Abstract–More and more often series compensation, non- standard generation and power electronic devices co-exist in a vi- cinity of EHV transmission lines. A typical scenario is a relatively long transmission line already with elements of HVDC and/or se- ries-compensation, interconnected to a new wind farm with sig- nificant capacity driven by non-standard machines. In some cases single-pole tripping is applied in order to maximize power trans- fer during reclosing, and to avoid synchronization of the islanded generation. During transients, in such systems complex interac- tions occur between series capacitors, multiple machines, and HVDC devices. This includes both the primary equipment, and the associated controls. As a result the system responds differ- ently compared to a traditional network fed from synchronous generators. Some protection techniques that are well-recognized and successfully applied in traditional configurations may per- form inadequately under such complex conditions. The paper lists protection concepts that must be carefully examined or tested before using in such a difficult application environment. It also proposes practical supervision methods that could be imple- mented via custom logic on a modern multi-function relay in or- der to solve some of the performance problems. Index Terms– Transmission lines, Distance protection, Single pole tripping, Directional protection, Phase selection. I. IMPACT ON THE PRIMARY NETWORK A. Symmetrical components and protection principles YMMETRICAL components and sequence network have been introduced as convenient means to analyze three-phase power networks. With the advent of computers, the application to analysis faded away, but symmetrical com- ponents are still foundations of many protection principles. Ground directional overcurrent functions, negative-sequence and neutral; extra directional supervision build into some dis- tance relays; polarizing the quadrilateral reactance line from the zero- or negative-sequence current; phase selection based on symmetrical currents are examples of protection principles relying heavily on symmetrical currents and voltages. Traditional power networks are usually symmetrical. Sin- gle pole tripping, or open pole conditions in general, may be the most important exception to the rule. With one pole opened, line relays would dynamically scale down on usage of symmetrical components as the latter appear due to the series unbalance (open pole) creating complex interactions with the plausible shunt unbalance (the second fault). Modern power systems incorporate more and more de- vices that differ considerably from the traditional arrangement of relatively high-inertia synchronous machines feeding a pas- sive, near symmetrical three-phase network. The non- traditional devices respond to system events in their unique way, typically quite differently compared with synchronous machines, lines and transformers. In particular, the expected relationships between the symmetrical components are af- fected, stretching the otherwise well-known and proven pro- tection principles. B. Non-traditional primary equipment Power electronic devices, converters and reactive power compensators; series capacitors; phase shifting transformers; two-phase Scott transformers; Variable Frequency Trans- formers (VFTs); non-traditional generators in large quantities such as in a wind-farm arrangement, all affect signals seen by protective relays, symmetrical components in particular. C. Impact of series compensation It is well recognized that series capacitors may cause ab- normal shifts in currents and voltages, up to the extent of “in- version” causing directionality problems. Negative reactance produced by capacitors even if partially bypassed by the Metal Oxide Varistors (MOVs), causes impedance functions to overreach making the application of directly tripping under- reaching zone 1 problematic. Subsynchronous oscillations oc- curring on low-current faults with the MOVs or by-pass switches not triggering create extra accuracy and security problems for protection. Asymmetrical MOV / gap operation creates a series unbal- ance in the six-port of series capacitors in addition to the shunt unbalance of the fault. Using Goldsworthy’s equivalent for the capacitor and its conducting MOV, or even a short for a gap / by-pass switch yields a complex equivalent with mu- tual coupling between the phases, or symmetrical compo- nents. Normally the three single-phase arrangements of ca- pacitors, MOVs, gaps and by-pass switches are not coupled, suggesting the positive-, negative- and zero-sequence imped- ances identical and the sequence networks decoupled. Phase Selection and Directionality Issues when Protecting Lines with Series Compensation, HVDC Devices or Non-Traditional Generation B. Kasztenny, Senior Member, IEEE, D. Finney, Member, IEEE, I. Voloh, Member, IEEE S B. Kasztenny is with General Electric, Ontario, Canada (e-mail: Bogdan.Kasztenny@GE.com). D. Finney is with General Electric, Ontario, Canada (e-mail: Dale.Finney@GE.com). I. Voloh is with General Electric, Ontario, Canada (e-mail: I.Voloh@GE.com). 1