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Many water and wastewater electrical systems are designed as radial systems, where a source at one end of the system supplies the distribution network and loads connect to the other end of the system. In radial systems, the path for fault current is readily defined. For those faults, the current does not flow through the paths between the fault and the loads, except for transient current from stored energy in rotating motors.

For improved reliability, or to increase load-carrying capacity, some systems are designed with multiple sources connected together. For example, two or more utility lines may be connected to a common substation bus or the facility may operate a methane-fueled cogeneration plant connected to the distribution system that is also supplied by a utility service. For such systems, fault current might flow through a protective zone for faults on either the source side or load side of that zone. Overcurrent relays may respond to faults in many zones of the system or even faults on the utility system.

It's typically not possible to develop settings that will provide the necessary sensitivity and selectivity for all fault locations on such a system. Directional overcurrent relays determine the location, upstream or downstream, of the fault relative to the zone that detects the fault current.

The directional measurement is made by comparing the measured current to a reference quantity, usually a voltage that does not change with fault location. Settings for a directional overcurrent relay are then made to meet sensitivity and selectivity requirements for faults in the desired direction.

If a water or wastewater facility operates generators in parallel with the utility system, directional overcurrent relays may be required at the service point to prevent the local generators from inadvertently energizing the utility system. If a fault occurs on the utility system, the local generation would continue to energize the fault, creating safety hazards for the general public and for utility crews making repairs.

In one example, the designation 52 is the IEEE Std. C37.2-1996 for a circuit breaker. The phase-time overcurrent relays are designated 51, and the ground time-overcurrent relay is designated 51N. If instantaneous phase or ground overcurrent protection were applied, the designations 50 and 50N would be added to the relay symbols.

There are three phase overcurrent relays and three CTs. The dotted line from the relays to the circuit breaker denotes the relays are wired to trip the circuit breaker on an overcurrent condition.

Voltage and Current Balance Protection

Three-phase power systems are designed and operated to achieve the most balanced voltage and current conditions possible.

In a balanced three-phase system, the voltages are of equal magnitude and their sinusoidal waveforms are displaced from each other by 120 electrical degrees, or one-third of a power cycle. The sinusoidal voltages also reach their peak values in a known repeated sequence. For example, the sequence might be phase a, phase b or phase c. With balanced voltages and balanced loads, the currents will also be balanced.

Unbalance of either voltage or current can be caused by a fault, an abnormal condition (such as unintended load imbalance or an open conductor), or reversed phase connections. Deviation of the magnitudes or phase displacement angles indicates an unbalanced condition.

Voltage and current unbalance beyond a small tolerance is particularly detrimental to rotating machinery such as motors and generators, causing damage by overheating even when an overcurrent condition does not exist. Voltage and current balance relays measure the three-phase voltages or currents and calculate the degree of unbalance. Pickup settings usually are expressed in percent of the rated voltage or current and a time delay is typically used for security to allow an unbalanced condition to be corrected by fault protection relays in other zones.

Voltage balance relays can detect unbalance between the source and the point where the relay is applied, but not between the relay location and downstream loads. Typical applications for generators and motor circuits are at utility service points, at switchgear buses to control the automatic transfer between alternate sources, and to motor control centers or switchgear buses that supply multiple motors. Current balance relays are typically used to detect current unbalance for individual circuits.

Voltage-Based Protection

Several types of protective relays measure voltage to perform specific protective functions. Overvoltage and undervoltage relays operate when voltage is higher or lower than a settable level. The settings must allow for normal voltage variation associated with changes in load and utility service voltage tolerance.

Typical applications are at utility service points, at switchgear used to connect generators to a system, and at switchgear buses to control the automatic transfer between alternate sources. Overvoltage relays also are used to detect ground faults on generators and systems designed with grounding impedance to limit ground fault current to very low values.

Overfrequency and underfrequency relays measure voltage to determine the system frequency. Deviation from the nominal frequency (60-Hz in North America) might be caused by the failure of a generator governor, but usually indicates a mismatch between the power input to system generators and the power consumed by the system loads and losses. Operation at off-nominal frequency causes fatigue and cumulative damage to generator steam turbine blades and frequency may be applied for steam turbine protection.

On a system level, off-nominal frequency may be accompanied by abnormal voltage and could indicate the system is becoming unstable. Frequency relays may be applied for system protection to avoid a total collapse by disconnecting certain loads or tripping certain circuit breakers to separate the system into islands. In this way, stability can more likely be maintained or recovery more easily managed.

Frequency relays may be applied at the utility service point for water and wastewater facilities having local generation. Often the utility requires such protection as part of the relaying package intended to prevent the local generation from energizing a fault on the utility system. For water and wastewater facilities with local generation, frequency relays may also allow the facility to separate from the utility and continue operating at least some of the processes in the event of a utility system fault and disconnection of service.

Synchronism check relays measure the magnitude and phase angle of the voltage across an open point where two sources are to be connected. These relays are not protective relays, but serve a supervisory role in controlling the operation of circuit breakers. The two sources may be utility services from separate substations. Often, one source is the utility service and the other source is a local generator.

To connect the sources, the three-phase voltages must be nearly equal, the frequencies must be nearly identical, and the phase sequence of the sources must be the same. Generator paralleling controls automatically adjust governors and exciters to achieve synchronism before paralleling the generator with a utility system. Connecting two sources that are not in synchronism may result in damage to rotating machinery because of torsional stress to the shafts, and damage to the tie circuit breaker and switchgear with associated risks to personnel operating the equipment.

Defining a zone of protection for voltage and frequency relays that may respond to faults or abnormal conditions for any location on a system is difficult. Voltage and frequency relays are usually applied as a last line of defense to separate systems at interface points. Time delays are used to allow isolation of a fault or abnormal condition by relays in a specific zone.

 

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