Considering a medium-voltage circuit breaker replacement?

Author: Doug Edwards

08/20/2018

The following article is an in-depth technical review of the design considerations to be mindful of when replacing air magnetic switchgear with roll-in vacuum replacement breakers. Specifically, this issue addresses the important design issues with Mechanism Operated Contacts (MOCs) for roll-in replacements.

 

Metal-clad switchgear (as described in IEEE Std. C37.20.2TM) may include cubicle mounted auxiliary switches operated by circuit breakers. The operation of these switchgear mechanism operated contacts (MOCs) by replacement medium-voltage vacuum circuit breakers, replacing air magnetic circuit breakers, introduces several operational issues. Siemens has addressed the operation of MOCs by replacement circuit breaker designs in a variety of ways, each specifically considering the original equipment's capabilities and limitations.

 

There are three operational issues critical to evaluation of the circuit breaker design and operation of MOCs:

  1. Maximum energy required
  2. MOC mechanical endurance capability
  3. Replacement circuit breaker interchangeability.

Four of the larger installed bases for air magnetic replacement circuit breaker designs and an electrical auxiliary relay solution are reviewed with respect to these operational issues. The solutions discussed are representative of the solutions used for the range of replacement circuit breakers as supplied by Siemens:

 

A. GE Magne-blast type AM

 

B. Westinghouse type DHP

 

C. Allis-Chalmers type MA and FA (used in type D switchgear) and types FB and FC (used in type F switchgear)

 

D. ITE type HK

 

E. Electrical Auxiliary Relay Solution.

 

The requirements for proper application, evaluation, and testing of designs are covered in standard IEEE Std. C37.59TM, "Requirements for Conversion of Power Switchgear Equipment."

 

IEEE Std. C37.59-2018TM excerpt: A.5 Additional MOC switch design verification

 

The design of a MOC switch for use with an air magnetic or oil circuit breaker may require much more energy to drive it than the MOC switch for a sealed interrupter circuit breaker. The operating mechanism of a sealed interrupter design may not have sufficient reserve power to meet its close and latch rating while operating the maximum number of MOC switch contacts that may be installed in the switchgear vertical section. The highest energy requirement, normally occurring during a circuit breaker closing operation, may result in a stalled circuit breaker mechanism or intermittent contact making when closed on a high fault.

 

Design verification:

 

a) The maximum energy required for closing should be determined by the converter with an adequate and stated margin to allow for practical field conditions when the lubrication may not be ideal. The justification for the adequacy of margin should be stated. 

 

b) The MOC switch on which the design is based should be stated with clear limitations of the number of contacts and the service condition of the complete assembly. If blocking/rejection interlock modifications are required to limit application (e.g., to restrict the number of MOC switch contacts or energy required), the revised blocking/rejection interlocks should be provided. 

 

c) Travel curves for both opening and closing should be taken that show velocity, timing, rebound, and bounce within accepted limits as defined and proven by power testing for the assembly. 

 

d) No load endurance testing to at least the "between servicing" intervals shown in IEEE Std. C37.06, IEEE Std. C37.13, or IEEE Std C37.14, as applicable for the equipment type and ratings should be conducted with no mechanical deterioration of any of the parts of the assembly that would prevent the circuit breaker assembly from immediately performing its rated duty cycle. 

 

e) Minimum short-circuit testing should include closing on maximum asymmetrical short-circuit current in accordance with the requirements of ANSI C37.50, IEEE Std. C37.14, or IEEE Std. C37.09 as applicable for the equipment type and ratings, and it should include a successful opening operation after a cooling period following a full-rating closing operation.

 

Operational issues - maximum energy required

 

The energy for operation of MOCs comes from the circuit breaker stored energy mechanism and is in addition to the energy required for the operation of the primary contacts. This "residual" energy requirement for the MOC can be substantial. The capability of the circuit breaker to meet its critical operating characteristics must be maintained.

 

The critical characteristics for a circuit breaker include:

  • Contact pressure
  • Contact bounce, contact rebound, and overtravel
  • Opening and closing velocities
  • Opening and closing times.

