Combined  Current and and Voltage Transformers

Medium & Low Voltage Instrument Transformers (CTs & VTs) - Frequently Asked Questions (FAQ)

• =/Q: Can metering CTs and protection CTs be used interchangeably?

  A: Absolutely not recommended!

  Metering CTs: Have a small core cross-section, designed to saturate easily. During a short-circuit fault, the core saturates quickly, limiting the secondary current and protecting connected meters from damage. However, it cannot accurately transform fault current, which could cause protection relays to fail to operate (fail-safe for meters).

  Protection CTs: Have a larger core cross-section with high anti-saturation ability. They can accurately transform high fault currents to ensure protective devices operate. However, their measurement accuracy at normal low currents is not as high as metering CTs.

• =/Q: Why must the secondary circuit of a VT never be short-circuited during operation?

  A: A VT's secondary voltage is low under normal operation, but its internal impedance is very low. A short circuit on the secondary side will cause a very high short-circuit current, burning out the VT windings. Therefore, fuses or miniature circuit breakers (MCBs) must be installed on the VT secondary side for protection.

• =/Q: How do I choose the right CT for my project?

  A: Consider the following key parameters:

  Rated Voltage: Must be equal to or greater than the system's nominal voltage at the installation point (e.g., 0.66kV, 10kV, 35kV).

  Ratio: Selected based on the line's rated current and the desired measurement range. Normal operating current should ideally be around 60-80% of the CT's secondary rated value.

  Accuracy Class:

  Metering CTs: Commonly 0.2S, 0.5, 1 class. Used for energy metering and instrumentation, requiring low error under normal load conditions.

  Protection CTs: Commonly 5P10, 10P20, etc. The "P" stands for Protection. The number (10 or 20) is the Accuracy Limit Factor (ALF), indicating the multiple of rated current up to which the CT maintains its accuracy, ensuring protective relays operate reliably during faults.

  Rated Output (VA): The total burden (impedance) of all connected instruments, relays, and cables in the secondary circuit must not exceed the CT's rated output capacity, otherwise error increases.

  Mounting Style: Window-type (bar-type), bushing-type, support-type, etc., chosen based on switchgear design and economics.

• =/Q: How do I choose the right VT for my project?

  A: Consider the following parameters:

  Rated Voltage: Primary rated voltage must match the system voltage (e.g., 10/√3 kV). Secondary rated voltage is typically 100/√3 V (for phase voltage) or 100V (for line voltage or open-delta connection).

  Accuracy Class: Metering常用 0.2, 0.5 class; Protection常用 3P, 6P class.

  Rated Output (VA): As with CTs, the total connected burden must not exceed the VT's rated capacity.

  Winding Connection: Commonly V/v (open delta), Y/y (star/star), YN/yn (with residual winding), chosen based on system connection and protection scheme.

• =/Q: Inaccurate energy meter readings – could this be related to CTs/VTs?

  A: Very likely. Common causes include:

  Incorrect CT/VT Ratio: The ratio marked on the device does not match the ratio configured in the meter/system.

  Reversed Polarity: Causes incorrect power flow direction calculation.

  Excessive Secondary Burden: Too many connected devices or long/thin wires cause the actual burden to exceed the transformer's rated VA, increasing error.

  Poor Connection or Minor Open Circuit in CT secondary: Causes low or fluctuating readings.

  Degraded Transformer Accuracy: Due to long-term overload or aging.

• =/Q: Protection relay maloperation (false trip) or failure to operate – could this be related to CTs?

  A: Yes.

  Maloperation: Multiple grounds in the CT secondary circuit introducing interference; or a protection CT with poor saturation characteristics distorting the current waveform during a fault, introducing harmonics that confuse the relay's algorithm.

  Failure to Operate (Nuisance Trip): CT ratio selected too high, making the fault current on the secondary side too small to activate the relay; or an open or short circuit in the CT secondary circuit.

• =/Q: What is a Current Transformer (CT) and a Voltage Transformer (VT or PT)? What is their primary purpose?

  A:       Current Transformer (CT): A device that proportionally converts a high primary current from a power system into a standard low secondary current (typically 5A or 1A). Its main purposes are isolation and measurement, providing a safe, standardized current signal for measuring instruments and protective relays.

  Voltage Transformer (VT or PT): A device that proportionally converts a high primary voltage from a power system into a standard low secondary voltage (typically 100V or 100/√3 V). Its main purposes are also isolation and measurement, providing a safe, standardized voltage signal for measuring instruments and protective relays.

