Power Quality Practitioner™

"Practitioner is one who brings an art or science to full realization"

The over current relay :

  • The over current relay is operate based on the fundamental current
  • In the fault fundamental current at phase A is 611A, 1.007 times of the capacitor’s rated current,the relay do not trip, even though the RMS of the fault current is 1.97 times of the rated current.


Solutions to Capacitor Bank Failures

  • Improve the capacitors relay with operating on the I RMS/V THD/I THD
  • Change the resonance frequency of the capacitor bank


The GPGC PQMS Future Plans for Expanding

The PQMS Phase II, covers:

  • Monitoring All the rest 500kV Substations Buses Voltage and the most Capacitors Banks
  • Monitoring Some Selected 220kV Substations Buses Voltage


The GPGC PQMS Future Plans for Integration ​

​Integrate data from some intelligent electronic devices (IEDs), including:

  • the measurement of the DC current of transformer neutral
  • the fault waveform captured by capacitors relay

Searching for the Cause of Capacitor Bank Failures


The fault was suspected to be caused by harmonic Resonance with the DC-bias Transformer as a harmonic source and the HVDC in GR mode is the reason cause the transformer operating in DC bias.

Date: 2004-05-25, Time 17:07 :  Voltage THD Measured during First Capacitor Failure

The reasons for the breakdown of fuses:

  • The RMS of fault current at Phase A:1195A,  1.97 times of the rated current 
  • The RMS of fault current at Phase C:1053A,  1.73 times of the rated current 

2004-05-29 20:59: Voltage THD Measured during Second Capacitor Failure

Transformers of DC Bias: Impacts

Transformer saturation by DC bias:

  • Resulting in Voltage THD increasing
  • Increased noise and oscillatory motion


Searching for the Cause of Capacitor Bank Failures: 2005/8/8 Fault Analysis

  • The same fault happen again on AUG 8,2005
    • 23 fuses were breakdown
    • 13 capacitors were damaged​
  • 2005/8/8 17:35 Three Gauge—GuangDong HVDC went to GR From Dual-Pole Mode


Wave Recording of V & I of the fault capacitor bank

Searching for the Cause of Capacitor Bank Failures: Voltage THD

Capacitor Bank Failure by HVDC in GR Mode


​Examples:


Date: 2004-05-25, Time: 17:07

  • Imbalance relay protection tripped a 35kV capacitor bank
  • Damage:
    • 16 fuse breakdown
    • 5 sets of capacitors damaged

Date: 2004-05-29, Time: 20:59

  • Another capacitor bank at the same 35kV substation exploded and no relay operated.
  • Damage:
    • 35 fuse breakdown
    • 10 sets of capacitors damaged. 
    • explosion of The discharge coil


Substation Capacitor Bank Failure by HVDC in GR

Searching for the Cause of Capacitor Bank Failures: Voltage THD