The layout of the MOC System may tailor the actuation point of the MOC, allowing the closing mechanism to develop momentum before force is required to operate the MOC system. Designs may allow for improved force profiles and thus improved margin in the operation of the circuit breaker.

 

The variation of forces in the MOC systems will affect the circuit breaker performance. The circuit breaker must maintain the main contact critical characteristics while also operating the MOC under worst case conditions. IEEE Std. C37.59-2018 specifies that the margin must be stated but does not detail how to simulate a marginal load or establish the margin. Margin is required to assure that the circuit breaker critical performance characteristics are maintained for less than ideal conditions in the field. Such conditions may exist due to poor maintenance or dirty environment. Since this performance margin does not address faulty MOC systems, the MOC system must be maintained in proper operating condition.

 

Original manufacturers often had various MOC mechanism design versions, and the load imposed by the MOC system may differ between design vintages. The operations of the circuit breaker must be evaluated to consider the various MOC designs, including a safety margin.

 

Justification for the adequacy of margin is accomplished by various MOC mechanism design versions, and the load imposed by the MOC system may differ between design vintages. The operation of the circuit breaker must be evaluated to consider the various MOC designs, including a safety margin.

 

Justification for the adequacy of margin is accomplished by various methods for the different manufacturers' equipment. MOC designs of the GE Magne-blast vertical lift (type AM) and Allis-Chalmers (MA, FA, FB, and FC) circuit breakers include contact blocks on a common shaft with a single return spring. The MOC designs for Westinghouse DH, DHP, and ITE HK circuit breakers include multiple contact block modules (tiers) with separate shafts. The Westinghouse switches each included a torsion spring to return the switches to their rest position. The ITE switches, although separate, include a common set of springs to return the switches to their rest position. Testing of the forces to operate these various designs must consider the addition of extra contact blocks or the addition of spring loads to provide margin in the operating forces.

 

Mechanical load endurance capability

 

The life of the MOCs in field applications is a concern for original circuit breakers as well as replacement circuit breakers. Use of vacuum circuit breakers with their increased velocity heightens this concern. The original manufacturer's designs had various capabilities, each specific design with its own limitations.

 

There are kinematic differences between the original air magnetic circuit breakers and replacement vacuum circuit breakers. The original air magnetic circuit breakers typically operated more slowly than vacuum circuit breakers. The increased speed of operation with vacuum circuit breakers may apply to the existing MOC that it was not originally designed to withstand.

 

The demonstration of the mechanical endurance to at least the "between service interval" requires evaluation of the original manufacturer's design. IEEE Std. C37.59-2018 does not clearly state the arrangement of the MOC system that should be tested.

 

Some manufacturers' designs do not have large differences between the arrangement and number of contacts provided (typical for those with simple addition of contacts on a single operating shaft - GE and Allis-Chalmers). Other manufacturers added additional tiers of contacts including return torsion springs (Westinghouse). These additional springs allow for absorption of the energy transmitted to the MOC system. The worst case test of the MOC's mechanical endurance capability is with the fewest number of contacts or return springs. The worst case conditions should be established and the mechanical endurance demonstrated as the worst case conditions.

 

The force applied to the MOC system is proportional to the square of the operating velocity. Therefore, if the velocity were doubled, the force imposed on the MOC system would be increased by a factor of four. This added force must be absorbed by the MOC system or by a special design of the actuation system on the circuit breaker.

 

The increase in force to the MOCs may be addressed in a variety of ways. Some original MOC designs may be sufficiently robust to withstand the increased velocity and force. Other original MOC designs may not. For these, either the MOC must be replaced with a new more robust system, the velocity and force that are applied to the MOC system must be reduced, or the mechanical MOC system replaced with an electrical auxiliary relay system.

