Power Line Communication Technology in Industrial ... - IEEE Xplore

2015 IEEE International Symposium on Power Line Communications and Its Applications (ISPLC)

Power Line Communication Technology in Industrial Networks Samuel C. Pereira and Alexandre S. Caporali

Ivan R. S. Casella

Department of Automation and Processes Control IFSP – Federal Institute of Science, Education and Technology São Paulo, Brazil [email protected], [email protected]

Engineering, Modelling and Applied Social Science Federal University of ABC Santo André, Brazil [email protected] In [3], a PLC solution was used for successfully replacing a wireless Modbus interface in a harsh industrial environment (open field). In this case, the old wireless transmitter was presenting many failures and frequent stopping of the plant. A BPLC (Broadband PLC) MODEM (HomePlug® standard) was used and reduced to zero the downtime of the plant.

Abstract— The PLC technology could be an additional alternative on the market for data transmission in industrial applications, like networks used to controlling machines and collecting sensor readings at field level. This paper presents a study about the industrial needs concerning data transmission, the sources of interference and attenuation for PLC signal in industry and the best technology available for these applications. The results of real communication tests with G3-PLC MODEMs working in a physically simulated industrial environment are presented. Different bands and subcarrier modulations were tested in order to recommend the one with best performance.

This paper, firstly discusses the industrial needs concerning data transmission and the obstacles for PLC technology in this field (section II). The characteristics of industrial networks protocols (data rate, response time, range, security issues and so on) and the sources of interference and attenuation for the PLC Signal in these environments are mentioned.

Keywords— Powerline Communication; Industrial Networks; PLC; G3-PLC; PLC in industry; Data Transmission.

In the second part (section III), the industrial needs are compared with the main PLC technologies available and the advantages of choosing G3-PLC standard are presented.

I. INTRODUCTION Data transmission via powerline network, when achieved in a robust and reliable way, even in harsh environments, presents advantages that might become viable, in certain conditions, its use as replacement of other means of transmission at field level. The main advantages in relation to its competitors (cables and wireless) are highly reduced installation time/costs (cables) and the ability to penetrate structure with a longer range (wireless technology). The use of PLC (Power Line Communication) technology has been mainly directed for Smart Grid applications and its potential is not exploited in the industry, where electrical cables and wireless technologies are employed for data transmission.

In the third part (section IV), real communication tests with a pair of G3-PLC MODEMs in a physically simulated industrial environment (procedure and results) are presented. Different frequency bands and subcarrier modulations were tested in order to recommend the best configuration for use in industrial networks. In the end (section V), the results will be discussed, focusing in the future of PLC in industry. II. DATA TRANSMISSION IN INDUSTRY The communication in industrial environment, at field level, operates in the lowest level and offer connections between simple industrial devices (sensors, actuators and motors) and high-level devices (computers, programmable controllers, distributed control systems and so on). According to [4], most of transmitted I/O data at field level has less than 1 byte. Regarding this feature, the necessary data rate at field level is not high, but one failure or delay in the communication between controllers and actuators may cause serious problems for industrial processes or even cause accidents.

PLC could be a helpful alternative for data transmission in industry, mainly in conditions where infrastructure is a problem (e.g. installing new communication systems for machines, controllers or sensors in old facilities or even communication with devices in farther locations with an already installed powerline network). The much lower installation time and costs plus the fact of not having to stop the plant for preparing the infrastructure make PLC an attractive solution for industry.

In the industrial environment, the main sources of interference and attenuation come from electric motors and their controllers (e.g. inverters). The electric motor is the core of the majority of industrial machinery and its role in the PLC channel must be taken into account. The motor control through inverters is also a problem due to the noise generated by power switching.

The available alternative technology with similar advantages, wireless, is already employed in the industry, and with publications dated from 1988 [1]. Nowadays, mature solutions like IEEE 802.11 standard, IEEE 802.15.4 and Bluetooth are offered [2]. However, according to [2], due to the difficult in achieving the timely and successful transmission of packets over wireless channel, this technology have not gained good acceptance on the factory floor.

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A. Characteristics of data transmission in industry The communication system at field level should be able to transmit real-time periodic data for monitoring all devices, sporadic data for alarm and non-real-time message data for network maintenance [5]. Regarding this situation, wired connections are recommended, ever since it offers low possibility of errors and information packet loss. In Table I are showed the main wired industrial network protocols and some popular wireless solutions with their respective data rate, transmission distance (range) and maximum number of nodes.

