Distributed Generation Drives System Planning

Network planning specialists at VSD — a distribution system operator in Slovakia and part of the European RWE Group — process applications on a daily basis for the connection of additional load and distributed generation. These connections result in the need for minor and, at times, major modifications to the distribution system.

The utility's internal guidelines for grid planning define the basic framework for such modifications. This often results in the need to quantify the impact of the distribution system on the newly connected load/generation and, vice versa, to assess the impact of these connections on the distribution system. In those cases, it is necessary to model the section of the distribution system either in very simple form using some of the available tabular calculators or in a more complex form using professional system planning software.

Planning Specification

In 2009, a team of engineers from the VSE Group, also part of the RWE Group, outlined the requirements for network planning software (NPS) from the user's perspective by defining both the engineering and IT architectures. This was in response to a massive update of VSE Group's business applications, network information and control systems — SAP, supervisory control and data acquisition (SCADA) and geographic information system (GIS) — installed from 2004 to 2009.

Systematic implementation of NPS was one of the last pieces of the puzzle when it came to VSE Group's IT tools. NPS is expected to replace the manual creation of network models, which is less precise, very time-consuming and not flexible with regard to maintenance needs and updates. This form of network modeling affected, in a negative way, the quantity and technical capability of network analysis.

The NPS now available on the market offers plenty of analytical and calculation functions that often exceed the quality and range of technical network data grid operators typically maintain. VSE Group's minimum requirements for NPS comprised the calculation of symmetrical load flows, symmetrical three-phase, and unsymmetrical single-phase and two-phase short-circuit currents.

Other voltage-related analytical functions included rationalizing the feeder measurements (PQ values), determination of the capacitive currents in the 22-kV network, contingency analysis in the 110-kV system and determination of low-voltage (LV) fuse ratings. The duration of large network model calculators of up to 100,000 nodes should be minimal (completed within a minute). However, these performance functions were not the main driver of this project, as they were already specified by the NPS.

The resulting technical specification focused mainly on data conversion and the consequential software processing. The data had to be convertible from any voltage level so users could analyze the high-voltage (HV) system and domestic load conditions. The large geographical HV/medium-voltage (MV) network models had to be capable of representing the grid, which has an area of 16,200 sq km (6,255 sq miles). Thus, it was essential for the NPS to have simple graphical representation of the results on the network diagram. This would enable users to distinguish individual feeder/substation-based supply areas or devices with some of the parameters beyond the area being studied.

Editing, exporting and printing functions completed the VSE Group's requirements package, which was used as a basis for the contract tendering process.

The source network database from the GIS had to be able to store three independent geographical grid models for the HV, MV and LV grids. The parameters for each of the grid models were specified as follows:

High-voltage grid
The HV model would represent specifically the 110-kV grid in the complete supply area of VSD. The (external) transmission system operator's 400-kV grid would be represented by the in-feeds connected to a 400-kV bus bar. The subsequent transformers' 400/110-kV and 110-kV overhead lines would be represented by electric parameters while bus bars and switching components would be converted from the GIS without any simplification. The end stations' 110/22-kV transformers would be represented as a load. The HV grid would be designed to operate as a meshed connected network; therefore, the contingency analysis would be used to identify bottlenecks in the grid.
Medium-voltage grid
The MV model would represent one of the eight subregions of the supply area, so, in practice, eight different MV models would be required. In-feeder models representing the short-circuit conditions in the upstream 400-kV and 110-kV grid would be connected to a 110-kV bus bar. Unlike in the HV grid, the 110/22-kV transformers would be modeled by the transformers' electrical parameters. The downstream 22-kV lines and 22/0.4-kV transformers would be modeled by standard parameters. Unlike in the 400-kV grid, the end stations would be modeled completely, including the 22/0.4-kV transformers, with loads to represent the outgoing 0.4-kV feeders. It should be possible to find the optimum transformer tap position and analyze the impact of the tap positions on the 110/22-kV and 22/0.4-kV transformers in a complex 22-kV model. The load trim function is important for this submodel to scale down the loads to real conditions on the system.
Low-voltage grid
The 0.4-kV model would represent a city or village. Apart from the 22-kV in-feeds, the whole downstream 0.4-kV grid would be converted from GIS with a ratio of 1:1, respectively. The end loads would represent individual customers and only standard-type parameters would be required to model the electrical parameters of individual elements. The dimensioning function would need to check the appropriate fuse ratings that protect the equipment and satisfy the 5-second disconnection limit required under fault conditions.

All three models (110 kV, 22 kV and 0.4 kV) would need the option to be virtually interconnected to allow for an analysis of mutual impacts between voltage levels in the most precise way.

Network Planning Software Selection

At the end of 2009, the VSE Group circulated a tender for the future supplier. A multi-criteria assessment was used to select the final supplier. VSE Group made the decision based on the total cost of ownership, including five years of maintenance, and the evaluation of technical requirements. Total cost and compliance with the technical specification were each assigned a 50% share.

In 2010, the first part of the contract, the business blueprint of the project, was elaborated in cooperation with all the interested vendors and approved. In 2011, the second part of the contract — this included modification to the existing GIS system, development and supply of the conversion tools and the NPS — was considered. This process resulted in the award of the VSE Group contract to Siemens Power Technologies International (Siemens PTI) in October 2011.

The Network Planning Software

The Siemens software package, containing the network planning software PSS SINCAL version 8.5, included the related software to convert network data, background graphics and, last but not least, the updater of network data. User requirements, collected from utility users around the world, are integrated into this NPS in the form of regular updates, which benefits both the software development as well as the customer. Two system enhancements within PSS SINCAL were the result of the VSE Group's recommendations.

