DCUs and/or IP gateways?

 

Data Concentrators and Telecom Gateways enable digital grids

Power Line Communications (PLC) have been widely adopted in many smart meter roll-outs where medium voltage (MV)/ low voltage (LV) transformers in Secondary Substations (SS) feed a large quantity of meters. This is  common for the grids in Europe. The network architecture of PLC-based metering system is different from the architecture of a RF-based system, or a metering system that relies on the telecommunication services provided by public cellular network operators.

As the telecommunication media are the power cables, smart meters connectivity depends on the LV grid topology. This implies that all smart meters attached to each LV grid segment that is linked to a MV/LV transformer naturally form a telecommunication subnetwork that is managed by the equipment installed in the SS.

Data concentrators installed in SS not only play a key role in any smart metering architecture but also are a key component for grid monitoring applications.

If we examine a PLC data concentrator, this complex device integrates several functions.  Firstly, several telecommunication technologies are supported. On one hand, a PLC coordinator or base node is responsible for managing the PLC subnetwork and providing secure telecommunication services to all smart meters attached to this subnetwork. On the other hand, it integrates a cellular interface so that a continuous connection is guaranteed to the central system.

To ensure this continuous connection, some data concentrators manufacturers, such as ZIV, add  the capability to its devices to set up an alternative connection through a backup public network operator so that whenever a network problem arises with the main public network operator, the connection with the central system is not lost, thanks to the backup operator. Secondly, a task scheduler is also included so that data can be regularly gathered from the smart meters (based on open protocols such as DLMS) and stored internally. All this data is delivered at scheduled times to the central system using web services. Additionally, either on demand requests coming from the central system or important spontaneous events delivered by the smart meters will be serviced based on their priorities.

Data concentrators monitor voltage and current levels at the LV side of the MV/LV transformer and additional monitoring of other parameters, such as power quality and service continuity. Besides, they can also detect faults and, in certain grid topologies, even MV high impedance faults. Both features, monitoring and fault detection, make of the data concentrator a key component in the new digital grid.

Certain Utilities consider that not all secondary substations require a data concentrator. Nevertheless, it’s important for them – due to economies of scale – to maintain the PLC telecommunication interface to all their smart meters. Those Utilities rely their smart metering architecture  on telecommunication gateways.As depicted below, telecommunication devices do not have to include either temporary data  storage or grid monitoring features. Such devices are plain telecommunication devices that integrate both, a PLC coordinator/base node and a cellular interface to ensure a continuous connectivity to the central system. It should be highlighted that PLC gateways do not need to be installed in secondary substations. They can be installed in different locations on the LV grid.  For instance, they may be installed in the LV fuse boxes, that are the boundary of the Utility’s private distribution network or even close to the smart meters. In this way, this device provides flexibility for many smart meter roll-outs allowing it to cope with difficult installation sites at a very reasonable cost.

An important component when deploying a smart metering infrastructure based on telecommunications gateways is the so-called virtual data concentrator. This Software component, that is installed in the Utility’s data center, is responsible for gathering the data from all smart meters accessible through the telecommunication gateways. In terms of telecommunication protocols, the virtual data concentrator accesses all the smart meters using the well-known DLMS protocol, which demands higher data usage to gather all metering data values. As telecommunication gateways do not provide temporary data storage, metering data availability greatly depends on the telecommunications network reliability. For this reason, features such as dual SIM for assuring a continuous link with central system are mandatory. Also, it is very important to know the connectivity status with all the smart meters attached to the subnetwork created by the telecommunication gateway. To do so, PLC technologies such as PRIME (http://www.prime-alliance.org/) have internal mechanisms (ALIVE messages) that are regularly exchanged between the telecommunication gateway and every smart meter in the subnetwork.

 

 

Data concentrators or telecommunication gateways?

Some utilities decide to use data concentrators or telecommunication gateways based on the size of the site, i.e. the number of customers associated with each SS. These Utilities therefore tend to use telecommunication gateways in low density areas, where a MV/LV transformer feeds up to 25 customers. For higher density areas, data concentrators are the preferred option due to its advanced grid monitoring functionalities, that provide valuable information related to the state of the LV grid (real time information).

Other utilities, independent of the number of customers, design their smart metering architecture only using data concentrators, as the added value provided by these devices, thanks to their grid monitoring functionalities, will pay off the additional investment.

However a PLC-based smart metering architecture may be based on both, data concentrators and telecommunication gateways. As stated above, from a logistics point of view, it is important that all smart meters share the same telecommunication interface, in this case, a PLC interface. This is a critical requirement to keep the deployment costs under control.

The challenge of meeting affordable costs in dispersed rural  areas with poor telecommunication connectivity possibilities and low-density SSs, can be improved with the flexibility provided by the telecommunication gateways. The additional functionality integrated in data concentrators make them a critical grid monitoring node that allows for enhancing current grid visibility by deploying innovative algorithms for different functions such as, customer phase and line detection, non-technical loss reduction or LV fault detection.

Both, data concentrators and telecommunication gateways, contribute to an evolved digital grid architecture, offering a high degree of cyber-security and allowing the use of existing PLC smart meters. A PLC interface in smart meters is seen by many utilities as a major requirement in keeping their smart metering system costs under control.

ZIV, since 2007, has been the preferred option of many Utilities that have relied on their PLC smart metering systems using ZIV data concentrators and telecommunication gateways. The reliability of its PLC telecommunications is key for maintaining a stable telecommunication subnetwork where smart meters are attached. The performance of the whole smart metering system relies on how stable the PLC subnetwork is. ZIV has proven this in all the deployments ZIV has participated in with their data concentrators and telecommunication gateways, which are also a key component to ensure interoperability with different smart meter vendors.

 

“An architecture based on both,  DCUs and Gateways,  ensures reliability and flexibility. DCU’s also provide additional LV grids monitoring functions”

by Txetxu Arzuaga – for Metering International Cover Article, Issue 5 – 2018

 

 

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