Power Flow

Directional relays can be found in facilities with their own on-site generating capabilities and a stand-by utility feed. Directional overcurrent protection is applied to prevent the plant from supplying power to the utility. If current flows from the plant to the utility, the Utility circuit breaker will open.

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67

Closed Breaker Tripped Breaker

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67 Current

Direction Current

Direction

Figure 41: Directional Overcurrent Protection in an Industrial Application

2. Operation

Electro-mechanical relays used magnetism to produce torque in the polarizing element and were designed to operate when the measured current and voltage were a specific phase angle apart.

These relays used the non-faulted voltages for polarizing signals. Figure 42 demonstrates all of the available phasors that the relay designer could use to obtain the desired Maximum Torque Angle (MTA). The maximum torque angle is the defining point for directional control and was typically fixed at 90° or 60° in electro-mechanical relays. A typical configuration (60º) used IA

current phasors compared to the V_BC voltage to detect an A-Phase fault; I_B compared to V_CA to detect a B-Phase fault, and I_C compared to V_AB to detect a C-Phase Fault. Figure 43 displays the operating characteristic of an A-Phase relay with a MTA of 60°. The dotted line is drawn 90°

from the MTA to indicate the zero torque line which is the transition between forward and reverse directions. Any A-Phase current phasor below or to the right of the dotted line is flowing in the positive direction and will cause the relay to trip if the current exceeds the pickup setting.

Current phasors above and to the left of the dotted line flow in the reverse direction and will never trip.

Figure 42: Standard Phasor Diagram

180 0

Figure 43: Directional Polarizing

Modern directional overcurrent protection starts with a polarizing element acting as an internal switch that turns the overcurrent element on or off. If the current flows in the trip direction, the overcurrent protection function operates as a standard overcurrent (50 or 51) element. If the current flows in the reverse direction, the directional element does not turn “on” and blocks the element from operating. The polarizing element can be an integral part of the directional-overcurrent (67) protection or it can be a separate element used to block or permit an independent 50/51 element. Basic polarizing elements use two separate signals (Voltage and/or Current) and compare the phase angle between the two signals to determine direction.

Traditionally, a phase to phase voltage was compared to a line current in the polarizing element but this configuration can cause nuisance trips under certain fault conditions. Some modern relays can use any voltage, current, or sequence component as the polarizing source to ensure reliable directional operation. Some relays even use a pre-fault value for directional control or the relay can choose the best option from a list of choices depending on the type of fault.

3. Settings

Typical settings for 67-elements are described below:

A) Enable Setting

Many relays allow the user to enable or disable settings. Make sure that the element is ON/Enabled or the relay may prevent you from entering settings. If the element is not used, the setting should be disabled or OFF to prevent confusion.

B) Pickup

This setting determines when the relay will start timing if the current flows in the correct direction. Different relay models use different methods to set the actual pickup and the most common methods are:

¾ Secondary Amps – the simplest unit. Pickup Amps = setting

¾ Per Unit (P.U.) – This setting could be a multiple of the nominal current as defined or calculated if the relay has setpoints for nominal current, Watts, or VA. It could also be a multiple of the nominal CT secondary.

Pickup = Setting x Nominal Amps, OR

Pickup = Setting x Watts / (nominal voltage x √3 x power factor), OR Pickup = Setting x VA / (nominal voltage x √3), OR

Pickup = Settings x CT Secondary (typically 5 Amps)

¾ Primary Amps – There must be a setting for CT ratio if this setting style exists. Check the CT ratio from the drawings and make sure that the drawing matches the settings.

Pickup = Setting / CT Ratio, OR

Pickup = Setting * CT secondary / CT primary

C) Curve

This setting chooses which curve will be used for timing. Be very careful to select the correct curve as there can be subtle differences between curve descriptions. Compare the curve selection to the coordination study to ensure the correct curve is selected

D) Time Dial/Multiplier

This setting simulates the time dial setting on an electro-mechanical relay to determine the time delay between pickup and operation in conjunction with the selected curve. ANSI curves usually have a time delay between 1 and 10. IEC time setting are typically between 0 and 1.

E) Reset

Electro-mechanical 51-element relay timing was controlled using a mechanical disc that would rotate if the current was higher than the pickup setting. If the current dropped below the pickup value, the disc would rotate back to the reset position.

Some digital relays simulate the reset delay using a linear curve that is directly proportional to the current to closely match the electro-mechanical relays. Other relays have a preset time delay or user defined reset delay that should be set to allow any related electro-mechanical discs to reset for proper coordination between devices.