Relaying and protection can be confusing-REALLY CONFUSING. Elaborate new ways to protect power systems are being invented every day. And, don't get me wrong, that's a good thing! We want to have the most sophisticated options available when it's necessary. However, for the majority of projects out there, only a few types of relaying schemes are really necessary to ensure a safe and effective power system design. This article will run you through some of the most common types of protection and when to use them.
Inverse-Time Overcurrent (ANSI Number 51): Inverse-time overcurrent is the oldest kind of protection in the book. Why? Because it's just a mimic of the behavior of fuses. Above some threshold (the long time pickup rating), the inverse-time relay has a trip curve. Larger values of current lead to fast tripping, while smaller values of current lead to longer tripping.
Instantaneous Overcurrent (ANSI Number 50): Instantaneous overcurrent is the simplest of protection schemes. When the current is greater than some value, the relay trips-easy as that! Instantaneous overcurrent and inverse-time overcurrent are often combined to offer more sophisticated time-current curves, like the one shown in Figure 1 below. Delays can be added to the instantaneous set point to provide a fixed time gap between the relay sensing an overcurrent and actually tripping.
Figure 1: 50/51 Integrated Trip Curve
Differential (ANSI Number 87): Differential protection is a clever way to achieve selective, high-speed fault clearing on upstream buses in a power system. The further upstream (closer to the grid) we go on a power system, the more the delays from 50/51 overcurrent protection will pile up. Sometimes, this leads to problems with arc flash ratings because faults on buses do not clear quickly enough. Differential protection focuses on a different approach than just the magnitude of the current flowing through the relay. With differential protection, multiple current transformers are used to measure current flowing into and out-of a bus, transformer, or similar piece of electrical equipment. Then, if there is a significant difference in those values, we trip the breaker(s) protecting this bus. The idea behind differential protection is that a fault at the protected equipment will be the only way to trip the breaker, allowing us to be very fast and selective. The area protected by a differential relay is known as the "zone of protection". Figure 2 shows an example of bus differential protection being used with three feeders and a main branch. The zone of protection is the area inside of the current transformer boundaries
Figure 2: Current transformers measure all the branches coming into and out-of a bus. The 87 relay will trip the breaker if an imbalance is detected.
Undervoltage (ANSI Number 27): Undervoltage may not represent an obvious concern for human safety, but it can pose real problems for system operation and consequently lead to dangerous/costly conditions. Equipment is always designed with a voltage range (e.g. +/-10% of some nominal value). During heavy loading conditions on the power system, voltages could dip below the allowances permitted by equipment. In this case, one may be required to shut down the power system to prevent damaging equipment. In particular, motors would be a common item for concern. When the voltage of a motor drops, it tends to draw more current. Outside of the permitted voltage range, this current may become large enough to damage the motor. Undervoltage relays are designed to trip with a voltage vs. time curve, just like a 50/51 relay does for current.
Overvoltage (ANSI Number 59): Just as undervoltage can be a problem, so can overvoltage. Overvoltage generally occurs during lightly loaded conditions. This problem can be magnified by voltage fluctuations on the grid, especially when transformers aren't equipped with on-load tap changing devices. Overvoltages can cause damage to equipment by breaking down insulation due to high electric field strength. It's more common to see undervoltage protection (27) than overvoltage (59), since overvoltage can usually be mitigated with proper transformer taps and knowledge of grid voltages. Overvoltage relays are designed to trip with a voltage vs. time curve, just like a 50/51 relay does for current.
Synchronism (ANSI Number 25): Synch check relays are essential where more than one separately-derived source could be operating together. For instance, say an emergency generator is going to turn on and run while voltage is still being supplied by the grid. Or, alternatively, a switchgear lineup could be fed from two separate transformers for redundancy and require synchronism between these sources. The purpose of the 25 relay is to make sure that whatever switching device allows the paralleled operation of sources switches in at the right time. Technically, the goal is to minimize voltage imbalance to prevent unintended current flow.
Figure 3: Two voltage waveforms are checked for synchronism. The vector difference (phase and magnitude) must be sufficiently small to allow sources to operate in parallel.
Mechanical Alarm and Failure (Multiple ANSI Numbers): Okay, this one's actually a catch-all for the various types of mechanical alarms that need to be tied into a well-designed protections system. Some examples, would be thermal overload protection on a transformer (ANSI Number 49) and transformer pressure alarms (ANSI Number 63). However, all kinds of other alarms could be applicable depending on the project design and the equipment involved.
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