On Relay Chatter Circuit Analysis Presented by James C. Lin PSA 2015 International Topical Meeting on Probabilistic Safety Assessment and Analysis Sun Valley, Idaho April 26-30, 2015 Outline of Presentation • • • • • • Introduction Seismic relay chatter circuit analysis Selected relay chatter considerations Selected vulnerable circuits Non-relay contact devices Conclusions 22 Introduction • Functions of relays in control circuits • Types of relays – Control and auxiliary relays • Electro-mechanical, pneumatic timing, and solid state relays – Protective relays • Electro-mechanical, solid state, and overload relays • Relays are typically not structurally damaged during a seismic event – Due to the low seismic ruggedness of the relay contacts, vibration induced actuation of certain relays could occur during an earthquake potentially resulting in an undesired equipment operation or change in equipment position/state • Inadvertently operate equipment • Could lock out electrical buses or DGs – The adverse effect of relay chatter is one of the causes of seismic induced failures evaluated in SPRA • To develop a comprehensive SPRA, the potential effects of relays on the performance of the accident mitigation components in response to the earthquake must be analyzed – An essential part of a rigorous SPRA 33 Seismic Relay Chatter Circuit Analysis • Identify relays or contact devices whose chatter during an earthquake can cause an adverse impact on the accident mitigation functions – Should evaluate the control circuits for all accident mitigation equipment considered for the response to seismic initiating events – Relay chatter-induced unavailability of the PRA components on the SEL • Should be performed by PRA components on the SEL – Evaluate control circuits associated with each of these components for chatter induced adverse impacts – Generally assumed that circuits that require simultaneous chatters of two or more relays in unison in order to re-position or change the state of a component are deemed improbable – In some circuits, contact chatter during the earthquake may temporarily cause selected mitigation equipment unavailable • Once the earthquake is over, the chatter would stop and the mitigation function would be automatically restored • Due to the insignificant impact on the PRA mitigation function, this type of relay chatter can be screened for further consideration. 44 Selected Relay Chatter Considerations • Solid state and timing relays – Solid state relays are not prone to chatter due to the lack of mechanically moving parts – Timing relays with settings greater than 1 second are not affected by chatter of upstream relays • Relay energy state – The seismic capacity of an energized relay is significantly stronger than that of de-energized relays – Normally energized relays can typically be screened out in the relay chatter analysis • Chatter frequency – If the chatter frequency of a contact device upstream of a relay is reasonably high, the duration in which the circuit path is complete may not be sufficiently long to allow the downstream device to actuate (e.g., relay to energize and its contacts to move, breaker to trip open, MOV to change position, etc.) – The amount of time required to energize the downstream device (e.g., breaker trip coil, motor winding, etc.), build up the electromagnetic field, trip open the breaker, cause the motor to move, change the MOV position, etc., is likely to be longer than the circuit path complete time during the contact chatter cycles – The circuits that typically would cause adverse effect on the PRA equipment mitigation functions are those that involve a seal-in circuit or lockout relays • Most likely adverse impacts – MOVs or SOVs inadvertently changed to undesired positions, tripping open of bus or equipment (e.g., pump) breakers, spurious opening of Safety Relief Valves/Power Operated Relief Valves (SRV/PORV), blocking of breaker closure or equipment start, and lockout of an essential bus or a diesel generator – In addition to tripping open the breakers, overcurrent protection may sometimes (e.g., diesel output breaker or essential bus overcurrent) also include lockout function by blocking the breaker from closure if the lockout relays are not reset – The typical overcurrent protection design for a piece of equipment such as a medium voltage pump includes ground fault overcurrent, instantaneous overcurrent, phase time overcurrent, etc. – The overcurrent protection for a diesel generator breaker could also include such other protection function as differential overcurrent, etc. 55 Selected Vulnerable Circuit • • • • MOV seal-in circuit Large pump overcurrent circuit Bus lockout circuit Emergency diesel generator lockout circuit 66 Example MOV Control Circuit 77 Example Control Circuit for 4.16-kV Pump 88 Example Circuit for Undervoltage Signal 99 Example Circuit for Bus