SCADA---professional-4

SCADA 101: SCADA System Architecture for Solar PV Plants

When we talk about Supervisory Control and Data Acquisition (SCADA) system architecture, we're referring to all of the hardware and software provided by the SCADA subcontractor as part of their turnkey scope of work.

 

In a solar PV plant, the SCADA architecture includes:

  • One or more master stations or Master Terminal Units (MTUs), which operators use to monitor the plant and interact with remote devices through a Human Machine Interface (HMI). For a solar plant, this will be a computer in the central monitoring station or control room running the SCADA software.
  • One or more remote stations, which can be Programmable Logic Controllers (PLCs) and/or Remote Terminal Units (RTUs). These are hardware systems—hardened for outdoor or industrial use—that communicate with substation Intelligent Electronic Devices (IEDs), sensors, HMIs, inverters, trackers and other devices. They can control the processes of these devices, gather their data in real time, and transfer it back to the master station.
  • The communications system, which is how the MTU and RTU, as well as all the different devices throughout the plant, connect and communicate with each other. This includes all of the networking hardware.

What is a SCADA network?

A SCADA network is a wired or wireless network that connects all of the devices on the solar site. It not only connects the remote stations with the master and allows them to transmit data back and forth, but it allows the various networked devices to communicate with each other. Typically these networks are integrated using fiber-optic cables.

 

Today's solar farms are far bigger than other types of power plants. They are spread out across acres of land. This requires a fiber network that is intelligently planned and laid out. This is a joint effort of the EPC contractor, who designs the physical layout and installs the fiber cables, and the SCADA contractor, who determines how all the devices will connect and communicate via the network.

 

The fiber network starts with a hub location, which in the case of a solar PV plant is typically the substation. It is located near the point of interconnection of the farm, where the master SCADA system equipment also resides. The fiber network is terminated into a patch panel within the master SCADA enclosure.

 

Given the vast size of solar PV plants, the fiber network must be divided into different sections called loops. The number of loops is contingent on how the site is spread out and how the construction company installs the fiber cables. The largest number of loops we have seen to date is 11 fiber loops in one plant.

 

Switches are a vital component of the network. A switch is a piece of networking hardware that connects other devices together. Multiple networked devices can be plugged into the switch, enabling them to communicate with each other via Ethernet. It is called a "switch" because it uses a method called "packet switching" to receive and forward data to/from the correct device.

 

Dispersed through the solar farm are small switches, which connect the network hardware as well as devices like inverters and trackers. These devices use either one or two Ethernet connections, depending on the manufacturer. The field switches will also connect to the fiber optic network via fiber jumpers.

 

From the field equipment, one or two fiber cables come back to the main root switch at the substation. All of the servers and all of the different types of substation IEDs are connected to that root switch and use this "fiber backbone" to communicate with the equipment in the field.

 

The root switch is a managed switch, meaning you can log into it via the SCADA server and monitor activities inside it. Some plant owners require manage switches at the field level as well.

 

Why is fiber optic cable commonly used for utility scale solar projects?

Transmitting distance and performance are the main reasons fiber optic cable is often used for utility scale solar plants.

 

deposit-photo

 

Solar farms typically cover a huge geographical area, and fiber is the best solution due to its extremely long transmission distance and resilience to electrical noise. It's truly incredible to see tracker or inverter data populate within a second, even though the devices are a mile or more from the substation.

 

Can a wireless radio network be designed instead? What are the pros and cons of going the radio route?

When it comes to networking speed, radio is nearly as fast as fiber. However, the main concerns with a wireless radio network are security and reliability.

 

With radios, there can be some environmental conditions that can cause disrupted signals or loss of communication. Modern radio networks implement new technologies to ensure better communication between two points. If one radio fails, it will automatically switch to a redundant radio so the communication path does not drop. However, radio signals are still susceptible to interference from different sources. For example, electrical or environmental anomalies can create strong magnetic fields, which risks a collapse of the whole radio network. As soon as that radio communication is lost, there is no way to remotely monitor the site's field equipment. The only way to know what's going on is to have someone physically onsite.

 

This can be a major issue because many O&M providers remotely monitor plants that are hundreds of miles from their operations center. If there is a lack of visibility to the site, it might require them to trip the plant offline, which will cause monetary losses for the site owners.

 

These are the major cons of a radio network. On the other hand, the main pro is the cost.

 

Radio networks are significantly cheaper than fiber to install. With fiber networks, there is considerable construction and material cost involved with installation such as trenching, cable-laying, termination, and testing. With radio networks, all that is required is to mount a pole and put the radio on the top.

 

What common communication protocols are used by the SCADA system?

Modbus protocol has been around for 40 years and is the most common protocol used for automation components, including those used in solar power plants. It is open source, and 80-90% of plant devices (inverters, trackers, etc.) talk Modbus protocol. If the SCADA system and power plant controllers can talk Modbus, it is easy to pull the data from the devices in real time.

 

DNP3 is another common protocol, primarly used to communicate between different substation devices in the SCADA system. DNP3 is a newer protocol that has become more widespread over the past 10-15 years. It has some good features, including a timestamp. With a timestamp of the data, you know exactly when and where the data is received, and whether it was good or bad quality.

