Integrating distributed energy resources into the smart grid - Part 2

In Part 1 we looked at how present and future demands are necessitating a transition to a smart grid, particularly the need to incorporate distributed energy resources (DERs) into the generation mix - and how these needs are presenting both safety and system management challenges.

Integration of DERs into the local distribution system, and ultimately into the entire utility generation, transmission and distribution system, is problematic for a number of reasons. First, the back feeding of power into the grid needs to be controlled to enable safe grid shutdowns for maintenance and repair work. Second, the availability of power from DERs needs to be monitored and controlled, to provide optimal benefit to both the grid and the customer.

Overcoming DER challenges with automation

Each DER site must have some level of automation to control and protect its specific type of power-generation equipment, and to interface this equipment to the grid. Automation systems will range from those that are very complex for control of a gas turbine to those that are very simple for control of a small wind power installation. IEEE standard 1547 makes reference to ‘interconnection systems’, and the automation system is part of the interconnection system.

DER automation systems typically consist of three main parts: the controller, the I/O and the operator interface. The I/O connects the controller to components such as valves, switches and motors. The controller monitors inputs, makes decisions based on its internal software program and adjusts outputs accordingly.

In many modern automation systems, the controller is actually an industrial PC, often referred to as a programmable automation controller (PAC) such as Advantech’s APAX product series. A PAC provides a ruggedised component that can be easily interfaced to both the utility and to various DER electrical and electronic components.

The operator interface is typically an LED monitor or a simple alphanumeric display that provides a view of the process being controlled. Most operator interface panels also provide a means of control and adjustment.

In addition to control of the DER’s power-generating equipment, automation must be provided for monitoring and control of the components used to interface the generating equipment to the local utility distribution system. These components include, but aren’t limited to, automatic transfer switches, synchronisation check devices and direct transfer trip relays.

Automation systems address the DER grid integration issues discussed above by performing control and monitoring tasks as listed and described below.

Monitoring

One of the main functions of the automation system is the monitoring of DER power output. Local DER automation systems provide information to the utility concerning the exact amount of power being produced. Power is typically measured by components such as current transformers and potential transformers, both of which are inputs to the automation system.

From the energy management system or utility point of view, customer side demand management and outage management are very important parts of the smart grid. To improve energy management, utilities need to monitor DER generation conditions such as power transitions and deviation conditions.

The automation system then processes these inputs to calculate power. Alternately, some type of smart meter can be used to calculate power. The meter can be packaged in familiar electricity meter housing, or it can look more like a panel meter.

In either case, the smart meter is typically interfaced to the automation system, which in turn is connected to the utility, usually via some type of wired or wireless ethernet connection.

Energy management

Another function of the automation system is to process requests from the utility. In times of peak demand, a utility might request the DER to run at full capacity, and may offer pricing incentives to do so.

For larger DERs, the utility often sends pricing information, allowing the DER automation system to perform make/buy decisions on a real-time basis. When utility power is cheaper than internally generated power, the DER automation system can reduce power output. When utility power is more expensive than internally generated power, the DER can run at full capacity.

These make/buy decisions most often apply to fossil-fuelled DERs such as gas turbines and diesels. These types of DERs incur relatively high operating costs, primarily for fuel. These types of DERs can also be stopped, started and ramped up or down relatively quickly.

On the other hand, renewable DERs must run based on the availability of sun, wind or other prevailing environmental conditions regardless of utility pricing signals, making them a more difficult resource to integrate into the generation mix.

Safe shutdown and synchronisation

When work needs to be done on local distribution systems, the utility can request a shutdown of the DER. In cases of safety-related shutdowns, the local automation system typically provides a fail-safe method to verify complete shutdown of equipment and correct status of all interposing switchgear.

Yet another function of the automation system is synchronisation of rotating generation equipment with the utility power system. This is a quite complex subject requiring the consideration of many power grid and DER variables. Modern automation systems, especially those employing powerful PACs, are best equipped to address this issue.

Integrating DERs with PACs

Until about 20 years ago, programmable logic controllers (PLCs) were at the heart of almost every automation system, and operator interface terminals were typically custom designed and vendor-proprietary displays.

Around 1990, PCs began to emerge as a viable option as the main controller in an automation system. When a PC was used for control, it was a natural fit to employ a PC monitor as the operator interface.

In response to market demand, suppliers re-packaged PCs into form factors suitable for industrial use. Operating specifications were improved in terms of allowable temperatures and shock and vibration levels - as was resistance to electrical noise and interference. Reliability was also improved at both the hardware and software levels.

The end result was the industrial PC, sometimes referred to as a programmable automation controller (PAC). The PAC is proving to be a good fit for DER automation applications for a number of reasons.

Connectivity

In most instances, a DER automation system must be connected to utility control and communication systems. The most popular communication network for this purpose is ethernet, and most PACs feature built-in ethernet support along with internet connectivity.

DER applications often require interface to a number of different electrical and electronic components including electric power meters and protective relays. It is relatively easy to interface a PAC to these components.

PACs typically feature multiple communication ports conforming to various utility industry hardware connection standards such as ethernet, Modbus RTU, DNP3 and others. The ability to communicate with different communications protocols was a low priority before the advent of microprocessor-based relays and meters, but now it’s a necessity.

By connecting to microprocessor-based devices via digital networks, PACs can download event data with millisecond precision, monitor real-time current and voltage, and perform remote maintenance testing.

Among industrial controllers, PACs also feature the widest range of discrete and analog I/O. This makes it relatively easy to interface a PAC in a hard-wired manner to the many different types of electrical components in a DER.

Programmability

Programming a PAC to interpret different communication protocols is relatively straightforward as most PACs can be programmed via a number of popular languages. Programming via different languages becomes a necessity in many DER applications not only for communications, but also for control. For example, integrating a DER with a utility’s power distribution system can be very complex in terms of synchronisation and other electrical power system factors, as described previously.

Again in contrast to PLCs, PACs can be programmed with popular languages such as VB and various varieties of C, making it possible to program even the most complex algorithms. Once programmed, these algorithms can be downloaded to the PAC for real-time and high-speed control.

For example, a PAC system with a WinCE platform and utility-specific I/O module (16 channels synchronised data acquisition and pulse I/O module) can generally fulfil DER monitoring and controlling functionality.

Speed and power

As algorithms become more complex, the sheer computational power of the DER automation system becomes important. Along with computing power, high speed is also often required so that the automation system can continually adjust DER operating characteristics to match the ever-changing electrical parameters of the utility distribution system. PACs excel in these areas, providing high speed and computational power by employing the latest in commercial PC technology.

Conclusion

DERs are here to stay and growing in popularity for both practical and political reasons. Integration of DERs into the utility generation, transmission and integration system presents numerous challenges. Modern automation systems, especially those powered by PC-based industrial controllers, are one of the best methods for dealing with these challenges and successfully integrating DERs into utility systems.

By Gary Frederich and Patric Dove, Advantech Corporation, Industrial Automation Group