transmission lines without measurements at the tap point (-s).

2. Problems Caused by Tapped Transformers

Figure 1 presents a sample system configuration with two parallel lines and several transformers tapped. The low voltage busbars may be interconnected although in most applications, the tap feeds a radial load. Note that if the LV busbars are networked, some means of isolating a fault on the LV side must be provided so as to prevent backfeed for a fault on the differentially protected line. It is assumed that the current differential protection system monitors the currents only in the main substations. Currents at the taps, either transformers or lines, are not available to the relays. One possible solution to the problem of protecting such configurations is to use directional elements on the tapped transformers to send a blocking signal to the line differential protection system. This approach actually combines the differential protection principle yielding good sensitivity but requiring high-bandwidth communications, with the directional blocking scheme that
may be accomplished using more traditional signaling channels. Depending on the availability of the CBs at the tap, the blocking signal should be sent for transformer and downstream faults (when the CBs are installed at the tap and a faulty transformer may be isolated from the tap) or only for faults at the low voltage bus and downstream (when the CBs are not available and a faulty transformer requires tripping the entire line). As the line differential system is not sensitive enough to provide sufficient protection for the transformer, a direct transfer trip must be actually sent by the transformer protection to the relays in the main substations. In any case, reliable communication is required from the tap (-s) to the main substations. This paper presents a solution that assumes that high-speed communication from the taps facilitating digital current differential protection is not available. It is assumed that the protection zone must cover the entire line including taps and a portion of the high voltage windings of the transformers. It is to be noted that the ability to protect a tapped power line without measuring all the currents in the zone will cost the user in terms of sensitivity and speed of performance.




The following sources of differential error signals need to be addressed to make the application possible:
Total load current of the tapped transformers and lines.
Faults on the low voltage side of the transformers.
Magnetizing inrush of the tapped transformers on top of the line capacitive inrush current.
External ground faults on the high voltage system causing a zero-sequence infeed from the line-side windings of the tapped transformers when connected in a grounded wye.
The following sections discuss these problems and proposed solutions.

3. Application of Differential Relays to Lines with Tapped Transformers
3.1. Unbalance Caused by the Load Current

During normal operation the tapped transformer (-s) will draw some load current. The total load current would appear to the line differential system as an error signal. Typically, the amount of load drawn from tapped connections is low as compared with the power transferred between the main substations. This provides the opportunity to restrain the differential relay by the bias current. However, as the number of taps increases (as high as five), the total load current leaking from the differential zone may become quite high. Also, when the line is initially energized, the entire load current is supplied from a single terminal and the differential error current will be equal to the restraining current and, as such, the biased characteristic does not help. One solution to the problem is to raise the pickup setting above the sum of the maximum total load current and the line charging current. It should be noted that the penalty for raising the pickup setting is loss of sensitivity.

As far as the charging current is concerned, the following should be taken into account. For long lines, the compensating reactors may be used, therefore the pickup setting should accommodate only the uncompensated portion of the charging current. Modern digital relays can apply an adaptive pickup setting based on the connection status of the shunt reactors. Alternatively, the shunt reactor current can be input directly to the relay and digitally subtracted from the total current. Digital relays can also directly compensate for the charging current by virtually calculating the charging current on-line using the terminal voltage and line parameters. Such compensation can attenuate the differential error current due to capacitive charging by a factor of ten. The charging current that is seen by the differential relay Πregardless of the compensation by the shunt reactors and/or relay compensating algorithms Πis shifted by 90 degrees or more as compared with the load current drawn by the tapped transformers. This allows for lower pickup setting because the net current is lower than the sum of magnitudes of the charging and total load currents.

When a line differential relay uses a traditional characteristic with separate boundaries created by slope and pickup settings, then only the pickup setting can prevent misoperation due to the leaking load current - increasing the slope does not help in single infeed situation. Some relays, however, combine the pickup and slope settings when creating the restraining current (for a given restraint level the higher the slope, the lower the pickup). This gives a chance for better sensitivity even when the relay is set to restraint on the total tap load current. Another solution to the differential error signal caused by the unmonitored load current is to apply distance supervision. The setting requirement is to overreach all the line terminals but avoid picking up on the load current. The overreaching distance elements will, however, pickup on close external faults on the HV system. Under such conditions, however, the differential system would see increased restraint current due to the through fault current. In addition, the large restraining current may be present only in the faulted phases while the differential error current due to the load is present in all three phases. All this may endanger security and the higher pickup/slope settings may be required despite the distance supervision.

3.2. Faults on LV Side of the Tapped Transformers

As the tapped currents are not included into the current balance monitored by the differential relays, faults on the low voltage side of the taped transformers would create a differential signal and, most likely, result in a misoperation. Distance supervision can be used to cope with this problem. The distance element should be set to overreach securely all the line terminals including tapped lines. But at the same time, they should not reach to the low voltage terminals of the transformers (Figure 2).