 

This is an important distinction from Modbus. If an end device stops providing new data, and you're using Modbus protocol, you may never know that data is bad because the data isn't time stamped. You won't know you're seeing old data. By using DNP3 protocol, you can detect that something has gone wrong with the device by looking at the timestamp and data quality.

 

For systems that Nor-Cal integrates, we typically use DNP3 as the protocol for critical devices like the substation devices, and then Modbus for the rest of the plant.

 

What hardware components make up a SCADA system?

SCADA system hardware includes servers, networking devices and field devices.

 

SCADA systems typically require two separate servers. One of which is for the HMI and data collection platform. It may sound complicated, but at a high level is a computer that collects all the real-time data that displays all of the information on screens, including any system alarms. It is the means by which the operator can see the entire plant and interact with the plant devices.

 

The second server is the historian server, which stores the plant data. The data can then be used for troubleshooting or performance monitoring. If there's ever a need to go back and query what the solar PV site was doing a year ago (required for some regulations), the historian server provides that functionality.

 

At Nor-Cal, we typically use Dell servers, which are industrial and hardened, for the SCADA and historian.

 

Other hardware includes the firewall, network switches, protocol converters, and controllers. Programmable Logic Controllers (PLCs) are used to control the electromechanical functions of the equipment and devices. We prefer to use the GE PLC because it has the automation and control features most plant owners and operators require.

 

All of this hardware is connected together using the system of network switches we described earlier, communicating via Ethernet connections and occasionally serial.

 

What SCADA software is typical?

Solar PV plants produce a massive amount of varied data. There is tracker data, inverter data, MET station data, internal tags in the controller, data from third parties, and data from the utility. All of this data concentrates into one SCADA platform. Not only must the SCADA system software be able to handle this huge amount of data, it must be able to understand different protocols.

 

We typically use a GE-provided system called CIMPLICITY for our systems. It's a well-known SCADA software that's been trusted for decades not only by the solar automation industry, but by the pharmaceutical industry and many other power generation companies. GE continually supports and improves the software. It can handle various types of protocols, including Modbus and OPC UA.

 

On some SCADA projects we use software called Ignition. Like CIMPLICITY, it can handle Modbus and OPC UA. Unlike CIMPLICITY, it can also handle DNP3. Not all solar plants require DNP3 protocol, but when they do, it's better to have this feature in the software.

 

What type of documentation can the SCADA subcontractor provide that depicts the system architecture components and how they're connected?

There are several types of documentation provided by the SCADA subcontractor:

  • Warranty documentation
  • Datasheets or cut sheets for the hardware and software
  • Bill of material or equipment checklist
  • Drawings of the SCADA architecture
  • Control narrative, which describes how the plant controls will work
  • Points list, which tells how many data points will be pulled from the whole site
  • IP address or networking list
  • Testing plans and reports

All of these documents play a vital role in overall solar plant management. They communicate how the plant architecture is laid out and how it will work.

 

At Nor-Cal, we have a designated specialist who handles all of these SCADA documents and makes sure they're submitted to the client.

 

Site Testing Documentation

The SCADA subcontractor provides a Site Acceptance Test (SAT) plan and then a SAT report, once the site commissioning is complete. This also includes a Factory Acceptance Test (FAT) plan and report.

 

The FAT documentation shows that the SCADA provider tested the equipment at their facility. Once the equipment is shipped to the site and the SCADA provider is done with all the commissioning, the SAT is the final step.

 

If in the future something isn't working at the plant, this documentation acts as a baseline. The owner or operator can refer to these reports and see if the device was working properly at that time.

 

SCADA Architecture Drawings

Earlier we discussed SCADA network architecture, with its layout of fiber, loops and switches. These network diagrams need to be properly depicted and provided as part of the SCADA documentation. That way, a person coming in two or three years after the plant is commissioned can learn how the network architecture is laid out. These drawings are called communication block diagrams or network drawings.

 

The SCADA subcontractor should also provide electrical drawings of the meteorological (MET) stations to aid in troubleshooting down the road.

 

Why is Nor-Cal is the best partner for your SCADA system?

Two words: "open architecture."

 

Our SCADA systems are based on open architecture hardware and software, eliminating the need for ongoing subscription fees, data access fees, and restrictive service contracts. You're not limited to proprietary solutions that may not do everything you need. We also give you the freedom to choose your own hardware and software. If you have a specific preference, we have no problem sourcing the product, doing the engineering and design and then providing it to you for your project.

 

For example, we typically use CIMPLICITY or Ignition SCADA software. However, we have customers who prefer Citect SCADA, and that's no problem for us. If a customer wants Allen Bradley PLC instead of GE PLC, we can make that happen. That does mean we need to make some changes to our SCADA architecture. We explain those changes, and if the customer is fine with that, we move ahead.

 

Today's solar PV plants are all about flexibility and scalability, and that's what we provide. If you are doing the SCADA architecture, you should absolutely go with Nor-Cal.

 

Schedule a call with us today to learn more!

 

Abhishek Dabholkar

Written by Abhishek Dabholkar