 

Replacement circuit breaker interchangeability

 

IEEE Std. C37.59-2018TM offers two definitions associated with interchangeability:

  1. Replacement interchangeable circuit breaker: a circuit breaker that utilizes all new parts, has been
    design tested to IEEE Std. C37.09 or to ANSI C37.50 or IEEE Std. C37.14 as applicable for the equipment
    type and ratings, and requires no conversion of existing switchgear to maintain proper operation.
  2. Replacement non-interchangeable circuit breaker: a circuit breaker that utilizes all new parts, has been
    design tested to IEEE Std. C37.09 or to ANSI C37.50 or IEEE Std. C37.14 as applicable for the equipment
    type and ratings, but requires conversion of existing switchgear to maintain proper operation.

Users certainly desire to have all circuit breakers be interchangeable without making adjustments or modification to the switchgear. Nevertheless, due to the condition or capability of the original MOC mechanism to accommodate operation by a replacement circuit breaker, upgrades may be appropriate.

 

Unless interchangeability is maintained, interlocks are required.

 

Operation of the MOC by vacuum circuit breakers can change the amount of contact bounce of the MOC contacts. Due to the higher operating velocities and under / over-travel, contact bounce may be induced. There are no specific limits required by the industrial standards.

 

The requirement in IEEE Std. C37.59-2018 is that "....differences shall be taken into consideration....to ensure total control circuit and indication coordination...." Siemens has adopted a limitation of four ms or less on MOC contact bounce. The fastest relay pickup and dropout times are well in excess of this criterion, assuring compatibility.

 

Synchronism of the MOC contacts with the circuit breaker main contacts must be evaluated. The timing difference for MOC contact operation and circuit breaker main operation is ideally zero ms. With the rotary MOC switches, for a closing operation the "b" contacts open prior to the "a" contacts closing, and for opening operation, the "a" contacts open prior to the "b" contacts closing. We have obtained field data for actual synchronism timing for original air magnetic circuit breakers and their MOCs. Actual synchronism timing of a main contact make to the MOC "a" contact make for circuit breakers with Siemens velocity mitigating mechanisms. For direct drive MOC systems (GE and Allis-Chalmers), the time differences are typically less than 10ms.

 

Meeting contact bounce and synchronism timing criteria are also considered critical characteristics for meeting the IEEE Std. C37.59-2018TM definition of "replacement interchangeable circuit breaker."

 

Circuit breaker design comparisons

 

There are several suppliers of medium-voltage circuit breakers, and each of these manufacturers has several design vintages of circuit breakers. For this discussion, four major manufacturers and their largest installed base circuit breaker types are discussed. These discussions are generally applicable to all solutions used for the various replacement circuit breakers offered by Siemens.

A. GE Magne-Blast Type AM

Maximum energy required

 

The original GE MOC (GE used term 52STA) was tested with the maximum number of contacts and an additional spring load added. Siemens successfully operated MOCs with an additional 33% spring load. The additional load provided by a spring is considered a reasonable simulation of additional load due to poor maintenance or dirty mechanisms. The critical characteristics of the circuit breaker were verified at the end of the mechanical endurance tests for verification of capability with the additional spring load.

 

MOC mechanical endurance 

 

The GE original MOC included a robust design that can withstand the higher velocities and forces of a typical vacuum circuit breaker. The GE original MOC does have a design characteristic where the vertical motion travels through an arc, and this arc is transmitted to the main shaft of the switch, resulting in fatigue and bending of the switch shaft over time. Siemens redesigned the operator mechanism for the MOC so that the vertical motion does not travel through an arc, thus eliminating the bending moment for the switch shaft. Siemens recommends that the GE MOC mechanism be replaced with the Siemens replacement mechanism. The GE switches are reinstalled on the Siemens MOC mechanism. The Siemens replacement MOC mechanisms should be installed in all cubicles at a given site.

 

The GE MOCs have a single spring for returning the contacts to rest position, thus the ability of the MOC mechanism is relatively independent of the number of contacts. Siemens successfully operated the original GE MOCs and the Siemens replacement MOC mechanism in excess of the "between service interval" without any velocity mitigating measures on the replacement circuit breaker.

 

Replacement circuit breaker interchangeability

 

Siemens has successfully completed mechanical endurance tests for the GE Magna-Blast type AM with the original MOC switches. Thus, full interchangeability is maintained.