Overcurrent 10 10 Chatter Vulnerability for Non-Relay Contact Devices • Circuit breakers – The main contacts are designed to withstand the magnetic forces resulting from very high short circuit current – The breaker main contacts are mechanically actuated by a linkage from the breaker and have not shown chatter vulnerability – The breaker auxiliary contacts are also typically not vulnerable to chatter because they are built into and mechanically linked to the breaker main contacts (except for a few models of low/medium voltage switchgear breakers) • Electro-mechanical contactors and motor starters – The main contacts of an electro-mechanical contactor/motor starter are designed to withstand the magnetic forces resulting from very high short circuit current, and have not shown chatter vulnerability – The auxiliary contacts for electro-mechanical contactors/motor starters are not designed to withstand short circuit conditions. However, they are mechanically actuated contacts (i.e., mechanically opened/closed when the contactor is actuated) which are inherently rugged and not prone to chatter (except for a few models) • Manually operated control switches – Rotary control switches are not vulnerable to chatter because vertical and horizontal seismic motion cannot cause the contacts to rotate – The non-rotary control switches are also not vulnerable to chatter because the contacts of these manually operated control switches are mechanically actuated which is inherently rugged for chatter 11 11 Chatter Vulnerability for Non-Relay Contact Devices • Limit, torque, and position switches – Actuation of a contact on Limitorque motor operators with a rotary shaft requires the rotary shaft to turn 90 degrees and thus are not prone to seismic chatter – A torque switch is a mechanical device and not prone to chatter – The position switch contacts for circuit breakers are mechanically operated and not prone to chatter (except for a few models) • Mechanical sensor switches – Include mechanical pressure, level, flow, and temperature switches – Are generally not vulnerable to chatter because they require the application of a reasonable force to change state – Mercury and sudden pressure switches on large power transformers are vulnerable to chatter 12 12 Summary of Non-Relay Contact Devices • Most of the non-relay contact devices are not prone to chatter and do not need to be evaluated in the relay chatter analysis • Chatter of only the following contact devices need to be considered – Mercury and sudden pressure switches on large power transformers – Small sensitive switches (e.g., operator micro switches) – Auxiliary contacts for motor contactors and low/medium voltage switchgear circuit breakers for a few models • Sudden pressure switches are mostly located in the control circuits for nonessential equipment – Chatter is, in general, not likely to result in adverse impacts on the accident mitigation equipment – However, sudden pressure relays can be activated as a result of earthquake induced pressure pulses causing lockout of auxiliary transformers and possibly loss of offsite power supply • Compile a list of all mercury switches (e.g., Mercoid and Magnetrol switches), sudden pressure switches, and micro switches in use at the plant – Switches associated with any components in systems not credited in the SPRA and switches that are located in non-seismically designed area of the plant can be screened out – For those switches retained, a circuit analysis similar to that for relays should be performed – Those switches with no adverse impact on the PRA components can also be screened – Only switches with non-negligible impacts on the PRA mitigation functions require a fragility analysis 13 13 Conclusions • The most significant adverse impacts can result from control circuits with seal-in and lockout designs – Seal-in feature can cause the MOV to spuriously transfer to an undesired position requiring operator manual re-alignment – Seal-in feature in the time OC protective circuits for the control of medium voltage breakers can trip off its electrical loads during a seismic relay chatter event • Feeder breakers for medium voltage pumps are generally provided with time OC protective function, which usually will only trip open the affected breakers but would not block them from re-closing – The seal-in design of the time OC relays in the control circuits for the supply breakers of essential buses is typically devised to cause a lockout relay to activate, tripping open these breakers and blocking from re-closing until manually reset, which in many cases are performed locally at the switchgear – For the control of EDG output breakers, many of the protective conditions are designed with a lockout signal which will not only trip open the EDG output breakers but also block it from re-closing until manually reset. 14 14 Thanks for your attention 15 15