Photo courtesy of ewz.
High voltage cables have to be prepared for future power flows and changing demands.
High voltage cables have to be prepared for future power flows and changing demands.
High voltage cables have to be prepared for future power flows and changing demands.
High voltage cables have to be prepared for future power flows and changing demands.
High voltage cables have to be prepared for future power flows and changing demands.

Switzerland's Largest City Plans 2050 Target Grid

Aug. 9, 2023
Switzerland’s ewz uses data, calculations and simulations to define cable requirements and start developing its 150-kV grid of the future.

As the distribution system operator for Zurich, Switzerland, ewz is developing its 150-kV grid to meet estimated demand in 2050. This grid of the future must also meet ambitious goals the city of Zurich has set for energy efficiency, sustainability and reduction of carbon emissions. To reach these goals, renewable energy (mainly photovoltaic systems), electric vehicles (EVs) and electric heat pumps have been identified as important focus areas. As the distribution system operator, ewz must provide the necessary infrastructure.

Additionally, a major change is planned for the network topology of Zurich’s 150-kV distribution grid. Today, four coupling substations connect the 150-kV grid to the national 220-kV grid. Two of the stations are in the south, far outside the city of Zurich. From these, energy is transmitted through the 150-kV lines to the city. A new 220-kV/150-kV coupling substation right at the border of Zurich will replace them. The type and number of cables connecting this new coupling substation to the city must be planned.

Regulations regarding magnetic field emissions are strict in Switzerland. Therefore, ewz decided the cable type for its future high-voltage grid must be changed. Instead of single-core cables, three-core cables will be used because they do not require additional shielding. To make this change, ewz had to define completely new cable requirements that would be suitable for several decades, handling power flows today and well into the future.

Maximum Cable Current

One aim was to achieve a rated cable current as high as possible with a given conductor diameter. Thus, cable load and loss factors — both derived from load profiles — were used. The load factor is the ratio of the average current per day to the maximum current. The loss factor includes thermal inertia.

For permanent operation mode at full capacity, the factors are equal to one. For partial load operation mode and limited times of high demand, their values are below one. If these operation modes are applied to the same cable, the partial load allows a higher ampacity of the cable. To increase the rated current for a given cable size, not only the rated current but also the load factor is needed. Therefore, the future load and future load profile needed to be estimated.

2050 Expected Demand

As electrical supply must be available in all conditions, the maximum load or production case was used for planning purposes. In Zurich, the maximum demand occurs during a cold and cloudy winter period, including high demand from electric heat pumps and electric vehicles but nearly no production from photovoltaics.
To estimate the city load in 2050, ewz used various scenarios with expected consumption and production for the future. The scenarios included a substantial load rise from increases in population, building density in the city and electric vehicles becoming 90% of vehicles on the road by 2050.

Thanks to Zurich’s municipal structure plan, ewz was able to evaluate the effects of an increased population. The city is divided into building zones, and the municipal structure plan determines the maximum building floor space for each zone. With the planned increase in building density, the maximum floor space of buildings also is expected to rise. The potential for city load growth is in the zones where the maximum allowed floor space has not been realised yet.

Furthermore, the municipal structure plan assigns domestic and commercial building utilization to the zones. For both purposes, load densities (volts-ampere/sq m) were defined and assigned to the building floor spaces of the zones (sq m). As a result, ewz was able to aggregate the load for domestic and commercial purposes for each zone. The expected EV loadings, with load management, were added to these zone loads. The final zone loads were assigned to the city substations.
To establish substation load profiles, the domestic, commercial and EV loads were aggregated proportionately with corresponding profiles. Considering EV load management, ewz estimated a proportion of EV charging would be shifted into the night.

All zones with their accumulated loads and profiles were assigned to city substations. The sum of the substation loads provided the maximum possible city load. In the next step, this maximum possible city load was adjusted to the expected city load from the reference scenario in 2050. The substation loads were weighted accordingly. The results for each substation were a maximum load and an aggregated load profile. After defining the substation loads and 24-hr profiles for 2050, the model was ready for use in the planning process.

Planning Requirements

To determine the critical cable loadings, planning requirements were established. Emphasis was put on outage conditions, during which the security of supply should still be guaranteed. These conditions strongly depend on the topology of the grid.

