Peculiar planning challenges abound in the development of distributed generation in special horticultural areas.
In the Netherlands, combined heat and power (CHP) plants are used extensively in areas designated for horticultural use and more are planned for the future. The installation of CHP plants was mainly heat driven, with the generated electrical energy being used for loss compensation and peak load reduction. However, by the mid-1990s, these units were producing carbon dioxide. By 2004, the production of crops in greenhouses requiring illumination started to increase rapidly, leading to a significant increase in the number of CHP plants. During the period of 2004 to 2008 in the Netherlands, the total installed power increased from 1000 MW to 2500 MW.
Local governments in the Netherlands designate rural areas for horticultural activities, and each greenhouse constructed includes a CHP plant. Therefore, the clustering of horticultural activities and the associated CHP plants results in the grid operator, one of which is Stedin, having to accept a high-penetration level of CHP plants on the medium-voltage (MV) distribution network.
Netherlands Legal System
To comply with the legal and regulatory framework in Holland, Stedin and other grid operators are obliged to accept a connection to the local distribution network within a period of 18 weeks from receiving the connection application. While acceptable for a single connection, this is a demanding standard for connecting multiple CHP units to MV networks that have limited capacity. These sources of distributed generation (DG) frequently require reinforcement of the MV distribution network and subtransmission grid system.
In some cases, it has been necessary to reinforce the high-voltage (HV) grid system. However, due to the time required to secure additional rights-of-way for system reinforcement, the legal obligations placed on the distribution system operators (DSOs) and transmission system operators (TSOs) are such that they cannot respond to connection requests.
The development of efficient MV grids in horticultural areas is a complex problem that starts with consideration of grid planning. Stedin follows a three-level approach to planning: consider the impact on the transmission system (level 1), followed by the impact on the subtransmission grid (level 2) and, finally, the connection to the MV distribution network (level 3).
As the construction period of a CHP is approximately nine months, provided the customer submits an application at the outset of the project, the connection of a single CHP unit or multiple units can be completed within the project time frame without reference to level 1 and level 2 considerations. Levels 1 and 2 are presented in detail, while the connection principles of level 3 are fully covered in the Dutch regulation.
To plan the grid system and distribution network, utilities need to develop, in advance, scenarios for specific geographical areas. The expected load growth and the number of CHP plants to be installed, together with technical and economic developments that could influence the load growth, are among the key issues that must be considered. In this way, the system bottlenecks for each scenario are identified.
Oostland II Project
Oostland is an area designated by the local authority for the development of horticultural activities. The area is already partially developed with greenhouses and CHP plants, with an installed capacity of some 200 MW already connected to the MV distribution network and significant further development and installation of more CHP plants scheduled for the next few years.
The initial phase of the project is a three-stage assessment of the impact of the anticipated total development on the existing electricity infrastructure. The first stage is consideration of the total estimated load, how existing substations can be extended, where new substations are required and interconnection to the HV transmission system.
The second stage includes studies of the existing subtransmission and distribution grid in the Oostland area. The information is then used in studies on the potential locations of CHP plants and where the DSO has to install a substation. Equally important is the substation selected for the interconnection to the subtransmission system.
At the third stage, the MV network planning stage, all these interacting decisions are considered prior to the design process that specifies the means of affording the physical connection.
For the grid-planning process, four scenarios are developed based on experience gained in comparable horticultural areas, new technological developments and an inventory of all connection requests received in the Oostland area:
Scenario one is the absolute maximum scenario in which the politics that promote the use of CHP plants for heat and electricity generation are considered. These strategies encourage the market gardener to supply heat for district heating outside the horticultural area. In the first couple of years, the demand for district heat grows, but the scenario allows for the expected fall in demand due to the insulation improvements in insulating houses and offices. This scenario allows for the district heating system to be abandoned. The maximum installed capacity of CHP plants is 940 MW.
Scenario two represents the maximum growth in the Oostland area, taking into account a reduction of the horticultural area during the period 2011 to 2015. However, it assumes the existing district heating will stay in service, but the demand for heat decreases as in the first scenario. The maximum installed capacity of CHP plants is 570 MW.
Scenario three is based on an inventory of the market gardeners in the Oostland area that has a growth potential of 10% per annum until 2015. In this scenario, the number of CHP plants decreases after 2015 due to new forms of sustainable heating techniques, for example, geothermal heating. The total power of CHP plants is 350 MW.