B. Westinghouse DHP

Maximum energy required

 

The Westinghouse 5-15kV DHP MOC systems, although similar, had various constructions with differing maximum forces required to operate the MOCs. The MOC system consists of a pantograph, a connecting rod, and one, two, or three tiers of switches. The switches each include a torsion spring with a significant spring constant. When operating the MOCs, these springs provide a substantial force against the circuit breaker and demand energy from the circuit breaker to successfully operate. Siemens successfully operated the MOCs with the maximum number of contacts (three tiers) with an additional 15% spring load.

 

The operation of the Westinghouse DHP MOCs with the maximum number of contacts and with an additional load is the worst case maximum energy test for the circuit breaker.

 

MOC mechanical endurance

 

For the ability of the MOCs to absorb the energy, the worst case for testing this MOC mechanism is the operation with the minimum number of contacts. The pantograph and connecting rod are vulnerable to the increased velocities and forces provided by vacuum circuit breakers. These limitations in the original MOC design demand mitigation of the velocities and forces applied to the MOC system.

 

Siemens mitigates the velocity, thus the resulting forces, applied to the MOC system by using a MOC-Saver (Siemens Patent) design. The Siemens MOC-Saver includes a bi-directional stored energy mechanism (bi-directional snubber) and a bi-directional hydraulic velocity controller module. The principal of operation is the same for both the open and close operations. The circuit breaker discharges the energy of the open or close springs and operates the circuit breaker main contacts. Energy is stored in a bi-directional snubber and this energy is then discharged to operate the MOC system. The velocity of operation is controlled by the velocity controller.

 

The bi-directional snubber allows the main contacts of the circuit breaker to operate at a different velocity from the MOC mechanism. The system is not fully coupled, thus assuring positive MOC switch actuation. Alternate designs that allow the circuit breaker to operate while not allowing the MOC system to operate introduce significant concerns with indication, control circuitry, and unknown mis-operation during the life of the circuit breaker.

 

No field adjustments are required for the bi-directional snubber or for the velocity controller regardless of the number of MOC switches being operated. The snubber and the velocity controllers are both bi-directional with independent factory set adjustments for open and close operations. The separate control of velocity allows the snubber to have sufficient spring constant to operate one, two, or three tiers of switches (and three tiers plus an additional safety margin load). The velocities are reduced to allow the system to operate the MOCs without field adjustment. This allows for uniform construction of the circuit breaker and full interchangeability.

The bi-directional snubber and velocity controller system has successfully operated the original Westinghouse DHP MOCs in excess of the "between service interval" with the number of MOC switches and with the maximum number of MOC switches.

 

Replacement circuit breaker interchangeability

 

The operation of the Westinghouse DHP MOCs with both the maximum and minimum number of tiers covers the issue of satisfactory operation across the full range of configurations. There are a variety of MOC rods, switches, and pantograph designs. Siemens tests bound the variety of arrangements by establishing a field measurable maximum force for operation of the MOC system. Also, the Siemens testing included operation of all four conditions:

  1. One tier of switches
  2. Two tiers of switches
  3. Three tiers of switches
  4. Three tiers of switches with additional spring load.

All of these arrangements are operated by the circuit breaker without adjustment of each condition. Thus, full interchangeability is maintained with Siemens MOC-Saver design.

 

Not all manufacturers offer full interchangeability. Some manufacturers require adjustment of the MOC spring depending on the number of MOC switches to be operated in a given cubicle. Such a design would not be interchangeable and, according to IEEE Std. C37.59-2015, would require interlocks.

 

Siemens' design provides sufficient power to operate both the circuit breaker and MOC with sacrificing the expected life of the breaker operator mechanism. The maintenance requirements for all Siemens medium-voltage replacement circuit breakers are among the lowest in the industry. Siemens 3AH operators recommended maintenance practice is to exercise the circuit breaker annually and perform maintenance every 10 years or 10,000 operations.

  • Nominal MOC force = 45 lbs (three tiers)
  • Maximum MOC force measured = 60 lbs (three tiers)
  • Maximum MOC force operated and Production Test force operated = 80 lbs.

