8/9/2019 05157821

1/4

8/9/2019 05157821

2/4

only takes one tenth of the whole investment. At the same time,considering the important role of SCADA system in PV powerplants, device redundancy can be seen as a feasible method tosolve equipment failure and improve the reliability of SCADAsystem.

This paper focuses on improving communication reliabilityof SCADA system in PV power plants through adoptingeffective security strategy and redundancy mechanism. Section

2 presents the component of SCADA system in PV powerplants. Section3 studies security communication mechanism indistributed PV power plants network. SCADA systemredundant structure is shown in section 4, and in this sectionredundancy switching mechanism is discussed. Finally, somefunctions of the SCADA system were verified through SCADAserver configuration software.

II. SCADA SYSTEM STRUCTURE

In this study, the structure of a grid-connected PVgeneration system is depicted in Fig. 2.The design scheme thatone DSP chip integrated system controller with local SCADARTU was adopted. This design scheme improved controlperformance and saved system costs. Control circuits, PWMdrive circuit, as well as signal conditioning circuit were omittedin Fig.2.

dci

a

b

c

T1 T3 T5

T T6

C U

dcV

U

U

4 T2

1

2C

D2

D1

SCADA-RTU

LCL

Filter

PT1

ACT1

DCT

PT

PWM1 PWM6

Port 1: Ethernet

(RJ45) Port 2: RS485

ACT2

A B

ACT3

temperature

irradiance

Fig.2. SCADA in grid-connected PV system

Local SCADA system in PV power plants is composed ofdata acquisition unit, RTU, and communications unit. TheSCADA system could measure and collect PV arraytemperature, irradiance, DC output voltage and current, inverteroutput AC voltage and current relay switch state and so on.Data acquisition unit consisted of current transformer (DCTand ACT) and voltage transformer (PT). The design of SCADAsystem communication unit depended on the selection of

communication method. At present in RES power generationsystem several communication methods were adopted inremote monitoring, such asRS485, Internet, GSM, GPRS, GPS,industrial fieldbus [8]-[10].

Although GSM, GPRS, and GTS have their uniqueadvantages, the high costs of investment cause thesecommunication methods not to be suitable for PV powersystems. The costs contain device investment and additionalcommunication costs. Internet WEB communication isextensively used in many areas, but it is also not suitable forsending control information in power systems for its high

communication latency (about 0.5-2s) and low reliability.Nowadays in industrial communication area, there are anumber of fieldbus (such as PROFIBUS. HART, FF.etc).Among these fieldbus, industrial Ethernet fieldbus displays itspredominant advantages in many fields for its hightransmission rate, strong compatibility and networkmanagement. Combining with actual network requirement ofPV power plants, our system adopted industrial Ethernetfieldbus which was based on TCP (UDP) / IP protocol as workcommunication bus. And RS485 was adopted as redundantcommunication bus at the same time. As shown in Fig.2, Port1was Ethernet communication interface (RJ-45). Port2 wasRS485 communication interface and A / B were RS485differential signal output.

III. COMMUNICATION SECURITY

A. Security communication strategyDistributed PV power plants are an open network

communication system .If processes are monitored andcontrolled by devices connected over the SCADA system then

a malicious attack has the potential to cause significant damageto PV power plants. A set of communication mechanismbetween SCADA RTU with SCADA server was presented inFig. 3.

SCADA-RTUSCADA

Server

Link request

Link respond

Date or Service

ACK R

KDC

Security certificate

Apply key Apply or Distribute Key

Device announcement

Security certificate

Request or Respond

Fig.3.Security communication mechanism

As shown in Fig.3, before data communication betweenSCADA RTU and server, a secure connection should beestablished to ensure that unauthorized entities cant gain entryinto the network. Firstly, RTU sent link request message toSCADA server and could not do any operation before gettingback link respond message from SCADA server. In order toidentify whether the RTU was a legal device or not, RTUrequested to the server for certification and applied for a newsecurity key. SCADA server identified the RTU through Keydistribution center (KDC) (KDC could be an independentserver or integrated with the SCADA server). If the RTU waslegal device, KDC distributed a new sub-key return to the RTU.After completing security certification operation, if the

equipment was identified as a security device, RTU began tobroadcast device announcement message. In order to improvecommunication quality of SCADA system, Confirmmechanism was provided. When RTU didnt receive ACK inone cycle after performing a data or service message sendingoperation, it should perform a retransmission and didnt need tore-establish connection.

B. security measureThe proposed communication mechanism involved three

security measures.

