Albert Ruiz

Andreas Sumper

J.-I Cairo

CiteSeerX

 C I R E D 22nd International Conference on Electricity Distribution Stockholm, 10-13 June 2013 
 
Paper 1242 
 
 
CIRED2013 Session 1 Paper No  1242      
CHANNEL REQUIREMENTS FOR LTE DUE TO MICROGRID MANAGEMENT SYSTEM 
IMPLEMENTATION 
 
 
 J.-I. CAIRO  Albert RUIZ  Andreas SUMPER 
 Research Institute for Energy of Research Institute for Energy of Dept. of Electric Engineering at 
 Catalonia (IREC) – Spain Catalonia (IREC) – Spain CITCEA (UPC) ETSEIB and IREC 
 icairo@yahoo.es aruiz@irec.cat asumper@irec.cat 
 
ABSTRACT 
This paper is presenting the repercussions that managing 
microgrids, when they are expanded within the power 
grid network, may take over telecommunication networks. 
In particular from secondary to tertiary control schemes, 
it is presented the kind of traffic that can be generated, 
and finally based on such traffic the capacity is 
calculated. Special interest in presented on the 4G LTE 
standard due to low latency and high data rate. 
INTRODUCTION 
The electrical power system, from generation, 
transmission and distribution, is facing a strong change in 
order to accommodate renewable energies of intermittent 
nature, together with the classical energies and the 
penetration of electric vehicles. All together is posing a 
strong challenge for the operation of such complex 
systems, and the communications requirements are one of 
the main topics in order to provide functionality. 
 
Economic, technological and environmental incentives 
are changing the face of electricity generation and 
transmission. Traditional scheme based on centralized 
generating facilities and kilometric aerial cables are 
giving way to smaller, more distributed and closer to 
consumer generation. In the last decades, the interest for 
Distributed Generation (DG) and Distributed Energy 
Resources (DER) and their integration in the utility grid 
has increased significantly. These two concepts, together 
with Microgrids, are usually confused. This is largely 
because there are no consistent definitions for them.  
 
In this paper, an work on microgrids is presented, based 
in experiences from the real microgrid built at IREC 
laboratoeies [1] (based power structure) and 
communication needs are abstracted in order to finally 
concentrate on the communications by use of 4G mobile 
systems. The paper is organized as follows: First, a brief 
descripcion of microgrids is done, defining the microgrid 
concept as basic part of the smart grid (SG). Following 
several requirements are described for the control 
strategies of these systems. From the understanding of 
such control techniques, it is derived the amount of 
information that is required based on real physical 
measurements. With the previous information, it is 
explained in next paragraph the message exchange 
betwenn microgrids and those are reproduced by the IEC 
61850. Finally in last sections of the paper, a description 
of the implementation of such communication 
requrements by means of the LTE technology and the 
real implemented work is descrived. At the ned, a list of 
related publications from the authors work is introduced. 
 
MICROGRID MANAGEMENT 
Microgrids might be understood as small-scale grids. 
Nevertheless, depending on the type and depth of 
penetration of distribution resources (DR) units, load 
characteristics, power quality constraints and market 
participation strategies, the required control and 
operational strategies of a microgrid can be significantly 
and conceptually different than those for conventional 
power systems [2-4]. The main reasons are the following: 
 
• Electronically coupled DR units barely have 
inertia. So, steady-state and dynamic response of 
these units are totally different than those of 
conventional large turbine generators. 
• Microgrids are inherently subject to a significant 
degree of imbalance due to the presence of 
single-phase loads and/or DR units. 
• A noticeable portion of supply within a 
microgrid can be from non-controllable sources 
(e.g. wind turbines, PV). 
• Storage units can play a major role in control 
and operation of a microgrid. 
• Economics often dictate that a microgrid must 
readily accommodate connection and 
disconnection of DR units and loads while 
maintaining its operation. 
• A microgrid may be required to provide specific 
power quality levels or preferential services to 
some loads. 
• In addition to electrical energy, a microgrid 
might be required to generate and supply heat. 
 
ISA-95 is the international standard for the integration of 
enterprise and control system. ISA-95 defines some 
models terminology to determine which information has 
to be exchanged. The time frame at upper levels might be 
days, weeks, month, etc., while at lower levels it might be 
seconds or milliseconds. 
 
