Protective Relaying for Water & Wastewater Treatment Plants (Part One)

Written by Keith Robertson, P.E., Schneider Electric WWCC

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Directional Power Protection

Power directional relays use voltage and current inputs to measure power. Either real power (watts) or reactive power (vars) can be measured, but for most applications, real power is the desired quantity.

Since power is the product of voltage and current, there is directionality associated with the measurement. Direction is determined by the phase angle between the voltage and current waveforms and power is transmitted (or "flows") from one point to another.

For example, power flows from a source to a load, into or out of a switchgear bus and into or out of a generator. For systems with multiple sources, power can flow in more than one direction. A power directional relay operates when power flow is higher or lower than a set pickup in a specified direction. The pickup and directional settings are selected to indicate an abnormal or fault condition.

At a water or wastewater facility with local generation, loss of the prime mover power input to the generator allows power to flow from the system into the generator and the generator acts as a synchronous motor. This condition mainly creates the risk of damaging the prime mover, so a power directional relay is applied to detect power flow into a generator on loss of the prime mover.

If a wastewater facility operates a methane-fueled cogeneration plant, the service agreement with the utility company may or may not provide for the utility to buy surplus power from the facility. If there is no agreement for the utility to purchase power, a directional power relay may be required at the service point to prevent transmission of power from the cogeneration plant to the utility system. This relay may be part of the required protection at the service point to prevent the cogeneration plant from inadvertently energizing the utility system.

If two transformers supplied by separate utility feeders are connected to a common bus at a water or wastewater facility, a fault on one utility feeder will allow current and power to flow from the other transformer through the common bus and back through the first transformer to the fault. Depending on the fault type, location and transformer winding connection, the current may be nearly zero and the only power flow may be that associated with the transformer losses. The direction of the flow will still indicate an abnormal condition.

Differential Protection

Differential relays operate on one of the fundamental principles governing behavior of electrical circuits - Kirchhoff's Current Law, which states that the sum of all currents flowing into an electrical node is zero. Stated another way, the current flowing into a node must equal the current flowing out of the node.

A differential relay measures the sum of all currents connected to whatever node is being protected. If the sum is not zero, there is an unintentional path for current flow that creates an unbalance between current entering and current leaving the node. The unintentional path is a fault between phases or between one or more phases and ground.

It is perhaps easiest to visualize differential protection applied to a substation bus, since a bus exactly fits a simple definition of an electrical node: a junction with current-carrying branches connected. For relaying purposes, other power system components also meet the definition of a node.

For example, a transformer can be considered as a node with high and low side conductors carrying current to and from the node. Motor and generator windings have leads on each end that carry current into and out of the windings. Transmission lines have two or more terminals where currents flow in and out. For water or wastewater facilities, differential relaying is typically applied to buses, transformers and rotating machinery.

A basic form of differential relaying consists of multiple current transformers with their secondary terminals connected in parallel. The parallel connection sums the currents from the individual current transformers. An overcurrent relay is connected to the CT secondary junction to measure the differential current.

The differential connection provides significant advantages over simple overcurrent relaying in terms of selectivity, sensitivity, and speed. Differential current transformer connections define the zone of protection. With a properly designed system, differential relays will not detect normal load current nor will they operate for faults outside the defined zone.

Since a differential relay does not respond to load current, the pickup can be very low to provide sensitive protection. Since the protection is inherently selective, the relay can therefore be allowed to operate with no time delay.

Such ideal characteristics depend on perfect current transformers, all perfectly matched. Practical current transformers have limits beyond which the measured current will not accurately represent the actual current. Current transformer errors will result in a false differential current in the relay.

For example, a fault might occur just outside the protected zone, but one or more current transformers may be unable to accurately reproduce the primary current in the secondary circuit. The sum of all the secondary currents will not equal zero and the differential relay may trip incorrectly, resulting in a loss of security and selectivity.

Differential relays and relay schemes are designed to accommodate current transformer error without loss of reliability. Several types of differential relay systems have been developed to maintain reliability when current transformer performance is not perfect and each type has its own operating principles and application requirements. Two of the most common systems that are used at water or wastewater facilities are the multi-restraint system and the high impedance system.

Multi-restraint differential relaying is based on the principle that current transformer error is likely to increase as the primary current increases. A high CT secondary current is more likely to be inaccurate than a low current. Currents from individual circuits connected to a protected zone are used to restrain operation of the relay. Higher individual currents restrain relay operation so that a higher differential current is required to produce a trip signal.

To obtain this characteristic, differential relays calculate a differential quantity and a restraint quantity.

I_d = | I₁ + I₂ + ...|

I_r = | I₁ | + | I₂ | + ...

Where I_d = differential current

I_r = restraint current

I₁, I₂, ... = individual measured current for each circuit connected to the differential zone

For some relays, the restraint quantity is calculated as I_r = Maximum of | I₁ |, | I₂ |, ...

The symbol |*| indicates the magnitude of a phasor quantity that has both magnitude and phase angle. The differential current is the phasor sum of the individual currents. The restraint current is the sum of the individual current magnitudes or the maximum of any individual current.

A typical differential relay requires the current I_d to be some fraction of I_r to produce a trip signal. If the fraction is constant for any value of I_r, the relay provides a constant percentage characteristic. If the fraction changes for different values of I_r, the relay is said to provide a variable percentage characteristic.

A lower percentage setting provides higher sensitivity for lower currents where current transformer error is expected to be small. A higher percentage setting provides better security for high fault current where current transformer error is likely to be significant and high sensitivity is not as important. Modern microprocessor-based differential relays usually provide settings for minimum pickup and two percentage settings for different ranges of I_r.

High impedance differential relaying uses the fact that a current transformer must develop a voltage to produce its secondary current. The current transformer characteristics place a limit on the voltage that can be developed.

For high impedance differential relays, the current transformer secondary terminals are all connected in parallel to perform the summation. Instead of measuring the differential current, the relay places a high impedance voltage measuring element at the junction of current transformer secondary connections. If no fault is present, the currents sum to zero at the junction and the voltage is zero. If a fault is present, the currents will not sum to zero and the current transformers will attempt to force current through the high impedance relay.

Rather than measuring the current, the relay measures the voltage produced by the current transformers. The voltage produced depends on the magnitude of the fault current. For a severe fault inside the zone, the voltage tends to approach the maximum the current transformers can produce. Because the measured quantity is voltage instead of current, this type of relaying is sometimes referred to as high impedance voltage differential relaying.

Current transformer error can result in a significant voltage produced for faults outside the protected zone. The voltage setting must be low enough for good sensitivity and high enough to avoid operation on the error voltage for faults outside the protected zone.

Next month we'll explore the types of protection applied to specific equipment installed at water and wastewater facilities and some typical criteria used to develop protective settings.

Keith Robertson is a staff power systems engineer with the Schneider Electric Water Wastewater Competency Center, 1010 Airpark Center Drive, Nashville, TN 37217, 888-778-2733, www.us.schneider-electric.com.

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