Scenario four is formulated to represent the minimum scenario in which economical developments discourage the installation of CHP plants and, beyond 2015, CHP plants are no longer used for horticultural applications. In this scenario, the capacity of the total new installed CHP plants is 160 MW.
Alternative Grid Designs
To determine the optimal solution for the connection of CHP plants in the Oostland area, alternative grid designs are derived. The starting point is the alternatives have to be within the grid operator's terms of reference. For level 1, the alternatives may be summarized in the following ways:
- Extension of an existing substation
- Construction of a new substation at the TSO level
- Control of power through high-voltage DC.
For level 2, the alternative solutions may include:
- Change of grid voltage
- Extension of the current grid philosophy
- Application of DC in MV grids
- Upgrade of grid components
- Construction of new substations at the DSO level.
The optimal solution can be defined once the project boundary conditions are defined. For each scenario, the alternative boundary conditions for the project have been decided as follows:
Physical space. Is there sufficient space to extend existing substations and build new substations?
Operation of the subtransmission grid. All grids have to be designed and operated to conform to current grid code with the consequence that Stedin has set the maximum number of transformers per substation site at four.
Energy transport capacity. This capacity has to be available within two years after receiving direction of an interconnection order.
The Dutch TSO has enforced some transport capacity limitations linked to exceeding system fault levels in some substations in addition to congestion of the transmission grid. An overview of a part of the existing transmission grid for the province of Zuid-Holland is presented that identifies the Oostland area.
The specified fault levels for substations 5 and 10 are exceeded, and grid congestion prohibits net energy export to the transmission grid from substations 2, 5 and 7. As these limitations are not within Stedin's area of responsibility, this limitation on energy transport capacity is considered a boundary condition.
For both levels per scenario, all alternatives are considered with the boundary conditions. At level 1, this resulted in two alternatives:
The capacity of substations 2 and 9 are used to connect CHP plants up to the minimum load without export capability. The CHP plants of the Oostland I project are already connected to substation 6, and this substation has to be extended. In 2013, a new TSO substation has to be in operation.
The capacity of substations 2 and 9 are used to connect CHP plants up to minimum load without export capability. In addition, substations 5 and 6 have to be extended. Also, in a different location than in the alternative one, in 2013, two new TSO substations have to be in operation.
At level 2, only one alternative has been selected. The existing 10-kV MV grid will be extended and a new 23-kV grid will be built using, if possible, a step-by-step plan. The steps that have to be taken per scenario show a great similarity. A major factor in the step-by-step plan is the period required to build a substation at the DSO level (two years) and to build a substation at the TSO level (four years).
Based on the previous considerations, grid operator Stedin decided on the following actions:
- Use scenario two as a reference
- Construct a new TSO substation
- Extend three existing DSO substations
- Construct five or six new DSO substations
- Introduce a new MV grid voltage level of 23 kV.
The actual development in the Oostland area will determine the rate and time scales for the new substations to be constructed.
Responding to Rapid Growth
The development of horticultural areas and the number of CHP plants installed has grown rapidly in the past two years. To provide all applicants with a connection to the local MV grid, and in order to facilitate all possible power flows, the grid operator must develop the grid in a proactive way.
Due to the rate of horticultural area development and the extended time scales associated with grid transmission projects, MV system development is limited to TSO substation extensions. The greatest challenge facing grid operators in the Netherlands is to complete the installation of DSO substations in phase with the development.
The commissioning of DSO substations can be easily postponed to respond to a late change in development. One of the largest risks that DSOs have to consider, which involves considerable capital investment, is the installation of an underground cable connection in advance that subsequently could prove to be installed in the wrong location.
Edward Coster (firstname.lastname@example.org) earned his BSEE degree from TH Rijswijk in 1997 and his MSEE degree from Delft University of Technology in 2000. He joined Stedin in 2000 as a senior specialist for network planning. In 2006, Coster joined the Electrical Power System Group at Eindhoven University of Technology on a part-time basis to start a Ph.D. research project. Coster obtained the Ph.D. degree in September 2010. His main fields of interest include distributed generation, power system protection, dynamic behavior and stability of power systems.
Dik van Houwelingen (email@example.com) received his MSEE degree from Delft University of Technology in 1985 and subsequently worked on research at the University of Twente before joining the Energy Board in Rotterdam in 1987. Since 2008, van Houwelingen has worked for the risk and portfolio management division of Stedin. His main interests include distributed generation, system development, cables and risk management.