For the city of Zürich, high-voltage cables were separated into two groups and different planning requirements were applied:

  • 150-kV cables connecting a coupling substation to the substations in the city — As these cables supply the city, they needed a high-rated current. Even if one of the connecting cables were to go out of service, the coupling substation should still feed its assigned load.
  • 150-kV cables within the city, linking the substations — As this part of the grid is meshed, these cables transport less energy and require a lower-rated current. If any cable were to fail, the grid should remain in operation and stay within the cable capacity limits. In Zurich, because of frequent urban construction, several cables at a time often must be put out of service to ensure safety. Depending on the location of construction sites and taking into consideration the possible outage of a cable, the worst-case condition is one cable supplies a substation.

According to the planning requirements, the number of cables connected to a substation was assumed:

  • For the new 220-kV/150-kV coupling substation, the number of cables in operation and the maximum coupling transformer power provided an estimated cable current. The number of cables was adjusted to make sure this cable current would be available.
  • For the substations in the city, it was determined in the planning requirements that one cable in operation should be capable of feeding the maximum
    expected substation load — thus resulting in an estimated cable current (which was lower than that of the coupling substation).

These estimated currents were verified with load flow simulations.

Load Flow Simulations

Load flow simulations for 24-hr profiles were performed according to the planning requirements. To switch the cables off and on for calculations, the load flow software was controlled by a script. The simulations for each cable in 24 hourly currents per day resulted in about several thousands of values in total.

To evaluate this large amount of load flow data, the calculated maximum current for cables was visualized in one distribution graph, from lowest to highest current. The same approach was taken for the cable load factor and cable loss factor of the cables. They were calculated from the 24 current values per day for each cable and case, and then shown as distribution.
These graphs were put together for cables connected to coupling substations on the one hand and for meshed cables in the city on the other hand. Then the distributions were compared to the respective estimated cable current. Results were especially interesting for the parallel cables, which lead from the coupling substations into the first city substations. About 50 % of load flow currents exceeded the estimated current. As these cables have different lengths and types — and, therefore, varying impedances — their maximum currents were highly unevenly distributed. It showed the estimated current did not account for any nonideal behaviour of the cables.

Again, it was the aim that rated current would be available and realistic. The final cable rating was chosen in a way that 90% of the currents from load flow calculations were smaller or equal. For the last 10% of load flow calculation currents above the chosen rated currents, it was decided additional measures would be investigated. For the meshed cables within the city, the load flow simulation resulted in a current distribution below or equal to the estimated current. Therefore, it was defined as the rated current for these cables.

With the chosen rated currents, cable load factors and loss factors from the 24-hr time load flow simulation currents were calculated. The rated load and loss factors were determined in a similar way to the rated current, so again, 90% of the calculated factors were below the chosen rated value. To have consistent factor requirements, the same load and loss factors were defined for all cables.

When analyzing the hourly currents for the cables within the city, no violations of the chosen rating were found.

The Target Grid

For 150-kV cables connected to the new coupling substation, which have different cable types and lengths, it became clear to ewz that load flow simulations were essential to perform because of the highly uneven current distributions. Thanks to the cable load and loss factors, determined from the hourly time simulations, a higher cable ampacity was achieved.

Based on the cable requirements, ewz developed a 150-kV target grid. It will serve as the backbone of the electric infrastructure to provide energy for a growing population and enable an increase in renewable energies and a reduction of carbon emissions in the city of Zurich.

Klaudia Madeira Bein ([email protected]) has a MSEE degree from the University of Karlsruhe. From 1994 to 2016, she worked as an electrical engineer for power plant system engineering at ABB, Alstom and GE in Switzerland. In 2016, she joined ewz, the distribution system operator of the city of Zurich. She is a senior specialist for grid development, and her responsibilities include the future 150-kV target grid.

Britta Heimbach ([email protected]) holds a MSEE degree from RWTH Aachen University, a PhD degree from Ruhr University of Bochum and a MBA degree in business administration. She has worked as a specialist for grid planning and asset management at RWE Energy in Germany from 1993 to 2008. Since 2008, she has been with ewz in Switzerland and currently heads grid development.

Juerg Dieter Bader, PhD, studied mathematics and physics at ETH Zurich. He was with ewz from 1995 to 2023, focusing on grid concepts and development. His  responsibilities included statistical models for long-term forecasting of electric power demand and renewable energy sources.

Andri J. Casura ([email protected]) gained a MSEE degree and a master’s degree in information technology from ETH Zurich. After dabbling a few years in protection, automation and project management, he achieved a master’s degree in management, technology and economics also at ETH Zurich. In 2015, Casura was appointed to the newly formed high-voltage division, which he leads into the future as a visionary.

Raffael La Fauci ([email protected]) has a MSEE degree and PhD degree in electrical engineering from ETH Zurich. He leads the grid concepts and development department at ewz, the distribution system operator for the city of Zurich, Switzerland.

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