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Reliability

Reliability of a protective relay system is defined in terms of two components: dependability and security. Dependability is the ability of the relaying system to always operate correctly for a fault or abnormal condition. Security is the ability of the relaying system not to operate when there is no abnormal condition present, or if the condition is temporary or should be isolated by relays in another part of the system.

As with speed and selectivity, dependability and security are opposing demands. Improving dependability tends to reduce security and vice versa. All protective relay system designs attempt to balance the competing requirements of dependability and security. Lowering the overcurrent pickup setting of a relay is a simple way to increase dependability. Adding time delay is a simple way to increase security.

Also, relay designs often include features that enhance both dependability and security. For example, restraint elements may be a security feature for a differential relay so the relay avoids tripping for faults that should be cleared by other devices. The same differential relay may also have an unrestrained overcurrent element with a high setting to maintain dependability.

A motor protective relay may have firmware routines that recognize when the motor is started and restrain tripping on the high starting current during a programmed interval. An overcurrent setting can be closer to the motor starting inrush current for better dependability while the restraint feature maintains security.

Zones of Protection

Visualizing an electrical power system as divided into zones of protection allows protective relays to be logically applied to achieve the goals of speed, selectivity and reliability. The concept is simple: each component is a separate zone requiring specific protection.

In a water or wastewater treatment facility, the protective zones might be the main transformer, medium voltage switchgear, medium voltage distribution line, unit substation transformer, low voltage switchgear, low voltage feeder, motor control center and motor. The zones are separated by switching devices that will operate to selectively isolate a faulted zone from the rest of the system.

Zones of protection are defined by the location of switching devices and the current and voltage transformers that provide inputs to the protective relays; therefore, the zones should overlap. For example, a main transformer supplies medium voltage switchgear through a main breaker. The transformer zone should include the medium voltage switchgear main breaker. The switchgear zone also should include this main breaker. Transformer relaying and switchgear relaying can both respond to faults at the switchgear main breaker.

While it may seem obvious, including the correct logic and wiring to trip all switching devices necessary to isolate a faulted zone is important. For example, if a transformer is equipped with high and low side circuit breakers, both breakers must be tripped by the transformer protective relays. Bus protective relays must trip the main, tie, and feeder breakers for the protected bus section. Auxiliary relays trip the required breakers and may provide a lockout function so that equipment is not inadvertently reenergized by operators after a trip.

Historically, individual protective relays have been designed to detect specific conditions: one relay detects overcurrent for fault protection, one relay detects undervoltage, one relay detects motor overloading, etc. and a package of individual relays provided the protection for a particular zone. This was a natural design because specific types of electromechanical elements or analog electronic circuits were required to perform the different types of measurements.

Modern protective relays are now microprocessor-based, while storing the various protection measurements and logic decisions in relay firmware. This means once the currents and voltages are converted to digital form, one microprocessor-based instrument running multiple algorithms can perform multiple protective functions and provide all the protection for a particular zone.

Types of Protection

Since the zone of protection consists of a specific type of component, the appropriate relay protection for each zone must be chosen. Here are the most commonly used types of protective relays and some ways they are designed and applied to balance dependability and security:

Overcurrent Protection

Overcurrent protection is perhaps the most basic form of protection and can be applied to any power system component.

As current transformers supply the current to be measured, if the current is above the pickup setting for the set time delay, the relay produces a trip signal. The zone protected by an overcurrent relay depends on the pickup setting. A lower pickup setting extends the zone and allows the relay to respond to faults farther away from its location. Since zones of protection should overlap, an overcurrent relay should be able to respond to faults in at least the next adjacent zone.

Time delay is used to allow the relays in the next zone to operate first and achieve selectivity. Overcurrent relays typically provide a settable time delay and an instantaneous function to trip with no intentional time delay. To maintain selectivity, the instantaneous pickup setting must be high enough so that it does not respond to faults in other zones.

In a three-phase system, separate overcurrent protection is provided for faults involving only the phase conductors and faults involving one or more phase conductors and ground. The ground current measurement can be made indirectly with current transformers connected to form the sum of the phase currents. This sum equals the ground current.

Another way to measure ground fault current on insulated cable circuits is to pass the insulated conductors through a specialized current transformer designed to accommodate the conductors in its window. With this arrangement, the summation is performed magnetically. Measurement can also be made directly with a current transformer installed at the location to which ground fault current returns in the circuit - typically a transformer or generator neutral terminal.

The advantage of including both phase and ground overcurrent protection is to create greater sensitivity. Overcurrent pickup settings for phase faults must allow for normal and emergency load current in the phase conductors. Under normal conditions, current in the neutral or ground is very small and may approach zero.

Ground overcurrent pickup settings do not have to accommodate load current and can be much lower (more sensitive) than phase overcurrent pickup settings. Ground faults typically have higher impedance in the circuit path than phase faults and the circuit may have current-limiting impedance intentionally added. The result is that for most locations on a system, ground fault current is less than phase fault current. Since most faults (80 percent or more) involve ground, sensitive ground overcurrent protection separate from phase overcurrent protection is desirable.