C. Allis-Chalmers MOC

Maximum energy required

 

The Allis-Chalmers' 5kV type D-Gear (MA circuit breakers) and 5-15kV type F-Gear (FA, FB, and FC circuit breakers) all use a single tier of MOC switches. The addition of MOC contact is a direct simulation of additional mechanical load.

 

Siemens has successfully operated MOCs with additional contacts.

 

MOC mechanical endurance

 

The Allis-Chalmers original MOC is a robust design that withstands the higher velocities and forces of a typical vacuum circuit breaker. Siemens vacuum replacement circuit breakers (MSV for D-gear and FSV for F-Gear) have operated the original Allis-Chalmers MOCs in excess of the "between service interval."

 

Replacement circuit breaker interchangeability

 

Siemens has successfully completed mechanical endurance test in Allis-Chalmers cubicles (D-Gear and F-Gear) without modification to the switches. Thus, full interchangeability is maintained.

 

  • Nominal MOC force = 25 lbs.

D. ITE

MOC mechanical endurance

 

The ITE type HK MOC mechanism consists of a MOC stirrup, a connecting rod, and one , two,  or three tiers of MOC switches. There are stops in the system that absorb the energy provided to operate the switches. There are stops in the back of the MOC stirrup assembly, in the connection of the connecting rod to the stirrup assembly, in a spring between the connecting rod and the cubicle, and by sheet metal tabs around the level arms operating the switches. Testing demonstrated that this system is not sufficiently robust to withstand the higher velocities of a typical vacuum breaker. End-users have reported incidents of MOC switches over travel of the sheet metal tabs at the MOC switches even with original air magnetic breakers.

 

Siemens first design used to mitigate the velocity and thus the resulting forces applied to the MOC system for replacement of the ITE HK circuit breakers utilized the addition of a MOC gear drive system. In 2012, Siemens updated the Siemens HKR replacement circuit breaker system to use the Siemens MOC-Saver system.

 

Operational requirements for the ITE HK MOCs are quite the opposite of the Westinghouse DHP and DH MOC systems. The ITE HK MOCs require a relatively low amount of force to operate while the Westinghouse DHP and DH MOC systems require a relatively high amount of force to operate. Correspondingly, the amount of force that the ITE HK MOC system can absorb is quite limited. The Siemens MOC-Saver system is able to limit the velocity thus allowing its use with the ITE HK MOC systems as well.

 

  • Maximum MOC force measured = 29 lbs, Maximum MOC force operated and Production Test force = 58 lbs.

 

 

E. Electrical auxiliary relay system

Auxiliary relay systems may be provided regardless of the type of circuit being replaced. The physical location in the switchgear can usually use the location of the existing MOC switches or be located in close proximity to the existing switches.

 

Multiple relays may be required to replace the total number of MOC contacts. The installation of auxiliary relays requires control scheme modifications for the operation of the control coils but once completed, the auxiliary relays allow full interchangeability of any circuit breaker. Auxiliary relay systems require an outage for installation of the system but the reliability of the operation of the circuit breaker, the MOC system, and the allowance for variability in circuit breaker while maintaining full interchangeability offset the coordination of a relatively minor outage. Such an outage can also be accomplished at a different time from the circuit breaker installation.

 

Maximum energy required

 

Instead of providing a MOC mechanism on the replacement circuit breaker, the use of auxiliary relays as replacement of the original MOC switches removes the operational energy requirement from the circuit breaker mechanism. This provides for increased reliability in the operation of the circuit breaker mechanism by removing the MOC parasitic load.

 

MOC mechanical endurance

 

The life of an auxiliary relay system as compared to the mechanical endurance capability of the mechanical MOC systems is a significant improvement and is the major advantage for this option. Mechanical MOC systems have a "between service interval" requirement of 1,000 or 2,000 operations in accordance with IEEE Std. C37.06-2009 (depending on circuit breaker rating). For relays, the operational life is well in excess of this type number. Customers may have concerns with leaving relays energized and thus latching relays are often selected. This reduces the risk associated with mis-operation during loss of control power.