B. Attenuation and interference sources for PLC signal in industry (electrical motors and inverters) As mentioned in section II, the main sources of interference and attenuation in industry, come from electric motors and their controllers (e.g. frequency inverters). According to [10], electric motors act as terminal impedance. This impedance (motor) depends on frequency and in the frequency band from 10 KHz to 30 MHz, it can vary from 1Ω to 10KΩ whereas the low voltage power cables (where the PLC signal is transmitted), in the same frequency band, presents impedance from 5Ω to 50Ω. It means there is impedance mismatch in the whole frequency range. As in [10], due to this impedance mismatch at the interface between power cable and electric motor, the input impedance measured on the input of power cable becomes frequency variant. It means that a branch cable terminated with an electric motor may be considered as a load impedance connected parallel to the power line channel. Fig. 1 shows the measured and simulated input impedance of a cable terminated by an induction motor.

For wireless solutions, the environment drastically affects the range. Wireless signal has problems for penetrating structures (the higher the frequency, the harder it is), e.g. the ELPRO® Technology in open field has a range of 5000 meters whereas in obstructed environment (walls, steel structure, concrete) has a range of 1000 meters [8]. The range of ZigBee® Technology may vary from 10 up to 100 meters [9]. In [3] the weather was affecting the wireless system used to transmit data from a Modbus interface in open field. According to [6], for real-time communication in industrial control, there are the following necessary features: •

Extent (covered area): It may involve a selfcontained section of a plant or several items sharing a network. The transfer of information over a larger area is likely handled by an alternative system such as a field-bus. So the extent is relatively localized [6];

•

Response time: Normally a response time within 1-2 seconds maximum would be essential for real-time control. A longer time cannot be acceptable [6];

•

Robustness: If the event of node/link loss existed for longer than a certain period, or occurred at the wrong point within an operational sequence of a process, it is potentially serious. Ideally, each node should have the ability to perform automatically in a safe distributed manner when the link/node is lost in networks [6];

•

Data security: Unless there is a risk of malicious external interference to the process under control, it is less important [6].

TABLE I. Type

Protocol Name

Data rate (Kbps)

Range (meters)

Nodes

Fieldbus Foundation®

31,25

1900

8-16 32

Profibus®

38,4

1200

93,75

1200

182,5

600

Other issue that should be considered is the transmission of the PLC signal through different circuits. Normally, the communication is made in the same circuit (same wires), but sometimes in different rooms with poly-phases circuit, it is very difficult to have a common circuit. According to [13], for BPLCs, the signal can cross over to the other phase without coupling circuits. It occurs due to the capacitance formed between the bus bars that transfer the signal to the other phase without coupling circuits. In [13], this ability of crossing over phases is not mentioned for the NBPLCs. III. IDEAL PLC TECHNOLOGY FOR INDUSTRY The lack of standardization was an obstacle for PLC technology in the beginning. According to [14], in 2011, there were about sixteen standards for BPLC and about eighteen for NBPLC. However, as in [15] and [16], in 2010 and 2013, respectively were ratified and published, the IEEE standards (international) for BPLC (IEEE P.1901.1) and NBPLC (IEEE P.1901.2). G3-PLC standard, developed by Maxim® Inc, was used as the basis for IEEE P.1901.2 [17]. In Table II, it is described the main specification of both.

32

500

200

125

500

250

250

500

100

R-Fieldbus [5]

2000

100

ELPRO® [8]

19,2

5000

95

ZigBee® [9]

250

100

64000

DeviceNet®

Wireless

According to [10], the frequency inverter is an effective source of conducting noise and a nonlinear load that distorts the PLC signal. In [11], it is mentioned that the most important noise in industry is the impulsive one, and it is mainly originated in the driver inverter power semiconductors. As showed in Fig. 3, the CSD (Current Spectral Density) of the noise decreases with higher frequencies, which means the inverter may be a major problem for NBPLC’s (Narrowband PLC’s).

CHARACTERISTICS OF INDUSTRIAL NETWORK PROTOCOLS [7]

Modbus®

Wired

The use of frequency inverters in industry has become very popular. With these devices, it is possible to control the motor rotation speed and the starting current of AC induction motors. They convert AC voltage in DC voltage and afterwards convert to AC voltage with controlled frequency. However, this procedure generates noise and impedance variation that might be an obstacle for PLC signal. Fig. 2 shows the noise generated by the frequency inverter when it is active.

