CC-Link selected for biogas cogeneration plants

A growing number of farmers are generating power from the biomass produced on their farms. This is often used around the farm to heat buildings and power machinery; alternatively it can be exported to the grid.

However, the systems must run reliably for years if they are to repay their initial investment and provide a stable power supply to the end users. In turn, this is only possible with reliable automation and control network systems that are also easy to operate.

German power plant specialist Dreyer & Bosse has built up a reputation for reliable autonomous cogeneration units that use CC-Link open fieldbus network technology for data communications and safety and monitoring functions.

Less than 15 years old, Dreyer & Bosse employs nearly 100 people who have installed 5000 anaerobic digesters throughout Germany. The total output is over 1800MW, which is enough to power 4,000,000 households.

Power plants

Biogas-fuelled power plants turn liquid manure, plant waste and special fuel crops into energy. The main component is a heat and power cogeneration unit that generates power and heat from biogas, which is the primary output of the biomass fermentation unit.

In Germany, operators of plants that generate electricity from renewable energy sources are paid guaranteed rates for the power they feed into the public grid, and they can also feed the heat produced into a local district heating network. British farms tend to be about the same size as German ones, and there is much interest in developing the idea. The large size farms of America's mid-western states means these are opting for bigger plants, while the smaller family-owned farms of France, Spain and the Mediterranean countries are adapting the concept to suit their circumstances.

Dreyer & Bosse approach is to buil custom-designed cogeneration units that are based on individual selection of standard components. Usually the generators are driven by biogas with dual-fuel diesel/biogas engines for back up to ensure reliability and availability. Everything is installed in a single container that is delivered ready for connection. Modern control technology and the CC-Link (Control and Communication Link) fieldbus network ensure that the plants are both reliable and economical to operate.

A typical system

A D&B system could, for example, consist of a 500kW cogeneration unit installed on a farm with 70 hectares (173 acres) of arable land and 5 hectares (12.5 acres) of pasture. The slurry will be a mixture of straw and manure. The entire system is controlled by a powerful compact controller from Mitsubishi Electric, supported by two smaller controllers from the same manufacturer. The main controller is a MELSEC FX3U PLC (programmable logic controller), which communicates with the system's various automation components via a CC-Link master module and standard serial ports. Integrated instructions and drivers for Mitsubishi automation components eliminate the need for complex and time-consuming controller programming.

Components include a graphical control panel for clear visualisation of all processes. This makes operation as easy as possible for the farmer, who is likely to be fairly non-technical. As a back-up, an industrial modem provides remote access so that service engineers are able to interrogate the plant without having to visit. Another critical component is the frequency inverter that powers and regulates the engine cooling system and the gas compressor on the two gas-fuelled engines.

A number of important system safety and monitoring functions are performed by air circuit breakers with CC-Link interfaces. These advanced switchgear components protect the generators against peak overloads, short circuits and power failures, and also handle the automatic network synchronisation with the public power grid. The fast-switching circuit breakers of Mitsubishi Electric's Super AE series act as the link between the gas engine and generator set of the cogeneration unit and the public power grid. The tripping characteristics of the circuit breaker's electronic generator protection module are configured for the connected generator system with a series of switches. Network synchronisation includes support for electrical remote control and is effected with an undervoltage tripping device in the circuit breaker combined with an integrated motor drive.

The circuit breaker's electronic trip relay is connected to the central controller via the CC-Link network. The measurement circuits integrated in the trip relay can monitor and analyse all the relevant generator parameters. For example, the system can check the current voltage, current and power levels in the low-voltage network and send them to the controller and the control panel. Intelligent modern circuit breakers like the Super AE series can handle these functions themselves because they have integrated network support and digital instrumentation modules; in older systems these features are not possible without connecting additional transformers, power transducers and a number of analogue-to-digital converters with the controller. The circuit breaker can also be switched on and off remotely via the CC-Link network, with the help of a peripheral I/O module with digital inputs and outputs. The digital inputs are configured as relay contacts and dimensioned to handle the currents of the circuit breaker's closing coil, shunt trip device and drive motor.

Simpler wiring

The CC-Link fieldbus network and the simple installation of the system components enabled the Dreyer & Bosse engineers to reduce significantly both material costs and wiring overheads. Resistance to electromagnetic interference was also a major factor for the choice of this technology; unlike some other fieldbus systems, the conformity tests for CC-Link compatible devices include comprehensive electromagnetic compatibility (EMC) tests, in addition to tests for correct functioning, ageing performance and resistance to damp, for example.

CC-Link's high network data transfer rates of up to 10 megabits per second and its deterministic performance ensure a continuous update cycle speed of just 3.9ms for all data. The network is configured with simple menus in the PLC programming software package, which conforms to the international IEC 61131-3 standard. No special device configuration files need to be created to set up the network: the parameters are read directly from the CC-Link master module by the PLC program using a standard function block. The PLC's processing speed matches the network transfer rates, making it possible to achieve the high levels of safety and reliability required for cogeneration plants.

Reliable operation and monitoring are also crucial for trouble-free operation, of course. All the electrical parameters of the low-voltage network can be displayed in detail on the system's clear graphical control panel. When a circuit breaker trips a message is triggered automatically and the operator gets detailed information from the trip relay that has activated via the CC-Link network. The last ten alarms and trip messages are also stored in an error buffer with timestamps for later analysis. Remote maintenance and remote monitoring of important parameters via modem are also supported, further adding to the safety, reliability and service-friendliness of the system - for example users can check the power being fed into the public grid remotely at any time.