 C I R E D 22nd International Conference on Electricity Distribution Stockholm, 10-13 June 2013 
 
Paper 1242 
 
 
CIRED2013 Session 1 Paper No  1242      
As in [5], it has been identified and classified microgrid 
controls into: 
• Level 3: to control the power flow between the 
microgrid and the grid. This control is referred 
as Tertiary control. 
• Level 2: to control power flow of each DR 
within the microgrid. It also includes a 
synchronization control to connect and 
disconnect the microgrid to or from the grid. 
This control is referred as Secondary control. 
• Level 1: to ensure that electrical levels into the 
microgrid are within required values. This one is 
the Primary control. 
• Level 0: to regulate output voltage and current 
of each DR. This is the Inner control. 
 
In reference [5] authors propose that Level 1 is 
responsible for power of each DR, and Level 2 is 
responsible for keeping electrical levels. Authors of the 
cited paper propose a different classification because their 
work focuses on droop-control for microgrids. 
 
The droop method permits to control DR converters 
without using any critical communication [6]. In this 
method, active and reactive power of each DR depends 
on the frequency and voltage at the bus bar respectively. 
Actually, this method is usually referred as P -ω and Q - 
V.  
 
In the droop method, active and reactive power, need to 
be calculated continuously because the electrical stability 
(frequency and voltage) depends on these values. The 
droop method achieves high reliability and flexibility. On 
the other hand, it has several drawbacks that limit its 
application. For instance, the conventional droop method 
is not suitable when the paralleled system must share 
nonlinear loads. 
IREC’s control for microgrid 
In this section it will be presented a control algorithm for 
microgrids that interacts with utility company and the 
electricity market. The strategy of the algorithm is the 
scheduling of generation, storage devices and loads, from 
forecasting measures and economic issues. The algorithm 
uses optimization methods. These methods are beyond 
the scope of this paper but can be consulted in Reference 
[7]. The Spanish regulatory environment for microgrids 
has been taken as reference. 
 
Currently, there are three main agents in the Spanish 
market: the generation company, the market operator and 
the retailer. More specifically, the aggregator would 
receive supply needs and generation offers from each 
microgrid then, with this information, could offer buying 
and selling bids in the different markets of the system by 
optimization of the overall benefit of the represented 
microgrids. After the participation in the market, the 
aggregator might communicate the set points for 
generation or buying of energy to each microgrid. In 
addition, the aggregator should provide the information 
about each microgrid to its corresponding distributor. 
 
 
 
Figures 1 General diagram of Spanish Energy Market 
with microgrids. 
 
Tertiary control 
The Tertiary control has a scope of 48 hours with periods 
of 15 minutes. It has two objectives. The first one is the 
economic optimization using a program based on a Unit 
Commitment problem. The second one is to improve the 
profitability of the supply and demand balance by 
interacting with the grid. These aims are realized taking 
into account daily forecasts regarding weather, energy 
price and demand data. On economic optimization, the 
signals for the controllable units are calculated allowing 
the system to find out an optimal unit commitment 
considering future values by exploring the price 
differences between on-peak and off-peak periods during 
a day. The final result is a schedule of the power outputs 
for each period within the optimization range.  
 
Secondary control 
The secondary control runs each 15 minutes with periods 
of 30 seconds (more or less depending on the time range 
to obtain the real data available), and it is in charge of 
power quality optimization and to minimize the average 
of all deviations compared to the tertiary control 
program. To do so, the aggregator must take into account 
current weather data, operation data of generation units 
while ensures the exchanged power with the grid and 
storages states of charge programmed above. Every 30s, 
for each DR of the microgrid, the aggregator reads P and 
 C I R E D 22nd International Conference on Electricity Distribution Stockholm, 10-13 June 2013 
 
Paper 1242 
 
 
CIRED2013 Session 1 Paper No  1242      
Q measurements and sends P and Q set points. 
According to IEC 61850-5, time transfer means the 
complete transmission of a message including necessary 
handling at both ends. Time counts from the moment the 
sender puts data content on top of its transmission stack 
up to the moment the receiver extracts the data from its 
transmission stack.  
From the same IEC 61850 standard, message length may 
vary from 1 to 1024 bits length. Figure 2 shows a typical 
capture of power measurement message reading. 
 
