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Noise Interference

Noise Interference. G Cameron Telecom NZ Ltd. NZCCPTS Noise Interference Guide. Good pragmatic overview Due to high variability in nature of: noise sources within power systems (frequencies, levels and time variation) coupling to telecommunications plant

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Noise Interference

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  1. Noise Interference G Cameron Telecom NZ Ltd

  2. NZCCPTS Noise Interference Guide • Good pragmatic overview • Due to high variability in nature of: • noise sources within power systems (frequencies, levels and time variation) • coupling to telecommunications plant • susceptibility of telecommunications plant need to approach each problem with an open mind a formularised approach will not always work.

  3. Electromagnetic Fields around Power Lines Below several hundred kHz • Structure attached • Quasi-static • Decompose into separate • magnetic, • electric (capacitive) and • conductive coupling mechanisms. • Earth return component of current is the primary concern

  4. Weak Coupling Assumption • Ignore the affect of telecommunications plant on current distribution in the power line • Allows us to decompose problem into • Calculate current distribution on power line independently of telecommunications plant using standard tools • Calculate induced voltage at site of telecommunications plant E = C x L x I x K

  5. Coupling of Noise to Telecommunications Plant • Inductive coupling approximately linearly proportional to frequency at road width separations • Susceptibility of telecommunications plant to noise is technology and frequency dependent • For telephone services a 2000 Hz noise current is approximately 40,000 x worse than a 50 Hz current of the same magnitude • Very small noise currents in power lines (10s of milliamps) can cause significant problems

  6. Electromagnetic Compatibility • Electromagnetic compatibility issue • Responsibility of source to limit magnitude of interference • Responsibility of receiver to limit susceptibility of their system to interference.

  7. Noise Conversion Mechanisms • Telecommunications cables use transverse signals over twisted pair cable • Telecommunications plant has very high symmetry to prevent induced longitudinal noise causing audible transverse noise • Balance > 60 dB if properly maintained – reject 99.9% of induced longitudinal voltage.

  8. Noise Investigation Techniques • Survey longitudinal voltages on telecommunication cables • Measure spectrum of induced noise • Rule in/out conductive coupling mechanisms • Obtain power system distribution plans, notes of recent changes • “Measure” current magnitude and flow in power network

  9. Psychological Aspects • Take telecommunications company time to recognise that there is a widespread problem • Telecom customers are not good at complaining to Telecom! • Telecommunication customers expectations are rising

  10. Case Studies • Dismantling of 110kV lines north of Temuka – 47th Harmonic Zero Sequence resonance • Lower Waitaki – interconnection of 110 kV network to upper Waitaki stations • HVDC Link • 1965 – Toll circuits over 10 mile long between Benmore and Haywards • 1965 – ripple control interference as far away as Blenheim • 1980s – noise in Fairton when in earth return mode

  11. Case Studies • Tiwai point 23rd and 25th harmonic causing noise at Omarama and ripple control problems in Otago and Southland. • Kataia CBD broken neutral • Westport and Cape Foulwind loss of one phase in interconnecting line • Ngahere Gold Dredge • Tuamarina ripple relay signal problem

  12. Case Studies • Taupo SWER lines

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