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Intro to Relays #1 - What are Relays, CTs & PTs?
Protective relays are a complex and essential part of electrical engineering, especially in the solar and energy storage industries. While they may seem intimidating at first, they don’t have to be! This three-part series is designed for non-engineers who want to understand the basics of relay systems used in renewable energy projects.
**Intro to Relays #1 – What Are Relays, CTs, and PTs? (Below on this page)**
**Intro to Relays #2 – ANSI/IEEE Relay Device Numbers**
**Intro to Relays #3 – What Does SEL Stand For?**
**What Is a Relay?**
In the context of solar and energy systems, a "relay" typically refers to a "protective relay." These devices monitor key parameters like voltage, current, and frequency in an electrical circuit. When something goes wrong—like an overcurrent or overvoltage condition—the relay takes action by opening or closing a switch to isolate the affected part of the system.
Historically, relays were mechanical, electromagnetic devices. Today, most modern relays are microprocessor-based, essentially small computers that can process data and make decisions in real time.
**Function of a Relay**
The main purpose of a protective relay is to quickly remove faulty equipment from service, preventing damage and ensuring the overall stability of the electrical system. They serve two primary functions:
1. **Prevent Damage:** By detecting abnormal conditions early, relays help prevent failures in critical components like inverters, transformers, and circuits.
2. **Mitigate Impact:** If a fault does occur, the relay ensures that it’s isolated as quickly as possible to minimize disruption.
Relays continuously monitor the system, looking for deviations in current, voltage, or frequency. When a threshold is exceeded, the relay sends a signal to a switching device—such as a circuit breaker—to take corrective action before the system is harmed.
These relays protect various parts of the electrical infrastructure, including:
- Solar PV and energy storage systems
- Buildings and facilities
- The utility grid
For example, an overcurrent relay can detect excessive current on a feeder line and trigger a circuit breaker to cut off power. A reverse power relay prevents backfeeding into the grid beyond safe limits, protecting both the solar system and utility equipment.
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**How Is This Different From a Circuit Breaker or Fused Switch?**
While circuit breakers and fused switches also interrupt the flow of electricity when current becomes too high, their functionality is limited to basic overcurrent protection. They lack the advanced monitoring and customization capabilities found in modern relays.
Relays offer several advantages:
- **Multi-Function Monitoring:** They can track current, voltage, frequency, and even power factor. In solar applications, this helps ensure compliance with grid standards.
- **Custom Programmable:** Each relay can be programmed differently based on project needs. The same model can be used across multiple projects but operate uniquely in each.
- **Modular Design:** Unlike circuit breakers, which are all-in-one devices, relays often consist of separate components like CTs, PTs, and a switching unit, allowing for greater flexibility and integration.
**What Are CTs?**
CTs, or Current Transformers, are used to measure current in a circuit. Since the actual current in a system is often too high for a relay to handle directly, CTs step it down to a safer, manageable level. For instance, an 800:5 CT reduces a current range of 0–800 amps to 0–5 amps, making it suitable for relay input.
**What Are PTs?**
PTs, or Potential Transformers, measure voltage and frequency in a circuit. Like CTs, they reduce high voltages to levels that can be safely read by the relay. This allows the relay to monitor and respond to changes in the electrical system effectively.
**Switching Device**
The switching device is what actually opens or closes the circuit. This is commonly referred to as “tripping†the circuit during a fault. In large-scale systems, this could be a vacuum circuit breaker or recloser, while smaller commercial systems often use circuit breakers with shunt trip options.
**Conclusion**
At a high level, the concept of relaying is straightforward, but the details can get quite complex. While project managers and developers don’t need to know every technical detail, having a basic understanding of how relays work can be incredibly helpful. That’s where experienced engineers like those at Pure Power come in. If you need assistance with relays on your next project, reach out to us today to learn more.