 

Replacement circuit breaker interchangeability

 

Since the control circuit modification is installed in the cubicle, the original circuit breaker and new circuit breakers are fully interchangeable.

Summary

Replacement circuit breaker MOC systems require that the circuit breaker operator mechanism have sufficient energy to operate the primary contacts and the parasitic load of the original switchgear MOC system. The operating velocities and the condition and design of the original MOC system may require either:

  • Special mechanisms on the circuit breaker to mitigate the higher velocities typical of vacuum circuit breakers

  • Replacement of the MOC system with a more robust MOC system

  • An electrical relay system in place of the mechanical system.

The interface for the replacement circuit breaker to the original switchgear and MOC system optimally should allow for full interchangeability.

 

Siemens replacement circuit breakers utilize a variety of systems to provide for reliable operation of the circuit breaker, reliable operation and life of the original MOCs, and full interchangeability.

 

In addition to the mechanical MOC systems, Siemens recommends consideration of an electrical auxiliary relay system for replacement of the original MOCs.

References

  1. Burse, Ted A. and Bowen, Jim, “Medium-Voltage Replacement Breaker Projects,” IEEE Transactions on Industry Applications, Vol. 38, No.2, March/April 2002. 
  2. Cutler-Hammer I.B.32-255-1F, “Instructions for Installation, Operation and Maintenance of Type VCP-W Vacuum Circuit Breaker.”
  3. Cutler-Hammer I.B.6513C80C, “Instructions for Installation, Operation and Maintenance of Type DHP-VR Vacuum Replacement Circuit Breakers for DHP Switchgear.”[4] “IEEE Standard Requirements for Conversion of Power Switchgear Equipment,” IEEE C37.59-2002.
  4. IEEE Std C37.06TM, IEEE Standard for AC High-Voltage Circuit Breakers Rated on a Symmetrical Current Basis—Preferred Ratings and Related Required Capabilities for Voltages Above 1000 V.
  5. IEEE Std C37.09TM, Test Procedures for AC High-Voltage Circuit Breakers with Rated Maximum Voltage above 1000VIEEE Standard Test Procedure for AC High-Voltage Circuit Breakers Rated on a Symmetrical Current Basis.
  6. IEEE Std C37.13TM, IEEE Standard for Low-Voltage AC Power Circuit Breakers Used in Enclosures.
  7. IEEE Std C37.14TM, IEEE Standard for Low-Voltage DC Power Circuit Breakers in Enclosures.
  8. IEEE Std C37.20.2TM, IEEE Standard for Metal-Clad Switchgear.
  9. IEEE Std C37.59TM, IEEE Standard Requirements for Conversion of Power Switchgear Equipment
  10. NRC Information Notice 2002-34, “Failure of Safety-Related Circuit Breaker External Auxiliary Switches at Columbia Generating Station,” November 25, 2002. 
  11. Siemens SGIM-9908B, “Vacuum Circuit Breaker (Vehicle) Type DPR 5kV to 15kV, Instructions, Installation, Operation, Maintenance.” 
  12. Siemens SGIM-9928D, “Vacuum Circuit Breakers (Vehicle) Type HKR 7.5kV to 15kV, Instructions, Installation, Operation, Maintenance.” 
  13. Siemens SGIM-9968, “Vacuum Circuit Breaker (Vehicle) Type FSV 5kV to 15kV, Instructions, Installation, Operation, Maintenance.” 
  14. Siemens SGIM-9988, “Vacuum Circuit Breaker (Vehicle) Type MSV 5kV, Instructions, Installation, Operation, Maintenance.” 
  15. SquareD, Group Schneider, 8501CT9601 December 1996, “Industrial Control Relays, Type X.” 
  16. Westinghouse Electric Corporation, “Medium Voltage Metal Clad Switchgear “Maintenance Hints”,” April 10, 197.

Mailing address

Siemens Industry, Inc.

7000 Siemens Road

Wendell, North Carolina 27591

United States