DEVELOPMENT OF PLC SYSTEM WITH LARGE CAPACITY AND HIGH RELIABILITY

In Kansai Electric Power Co., Inc., smart meter data is transmitted from meter to the concentrator by RF mesh system or PLC system. We mainly apply PLC system to multi -dwelling buildings. In order to extend the coverage and improve usefulness of PLC system, we developed new PLC system especially for the multi-dwelling buildings receiving electricity at 22kV, which have sub transformer chambers.


INTRODUCTION
In recent years, smart meters have been introduced in many countries.The Kansai Electric Power Co., Inc. (KEPCO) started basic research to realize smart meter system in 1999, and started installation in 2008.As of December 2016, KEPCO has implemented approximately 7 million smart meters, and will have completed smart meter installation in 2022.In Japan, smart meter must have the function to measure 30-minute interval consumption data and transmit those data every 30 minutes.To realize those requirements effectively, as communication technologies, we apply RF mesh technology and PLC technology.PLC system is applied to multi-dwelling buildings which receive electricity at 6.6kV or 22kV.In the case of applying PLC system to multi-dwelling buildings receiving electricity at 22kV, concentrator for PLC system must be installed in each sub transformer chamber due to the capacity limitation of our PLC system caused by its transmission speed.In order to solve this problems, we developed new PLC system.

Smart Meter
Fig. 1 shows the smart meter communication network in KEPCO.For residential customers, KEPCO mainly apply RF mesh system and PLC system; either one of communication system (unit) is embedded in each smart meter [1].In RF mesh system, the data measured at each smart meter is transmitted to concentrator through neighbouring smart meters.However, in the case of multidwelling buildings, RF mesh system is not suitable communication technology due to the difficulty of penetration.Therefore, for multi-dwelling buildings, PLC system is mainly applied to the smart meter communication system.In our PLC system, the data measured at each smart meter is transmitted to concentrator through neighbouring smart meters, too.

PLC System for Smart Meter
PLC system is applied to multi-dowelling buildings, but there are two types of multi-dwelling buildings based on their receiving voltage.One is receiving electricity at 6.6kV; 6600V/200V transformers are installed in a transformer chamber (shown in Fig. 2).This method is generally applied to middle scale multi-dwelling buildings.The other is receiving electricity at 22kV; 22000V/400V transformers are installed in a transformer chamber and 400V/200V transformers are installed in sub transformer chambers to distribute electricity to each customer (shown in Fig. 3).This method is generally applied to large scale multi-dwelling buildings.
Our PLC system adopts "ITU-T G.9905" [2] as routing algorithm so that the necessary time to collect 30-minute interval data is relatively short in spite of its communication speed is low.Nevertheless, in order to collect all the 30-minute interval data every 30 minutes, the maximum capacity of the concentrator must be limited to 180 smart meters.This number is not enough to apply our PLC system to large scale multi-dwelling buildings.In addition, concentrators are connected to secondary side of 400V/200V transformers; we must install many concentrators on secondary side of each sub transformer and optical fiber to each sub transformer chamber.This causes expensive construction cost and complicated system configuration.
In order to solve these problems, we attempted to develop new PLC system that the concentrator has the ability to accommodate the larger number of smart meters and can be connected to secondary side of 22000V/400V transformer directly.

DEVELOPMENT SUBJECT
In order to realize the required PLC system, there are mainly 3 development subjects as follows:

OUTLINE OF DEVELOPMENTS 4.1 Development of Coupler for 400V Wiring
To suppress the cost of newly developing coupler, we made use of the conventional coupler case, instead of that, changed internal components, such as fuse, varistor, resistance elements.

Increase in Communication Speed
The increase in communication speed was required to collect all the measured data every 30 minutes in Largescale multi-dwelling buildings, which might have as many as several hundred residences.Therefore, we changed communication specification to increase the concentrator capacity.In conventional our PLC system, 3 to 5 concentrators are needed to collect that amount of data.We set the development goal: new concentrator's capacity should be more than 500 smart meters.In our PLC system, securing stable communication between concentrator and the first smart meter is essential; in our conventional PLC system, we have confirmed the stable communication between these two.Accordingly, in order to realize our development goal, we decided to enable the communication speed variable between lowspeed communication and high-speed communication from the first smart meter to other smart meter ahead.To be concrete, in case of poor communication environment, low-speed communication is selected, or in other cases, high-speed communication is selected.As a result, even if the number of hops increases, communication time should be supressed.Fig. 5 shows the concept of the newly developed PLC system. 10 communication speed options are added as in High-speed communication mode, as shown in Table 1.
Paper 0302 CIRED 2017 3/4 Furthermore, in the case we just simply increase the communication speed, there is a possibility that the system could not collect smart meter data every 30 minutes because of the increase in data volume.Therefore, in addition to the increase in communication speed, we performed data compression reviewing the meter data frame structure.Consequently, we could expand the capacity of concentrator from 180 smart meters to 540 smart meters.In addition to that, we confirmed the data collection time for 540 smart meters is reduced by approximately 90%.

Evaluation of Signal Attenuation
Newly developed PLC system is required to have the ability to communicate through the longer distance via sub transformer, between the concentrator and first smart meter.In order to confirm the communication reliability, we measured signal attenuation at several actual facilities.The facilities and the measurement results are shown in Table .2,and in Fig. 6, 7.  the measured values are within the tolerance.Even if the signal attenuation deviates from the criteria: communication environment is poor, the system automatically select the suitable communication mode, that is the Low-speed mode.
Based on these verification results, we started introducing newly developed PLC system to actual multi-dwelling buildings which receives electricity at 22kV, and we confirmed stable communication.

Fig. 5
Fig. 5 Concept of the newly developed PLC systemTable. 1 Communication speed options

Table . 2
Result of signal attenuation