The quality of the distribution system model has a direct, significant impact on the quality of the analysis; it is even more important than the features of the software. Furthermore, the time spent on creating a model is a critical factor.

The project consisted mainly of the creation of a conversion tool representing the interface between two standardized software products. Simply, the conversion tool reads the data in the GIS and translates it into the data format language of NPS.

Apart from the GIS, the conversion tool does not communicate with any other system, such as SCADA or SAP. During the conversion, auxiliary information (databases) is added to the converted GIS database. With this feature, it is possible to enter data from the in-feeder database in addition to the equipment and protection databases, including the MV and LV network electrical parameters not available in the GIS.

In addition, the recorded power flow measurements database is important as it contains measurements from MV feeders (that is, the most downstream locations in the network, where load and generation profiles are systematically and continuously measured). The measured MV data is used in NPS to scale the MV and LV loads (represented by their maximum or installed power values) and model the real conditions in a grid as precisely as possible.

These auxiliary databases are prepared by VSE Group administrators. This proved to be much simpler in the creation and maintenance of the interfaces than other systems.

The conversion tool, besides mapping an auxiliary database, also creates additional information. For instance, for HV and MV networks, the individual element names are predefined within each business or technical system, but this is not applicable for LV grids. Therefore, the tool traces line segments during the conversion process and assigns them names according to the LV feeders in the MV/LV substation.

In the solution, the conversion tool is integrated in the GIS system, thus the GIS is used as a middleware when accessing the source database. Therefore, the entire data selection process for future conversions is very interactive. The user can select any area using standard tracing and selection tools in the GIS and then convert it to PSS SINCAL.

With this automated conversion of data, the user is able to create an exact distribution system model within a short time. In the past, the modeling of one of the VSE Group's MV subregions, approximately one-eighth of the VSE Group's complete system, took up to 500 hours. Today, the newly automated conversion solution creates a more precise grid model of similar size significantly faster, in a maximum of 3 hours. This new solution has the potential to reduce the time for creating a distribution system model by some 99%. It is evident the systematic database of technical data in GIS marks an important milestone for the improvement of data modeling and grid analysis.

Planning Software Solution

The solution implemented at the VSE Group has improved the quality of network analysis significantly. The grid planning specialist now can spend more time analyzing the grid instead of manually creating and maintaining the network model. Because it is a 1:1 conversion from GIS, the results of different calculations can be mapped easily with other systems and linked to equipment in the real network. For example, currently, the load flow results for MV levels are used as one of the criteria to prioritize equipment maintenance in the utility's internal software related to optimized maintenance.

The available load capability calculated by NPS serves the operational grid planners in their daily work. Three-phase maximum short-circuit currents are used for the dimensioning of lines, bus bars and transformers. Single-phase short-circuit currents are used for the dimensioning of earthing systems and neutral point impedances (for example, Petersen coil or resistor).

The main driver of the project was to be able to analyze and design the so-called HV and MV target networks. Target network means the future design of the network optimized from the capital and operational costs point of view. This type of network should not contain any redundant equipment, but it should still satisfy the N-1 reliability criteria on the HV and MV level.

Currently, the VSE Group is in a transitional phase, using NPS to quantify the target network design proposed by the planning specialist according to the utility's grid planning guidelines. The resulting diagrams of the target network can be exported in some vector picture formats, making it possible to publish it using standard picture viewers for all specialists involved in the entire network planning process and operations, without the need for them to be familiar with the functionalities of NPS.

The next step will probably be the export of data from PSS SINCAL to a special optimization tool developed by the RWE Group that will enable the full process of defining the target grid to be fully automated.

What the Future Holds

In the future, driven by the increasing penetration of distributed generation, particularly photovoltaic and cogeneration, it can be envisaged that the unbalanced power-flow calculations and dynamics available in PSS SINCAL will be used in the VSE Group's system development studies.

The solution adopted by the VSE Group was developed according to the technical specification through the mutual cooperation of experts from Siemens PTI, L&Mark, ArcGEO and the VSE Group. It was approved by successful system acceptance tests in October 2011, creating a solid basis for grid planning support in the VSE Group.

Acknowledgement

The authors wish to acknowledge the excellent services provided during the project and the support given to this article by Dr. Thomas Bopp and Vladimir Kanas of Siemens PTI. Similarly, Janos Drienyovszki and Robert Vuleta, both of L&Mark, and Marián Marcincák of ArcGEO provided invaluable contributions.

Jozef Tomcik ([email protected]) studied power energy at the Technical University of Košice before joining the VSE Group in April 2004. Since July 2007, Tomcik has been working for the distribution operator as a specialist in grid calculations. Furthermore, he is participating in some external working groups, including CIRED's grid development and Eurelectric's NE T&D Interface.

Peter Mento ([email protected]) studied electrical engineering at the Technical University of Košice and joined the VSE Group in July 1997. Since January 2008, Mento has worked for the distribution operator as a specialist in grid calculations.

Jaroslav Serdula ([email protected]) studied power energy at the Technical University of Košice and joined the VSE Group in July 1995. Since September 2010, Serdula has worked as a specialist in the department for the renewal and development of medium-voltage and low-voltage grids.

System Statistics

The VSE Group, part of the European RWE Group, comprises a number of companies, one of which is VSD, the distribution system operator in Slovakia. Annually, the utility distributes 3,800 GWh of electrical energy to a geographical area equivalent to one-third of eastern Slovakia, some 16,200 sq km (6,255 sq miles). The distribution system supplies more than 610,000 households through 34 110/22- kV substations and 6,000 22/0.4-kV stations. The total length of the 110-kV, 22-kV and 0.4-kV overhead lines and underground cable networks is 21,000 km (13,049 miles).