Technical Article

An Introduction to Protective Relays for Solar-Plus-Storage Systems

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Electrical relays, protective devices used to switch power on or off for parts of a circuit, have been integrated into circuits for nearly two hundred years. The first example of a relay dates back to the mid-nineteenth century, when Joseph Henry used a small electric signal to activate an electromagnet and close a switch, establishing that high-power circuits could be controlled with low-power devices.

Electromechanical relays were ubiquitous up until 1982, when Schweitzer Engineering Laboratories (SEL) released the first static, steady-state relay. Since then, SEL has become a dominant supplier in the protective relay space, with competitors such as Siemens, Basler, and Mitsubishi chipping away at market share. In this article, we’ll explain how protective relays work, review some of the most common relay functions for solar and energy storage systems, and provide best practices for relay programming during project development.

How does a protective relay work?

Protective relays monitor voltage, current, or frequency and respond to abnormal conditions by opening or closing a switch to isolate parts of a circuit. Based on their switching mechanism, relays can be divided into two categories: electromechanical and static.  

Electromechanical protective relays use moving parts to open and close switches. A small electrical current is used to charge an electromagnetic coil, generating a magnetic field that controls the position of a spring-operated switch by either pulling the switch away from a contactor (opening the circuit) or pulling the switch toward a contactor (closing the circuit). 

Electromechanical relays are a tried and true technology. They are inexpensive and will work for both AC and DC circuits. While they may not be as durable or fast as static relays, electromechanical relays are a cost-effective option suitable for many applications.

Static relays use electrical signals to control solid-state semiconductor switching devices such as diodes or transistors. The static relay receives an input signal, processes it, and decides whether to open or close the circuit. No moving parts are required. 

Most utilities prefer static relays because they are generally faster to operate, longer-lasting, and more precise than electromechanical relays. Check your utility’s electrical service requirement (ESR) documentation or interconnection application to see if your utility requires a specific relay type or model. 

The relay (whether electromechanical or static) will have a default position: normally open and normally closed. 

Normally open relays keep the arm of the switch in the open position during normal operation. When energized (electromechanical) or triggered (static), the relay closes the switch to allow current flow.

Normally closed relays are the opposite. When energized (electromechanical) or triggered (static), they open, preventing current flow.

Determining the default position is an essential part of project engineering. Ask yourself: What do I need the failsafe position to be if the relay loses power? 

For example, utilities may require a normally open relay contactor for grid isolation, so the system is isolated by default, even if the relay and isolation contactor or breaker fails. On the other hand, devices with emergency stop buttons—commonly found on diesel generators—will typically use a normally closed relay contactor. During normal operation, the contactor remains closed, allowing power to flow. Pressing the button opens the circuit, cutting power and shutting down the generator. 

Always consider what happens when power is lost.  If opening the contact enhances safety, normally open is preferred.  If closing the contact enhances safety, normally closed could be the right choice, but must be justified with your control logic.      

How is a protective relay programmed?

Relays use voltage, current, and frequency set points to initiate an action, and can perform a wide range of functions — from grid isolation to load shedding to turning on a backup generator. The ANSI/IEEE C37.2 standard designates numbers and acronyms to describe common relay functions, providing a useful shorthand to list on plan sets and other engineering documents.

Let’s walk through an example of a grid-tied solar-plus-storage system capable of islanding. In this case, we are using an SEL 751 feeder protection relay to detect a grid outage and then initiate a method of grid isolation, such as a motorized breaker. 

First, we’ll set a few thresholds to detect grid loss using the Undervoltage, Overvoltage, Frequency, and Directional Power functions.

^{Configure Undervoltage (27) and Overvoltage (59) protection to detect grid loss.
27P: Pickup at 0.88 pu; Time Delay: 2 seconds.
59P: Pickup at 1.1 pu; Time Delay: 2 seconds.}

^{Set Frequency (81) thresholds for grid loss detection.
81U: Pickup at 59.3 Hz, Time Delay: 2 seconds.
81O: Pickup at 60.5 Hz, Time Delay: 2 seconds.}

^{Set the Directional Power (32) to detect reverse power flow from the grid.
Forward Power Threshold: 5% of ESS-rated power.
Time Delay: 3 seconds.}

Next, we’ll program auto-islanding logic with SELLogic. If voltage or frequency falls outside the set points established above, the SEL will trip the motorized breaker to island the system.

^{Set the breaker trip values.
OUT101 = 27P OR 59P OR 81U OR 81O}

^{Assign the relay to trip the breaker when one of the OUT101 conditions are met.
OUT101 = TRIP}

Set points are most often established by the IEEE 1547 interconnection standard, local utility, and/or related calculations (e.g., grounding transformer calculations). While modern inverters might be capable of performing similar grid isolation functions, most utilities will prefer the granularity, reliability, and proven track record that relays offer. 

How does relay programming fit into project development?

As with any grid-tied solar or solar-plus-storage project, best practice is to become familiar with the local interconnection and manufacturer-specific requirements as early as possible. Reach out to request ESR documentation and interconnection applications from the utility. Once the equipment is defined, engage the manufacturer(s) to determine if they have any relay requirements. Collaboration is key, and some relay suppliers will have in-house teams dedicated to helping with the whole process. 

For more tips, tricks, and insights into relay best practices, watch our Ask Mayfield Anything webinar ‘Relays for C&I Microgrid Control’ linked below.

Mayfield Renewables provides design and engineering services for solar-plus-storage systems, including systems that require the integration of protective relays. Contact us today for a consultation.