Figure 2. Capture of a “Read dataset P-Q” message.  
 
LTE FOR MICROGRIDS 
With the increasing demand for mobile broadband 
services with higher data rates and quality of service 
(QoS), in 2004 started the study of Long Term Evolution 
(LTE). The goal was to select a technology that would 
keep 3GPP's Universal Mobile Telecommunications 
System (UMTS) at the forefront of mobile wireless well 
into the next decade. Key project objectives where 
defined in the following areas: 
 
• Peak data throughput 
• Spectral efficiency 
• Flexible channel bandwidths 
• Latency 
• Device complexity 
• Overall system cost 
 
Broadband wireless mobile communication technologies, 
such as 3G and 4G, have many inherent advantages. Both 
3G and 4G are bidirectional communications systems 
with wide coverage, making them suitable for widespread 
terminal access and remote control. 4G systems are faster 
than previous 3G systems, allowing at least 50Mbps 
uplink, and 100Mbps downlink or more. Such high data 
rate should be fully capable to meet Smart Grid 
information transfer requirements. LTE is expected to 
provide smaller transmission and connection delays, 
increased bit rate (even at cell edge), better spectral 
efficiency, transparent mobility toward any network and 
reasonable mobile power consumption. 
 
Some important systems/parameters to cope within LTE 
that can be modeled and specified for microgrids 
management are the scheduling schemes and the latency 
requirements. 
 
 
 
 
Figure 3. Communicating management units within 
microgrids using LTE subsystem.  
 
 
 
Latency and channel requirements 
Communication latency among distributed intelligent 
agents can have significant impact on high level 
capabilities of a smart grid installation, in particular any 
protection and coordination. Table I (from reference [8]) 
shows some requirements for a Wide-Area Measurement 
System for Data Delivery (WAMS-DD). It is remarkable 
that the network must be decoupled from the global 
Internet because it might not be able to meet the latency 
requirements of the SG. This would potentially result in 
security vulnerability for the SG. To be used for the 
second order headings. 
 
LTE scheduling schemes 
OFDMA selected for downlink (DL) radio access scheme 
due to its robustness to multipath fading, higher spectral 
efficiency and bandwidth scalability (bandwidth is 
divided in multiple sub-bands denoted as resource block 
(RBs), the minimum scheduling resolution in the time-
frequency domain). Multiple access in DL OFDMA is 
achieved by assigning different frequency portions of the 
system bandwidth to individual users based in their 
channel conditions. In order to schedule resources to the 
different users on the DL, the base station (or eNodeB) 
needs channel quality reports from the individual users. 
The feedback reports from the individual terminal users 
to the base station are sent in the form of a channel 
quality indicator (CQI) that is obtained using reference 
 C I R E D 22nd International Conference on Electricity Distribution Stockholm, 10-13 June 2013 
 
Paper 1242 
 
 
CIRED2013 Session 1 Paper No  1242      
signals trasnmitted from the base station.The packet 
scheduler at the base station uses the CQI feedback from 
individual users to perform an RB to user assignment 
every transmission time interval (TTI of 1 ms in LTE) 
according to the base station selected scheduling policy. 
The scheduler also determines the data rate to be used for 
each user in each subframe and can perform rate 
adaptation by using adaptive modulation and coding 
(AMC) in different subframes. Such fast channel 
depending scheduling in both time and frequency domain 
multiplexing is referred as frequency-domain packet 
scheduling (FDPS), which can improve by 40-60% the 
system capacity over time-domain only scheduling. 
 
 
 
TEST SET-UP 
The test on IREC labs, is conducted using LTE 
connectivity in the vicinity of the center. In less that 200 
m straight line, there is the Telefonica tower. The 
modems are used to measure round trip time (RTT), 
connected into two computers via USB, by using the ping 
command. Pinging means the use of standard 4 packets of 
64 bytes to the IP address specified. 
 
