Learnings

Outcomes

The key outcomes for each of the work packages are summarised below including the learning from the literature, modelling and hardware testing carried out as part of this project:

The first work package reviews existing protection philosophies & arrangements, network modelling, fault level analysis and TW line protection products and techniques. A detailed analysis of the proposed model, parameters used, fault level studies and source to line impedance values for the proposed network is conducted. The study of source to line impedance ratio under varying fault level can be used to indicate the limitation of the operating performance of distance protection. As expected, distance protection is affected, i.e. either by increased operating time or non-operation. Some improvements may be achieved by using permissive intertripping schemes with weak infeed logic. Likewise, overcurrent protection is highly affected.

The network modelling to evaluate the fault contributions from the sources has been validated and matched with NGET internal data. Using the validated model, various scenarios for high and low fault level, have been simulated and the results were recorded as COMTRADE files. These signals were then injected into the physical relays in order to assess the operating performance of TW line protection. Various techniques such as disconnecting parallel lines and reducing generators’ fault level has been considered to evaluate the operating performance of the relay. The test scenarios include normal conditions for three phase & phase-phase faults at non-zero crossing, and phase-phase faults at zero voltages to evaluate the performance of TD21/32 elements, where testing will be considered at different fault inception points.

The simulations were based on a double circuit feeder in the south of the UK. The Thevenin equivalent technique- via n-1 strategy was used to determine the fault level contribution from each source.

The key benefit of the test setup has been the capability to superimpose three phase voltage and current traveling wave pulses onto the transient signal provided by the test set with nanosecond accuracy.

As part of work package 2, a number of protection functions were successfully tested:

TD21 – the incremental quantity distance element demonstrated high speed operation but is affected by zone reach issues similar to conventional distance protection. Higher fault resistance values can lead to the relay under-reaching. The TD21 element was found to operate for fault inception angles greater or equal to 10 degrees but a more detailed study is required to explore the operational limit.

TD32 and TW32 – incremental quantity directional element and travelling wave directional element

The TD32 element can help in a Permissive Overreach Transfer Trip (POTT) scheme to supervise the received permissive signals. In the POTT logic, the TW32 directional element is provided to accelerate permissive keying since a speed advantage of about 1 to 2 ms over the TD21 element can be achieved. However, in some conditions ETW32 can not be enabled, i.e.:

(a)   Lines with a TW propagation time shorter than 50 μs

(b)   Hybrid lines with overhead and underground cable sections

(c)    Multi-terminal lines  

TD50 - Incremental-Quantity Nondirectional Overcurrent Supervision is provided for the TW87 element.

TD67 – Directional overcurrent supervision for the POTT scheme

TW87 – Travelling Wave differential protection scheme was tested for in zone and through-fault scenarios with phase to ground and 3 phase faults.

More detailed testing was carried out for weak and strong fault-infeeds from both ends of the test circuit varying fault location, fault inception angle, fault resistance (where applicable) and fault type (phase to earth or 3 phase). Mutual coupling with a parallel line was also investigated. The results for TD21 and TW87 protection elements were analysed in detail and are available in the final report.

With regards to the telecommunication requirements for travelling wave differential schemes some testing has been carried out including end to end delay, differential delay and error rate. As the relays operate on the assumption that a direct fibre or Wavelength Division Multiplexing (WDM) connection is provided any differential delay on the out and return communication paths between the relays at each remote end will lead to measurement inaccuracies. Absolute propagation delay and error rates can be tolerated up to a given threshold and communications services need to deliver this minimum performance. Differential delay needs to remain as low as possible as performance will degrade with increasing differential delay.

Recommendations for further work

Based on the findings from this work, further assessment of the suitability of travelling wave protection as part of a field trial are required in order to:

• update the business case and compare alternative solutions,
• confirm availability of communication circuits meeting the minimum requirements and
• review capability of switchgear to clear faults quicker (in particular DC offset)

Some of these objectives could be achieved as part of a trial on a line with frequent lightning activity combined with further system modelling.

Lessons Learnt

The loss of PDRA resource is always a key risk to consider and some additional contingency will be added for future projects. Likewise, where procurement activities are part of the project some additional contingency in terms of timing and budget will be considered for future projects.

Longer term collaboration with experienced researchers has been a clear success factor for this project and the learning will be carried forward into future work.

The lessons learnt from this project in terms of the technical content are included in the project outcomes section below.

Dissemination

The findings from this project will be published as part of a research paper contribution to the 16th International Conference on Developments in Power System Protection (DPSP), 7 – 10 March 2022, Newcastle, UK.