• =/Q: What is the “Ratio”? How is it understood?

  A: The ratio is one of the most important parameters of a transformer, indicating the conversion proportion.

  CT Ratio: For example, "600/5 A". This means when the primary current is 600A, the secondary output current is 5A. The ratio Kn = 600 / 5 = 120.

  VT Ratio: For example, "10kV/100V". This means when the primary voltage is 10kV, the secondary output voltage is 100V. The ratio Kn = 10000 / 100 = 100.

• =/Q: What should I look for during routine inspections of instrument transformers?

  A:Abnormal Noise: Buzzing, discharge sounds, or intense humming (could indicate insulation breakdown or internal loosening).

  Overheating: Check temperature by hand (caution!) or with an infrared thermometer. Abnormal temperature rise is a concern.

  Unusual Smell: Burning smell, ozone odor.

  Physical Appearance: Check for cracks, contamination, or discharge traces on porcelain insulators; oil leaks or abnormal oil level in oil-filled units; aging or cracking on silicone rubber housings.

  Secondary Circuits: Check for loose or corroded terminal connections.

• =/Q: What is the working principle of CTs and VTs?

  A: Both operate on the principle of electromagnetic induction.

  CT: Works like a "step-up" transformer. It has very few primary turns (N1) connected in series with the circuit, and many secondary turns (N2). According to I1 / I2 = N2 / N1 = Kn (Ratio), it transforms a large current (I1) into a small current (I2).

  VT: Works like a "step-down" transformer. It has many primary turns (N1) connected in parallel with the circuit, and fewer secondary turns (N2). According to U1 / U2 = N1 / N2 = Kn (Ratio), it transforms a high voltage (U1) into a low voltage (U2).

• =/Q: What should I do if I discover an open circuit in a CT secondary?

  A: This is an emergency! Respond immediately:

  1.  Report: Immediately inform the system operator (dispatcher) and the responsible person.
  2.  De-energize: Request a power shutdown to handle it ASAP. If de-energizing is impossible, try to reduce the load current.
  3.  Isolate: Establish a safe perimeter to keep others away.
  4.  Rectify: A qualified person, wearing appropriate PPE (insulating gloves, boots), must use insulated tools to safely short-circuit the secondary terminals at the CT's terminal block. If the open point is obvious and easy to fix, repair it immediately.
  5.  Crucial: Never attempt to handle this with bare hands!

• =/Q: What should I do if I find a VT secondary short circuit or blown fuse?

  A:

  1.  Disable Related Protections: Isolate protections that might maloperate due to loss of VT signal (e.g., undervoltage protection, distance protection).
  2.  Investigate: Locate the short circuit or fault cause. Check the secondary circuit for obvious shorts or burn marks.
  3.  Replace: After confirming the fault is cleared, replace the fuse with one of the exact same rating and type.
  4.  Restore: Re-energize the VT and then re-enable the protections that were disabled.
  5.  If the new fuse blows again, the fault is still present. A thorough investigation of the circuit or the VT itself (for internal faults) is required.

• =/Q: Why must the secondary circuit of a CT never be open-circuited during operation?

  A: When a CT secondary is open, the primary current becomes entirely magnetizing current, causing the core to saturate heavily and overheat drastically. Simultaneously, a very high voltage (thousands of volts) is induced across the secondary terminals. This is extremely dangerous and can break down insulation, damage equipment, and seriously threaten personnel safety. When changing meters in a CT circuit, the secondary terminals must be reliably shorted with a shorting link or wire first.

• =/Q: Why can’t we measure high currents and voltages directly with meters?

  A: For two main reasons:

  Safety: Medium and low voltage systems can have currents up to thousands of amperes and voltages up to 35kV. Direct measurement is extremely dangerous for personnel and equipment. Transformers isolate the high voltage and current on the primary side, providing a safe, low-energy signal on the secondary side.

  Economy & Standardization: Manufacturing meters that can directly withstand high voltage and current is very costly and results in bulky devices. Transformers allow all secondary instruments and protective devices to be designed to uniform standards (5A, 100V), enabling standardization and economy.

• =/Q: Why must the CT secondary side be reliably grounded, and why only at one point?

  A:Reason for Grounding: To prevent primary high voltage from breaking through insulation and entering the secondary circuit, endangering personnel and equipment.

  Reason for Single-Point Grounding: Multiple ground points create ground loops. Potential differences between different ground points can cause circulating currents in the secondary circuit, leading to increased measurement errors and potentially causing protection relays to operate incorrectly (maloperation).