Inspector

The RE board system is a subsystem of the Inspector. It is responsible for calculating earth resistance and loop impedance based on Neutral, Line (Phase 1), and Earth signals.

• The RE system runs with the following sequence (Non-Tripping mode):

1. The main board sends a data request frame to the RE board.
2. RE system sends Acknowledge frame to Main board to validate it received a request from main board. 
3. RE system calculates required data using two techniques:

A.   L/N shorting Technique: 
This technique aims to measure ZL (Line Impedance) and ZN (Neutral Impedance) by following the next steps:

1. Signal Control and Sampling:
   
   Measure the instantaneous unloaded and Loaded voltages of Line and Neutral, VL(Unloaded), VN(Unloaded), note that all voltage measurements on LLRE are with respect to Earth.
   
   

• Drive both phase control relay and the safety relay high.
• Capture LE (Line to Earth) and NE (Neutral to Earth) instantaneous voltages with sampling rate (4882.8125 Ksps).
• Compare each two successive samples of Line samples to detect the Second-Zero-Crossing of Line instantaneous voltage signal by comparing the sign change between each two successive samples.
• At the 10th Second-Zero-Crossing, store Half-Cycle number of samples of Line and Neutral to get the Unloaded voltages.
• At the 11th Second-Zero-Crossing, give a driving pulse to the Thyristor of a time between (15-20 millisecond) to conduct the L/N Shorting technique while storing Half-Cycle number of samples of Line and Neutral to get the Loaded voltages.

2.   Signal calculations:

     Perform calculations to determine the required ZN and ZL values.
      Find VL(Unloaded_RMS), VN(Unloaded_RMS) by doing the following:

• Calculate average values of LE and NE unloaded voltage samples and LE and NE loaded voltage.
• From average values calculate RMS values.
• Using calculated average values calculate VSOURCE_RMS and ISHORTING_RMS values according to these equations:
  
  
  
  
  Where 24 Ω is the shorting resistance value.
  
  
• Using the above calculated values calculate ZL and ZN according to these equations:

B.   Current injection technique:

This technique aims to calculate the accumulated (ZE + ZN) impedance.

1. A small test current is injected between Neutral (N) and Earth (E).
2. The injected current flows through the earth path and returns through the neutral conductor, forming a loop that includes the earth impedance and the neutral impedance.
3. The resulting voltage between Neutral and Earth is measured.
4. Using the injected current and the measured voltage, the system calculates the accumulated impedance:

  Z total = ZE+ZN​

5. Since the injected current is kept below the RCD tripping threshold, the RCD does not trip during the measurement.

• The RE system runs with the following sequence (Tripping mode):

1. Main board system sends Data request frame to RE board.
2. RE system sends Acknowledge frame to Main board to validate it received a request from main board.
3. RE system calculates required data using two techniques:

A.   L/E shorting Technique:

This technique is used to measure ZL (Line Impedance) and ZE (Earth Impedance).

1. Signal Control and Sampling:
   Measure the instantaneous unloaded and Loaded voltages of Line and Neutral, VL (Unloaded), VN (Unloaded), 
   note that all voltage measurements on LLREare with respect to Earth.

• Drive both phase control relay and the safety relay high.
• Capture LN (Line to Neutral) and EN (Earth to Neutral) instantaneous voltages with sampling rate (4882.8125 Ksps).
• Compare each two successive samples of Line samples to detect the Second-Zero-Crossing of Line instantaneous voltage signal by comparing the sign change between each two successive samples.
• At the 10th Second-Zero-Crossing, store Half-Cycle number of samples of Line and Earth to get the Unloaded voltages.
• At the 11th Second-Zero-Crossing, give a driving pulse to the Thyristor of a time between (15-20 millisecond) to conduct the L/E Shorting technique while storing Half-Cycle number of samples of Line and Earth to get the Loaded voltages.

2.   Signal calculations:

      Do some calculations to measure the required ZN and ZL values.
      Find VL(Unloaded_RMS), VN(Unloaded_RMS) by doing the following:

• Calculate average values of LE and NE unloaded voltage samples and LE and NE loaded voltage.
• From average values calculate RMS values.
• Using calculated average values calculate VSOURCE_RMS and ISHORTING_RMS values according to these equations:
  

Where 24 Ω is the shorting resistance value.

• Using the above calculated values calculate ZL and ZN according to these equations:

B.   Current Injection Technique

1. Signal Control and Equations for Generating Injected Current:

• Inject a known very small AC current -to avoid RCD tripping- directed from neutral to earth through:
• Connecting the Neutral point to the Current Injection point using LN, N-PE Impedance Switch Relay.
• Generating the injected current by:
• Generating a voltage through the external DAC that has the following formula:

• As this voltage will be converted to a current signal that is described by following formula:

2.   Signal Capture and calculations:

Capture Neutral Signal and do some calculations to measure the required ZE value as follows:

• Measure Neutral Voltage instantaneously and capture specific number of samples per cycle according to system memory limitation and value accuracy.
• Calculate VN_Injection_RMS by using the RMS formula from the sampled Neutral Voltage Value.
• Calculate ZE from the following equation:

• Using the calculated parameters from the previous two techniques, prepare the required data to be sent.

• Send measurements results frame to main board.
• Reinitialize system and wait for another request data frame to run the whole sequence again.

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