Part 1 - Protection Relays

Building a solar project that connects to the grid introduces a whole new scope of medium voltage (MV) equipment, commissioning, and knowledge that most people would not be familiar with unless they have worked for a utility in the past. Even experienced electricians are not typically familiar with MV since they do not encounter it even with commercial or industrial projects that typically have a maximum voltage of 480V, up to 600V.

MV equipment is classified as any voltage higher than 600V; however, in practice, 4,000 volts (or 4kV) to 65,000 volts (or 65kV) is the typical range for MV equipment. Depending on the part of the country you live in, different utilities standardize on different common MV voltage classes. Where we are located in North Carolina, we typically see distribution MV classes of 12.47kV, 22.86kV, and 34.5kV. While there are other voltage classes out there, they are less common.

Due to the potential danger associated with faults at these high voltages, one of the equipment types used is called a protection relay. If you've never heard this term before, you may have heard a related name, "recloser." While these names are not the same, they are related and are often used interchangeably, incorrectly.

Let's start with understanding the term "relay," and then we'll move on to "protection."

A relay is an electro-mechanical device that will switch something on or off when it is energized. Traditional relays have a coil of wire wound around an iron core and form an electromagnet when an electric current flows through it. When the electromagnet turns on, a switch can be pulled (or pushed) from one position to another. This switching action can then be used to turn things on or off. The clicking sound you hear when using your car's turn signal is a relay switching on then off (the switch open then closed).

Decades ago, special relays were used in early computers called vacuum tubes. In modern times, vacuum tubes have evolved into special relays called transistors. Although these devices are different sizes and use different technologies for switching, they are all different types of relays.

So, in summary, a relay is a device that switches something when it is energized (or receives a signal).

Protection describes the function of the relay. Our "protection relay" is intended to provide protection when it switches. In this case, "protection" refers to the utility grid and any equipment connected to it. The purpose of the protection relay is to safeguard the grid and connected equipment when it senses a fault.

Sensing a fault is more complicated than simply being energized or receiving a signal. To detect a fault, our protection relay has to have some smarts to know when there is a fault condition and what to do about it. If our relay switches every time there is a fault (big or small), that switching may result in unnecessary power outages or critical equipment turning on and off when it's not really necessary. Our protection relay, therefore, uses a very fast computer to monitor the power quality of the grid it is connected to and analyzes those conditions based on a set of programmable rules to determine when a switching event should take place or not.

These programmable rules are called "protection elements." Protection elements evaluate specific electrical fundamental measures such as voltage, current, and frequency. Each protection element can be configured for sensitivity based on the magnitude of the measurement as well as the duration of the measurement's value. Said another way, if voltage goes too high for too long, switch!

Multiple protection elements are typically used in coordination to define an acceptable electrical operating envelope. When the grid power quality operates outside of this operating envelope, the protection relay's elements are triggered, and the relay can take action to prevent a dangerous fault event.

Protection relays are just the monitoring and analysis component, a type of special computer. To actually protect the grid and 'switch,' the protection relay needs to be paired with a properly rated disconnect device. Disconnects that can be remotely controlled to open or close are referred to as breakers (for 600V and below applications) and vacuum fault interrupters (VFI) at higher voltages. The selection of breaker or VFI will depend on the specific voltage, current, and service duty the application requires.

An example of how this all works together can be described by a common event during a storm: a branch falling across two overhead lines.

When a branch makes contact with two overhead lines, the branch creates a short in the circuit. This electrical short causes a voltage imbalance and a high current load on the two affected lines. The relay's protection elements would measure these various abnormal conditions and determine there was a fault. When a fault condition is detected, the protection relay triggers the disconnect to open in a special event called a trip. Once a trip takes place, the grid is disconnected to isolate the fault, and power is lost for any customers along that circuit.

This is the point where the term "recloser" comes from. To re-close is when the protection relay attempts to restore the circuit by closing the disconnect that tripped to see if the fault still exists. If the fault has cleared itself (perhaps the branch fell off the lines in the storm's wind), the disconnect will stay closed, and customer power is restored. If the fault still exists, the protection relay will trip again and then attempt to re-close again. The number of re-close events and the time between these events is also a programmable configuration in the protection relay.