Intelligent Power and Motor Control Centre (iPMCC)

iPMCC is a system integrating intelligent Motor Protection Relays (IMPR) in a highly dependable Power and Motor Control Centre switchboard. Connectivity to the supervision and control system is provided through an industrial communication network.
This solution is particularly used in large industrial sites and infrastructures, with continuous or hybrid process, and whenever continuity of service is a priority.

intelligent Motor Protection Relay

IMPR is the key component of an iPMCC offer. It is a microprocessor controlled device. Motor monitoring and protection is performed based on measurements from sensors, such as current transformers, voltage transformers (embedded or external), thermal sensor, earth leakage detector,… From these measurements and the settings, it determines fault conditions or potential risks for motors and operators.According to the motor protection model, an IMPR has the capability to detect many kinds of faults. It is a great improvement compared to thermal relay protection. Moreover, many complementary functions can be implemented by an IMPR: monitoring, alarming, fault recording, statistics, communication, etc…

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                          1: TeSys T motor protection relay with communication capability
                          2: extension module with voltage measurement
                          3: phase current sensors
                          4: earth leakage detector
                          5, 6, 7: Human Machine Interface

Fig. N73: Example of motor control and protection architecture

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Motor Control Centre
A Motor Control Centre (MCC) is an electrical switchboard which groups all motor starters of a process, in order to build a centralised installation. Motor starters management centralisation is requested in many industries and infrastructures, in order to facilitate operation and maintenance. Withdrawable MCC functional units (FU) are used in critical applications, as they are more convenient to manage in case of fault. The faulty motor starter can be replaced quickly, without shutting down the whole switchboard.
Fixed or disconnectable FUs can be used in less critical applications.

MCC -type ASSEMBLIES must be full-compliant to IEC 61439-1 and 61439-2 standards  to guarantee availability, safety and reliability of the application. In an iPMCC configuration, design verification, especially temperature rise test, is essential because the IMPR (electronic device) is more sensitive to heat. Furthermore, MCC should provide a dependable and reliable communication bus connection.

A MCC is different from a universal cabinet in the way that a universal cabinet can only be used to accommodate a group of few motor starters. It has lower electrical characteristics requirements, and it does not provide the separation between motor starters in different functional units. Therefore, in an universal cabinet complete shutdown will be necessary before maintenance operations or any reconfiguration of the starters.

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Fig. N74: Example of iPMCC: Okken switchboard and drawers by Schneider Electric

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iPMCC offers great advantages at all the project stages: design, execution as well as operations.

• Improved project efficiency

  - Reduction of engineering as starters are more standardised over a wider power range,
  - Reduction of on-site wiring time thanks to field buses,
  - Reduction of set-up time thanks to download of parameters.

• Reduced commissioning time

  - Better understanding of the process reactions thanks to detailed diagnostics and statistics,
  - Faster error fixing and bug tracking,
  - Easier fixing of process start-up problems.

• Improved Continuity of Service

  - Better protection of motors and loads by using more accurate sensors and more accurate motor protection models,
  - Reduced untimely downtime with alarms giving time to fix the problem before tripping occurs.

• Reduced operating and maintenance cost

  - Less downtime,
  - Faster problem fixing,
  - Less spare parts stock,
  - Preventive maintenance strategy

• Reduced evolution cost and time

  - Simplified engineering,
  - No wiring required,
  - Simplified set-up,
  - Easier Process tuning and commissioning.
A complete iPMCC offer concentrates the knowledge and experience of electrical distribution, motor protection and control, automation and installation. This is why only a few leading companies in electrical distribution and automation can propose this offer.

Communication

An iPMCC configuration is made of lots of motor starters. In order to supervise the system, it is necessary to send information such as motor status, current values, alarms, etc… The traditional wire-to-wire connection is not an efficient and cost-effective way when there is a lot of data to be transmitted. Today, transmission via a communication network is the preferred way.
The communications need the support of a common language, which is the communication protocol. The following chart shows the protocols commonly used at different levels of an industrial communications networks. At the moment, the most popular device bus protocols are Modbus SL, Profibus-DP and DeviceNet, while Ethernet TCP/IP is growing very fast.

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Fig. N75: Different communication protocols

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Modbus
Modbus is a message handling structure introduced by Modicon in 1979. Modbus is an application level protocol based on the OSI model. It is independent of the physical layer. 

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Fig. N76: Modbus architecture

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Modbus SL (Serial Line)
Modbus can be implemented on RS232, RS442 or RS485 links as well as other media like Ethernet. Modbus RS485 has been the most common protocol in the world. It supports communications speed up to 115kbps, but most devices support only communication up to 19.2 kbps.