64 30

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TABLE II.

SPECIFICATION OF IEEE P.1901.1 AND IEEE P.1901.2

Specification

P.1901.1

P.1901.2 (G3-PLC)

Frequency Range Data Rate (maximum)

2-28MHz

3-500KHz

500Mbps

500Kbps

AES-128-bit Encryption

AES-128-bit Encryption

1500 meters

Over 6 kilometers

OFDM

OFDM

Yes BPSK, QPSK and QAM (16/64/256/1024)

Yes D8PSK, DQPSK, DBPSK and ROBO

Security Range (Maximum) Modulation ROBO Mode Subcarrier Modulation Type

Comparing the specification of both standards with the specification of industrial network protocols in Table I (section II), it is possible to notice that G3-PLC standard (IEEE 1901.2 compliant) has enough data rate for field level communication in industry applications with the advantage of having a much longer range, which may be helpful (industrial plant far from the control center). In [3], the machine had a data rate of 2Kbps and the PLC MODEM for transmitting its data had 50Mbps. In this case, it was necessary to use a repeater after 200 meters, which would not be necessary with a G3-PLC MODEM. Other advantage is the ability of crossing voltage transformers without using coupling circuits (impossible with BPLCs).

Fig. 2. Noise in the power line channel with inverter in idle state (upper graphic) and active state (lower graphic) [10].

G3-PLC standard has four subcarrier modulation types in which it is possible to have different data rates. In ROBO (Robust Operation) mode, which has lower data rate, the information is repeated four times in the same packet, improving the robustness, and as in [18], provides communication even in conditions with negative signal to noise ratio (noise level higher than the signal level). G3-PLC operates in frequency bands destined for NBPLC by CENELEC (Europe), ARIB (Japan) and FCC (USA). This standard presents useful characteristics for industrial usage (robustness, enough data rate, longer range in comparison with BPLC standards plus the ability of crossing voltage transformers) and it was the one used in the tests of section IV.

Fig. 3. CSD of the noise generated by frequency inverter in three different configurations [12].

Fig. 1. Measured and simulated input impedance of a power cable (type MCCMK 3x16+16, length 9,7 meters) terminated with an induction motor (15KW, 4 poles) [10].

Fig. 4. Floor plant of Electrical Machines Laborarory (dimensions in meters).

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IV. G3-PLC TESTS IN A SIMULATED INDUSTRIAL ENVIRONMENT

AB connected to primary and MODEM RX to secondary): For testing the ability of G3-PLC signal in crossing transformers without using coupling circuits; 6) Three-phase electric motors (M1-M6) in ON and MODEM RX isolated with an 6Km simulator circuit: In order to test the communication for longer distances, it was created an equivalent circuit of a 6Km power line (for calculating the values, the characteristics of the cable informed by the manufacturer were used).

A. Tests Method For simulating an industrial environment, it was used the Electrical Machines Laboratory of the Federal Institute of São Paulo (plant detailed in Fig. 4). This laboratory has six benches and it was installed one three-phase induction motor in each one (M1 to M6 - direct start with contactor). During the tests, the G3-PLC transmitter MODEM (TX) was connected close to M2 and the receiver MODEM (RX) to M1. In Fig. 4, it is detailed the physical position of motors and in Fig. 5, the electrical diagram. Different configurations were used for simulating the industrial environment and the possible obstacles for PLC signal.

The chosen frequency bands were FCC (145KHz – 478KHz), CENELEC A (36KHz – 90KHz) and CENELEC BC (98KHz – 137KHz). The FCC band, the wider one, allows a higher data rate (larger number of subcarriers). Moreover, as in [19], noise has greater power in lower frequencies, being challenging for NBPLC operating in frequencies lower than 150KHz. That is the reason the tests were performed in CENELEC A and BC (CENELEC A is particularly located in the frequency range with higher amplitude according to the measurements performed).

The G3-PLC MODEMs are based on Max2992 (G3-PLC Transceiver) and MAX2991 (AFE - Analog Front End), both from Maxim® Integrated Inc. It was used a Maxim software (Maxim Simple Connect) for performing communication tests in different configurations. With this software is possible to configure the MODEMs RX and TX (frequency band, gain, subcarrier modulation and packet size) and to monitor the communication between them (data rate estimation and number of received and lost packets).

