Customer Minutes Lost is One of the Key Regulatory Indicators of supply reliability used in the United Kingdom to monitor the performance of distribution network operators. To minimise customer minutes lost, it is important to install power-system protection that can identify the location of a fault and efficiently isolate any faulty section in the high-voltage network. To maintain the integrity of the complete network, many distribution network operators have adopted maintenance policies based on the well-established condition-based maintenance or reliability-centered maintenance strategies.

LIVE TRIP TEST

The maintenance procedures of EDF Energy Networks — whose service territory covers east of England, London and the southeast United Kingdom — specify periodic live trip testing on all high-voltage circuit breakers. This is conducted without interruption to supplies by closing the “normal open point” on the radially designed high-voltage networks prior to testing. The source circuit breaker is opened often through the local protection relay or by manual intervention, and standard trip-coil signature analyser instruments record the characteristics of the trip operation. Alternatively, a manual-type secondary injection test set is used in parallel with the secondary current circuit to trip the circuit breaker through the overcurrent relay. This method is not very precise as it only confirms the relay does in fact work, and there is a risk of damaging the relay due to excessive current.

Therefore, EDF Energy Networks saw a need for a test method capable of injecting a predetermined and accurate current into a single phase of the relay on a live system with a varying load current. The problem was discussed with Relay Engineering Services Ltd. (RES), a U.K. company specialising in computerised test equipment for protection relays and power-system simulation. According to RES, the test method would require an external current sensor to measure the relay current and would need to synchronise any injection, in terms of frequency and phase, with the existing load current.

THE NEW APPROACH

The development of an “on-load injection set” had the potential for providing an effective combined relay and circuit-breaker test set. Moreover, the test set — if equipped with relay timing, trip-coil profile analyses and battery monitoring — could establish the condition of the complete system, protection and circuit breaker simultaneously.

It is important and valuable in terms of condition monitoring to record the circuit-breaker condition data from the “first trip” on a unit that may have remained without a change of status for many months or even two to three years. The proposed injection method offered the opportunity to record the condition data for the complete system, including the relay, operating just as if a system fault had occurred.

This method of testing properly records the sluggish first-trip performance associated with faulty circuit breakers as well as with electromechanical relays. A test procedure capable of identifying the exact state of “readiness to trip” for the entire system must be performed on a live, undisturbed circuit through a secondary current injection.

ON-LOAD INJECTION SYSTEM

RES developed a test set that uses a software-controlled injection system. The digital signal processor is used as the software-controlled feedback loop, which constantly adjusts the injected current so that the clip-on Hall sensor sees the programmed current, adapting its parameters according to the burden presented by the secondary circuit.

The software must lock both phase and frequency to the existing load current, and start the injection testing at the current zero-crossing to ensure the injected current remains in phase with the load current for the duration of the test. The power supply used to inject the secondary-circuit current employs proven switching amplifier technology. These amplifiers have a very high efficiency, an important consideration in view of the large currents and voltages that may be required.

DATA ACQUISITION

RES completed the feasibility studies required for this load-injection test system before developing a completely portable instrument incorporating all the required test facilities. The unit, called PCA2 (protection condition analyser), includes a touch-screen user interface and extensive data acquisition features to capture and record events occurring before, during and after the injection. Data to be recorded includes the breaker trip-coil current profile in addition to relay current waveforms and several voltage/contact input channels.

By using an internal current sensor, the load current during all periods can be derived and recorded. This is important as the opening time of the circuit-breaker contacts can be established from the collapse of the load current. The injection set also offers the opportunity to terminate the fault injection when the load current collapses to avoid activating any circuit-breaker fail protection or if the load current increases above a predetermined value (that is, an actual fault situation).

The period between the start of the injection and the start of the trip-coil waveform is the relay operating time. The time from the start of the trip-coil waveform to the collapse of the derived load current gives the operate time of the main contact. Further automatic analysis of the trip-coil current waveform is performed to provide timing and condition data. These outputs are similar to those available from other circuit-breaker test sets; therefore, they are familiar to users of this type of equipment.

Additional voltage/contact inputs are included on the instrument to record battery voltage and any other values the user wishes to monitor. Similarly, additional current sensors can be connected to these inputs to monitor secondary currents on other phases or vibration transducers. The inputs can be used for direct connection to circuit-breaker main contacts during off-line testing as well. As the inputs record the actual voltage across contacts and trip coils, the instrument records more valuable data than just the on/off status.

TEST-DATA ANALYSIS AND REPORTING

PCA2 has several modes of operation in addition to the main automatic test that performs on-load injection with synchronised data acquisition. Its manual test screen offers a real-time oscilloscope-type display with root-mean-square measurements of all current/voltage/contact inputs, simultaneously with injection/relay testing features.

To start the automatic test, the current required and test duration need to be set, or alternately, the relay curve, setting and time multiplier can be entered, leaving the instrument to select the multiples of the relay setting to test. The integral time-delay feature allows the user to move a safe distance from the circuit breaker being tested. On completion of the automatic test, a scope-type display of the recorded data appears.

All recorded data can be saved to the internal flash memory or to a removable USB flash memory device. The IEEE COMTRADE file format was selected due to its wide use within the supply industry. Oscillographic records can be viewed on a PC using any of the many COMTRADE viewers available. A special PC viewer also has been developed that is more appropriate for producing test reports and comparing historical results.

USE IS SPREADING

EDF Energy Networks is nearing the end of trials with the portable instrument, which is proving to be an excellent innovation for assessing the condition of substation protection and switchgear panels. The use of real secondary-injection currents on a live system closely simulates the condition that occurs in the event of a system fault. A single overcurrent trip is used to record the first-trip operation and, while the circuit breaker is open, further testing to time other protection features can be completed (for example, auto reclosing systems).

Increasing interest is being shown in this new portable test set and other U.K. utilities are now using the unit.

ACKNOWLEDGEMENT

The author wishes to acknowledge the help and support given by Marcio Linhares, R&D director of Relay Engineering Services, in preparing this article.

---

Stuart Clark was awarded his Higher National Diploma in electrical and electronic engineering by North Staffordshire Polytechnic. Since joining Eastern Electricity Board in 1988, he has held several engineering positions in system design, construction, maintenance, protection and metering at voltages ranging from 132 kV to low voltage. He manages a team of field engineers in the Peterborough and Kings Lynn areas of EDF Energy Networks and coordinates protection maintenance for the East of England area. stuart.clark@edfenergy.com