DC Microgrids for Commercial or Industrial Buildings

DC microgrids have become increasingly popular in recent years. Although they offer various advantages, certain challenges must be faced.A fully operational bipolar DC microgrid with a nominal voltage of ±380 V has already been in service at the Fraunhofer Institute for Integrated Systems and Device Technology IISB in Erlangen for several years and is continuously expanded. It serves as a platform for various DC grid technologies developed at the IISB, e.g. DC/DC- or AC/DC-converters, safety components and battery storage systems. As a living lab, the platform is open for partners from research and industry.

The DC microgrid supplies large parts of the building infrastructure, from office workstations and lighting to laboratories and charging stations for electric vehicles. Power is fed from PV strings with different orientations (south and east/west) on the roof, and a maximum combined power of 43 kW. MPP tracking is performed by non-isolated custom DC/DC-converters for each string orientation; MPP voltage for the east/west strings with 20 modules is typically around 700 V, while the strings with a southward orientation and 12 modules are about 400 V.

Energy Storage

Excess energy can be stored in three lithium-ion battery systems developed in-house, each with 20 kWh energy content and a maximum power of 100 kW. Battery system voltage ranges from grid voltage up to approximately 600 V when fully charged. The batteries are coupled to the grid with non-isolated converters as well.

The converters for the PV and the batteries have an efficiency of over 99%. Bidirectional, isolating AC/DC-converters with a combined power of 96 kW can feed stored energy back into the AC grid, e.g. for peak-shaving purposes. When needed, power can be sourced from the AC grid. It is also possible to store energy as hydrogen, or in a liquid organic hydrogen carrier (LOHC).

An overview of the research DC microgrid is shown in Figure 1. The grid covers well over 1000 m² of office and laboratory space in various building parts, with cable lengths up to 100 m to outdoor installations like DC charging or photovoltaic. The realization of the DC microgrid in its current state is part of the publicly funded Bavarian research project “SEEDs”.

 

Figure 1: Schematic overview of the DC microgrid at Fraunhofer IISB

 

Microgrid Application

Since the microgrid is in operation around the clock, the necessary safety components and switchgear are off-the-shelf devices for DC, rather than prototypes. As one can imagine, the range of suitable products is rather easy to survey.

Because the grid is solely fed from power electronic converters, the fault current in the grid is limited and mainly sourced by the output capacitors, thus limiting the fault energy to considerably small values, often insufficient to trigger conventional safety devices [1][2]. Of course, one could simply increase the fault energy by adding more capacitance between the grid’s poles. Considering fault energy as an unwanted attribute in a distribution system, a more elegant solution to the problem is desirable.

 

Figure 2: Mixed AC/DC-power monitoring system

 

AC and DC Power Monitoring System

Instead of relying on energy input to trigger a physical reaction, e.g. melting a fuse wire or activating a solenoid, directly measuring the current and taking appropriate action when a fault is detected yields various advantages.

The sensitivity can be set closer to the low fault energy levels in DC microgrids and may also be adjusted well after installation, without the need to exchange components. This also allows for increased selectivity of protection devices, reducing the risk for downtime in the grid. Besides the improved safety function, the data from the current measurement equipment can also be used to monitor the microgrid status.

For some application areas like electric vehicle charging or smart metering, an accurate and well-defined measurement of the energy consumption is needed. Also, the efficient control of different electrical energy sources - like local photovoltaic plants, fuel cells, battery storage or the AC grid – requires a precise measurement of the electrical power flow to optimize overall efficiency and thus to reduce cost, total energy consumption and CO2 footprint. To achieve this, an accurate AC and DC power monitoring system was developed by Siemens (see Figure 2).

 

Sentron PAC 4200

The key component of this monitoring system is the Sentron PAC 4200 which incorporates two important functions.

First, the Sentron PAC 4200 is used, in combination with an AC current transducer (Siemens 7KT1201), to measure the power fed from the AC grid with an accuracy better than 0.2% according to IEC 61557-12.

Second, it acts as a communication gateway between any RS485 devices, e.g. the integrated DC meters (AcuDC240) and the PC-based data storage-, handling- and visualization system, which was realized with the Siemens “Power Manager”-software.