8/9/2019 05157821

3/4

1) Security certification. Security certification performed theoperation of validating Security License. Security Licenseincluded device authority (License ID,Authorized device list,)and key information (key type, key length). It was used toidentify the device privileged.2)Encryption. Data encryption can prevent data destruction or

illegal wiretapping. Ethernet data packet encryption process inSCADA system is shown in Fig. 4. AES (Advanced Encryption

Standard) algorithm was adopted to encrypt Ethernet mediaaccess control access layer protocol data unit (MAC-PDU).And XOR algorithm was used to encrypt application layerprotocol data unit (APL-PDU). Both these two encryptionalgorithm carried with transmission key distributed by KDC.Thus the encrypted PDU could avoid malicious damage orillegal eavesdropping.

AES

encrypt

KDC

MAC-PDU PDU APL-PDU

Transport Key XOR

encrypt

PDU with Key Fig.4. Data packet encryption process

3) Role based access control. In Power generation systems,different users should have different rights. Role based accesscontrol (RBAC) strategy was introduced. Three roles(administrator, operator and ordinary user) were set in thissystem. Ordinary users could only view parameters of thesystem and didnt have the authority to change the parameters.But operators could modify the parameters. Systemadministrator was the highest authority owner, who took chargeof SCADA system control strategy, and had the rights of adding

or reducing users number and their privileges.IV. REDUNDANCY MECHANISM

To further enhance the reliability of SCADA system,redundant technology was presented.

A. redundancy topology

SCADA system redundant topology in PV power plants isshown in Fig.5. Redundant system consisted of two sets ofredundant bus (based on TCP / IP Ethernet fieldbus and basedon Modbus RS485 bus). The active device and alternatedevice should be produced by the same manufacturers and bethe same type. Type and communications port number ofdevice could be assigned by SCADA Server configuration.

Under normal circumstances, active device was running. Andwhen this device was malfunction, alternative RTU wasenabled through redundancy switching operation. Ethernetand RS485 Bus switched through different port (Port1 andPort2). The realization of network redundancy and deviceRedundancy switching operation depended on correspondingredundant services.

B. redundancy services

In order to achieve redundancy switching operation, fourservices was defined as follows: Redundant state

announcement service, Synchronization request service,Synchronization service and Device Switching service [11].These services aimed at industrial Ethernet fieldbus, not forRS485.

1

2

1

2

1

2

RS485 Centralized

ControllerGateway

...

PV1

Port 1 TCP/IP

Port2 Modbus

...

1

2

...

PV2

Switch

Fig.5. SCADA system redundant topology

Redundant state announcement service(R_DeviceActiveAnouncement) is broadcasted in every cycleby active device. Synchronization request service(R_SynRequest) is sent by new device to request configurationinformation and operating information. Synchronization service

(R_Syn) implements the function that active device sendsconfiguration and operating information to new access device.When the active device was malfunction, alternate devicebroadcast device switching service (R_DeviceSwitch) message.And the service included PD Tag, active device IP and faileddevice IP.

C. Redundancy switching

1) Device redundancy switching

R_SynRequest

R_DeviceSwitch

Active device Alternate device

R_DeviceActiveAnouncement

R_Syn

Fault device

Active device

Work

Device message

...

T

T

T Fault

R_DeviceActiveAnouncement

R_SynRequest Resp(+)

R_Syn Resp(+)

Fig.6. Redundancy switching mechanism

Active RTU, alternate RTU and their communication portcan be assigned by SCADA configuration server when devicespowered on .Device switching mechanism was shown in Fig. 6.In normal condition, alternate RTU sent Synchronizationrequest service message (R_SynRequest) to active device.

Active device gave a respond back to alternate device afterreceiving this message, and then called redundantsynchronization service (R_Syn) which was used to send todevice information and data to the alternate device. Activedevice should broadcast a device announcement servicemessage (R_DeviceActiveAnouncement) in eachcommunication cycle T. This message included time stamp. Ifalternate device did not receive this message in two cycle(check the time-stamp), device redundancy switching service(R_DeviceSwitch) would be triggered immediately. Alternatedevice became operating device, and the active device was

8/9/2019 05157821

4/4

defined as fault device.2) Network redundancy switching

Available communication network is a prerequisite fordevice redundancy. Aiming at network failure, RS485 serialcommunication bus was designed as redundant network bus inSCADA system. Generally, all messages should be sent in TCP/ IP Industrial Ethernet fieldbus in PV power plants. In eachCycle (T) RTU sent a device announcement message in

communication network. If configuration server didnt receivethis announcement in four communication cycle (double timeof redundancy switching time), the server judged that the activeEthernet was malfunction and switched to RS485 busautomatically. And communications port switched from Port1to Port2. RS485 bus based on Modbus communicationprotocol followed "inquiry - response" mode. In this modeConfiguration server sent inquiry command to RTU and waitedto receive information from RTU.

