Surge Protection Applications in Process Industry

Main keywords for this article are Surge Protection Applications. Substation Surge Protection. Switching Surges on MV System. Motor Surge Protection. Surge Protection of LV Systems.

Surge Protection Applications in Process Industry

Main Substation Surge Protection

  • Overhead transmission lines and outdoor substations shall be shielded to prevent direct lightning strokes. However, shielding does not prevent induced lightning surges due to nearby strikes. In addition to induced lightning surges, substations are subject to switching surges. Surge arresters shall be required to protect the equipment from the over voltage caused by surges.
  • A safety factor of protective margin between established impulse capability of apparatus insulation and the protective level provided by arresters shall be provided. The recommended minimum protective margin shall be 20 percent for impulse coordination and 15 percent for switching surge coordination.
  • It is very important that the lowest practical ground resistance be obtained and that the connections between arrester ground and terminal and the protected equipment, be as short as possible. Additionally, ground interconnections between these two points are often employed, to place an arrester in closest practical shunt with insulation to be protected.
  • Substations connected to overhead transmission lines shall have a set of arresters installed at each line terminal. The arresters installed at this point shall be intermediate class MOV arresters.  
  • Each high voltage transformer shall have arresters installed on the primary bushings. These arresters shall be station class. If the low voltage bus or distribution has lightning exposure, station class lighting arresters shall also be installed on the secondary bushings.
  • Surge protection studies, equipment selection and installation shall be coordinated with the local power utility company.

 

Underground Line Terminals

a. Arrester does not need to be installed in 13.8 kV and 34.5 kV substation switchgear when the line entrances are underground cables that do not terminate at an overhead system.
b. On 115 kV and higher voltages, arresters shall be installed at both ends of long length of underground cable which connects two substations.
c. For all voltage levels, when the line entrances are underground cables that terminate at an overhead system, arresters shall be installed at the underground-to-overhead transition point. Grounding of transition point arresters to the substation ground through low impedance path shall be ensured.

Switching Surges on MV System

  • Industrial power systems without overhead line exposure are more likely to  experience switching surges than lightning induced surges. Vacuum switchgear which chops the current; switching of large capacitor banks; and restrike of some medium voltage switchgear can create switching surges on the industrial power system. 
  • Switchgear manufacturer’s recommendations shall be followed for application of surge protection on switchgear. In general, surge protection shall be required on the vacuum type switchgear feeder terminals. 
  • Commonly applied voltage ratings of MOV arresters on distribution systems are listed for reference in Tables I and II 

Motor Surge Protection

  • Experience has shown that surge protection is not usually required on 480 V  motors. Surge protection shall be provided for medium voltage motors. See SES E06-S02 and E06-S03.
  • The surge protection package for motors shall be designed by the motor manufacturer and purchased with the motor.
  • The surge protection equipment shall be mounted as close to the motor terminals as physically possible, to minimize the length of connecting cables.
  • The standard protective circuit for rotating machines consists of arrester and capacitor located near the machine terminals. The function of the arrester is to limit the magnitude of the voltage to ground, while the capacitor lengthens the time to crest and rate of rise of voltage at the machine terminals. 
  • Basic winding design patterns of motors and generators involve large capacitance coupling between the conductor of the winding of each coil and the grounded core iron that surrounds it. A fast rising surge voltage at the motor terminal lifts the potential of the terminal turn. The turns deeper in the winding are constrained by the large capacitance from coil to ground and delayed in their response to the arriving voltage wave. The result is severe voltage stress on the turn to turn insulation of the terminal coil. It is the protection of the turn insulation that becomes critical in avoiding failure in multiturn stator windings of ac motors and generators.
  • Rotating machines, have relatively low impulse strength and have no established, standardized BIL’s. Rotating machines do, however, have standard high-potential test voltage values which have become important in the application of surge protection. An IEEE working group report contains a proposed voltage time boundary where the maximum impulse voltage is 1.25 times the crest of standard high-potential test voltages. The rate of rise of the surge voltage across the motor winding terminals is limited by the charging rate of the capacitor. Special protective capacitor units are designed for this purpose with low internal inductance, that control the rate of rise of incident over voltage to protect the turn to turn insulation. 

Adjustable Speed Drives (ASD)

  •  Inverters used to supply adjustable speed drives do not produce sinusoidal  output voltage waveforms. In addition to the lower order harmonics, these waveforms have superimposed on them steep-fronted, single amplitude voltage spikes. The motor stator winding is subjected to the resulting dielectric stresses.
  • Motors for inverter service shall be specified as special purpose motors in accordance with NEMA MG-1 Part 31. Permissible voltage spikes for these motors shall be as specified in NEMA MG-1 Part 31.
  • Adjustable speed drives shall be specified with filters necessary to limit the output voltage spikes to an acceptable level for the motor.

Surge Protection of LV Systems

  • The increased use of sensitive electronic equipment, for example computers,  programmable logic controllers and distributed control systems requires carefully designed surge protection for low voltage systems.
  • Packaged surge protection systems shall be utilized to protect sensitive electronic equipment. The protection system shall be specified to meet the requirements as defined by the manufacturer of the protected equipment.  
  • The application shall conform to the recommendations of IEEE C62.41 and C62.43.
  • The surge protection package shall be designed, manufactured and tested in accordance with NEMA LS-1 and UL 1449.

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