For precise measurement of the DC energy consumption, the biggest challenge is the accurate current measurement with the least possible offset to avoid an error accumulation over the long measurement periods, which are necessary to evaluate the power consumption of the DC microgrid building demonstrator. It was found that only high-quality triple core fluxgate current transducers like the IT 60-S from LEM can fulfill these requirements.

 

Figure 3: Block diagram of the Ultrastab IT-series fluxgate current transducer

 

IT-series Current Transducers

These IT current transducers are high accuracy, large bandwidth DC-transducers using fluxgate technology without Hall generators. The magnetic flux created by the primary current IP (see Figure 3) is compensated by a secondary current. The zero-flux detector is a symmetry detector using two wound cores connected to a square-wave generator.

The secondary compensating current is an exact representation of the primary current. The Ultrastab IT-series combines all the requirements for power measurement current transducers. They can measure over a wide operating temperature range from −40 to +85 ̊C. Offset and linearity are excellent: over the whole temperature range, offset is between 36 and 400 ppm and linearity between 8 and 12 ppm; the values depend on the model used.

An accuracy of 1ppm is equivalent to 0.0001%. Since the offset is so small, the transducers can be used from a few Amperes up, and just one model can cover the entire current measurement range; other transducers using different technologies would require the use of several transducers to cover the same current range while simultaneously keeping this accuracy level. This yields a non-negligible cost advantage.

 

Figure 4: Comparison of AC and DC efficiency over one workday

 

Monitoring and Data Logging for DC Microgrids

One area of application for precise current sensors with a broad operating range is monitoring and data logging for DC microgrids to evaluate their efficiency. The operation of such a microgrid is usually more efficient than a comparable AC grid. This is mainly because DC/DC-converters require a smaller number of components and less filtering, which are both a source of power losses.

Since most IT and office devices, lighting or e-mobility applications are inherently DC, as well as most regenerative energy sources and storage equipment, their combined use in a microgrid seems self-evident.

 

AC and DC Efficiency

An example comparing AC and DC efficiency on the basis of a typical office building is shown in Figure 4. In both cases, the same type of sources and loads were used. In case of insufficient energy input from the photovoltaic plant, the DC microgrid draws power from the AC grid. Averaged over the whole day, DC operation yields a gain in efficiency of approximately 3%. An additional percent can be gained in phases with high local power generation when input from the AC grid is low, in this case around noon.

Besides the increased power efficiency of a DC microgrid compared to an AC-grid, the easier integration of energy storage systems can add more benefits. The experimental findings presented in Figure 4 demonstrate how the extensive use of locally generated energy can lead to improved DC microgrid efficiency.

To further decrease power delivered from the AC grid, any excess generated (e.g. by photovoltaic) can be stored in local energy storage and used to compensate times with reduced generation (cloudy sky, calm wind) or even AC power failures.

 

DC Microgrids to Improve Energy Efficiency

An optimal sized battery storage allows the minimization of AC grid input for most days of the year. It is estimated that this yields an additional advantage in efficiency of approximately 1% to 3%.

Overall, DC microgrids can help improve the energy efficiency of commercial buildings and reduce the cost of installation as fewer components are needed. Integrating current measurement with DC sensors can be beneficial for the functionality of safety devices while simultaneously providing data for grid monitoring. Efficiency, resiliency and percentage of energy own consumption can be maximized by incorporating local energy storage into the microgrid.[3]

 

References

1. P. Meckler, F. Gerdinand, R. Weiss, U. Boeke and A. Mauder, “Hybrid switches in protective devices for low-voltage DC grids at commercial used buildings,” ICEC 2014.

2. J. Kaiser et al., “Grid behavior under fault situations in ±380 VDC distribution systems,” ICDCM 2017.doi: 10.1109/ICDCM.2017.8001035

3. R. Weiss, L. Ott and U. Boeke, “Energy efficient low-voltage DC-grids for commercial buildings,” ICDCM 2015.doi: 10.1109/ICDCM.2015.7152030