The gain of TX MODEM was adjusted to 6dB, the maximum rate, as it was expected strong attenuation of PLC signal. This setting is related to the voltage gain (in dB) of the internal driver in the last stage of the AFE interface (MAX2991 from Maxim®) that deliveries the TX signal to the power line.

The tests consist in sending 1000 packets (size: 200 bytes each packet) from TX (packets generated internally in TX) to RX and monitoring communication. The software informs the data rate and the number of lost packets. Afterwards the Packet Error Rate (PER – average of transmitted packets that are not detected correctly) will be estimated through: PER%= Number of lost packets / Total number of sent packets * 100

The tests were performed with D8PSK, DQPSK, DBPSK and ROBO subcarrier modulations. An oscilloscope Agilent TDS2012B in FFT mode connected to the power line (close to the TX MODEM) monitored the frequency spectrum amplitude. The results were exported to PC in order to create the graphics showed in Fig. 6 and 7. These graphics contain the ASD (Amplitude Spectral Density) in dB (0dB = 1VRMS) of the existing background noise in the frequency range of the NBPLC (0 – 500KHz).

(1)

For ensuring reliability, the test is repeated three times (five seconds after the last test is just finished) and the average PER of these three tests is informed. The tests were performed in the following configurations (these configurations and characteristics of used devices are in electrical diagram of Fig. 5):

B. Results and comments The results obtained in the tests are showed in Table III. The results for D8PSK subcarrier modulation were omitted, because in all tests, 100% of packets were lost. Only the circuit breakers in the path between MODEMs (D2, Q2, Q1, and D1) were enough for causing the loss of all packets. According to [20], circuit breakers may cause, approximately, 5dB of attenuation (each one).

1) Three-phase electric motors (M1-M6) in OFF: For determining the effect of motors in the PLC signal; 2) Three-phase electric motors (M1-M6) in ON: For determining the effect of electric motors in PLC signal, comparing with configuration 1; 3) Three-phase electric motors (M1-M6) in ON and MODEMs connected to different circuits (TX connected to phases AB and RX connected to phases AC): For testing the ability of G3-PLC signal in crossing over from one phase to other without coupling circuits, because in industry, different power line spots may be from different circuits; 4) Three-phase electric motors (M1-M6) in ON and the motor M2 (close to TX) controlled by a frequency inverter: For determining the effect of a frequency inverter in PLC signal, as in section II, is informed that inverters are the main source of impulsive noise in industry; 5) Three-phase electric motors (M1-M6) in ON and MODEM RX isolated with an iron core transformer (phases

The electric three-phase motors did not affect the communication between the MODEMs. It is possible to see comparing Config. 1 (motors OFF) and Config. 2 (motors ON) results. It was even possible to notice a slight improvement in PER for CENELEC-A band. In Fig. 6, where the noise ASD of both conditions are compared, it is not noticed any significant difference. The tests proved that G3-PLC signal is able to cross over from one phase to another one without any signal coupling. As showed in the results of Config. 3, the DQPSK subcarrier modulation did not work in all bands but in ROBO mode, no difference was noticed in all bands (comparing with Config. 2, where MODEMs are connected to same phases).

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Fig. 5. Electrical diagram containing all configurations.

In the last configuration (Config. 6), an equivalent circuit of a 6 Km transmission line was connected to the circuit for testing the data transmission for longer distances. The results obtained were satisfactory as the PER for CENELEC-BC and FCC bands were very low.

The frequency inverter was the device/condition that mostly affected the communication, as showed in Config. 4 results. It was noticed packet error rate increasing in all bands. However, it did not affect PLC signal with ROBO subcarrier modulation in bands CENELEC BC and FCC. Monitoring the frequency spectrum of the power line, it was possible to notice a slight noise amplitude increasing in the range from 32KHz to 200KHz, as showed in Fig. 7.

In all tests, the CENELEC-A band had the worst performance and it was due to the higher amplitude noise over its frequency range, as showed in Fig. 6. The CENELEC-B band was the one less affected by the obstacles inserted in all configurations. However, its low data rate (15,1Kbps at maximum) comparing with the characteristics of the industrial networks in Table I is not enough. The FCC band presented the best performance, with good data rate (30 – 35Kbps) and low PER (