V. EXPERIMENTAL TESTS

The complete three-phase PV generation system was set upin laboratory as shown in Fig.7. PV modules were placed onthe roof of our department. RTU in SCADA collected data andsent to remote PC (configuration server).

Fig.7 Experimental set-up in laboratoryFig.8 shows security access monitoring interface of

SCADA system. Before operating in this interface, the usermust enter his/her user ID, password, and priority. If either userID or password is wrong or incorrect, the user was refused toaccess. Fig.9 shows the page of devices running state. ST2bakis a redundancy device of ST1.The parameter andcommunication state of ST2bak, such as device cycle, activetag, enable operate time, can be seen in Fig.9.

Fig.8. Security access interface

Fig.9 Redundancy device running stateRedundancy device switching process was shown in

Fig.10.The service messages were captured through a protocolanalysis softwareEthereal. When active device (128.128.2.17)was malfunction, alternate device (128.128.2.15) was switchedto be active device and sent device announcement messages.

Fig.10 Device switching process

VI. CONCLUSIONS

In this paper, a complete SCADA system of PV powerplants has been present. Concentrating on the communicationreliability of SCADA system, security communication strategyand redundancy mechanism have been provided. Securitycommunication strategy ensured reliable communicationbetween SCADA RTU and server. It can avoid the system

being disturbed or breached by invalid message. Simultaneity,the realization of redundancy mechanism improved thereliability of SCADA communication network. The proposedtwo methods are used in grid-connected PV generation systemin Laboratory and could be effectively employed in RESremote communication areas.

REFERENCES[1]J. M. Carrasco, L. G. Franquelo, J. T. Bialasiewicz, E. Galvn,R. C. Portillo,M. . Martn Prats, J. E. Len, and N. Moreno-Alfonso,Power-electronicsystems for the grid integration of renewable energy sources: A survey,IEEETransaction on Industrial Electronics, Vol.53, No.4, pp.10021016, Aug.2006.[2] Benghanem, M.Maafi, A.Data acquisition system for photovoltaic systems

performance monitoring, IEEE Transactions on Instrumentation andMeasurement, Vol.15, No.1, pp.30 33, Feb. 1998.[3] Hamoud, G. Chen, R.-L. Bradley, Risk assessment of power systemsSCADA, IEEEPower Engineering Society General Meeting, 2003,Vol.2, Jul.2003.[4]Zimmermann, C.G, The Impact of Mechanical Defects on the Reliability ofSolar Cells in Aerospace Applications,IEEE Transactions on Device andMaterials Reliability,Vol.6,No 3,pp.486-494, Sep. 2006.[5]Calogero Cavallaro,Angelo Raciti,Antonino Torrisi, Reliabilityimprovement of photovoltaic power conversion systems by an optimalremote-management controller, Fourth IEEE International CaracasConference on Devices, Circuits and System, Aruba, Apr 17-19,2002.[6] Chan, F, Calleja, H ,Reliability: A New Approach in Design of Invertersfor PV Systems.10th IEEE International Power Electronics Congress, pp.16,Oct. 2006.[7] Vinay M. Igure, Sean A. Laughter, Ronald D.Williams, Security issues in

SCADA networks, Computers & Security.Vol.25, No. 7, pp. 498-506, Oct.2006.[8] Krauter, Stefan,Depping, Thomas. Satellite monitoring system for remotePV-systems, Conference Record of the IEEE Photovoltaic SpecialistsConference, 2002, pp. 1714-1717.[9]Gagliarducci, M,Lampasi,D.A,Podesta, GSM-based monitoring and controlof photovoltaic power generation , Measurement: Journal of the InternationalMeasurement Confederation, Vol.40, No.3, pp.314-321,Apr.2007.[10] Wang Li,Liu Kuo-Hua, Implementation of a web-based real-timemonitoring and control system for a hybrid wind-PV-battery renewable energysystem,Engineering Intelligent Systems, Vol.15, No.2, pp.99-105, Jun. 2007.[11]IEC 62409: Specification of EPA system architecture and communicationfor industrial measurement and control system[S], 2006.