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Historian Data Trends

SCADA 101: Local Historian Overview

Over our past few articles, we've been covering the basics of Supervisory Control and Data Acquisition (SCADA) systems for solar PV sites—how they work, why they’re important, and the major components that make them up.


The historian is a crucial component of the SCADA system, responsible for storing and logging site data. Let's start by covering some terminology, then delving into historian capabilities, features and applications.


What is the difference between SCADA, HMI, Historian, Alarming and Reporting?

SCADA is the overall system that monitors and controls all the different networked devices at the solar PV site (inverters, trackers, substation devices, MET station sensors, and other equipment). The Human Machine Interface (HMI) and historian are two major components of the SCADA system.


The HMI is the user interface that allows operators to view real time plant data. It is also the means by which the operator gives commands to the Power Plant Controller (PPC). The HMI displays alarms to alert operators to any unusual behavior from the plant. You can read about HMI platforms in depth in our HMI for Solar PV article.


The historian is responsible for storing and logging all of the data that the SCADA system aggregates. It allows operators and stakeholders to look at historical data for the plant.


A historian can also have reporting capability. It can generate manual or automated reports containing different sets of data and show what happened at the plant over specified periods of time.


What is a local historian? How does it fit into a local SCADA system architecture?

A "local," or "onsite," historian system runs on its own server or PC at the plant. This server or PC is connected to the SCADA network, and has a data link to the HMI software. The historian looks at the real time data from the HMI software and stores it on its local hard drives.


There are also cloud-based historian systems, where the data is stored offsite on Internet-based servers.


When is a historian recommended or required as a part of a local SCADA system?

At Nor-Cal, we recommend that any solar PV plant large enough to have a full local SCADA system (typically 20 megawatts or above) also have a local historian. This is a typical Owners' requirement for sites of this size.




Owners, operators and O&M providers often require a cloud-based third-party historian system to store data offsite, but also require the ability to pull data locally from the site. If there's ever a need to go back and query what the plant was doing a year or two ago (required by some regulations), a local historian can provide that functionality. A full local historian also allows for in-depth analytics and reporting.


How are historians used in DAS versus SCADA systems?

Smaller sites with Data Acquisition Systems (DAS) instead of SCADA typically don't require a local historian, as they have far fewer monitored devices and subsequently less data. They also are not subject to the same regulations and requirements as large utility scale plants. DAS sites can usually get by with only a cloud-based historian. However, some DAS site owners do also require a local historian.


Do local DAS systems for Distributed Generation (DG) and Commercial & Industrial (C&I) sized projects require data logging/historian capabilities?

They definitely need those capabilities. However, it's not as important for the data to be stored onsite for long periods of time. In most cases, the data is stored long-term through a third-party, cloud-based historian platform. The data can be stored onsite for short-term periods, usually a week or two, as opposed to a full local historian that can store multiple years of data.


How might the historian data be used differently for a DAS system as opposed to a SCADA system?

This depends on the level of analytics needed for the site.


Smaller sites with DAS have far less equipment and typically don't need the analytics or reports to be as in-depth. They will tend to be more "hands-off" in terms of operation. Larger sites with SCADA are complex and need to be closely monitored. This requires the in-depth analytics and reporting that a full local historian can provide.


system of rooftop solar panels


What are the hardware and software requirements for local historians?

There are two main factors when it comes to historian hardware requirements:

  • The hard drive space
  • The processing power of the machine on which the historian is installed

The hard drive space determines how much data can be stored and for how long, while the processing power determines how many data points the historian can handle.


Each networked device or piece of equipment at a solar PV site has data points (also called tags) that the SCADA system pulls data from and the historian logs and stores. The bigger the site, the more devices and equipment there are—meaning more data points or tags.


Historian software is typically tied to the site's point or tag count. The more tags, the bigger the historian software package you will need. For example, there may be different levels of software licensing you would buy for a 50,000 tag site versus a 20,000 tag site.


