Numerical protection relays offer the ability to alarm for many abnormal operating conditions, including the health of the relay. Below is a list of typical conditions to alarm for due to an abnormal condition:
- Phase undervoltage
- Loss of potential or blown fuse(s)
- Relay firmware or hardware failure
- Relay access
- Over or under station battery DC voltage
- Trip coil monitoring
The purpose of this article is to demonstrate how to set a numerical protection relay to alarm for some of the conditions listed above.
ABNORMAL POWER SYSTEM OPERATING CONDITIONS
Overexcitation and phase undervoltage are two such conditions taken from the list above.
Overexcitation occurs in a transformer when the magnetic core saturates. Stray flux is induced in nonlaminated components and causes overheating. Numerical transformer relays with phase voltage inputs can detect this condition by measuring the volts per hertz. A typical alarm threshold is when the measured volts per hertz reaches 105% of the nominal value.
Assume the nominal voltage measured by the relay is 120 volts secondary line-to-line and the power system operates at 60 Hz.
V/Hznominal = 120 volts/60 Hz
V/Hznominal = 2 V/Hz (1 per unit)
105% V/Hznominal = 1.05l(2 V/Hz)
105% V/Hznominal = 2.1 V/Hz
Use the Level 1 volts/hertz element as an overexcitation alarm (Figure 1). Set the pickup equal to or greater than 105% but less than the minimum pickup of the Level 2 element if the relay is also programmed to trip for this abnormal condition. Use a time delay of 1 second to allow time to ride through transient conditions.
Assign the time-delayed Level 1 volts/hertz element to an output contact, which closes on operation to signal the control center this condition exists.
Phase Undervoltage (27)
Generators are usually designed to operate continuously at a minimum voltage of 95% of its rated voltage, while delivering rated power at rated frequency. Operating a generator with terminal voltage lower than 95% of its rated voltage can cause undesirable effects such as reduced stability and import of excessive reactive power from the grid to which it is connected. Use the methodology demonstrated in the previous example to set this function to alarm with a fixed time delay.
EQUIPMENT FAILURE CONDITIONS
Loss of potential, relay failure, relay access, and trip circuit monitoring are the conditions covered here.
Loss of Potential (LOP) or Blown Fuse(s)
Loss of potential occurs due to blown potential fuses or by the operation of molded case circuit breakers. The basic principle used to detect this condition is voltage unbalance in the presence of no current unbalance. For example, there is more than a 25% drop in the measured positive-sequence voltage with no corresponding magnitude or angle change in positive-sequence, negative-sequence, or zero-sequence currents. Loss of potential to a relay can cause voltage-dependent protection functions such as phase undervoltage and distance elements to misoperate. It is very important to alarm for this condition so the fuses can be replaced to restore proper operation of any affected voltage-dependent protection functions. Use the methodology demonstrated in the first example to set this function to alarm with a fixed time delay.
Note: It is a good practice to use this function to block the voltage-dependent protection functions while the condition exists. For example, to supervise phase undervoltage protection elements by using LOP, use the following trip equation:
TR2 := . . . OR 27P2T AND NOT LOP
TR2 ≡ 2nd Trip Equation
27P2T ≡ Level 2 Phase Undervoltage Element
LOP ≡ Loss of Potential
One relay manufacturer uses the relay word bit HALARM to signal self-test problems. HALARM is pulsed for hardware self-test warnings and is continuously asserted for hardware self-test failures.
Program a normally closed output contact to open if the relay detects a relay hardware failure as follows:
OUT301 := HALARM
The control center senses a relay failure when this output contact is open.
The same manufacturer mentioned above uses the relay word bit SALARM, which is pulsed, for software programmed conditions such as setting changes, unsuccessful password entry attempts, and password change. It is very important to monitor these conditions due to the need for cybersecurity.
Program a normally open output contact to close if the relay detects any of these conditions has occurred as follows:
OUT302 := SALARM
The control center senses the alarm when this output contact pulses.
Trip Circuit Monitor (TCM)
If the trip coil fails for a circuit breaker or lockout relay (86), then personnel should know this condition exists immediately so that it can be quickly corrected. Figure 2 illustrates the external connections for a trip circuit monitor function.
This function should be programmed to block when the breaker is open, as indicated by the 52b contact input. If the TCM is monitoring a lockout relay, use a lockout relay (86) contact input to block this function when the lockout relay is tripped.
When the output contact is open, and continuity exists in the trip circuit, a small DC current flows, which activates the trip circuit monitor input. If the trip circuit is open, and the output contact is open, no current flows and the trip circuit monitor input is deactivated.
An output contact that is welded closed will also cause the trip circuit monitor input to deactivate, indicating failure of the output contact. When the output contact is closed, no current flows in the trip circuit monitor input. If the relay has issued a trip command to close the output contact and trip circuit monitor input remains activated, this is an indication that the output contact failed to close.
The output of the trip circuit monitor function should be programmed as an alarm to alert maintenance personnel.
This article demonstrates how to program numerical protection relays to alarm for specific abnormal operating conditions, including the health of the relay. By using these alarms, it is possible to quickly address such issues and restore the integrity of the power system.
Steve Turner is in charge of system protection for the Fossil Generation Department at Arizona Public Service Company in Phoenix. After working with Beckwith Electric Company, Inc. for 10 years, Steve spent two years as a consultant in San Diego. His previous experience includes positions as an Application Engineer at GEC Alstom and in the international market for SEL focusing on transmission line protection applications. Steve also worked for Duke Energy (formerly Progress Energy), where he developed the first patent for double-ended fault location on overhead high-voltage transmission lines and was in charge of all maintenance standards in the transmission department for protective relaying. Steve has BSEE and MSEE degrees from Virginia Tech University. He has presented at numerous conferences including Georgia Tech Protective Relay Conference, Western Protective Relay Conference, ECNE, and Doble User Groups, as well as various international conferences. Steve is a senior member of IEEE and a member of the IEEE PSRC.