The PQSensor™ MkVI is a device designed for the installation and commissioning of equipment that measures currents flowing in the capacitor divider circuit and in the primary winding of the CVT wound transformer (electromagnetic unit or EMU). It is used to make accurate measurements of harmonic levels and transients on high-voltage power networks.
Function Description
The PQSensor™ consists of three separate components: the Measurement Unit (MU), the Signal Conditioning Module (SCM), and the Interconnecting Cable.
The MU's primary function is to measure currents in the capacitor divider circuit and the primary winding of the CVT wound transformer. It is installed in the CVT secondary terminal box. The MU replaces existing links in the CVT secondary terminal box that typically connect these circuits directly to ground. The MU HCCT input circuit replaces the capacitor divider earth link, and the MU LCCT input circuit replaces the EMU primary ground link. The MU is also equipped with a test circuit for on-site testing and calibration.
The SCM is a weatherproof IP65 enclosure designed to be mounted on the structure of the CVT. It processes the signals from the MU. The calibration process involves adjusting the SCM's output so that it delivers its Nominal Output Voltage when the CVT is energized with nominal substation voltage.
The Interconnecting Cable connects the MU to the SCM, transmitting signals between them. It is a steel wire armour (SWA) cable containing two individually screened twisted pairs and a collective screen.
Important Technical Specifications
Measurement Unit (MU):
- Dimensions: 66mm (w) x 66mm (d) x 81mm (h).
- Mounting: Equipped with three M3 mounting holes on the base, compatible with Phoenix UTA98 DIN rail adapter (provided) or custom mounting plates.
- Input Circuits: HCCT and LCCT input circuits are low impedance circuits with guaranteed continuity.
- Input Polarity: HCCT2 and LCCT2 inputs must be connected to ground.
- Test Circuit: Included for on-site testing and calibration.
Interconnecting Cable:
- Standard Length: 3 meters (lengths up to 5m available on request).
- Termination: Pre-terminated at both ends with appropriate glands for connection to the CVT secondary Terminal Box and the SCM Box.
- Conductor Area: Twisted pair conductors are stranded with 0.5mm² nominal conductor area (typically 16/0.2 stranding).
- Screening: Two individually screened twisted pairs with a collective screen.
Signal Conditioning Module (SCM):
- Enclosure: Weatherproof IP65.
- Dimensions: Approximately 260mm x 160mm x 90mm.
- Mounting: Typically installed vertically on the CVT support structure with cable entries on the bottom face. A solid ground connection between the SCM enclosure and the CVT structure is essential.
- Supply Voltage: Universal supply, 110V ac, 230V ac, or 80-250V dc. Not polarity sensitive for DC input.
- Ground Connection: Pin 3 of the user connection terminal block must always be grounded.
- Nominal Output Signal: Approximately 60V ac (63.5V is a specified nominal output voltage).
- Input Impedance for Recording Equipment: Intended for use with recording equipment having an input impedance greater than 1Mohm.
- Operating Temperature: Rated for temperatures up to 65°C.
Usage Features
Installation:
- Measurement Unit (MU): Installed in the CVT secondary terminal box. The MU is designed to replace existing earth links in the capacitor divider and EMU circuits. A marked-up CVT schematic is crucial for identifying connection points and ensuring correct installation. The MU can be mounted on a DIN rail or with a custom bracket.
- Signal Conditioning Module (SCM): Mounted on the CVT support structure. In hot climates (above 40°C), it is recommended to mount the SCM in a shaded location, possibly using a mounting plate at least 500mm wide on the shaded side of the CVT structure, or providing a sun shade.
- Interconnecting Cable: Connects the MU and SCM. It should be securely fixed to the CVT structure to prevent loose sections.
- Supply Voltage and Output Signal Connection: Made using stranded cable with a minimum size of 1mm².
Calibration:
- Factory Calibration: The PQSensor™ is calibrated at the factory with the MU installed in the CVT and the SCM connected. This involves energizing the CVT with nominal substation voltage and adjusting the SCM output voltage to match the PQSensor Nominal Output Voltage.
- Site Calibration (Retrofit): For existing CVTs, a calibrated voltmeter is used to measure the SCM output voltage when the CVT is energized. The output voltage should be adjusted to equal the Nominal Output Voltage as given in the PQSensor™ Test Report. If the substation primary voltage differs from nominal, the expected PQSensor™ output voltage can be scaled accordingly.
Safety Precautions:
- Lethal High Voltages: Installation and commissioning involve working with equipment where errors can result in exposure to lethal high voltages. Only personnel with necessary knowledge and training should perform the installation.
- Safety Symbol: Pay particular attention to sections marked with the high voltage symbol (a lightning bolt in a triangle).
- Earthing:
- The CVT input must be solidly earthed at all times during installation work.
- The CVT secondary terminal box should never be opened unless the CVT is solidly earthed.
- The bottom end of C2 and the EMU must be solidly earthed before energizing the CVT.
- The HCCT2 and LCCT2 inputs must be connected to ground.
- The SCM enclosure must have a solid ground connection to the CVT structure.
- Pin 3 of the PQSensor™ user connection terminal block must always be grounded.
- Voltage Presence: The nominal output signal (60V ac) can be present at any time the PQSensor™ is energized. Appropriate safety precautions must be taken.
- Connection Checks: After MU installation, HCCT and LCCT input terminations must be carefully checked for correct and secure connections. Incorrect connections can result in lethal voltages in the CVT secondary terminal box when energized. Consult a qualified technical expert if in doubt.
Maintenance Features
Installation Verification:
- Continuity Test: After MU installation, a secondary injection test (e.g., using a Sverker) should be performed. Inject 1A ac between the bottom of the capacitor stack (HF) and ground, and measure the voltage drop. This voltage should be less than 40mV ac (equivalent to 40mΩ). Repeat the test between the bottom of the EMU and ground, where the measured voltage should be less than 60mV. These tests ensure the integrity of capacitor and EMU earth paths.
- Wiring Check: After energizing the CVT, verify the wiring by applying a suitable supply voltage to the SCM and injecting a secondary test current. Measure the PQSensor™ output voltage, which should be within 2% of the Nominal Output Voltage.
- Site Verification for Factory Calibrated PQSensor™: For factory-calibrated units, no site calibration is required. The output voltage value should be checked after the CVT has been energized. If the substation primary voltage differs from nominal, the expected PQSensor™ output voltage can be scaled accordingly.
Documentation:
- Calibration Label: Record calibration results and the CVT serial number on the label inside the PQSensor™ to ensure it is installed on the calibrated CVT.
- Test Reports: Sample test reports are provided for the Measurement Unit and the PQSensor™ (SCM), including input continuity check, hi-pot test, MU ratio and phasing check, HCCT and LCCT injection errors, and frequency response. These reports document the performance and calibration of the device.
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