Historian software platforms also differ in their reporting and analytics capabilities, so owner requirements are a factor there. Every brand and type of historian software is different in terms of the formats they can report in and their specific reporting features. For example, GE Historian does not have automatic reporting capabilities built in, while Canary Labs Historian does.


What are some common historian capabilities and features?

There are three common capabilities:

  1. Database queries. This means being able to "ask" the historian to pull raw data for a set of data points over a specified time period and put it into a spreadsheet. This is a core capability of any historian, although the query method can vary by software platform. There are also custom database query functions which can produce calculated data—for example, the average numbers over a time period, minimum and maximum, etc.
  2. Data trends. This involves extracting data and putting it in a graph to visualize data points over a specified period of time. Owners and operators can use this information to more easily uncover and analyze trends in site performance.
  3. Reports generation. This is a core capability that combines the two above. The historian can put trends and raw data into a report that runs on a specified schedule. This might be once a month or once a week depending on owner and operator requirements.

Historian Data Trends

What is the difference between a local database and a true historian?

A local database contains raw data. One example of this type of database is Microsoft SQL, which stands for Structured Query Language. In order to go in and pull data from this kind of database, you have to "speak" that language. Getting the data and turning it into useful information is a more involved process.


A true historian uses what’s called a historical database. The historian processes the raw data before it goes into the database, making it much more user-friendly to process data.


How often is data logged within the historian?

Typically, data is logged at one-second intervals, however there are some limitations.


If there are devices or equipment that can't report every second, the historian will still log their data at one second, but will "interpolate," or keep the same value over that period of time. The available hardware capacity is another consideration. If there's not enough storage on the historian's hard drive for one-second data over two years, the site owner may require instead to log five second data for two years, or 15 seconds.


How can local historian software be used to help troubleshoot system or device issues at a solar PV project site?

The historian helps track problems that build up over periods of time. By monitoring changes in value for certain data points over days, weeks, and months, operators can see if the correlating systems and devices are developing problems that need maintenance attention. If a device malfunctions suddenly, its historical data may offer visibility and insight into resolving the problem.


What is the difference between performance analytics and historian data?

reviewing data at a solar plant


Historian data is, as we covered earlier, raw data pulled in from the field devices and other site equipment. Custom queries and formulas can turn that raw data into calculated data that is more useful—for example, trends and averages over time.


Performance analytics use that raw and calculated data to provide insights into site health and performance. You can read more about how performance analytics software turns historian data into actionable insights in our Solar Performance Analytics article.


Does local historian software help with SCADA commissioning activities?

It does! At Nor-Cal, we use the historian throughout the commissioning process to track down problems and make sure the system is working properly. For example, if we need to tweak the scaling or the offsets on a data point, we use the historian to keep track of that data over the entire period or portions of commissioning.


The historian can be used to hold data to make sure the control portion of the site is working properly. When we're doing control testing on a site, we do a lot of tuning and tweaking in the power controller's logic. The historian capability allows us to view the historical data for certain points that pertain to the controller, which helps us with that tuning process.


Does it help with performance testing or capacity tests?

Yes. The performance and capacity tests are often run by the EPC or the owner, and they may ask the SCADA integrator to pull historical data to prove that the site plan is performing as expected. This helps to track and resolve any issues related to the plant’s performance or capacity.


How can Nor-Cal help determine local historian requirements for an owner’s upcoming project?

We draw from our many years of project experience to help owners with site specifications. We help owners determine how many years of data they’ll need to store locally, along with the data logging capability (one second, five seconds, etc.), and recommend historian hardware and software that will meet those requirements.


With over 7GW worth of successfully commissioned projects to date, we can help owners determine not just the best historian for their solar PV project, but the best overall monitoring and controls system. Our turnkey solutions are open source and completely customized to handle current and future needs.


If you have a solar PV project and want to discuss historian recommendations or any other aspect of the system, schedule a call with us today.


Troy Morlan

Written by Troy Morlan