Transfer speed from A to B is defined as the maximum 
quantity of data bits (payload) that can be transmitted 
from A to B with a HTTP connection over a TCP/IP 
during a unit of time (sec.). Control data necessary for the 
transmission such as TCP and IP heading are not 
included in this indicator. Latency between A and B is 
the required time in order to be able to send a packet, and 
is measured as the half of the time necessary to send and 
receive an ICMP (Internet Control Message Protocol) 
Echo/Reply between A and B. Such time includes the 
network necessary packet transport time as well as the 
processing time of the parquets between A and B 
When considering a message average length of 200 bytes, 
a latency of 50 ms, and average sending message rate  of 
1 Hz, for an LTE uplink of 75 Mbps, we can transmit: 75 
Mbps x 1s = 75 Mb or roughly 7,5 MB, and for a 200 
byte message, we can have until 37.500 remote terminal 
units transmitting from the microgrids established in the 
coverage area.  
Acknowledgments 
This work was supported by the EIT and KIC-Innoenergy 
under the project KIC-INSTINCT. 
CONCLUSIONS 
In this work, it has been  deployed within an nemulated 
microgrid at IREC facilities, a management algorithm 
that is sending consigns into the load, generation and 
storage units, that is the basis for obtaining the traffic 
requirements of their elements.  
 
Several control options are being presented and one of 
the most faburable communication protocols is explained. 
Bsed on these premises, a volum of traffic can be 
estimated that needs to be feeted within the LTE 
estandard for future mobile networks. Latency is being 
presented as well as scheduling, and finally due to high 
data rate capacities is concluded that high volume of 
deveices could be managed. 
 
REFERENCES 
 
[1]  Manuel Roman-Barri, “Experience on the 
implementation of a microgrid project in Barcelona” 
IEEE ISGT Europe, 2010 
[2]  T. Ackermann, “Distributed generation: a definition," 
 Electric Power Systems Research, vol. 57, issue 3, 
pp. 195-204, April 2001. 
[3]  G. Pepermans, “Distributed generation: Definition, 
benefits and issues," Energy Policy, vol. 33, issue 6, 
pp.787-798, April 2005. 
[4]   R. H. Lasseter “Microgrid: a conceptual solution," in 
Proc. IEEE 35th Annual Power Electronics 
Specialists Conf. PESC 04, vol. 6, 2004, pp. 
4285{4290. 
[5] J. M. Guerrero, “Hierarchical control of droop-
controlled ac and dc microgrids|a general approach 
toward standardization," IEEE Transactions on 
indusctrial electronics vol. 58, no. 1, pp. 158-172, 
2011. 
[6] Zhihao Zhang, “A load-shading control scheme for a 
microgrid with a fixed frequency inverter”, Electric 
power systems research 80 (2010), 311-317.   
[7] L. Igualada-Gonzalez, “Optimal managment of 
microgrids," Master's thesis, Department of 
Statistics and Operations Research, UPC, 2011. 
[8]  David E. Bakken,  “Smart generation and 
transmission with coherent, real-time data”, 
Proceeding of the IEEE, June 2011, Vol.99, No.6 
TABLE I 
DIFFERENT LATENCY AND DATA SENDING REQUIREMENTS WITHIN THE 
DISTRIBUTION POWER GRID 
Rate (Hz) Latency (ms) Quality WAMS-DD Deadline 
120-720 5-20 Ultra High Across grid or 
multiple ISOs 
< 5s 
60-120 20-50 High Within an 
ISO/RTO 
1 min 
30-60 50-100 Medium Between few 
utilities 
1 hr. 
1-30 100 – 1e3 Low Within single 
utility 
1 day 
< 1 > 1e3 Very Low Within 
substation 
> 1 day 
 
 


CHANNEL REQUIREMENTS FOR LTE DUE TO MICROGRID MANAGEMENT SYSTEM IMPLEMENTATION

http://citeseerx.ist.psu.edu/viewdoc/download;jsessionid=AB2E719B7B9D621649932F97FC9DFA0D?doi=10.1.1.1053.500&rep=rep1&type=pdf

CHANNEL REQUIREMENTS FOR LTE DUE TO MICROGRID MANAGEMENT SYSTEM IMPLEMENTATION

Abstract

Similar works

Full text

Available Versions

CiteSeerX