Station Applications
4.5 Description of the results
4.5.1 Unit-level mandatory functions
This category is the most traditional category in substations. The functionality in this category should be selected so that the most important features of the unit are secured at all stages. These functions should not rely on external communication, so that safety is guaranteed even if communication is lost. For smaller stations with less stringent requirements, this category might be the only one needed. This category alone should fulfill the most critical requirements. The functions belonging to this category are presented in Figure 4.1, and briefly described below.
• Protection
– Overcurrent protection, instantaneous and delayed (PIOC, PTOC) – Earth fault protection: Non-directional (EFPTOC)
• Control
– Circuit breaker control and operation (CBCSWI) – Auto-recloser (RREC)
• Supervision
– Breaker failure protection (RBRF)
4.5. Description of the results
4.5.2 Unit-level optional functions
Because optional functionality is rarely essential for network safety, this category is not always needed at the unit level, although an extensive library of these functions is available in modern protection and control IEDs. This includes functions which do improve the protection by, for example making it more selective or more accurate, but the network safety in general can also be guaranteed with only mandatory functions.
This may also include functions which are mandatory for a limited number of special cases, but are not always mandatory.
The functions belonging to the unit-level optional functions category are presented in Figure 4.1 and briefly described below.
• Protection
– Over/under frequency and voltage protection (PTOF, PTUF, PTOV, PTUV) – Over/under power protection, thermal overload protections (PTTR) – Earth fault protection: Directional (DEFPTOC)
– Transient earth fault protection (PTEF) – Distance protection (PDIS)
– Undercurrent protection (PTUC)
• Control
– Disconnector operation (DISCSWI) 4.5.3 Station-level mandatory functions
Currently, this category is not normally present at all in substations. All such func-tionality resides at the unit level (feeder bays, transformers, generators, etc.) and the station-level equipment is only used as a gateway for accessing these unit-level IEDs.
The drawback with this approach is the update cycle, as has already been mentioned.
The purpose of this category is to complement the functions at the unit level, rather than to replace them. When the unit-level protection can operate without the station-level protection, station-level functionality can be updated without interrup-tions. This will also extend the life cycle of the unit-level functions, as most of the update measures are carried out at the station level.
The functions belonging to station-level mandatory functions are presented in Fig-ure 4.1, and briefly described below.
• Protection
– Differential protection - bus bar (PDIF)
– Sensitive directional earth fault protection - intermittent faults (PSDE) – Phase discontinuity protection (DISPTOC)
– Protection against faults with low fault current magnitude: e.g. high impedance earth faults [Tengdin, 1996] [Nikander, 2002] (PHIZ)
– Islanding operation and Loss-of-Mains protection when islanding is not allowed [Rintamäki and Kauhaniemi, 2009] (dgoper)
• Control
– Interlocking (CILO)
• Other
– Fault locator (RFLO)
– Disturbance recorder (RDRE)
– Cyber security aspects [Nartman et al., 2009] (cybersec) – Station-level self-supervision (selfsup)
As noted earlier, this category is very close to the ’unit-level optional’ functions category and in situations where both unit- and station-level devices are available many protection functions from that category could also be allocated to this category.
4.5.4 Station-level optional functions
The functions belonging to station-level optional functions are presented in Figure 4.1, and consist mainly of functions which are not essential for the safety of the substation, but important for increasing the efficiency of the network and fast reaction to fault situations. This category also contains functions which are not yet mandatory, but can be expected to be mandatory soon. These functions address issues which are not yet common in the network (at least not in Finland), but which will gain in importance if the future challenges outlined in section 2.3 become a reality. These items are listed briefly below, and have been explained in more detail in section 2.3.
4.6. Chapter summary
• Post-fault power restoration [Mekic et al., 2009] and self-healing networks in general [Rasmunssen, 2009][Manner et al., 2011] (flir)
• Load shedding [Apostolov et al., 2007] (loadshed)
• Automatic recalculation of protection parameters based on topology and DG changes, adaptation of protection [Oudalov et al., 2011] (adapt)
• Advanced condition monitoring and Asset management support, CBM/RCM [Angel, 2003] (cbm)
• Reporting support (reporting)
4.6 Chapter summary
The functional division presented here focuses on increasing the life cycle of the substation automation system by utilizing station-level data processing. Initially, the station-level functionality criteria are defined. When the function requires commu-nication, but does not have strict requirements for response times, station-level im-plementation can be justified. Complex functionality requiring additional CPU per-formance and anticipated updates in the near future are also clear indications for station-level functionality. Based on the defined criteria, an example case of both existing functionality and current research topics was divided between the unit level and the station level.
The difficulty in the method is the numerical representation of the criteria, which are also case-specific, depending on the substation and the network. Furthermore, new algorithms can change the location of a particular function entirely (e.g. the in-troduction of a new, more accurate method, which may need more communication than traditional methods). Numerical values for, e.g. ’communication requirement’
or ’function maturity’ are very difficult to determine. For this reason, fuzzy logic methods and fuzzy c-means clustering were used, which provide a suitable toolset for handling these types of attributes. The focus of this chapter is more on the method than on the final categorization. In this thesis, the method was applied to Finnish electricity distribution substations. When similar categorization is needed for a par-ticular substation and/or application area, a new list of functions can be defined with new numeric values for the criteria. With this updated data, new categorizations can be obtained using the method presented here.
A future addition to this method would be a definition of the financial benefit of each function, such as the effect of the function on life-cycle costs. This could be added as an additional criterion, with a rule ’if a function has high financial benefit, it is mandatory’, but it could also be added as a third axis to the derived plane. This would help in visualizing the implementation roadmap for adding new functionality to a substation, highlighting those functions with the highest financial benefit, and showing them in the appropriate category. The challenge in this is in evaluating the financial benefit of each function. The life-cycle cost evaluations in section 2.5.5 have already shown the problem of insufficient statistics, the relevant different fault cases, and their root causes. Therefore, a more detailed analysis of fault cases and their root causes is also a future research topic. If the more detailed fault analysis proposed in the summary part of Chapter 2 could be conducted, the same data could be used here, connecting the results from Chapters 2 and 4 together more completely.