Motor Surge Suppressors and Cable Considerations

Transcription

1 Motor Surge Suppressors and Cable Considerations Understanding the growing need for motor surge protection in SA Arveen Gobind Pragma Condition Monitoring (Martec)

2 Agenda Introduction Network Motor Surge Protection System Motor and Breaker Surge Suppressor Cables Case Studies Conclusion Questions

3 Introduction Need for surge suppressors

4 Introduction Transients Load Shedding Loads are disconnected and reconnected and the transient surge suppressor protection needs to operate much more frequently Green energy Cannot be depended upon at all times. The network needs to be supported by generators and UPS facilities in order to supply this shortfall, Hence increased switching operations are experienced Non-linear loads Capacitors, chokes, VSD, UPS Breaker Switching Pre-strikes Re-strikes Fault clearing Protection Trips Lightning

5 Motor BIL Rated voltage Rated lightning-impulse voltage withstand (peak) Rated steep-front-impulse voltage withstand (peak) UN UP UP' kv kv kv 2, , , , , kv motors: Rated lightning impulse withstand voltage (1,2/50 μs wave) = 31 kv peak, main insulation Rated steep-front impulse withstand voltage (0,2 μs wave) = 20 kv peak, interturn insulation Rated power frequency withstand voltage (50 Hz) = 15 kv rms.

6 Surge Suppressor Protection Intervention 11 kv System Without Suppressor Measurement Criteria: Amplitude Clipping Envelope Decay time Re-strike Elimination Successful Result of Surge Suppressor Intervention

7 Surge Suppressor Protection Intervention Normal mains frequency conditions: the impedance of the capacitive elements is very high, effectively "disconnecting the resistive components from the system. High frequency transient conditions: the impedance of the capacitive elements is low with respect to the resistive elements, effectively "inserting" the resistive components in the power system as a cable-terminating network. Voltage refraction and reflection of steep wave-fronts are minimised, voltage doubling (or even higher) is avoided, and high frequency re-strike are eliminated by surge suppressor effectiveness. On-load tripping events (and operator related switching) transients are minimised, maintaining the winding integrity.

8 Surge Suppressor Suppressor Cables Connections Surge suppressors with porcelain bushings require the use of flexible connections to the bushings to avoid vibration or shock being transmitted to the bushings, in service and during transportation. Care should be taken not to over tighten the nuts on the bushing and earth studs. Torqueing instructions must be adhered to, avoiding cracks and stress points. Electrical Discharge may occur should these be ignored, hence weakening the insulation. The resistance of the suppressor circuit will be altered and poses risk of malfunction operations.

9 Surge Suppressor Suppressor Cables Unlike installing medium voltage cables which are screened and are installed to the norm, a nonscreened connection cable should be installed with a clearance distance between phases and earth to avoid complications with discharges.

10 Surge Suppressor Suppressor Cables The discharge effect is the ionization of air around the conductor due to the electric fields generated by high voltage and can lead to deterioration of the silicone insulation system of the cables. To eliminate and reduce the problems the minimum recommended installation distances "d" (mm) between phases and to ground should be followed. Supplied by Nexans: Global experts in cables and cabling systems

11 Surge Suppressor Suppressor Cables Flexible Cables In the case of surge suppressors with flexible insulated connection leads must be directly connected to the machine terminals Typical Surge Suppressor Cable Example

12 Surge Suppressor Suppressor Cables Clearance Contraventions / Poor Installations A call for further research

13 Case Study Surge Suppressor Installation on Mill Motor A 6.6kV Mill motor experienced a winding failure during switching operations, which called for an investigation with recommendations to prevent recurrence. Upon inspection, an inter-turn fault on the stator windings was reported. The investigation proved that the installation of a surge suppressor at the motor terminals reduced high frequency switching transients and the effects experienced by the motor windings. Without Surge Protection With Surge Protection

14 Case Study Surge Suppressor Leads Investigation The cables are connected to the transient overvoltage surge suppressor and the required mains supply of the motor or terminations. This is for the protection against steep wave-front, short rise-time, high magnitude transient voltages generated by switching sources. Cable identification and type comparison tests for partial discharge of various voltage ranges Old Silicone Cable New Silicone Cable Siwo-Kul Cable kv Rating of Cable

15 Case Study Surge Suppressor Leads Investigation Partial discharge activities during different voltage ranges from 2 kv to 6,6 kv using controlled voltage measurements were recorded in the frequency and time domains. The cables connecting the surge suppressors will be at system voltage during operation and if not installed correctly will produce partial discharge that can deteriorate the insulation system and lead to failure over time.

16 Conclusion The need for motor surge suppressors are more now, due to the complexities imposed on SA s network. The installation process must be followed with strict quality control. The effectiveness of the implementation of surge suppressors are justified from the tests performed. Electrical Discharge monitoring is of utmost importance in ensuring a reliable protection system. The surge suppressor and leads must form part of a motor routine inspection Further investigation will be done as I am now presented with a motor protection evaluation project incorporating surge suppressors. On-line technologies will be used to detect electrical discharges and trended over time. The effects on surge protector leads will be further monitored with focus on clearances and routing.

17 Thank you LinkedIn Facebook Twitter Arveen Gobind Eng Consultant, Pragma Condition Monitoring / Martec arveen.gobind@pragmaworld.net