Modbus RS485 is a low cost communication implementation, and it has the largest installation base and supplier network. The weak point of Modbus is the transmission speed (since it is limited by serial line speeds) and maximum number of devices. Modbus may face some problems in the application of very large industrial site, but it is still an economical and reasonable choice to the majority of motor protection systems.

Modbus is based on a Master/Slave concept. One device is the master and sends request to read or write data to each slave in turn. Slave answers to requests from the master. Even though you can have many devices connected to one serial line only one device can talk at a time.

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Fig. N77: Modbus SL architecture

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Modbus/TCP
Modbus/TCP is an excellent choice for large sites applications. Modbus/TCP uses the standard 10Mbps Ethernet media in physical layers to carry the Modbus message handling structure. It offers very fast speed and big number of devices in one network; it is easier to integrate MCC into the Local Area Network (LAN) of a company, so it is the choice of more and more customers.

Unlike Modbus SL, Modbus/TCP works on a Client/Server concept:

• A client initiates the requests and a server answers,
• Any device can be a client or a server,
• Many devices are both client and server at the same time,
• A network can consist of many clients.

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Fig. N77a: Modbus SL architecture 

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Many clients can send requests at the same time and many servers can answer at the same time:

• A client can talk to multiple servers at the same time,
• A server can answer multiple clients at the same time,
• Ethernet switches take care of packet delivery to all a devices at the same time.

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Fig. N78: Typical communication architecture

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Differences between Modbus/TCP and Modbus SL:

• Devices can be a client and a server at the same time.
• Everyone can talk at the same time: multiple devices can initiate communications, not just one. Increases system response time by parallel communications.
• Multiple requests can be sent from one device to another without waiting for the first request to be answered. A new piece of data is added to the Modbus frame called the Modbus Transaction identifier to allow a response to be matched to a specific request.
• The Transmission speed is much increased:10Mb, 100Mb, 1Gb etc.
• The transmission media is much more flexible and costs are lower: fibre, radio etc.
• The number of nodes on a single network is almost unlimited: maximum recommended is around 300, but routers can be used to join several networks.

Modbus I/O Scanning
Modbus I/O Scanning is a feature in Schneider Electric Programmable Logic Controllers (PLC) which allows simple Modbus transactions with a simple setup screen. It is only requested to set the address, poll time and data to read and/or write.

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Fig. N79: Modbus SL architecture

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Profibus
Profibus is a protocol introduced by a fieldbus working group in 1987. This group consists of 13 industrial organizations and 5 research institutes. Now, Profibus is managed by a user group which includes manufacturers, users and researchers.
Profibus-DP is the version of Profibus used at device level. It has been a successful protocol in the last decades, especially in Europe. Profibus-DP is a protocol with high transmission speed. It supports the communication up to 12 Mbps, but actually 1.5 Mbps is the most practical maximum value in applications because it may need special transmission media and it should be implemented in a short distance to achieve the transmission speed up to 12 Mbps.

DeviceNet
DeviceNet is a protocol based on CAN, which is a protocol widely used in the automotive industry. ODVA (Open DeviceNet Vendor Association) takes now the responsibility to promote and provide technical support to DeviceNet specification.
ODVA is an international association comprised of members from the world's leading automation companies. Collectively, ODVA and its members support network technologies using the Common Industrial Protocol (CIP™). These currently include DeviceNet^TM, EtherNet/IP^TM, CompoNet^TM and the major extensions to CIP — CIP Safety^TM, CIP Sync^TM, and CIP Motion^TM. ODVA manages the development of these open technologies and assists manufacturers and users of CIP Networks through tools, training and marketing activities.
DeviceNet provides communication with 3 possible speeds: 125, 250 or 500 kbps, which depends on the bus length and cable as well as product consumption. The maximum number of devices is 64, including master devices. The bus length is limited to 100m at 500 kbps.

Synthetic view
The following table shows a short (non-exhaustive) comparison of these protocols:

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	Modbus SL RS485	Profibus-DP	DeviceNet	Modbus/TCP
Speed	up to 115 kbps	9.6 kbps to 1 Mbps	125, 250 or 500 kbps	10 / 100Mbps / 1Gbps
Max. distance without repeaters	1300 m	100m at 12Mbps 1.2km at 10kbps	100m at 500kbps 500m at 125kbps	Twisted pair: 100m Optical fibre: 2000m
Max. number of devices	32: 1 master and 31 slaves	126: mono or multi-masters, 122 slaves max with 3 repeaters	64: 1 master and 63 slaves	64 with I/O scanning; no limit with others
Max. distance with repeaters	Depends on the type of repeater	400 to 4800m according to speed	Depends on the type of repeater	10km optical fibre

Fig. N80: Comparison of communication protocols