Instrument Transformers in Power System Protection

Electrical Substation | Source: General Electric

Protection or measuring devices require data on the electrical rating of the equipment to be protected. To obtain this data, following intermediary devices are needed:

  • voltage transformers (VT)
  • current transformer (CT)

These devices fulfill the following functions: 

  • reduction of the value to be measured (e.g.1500/5 A)
  • galvanic isolation
  • providing the power required for data processing and for protection operation itself. 

Current Transformers

The CTs are characterized by the following values. 

CT voltage 
This is the operating voltage applied to the CT primary. Note that the primary is at the HV potential level and that one of the secondary terminals is generally earthed. As for other equipment, the following is also defined : maximum1 min. withstand voltage at standard frequency maximum impulse withstand voltage. e.g. for 24 kV rated voltage, the CT must withstand 50 kV voltage for 1 min at 50 Hz and 125 kV impulse voltage. 

Rated transformation ratio 
It is usually given as the transformation ratio between primary and secondary current I1/I2. Secondary current is generally 5 A or 1 A. 

Current Transformer | Source: Nicore Magnetic Cores

Accuracy level 
It is defined by the composite error for the accuracy limit current. e.g. 5P10 means 5% error for 10 In 10P15 means 10% error for 15 In 5P and 10P are the standard accuracies classes. 5 In, 10 In, 15 In, 20 In are the standard accuracy limit currents. 

The accuracy limit factor 
is the ratio between the accuracy limit current and the rated current. Class X is another way of specifying CT characteristics based on "knee-point voltage" (fig.1, CT response in the saturated state).

Accuracy level power
Secondary power at rated current for which the accuracy level is guaranteed. Expressed in VA, it indicates the power that the secondary can deliver for its rated current, while respecting the rated accuracy class. It represents the total consumption of the secondary circuit, i.e. the power consumed by all the connected devices as well as the connecting wires. If a CT is loaded at a power rating lower than its accuracy level power, it's actual accuracy level is higher than the rated accuracy level. Likewise, a CT that is loaded too much loses accuracy. 

Admissible short time current 
Expressed in RMS kA, the maximum current admissible for 1 second (Ith) (the secondary being short-circuited) represents CT thermal overcurrent withstand. The CT must have the capacity to withstand short-circuit current for the time required to clear it. If the clearing time t is other than 1 sec., the current the CT can withstand is Ith / Vt. Electrodynamic withstand expressed in peak kA is at least equal to 2.5 x Ith.

Normal values of rated currents
At the primary (in A) 10 - 12.5 - 15 - 20 - 25 - 30 - 40 - 50 - 60 - 75 and multiples or decimal submultiples. 

When subjected to very strong current, the CT becomes saturated, i.e. the secondary current is no longer proportional to the primary current. The current error which corresponds to the magnetization current becomes very high.

CT Saturation Point or Knee Point

Knee Point Voltage - This is the point on the current transformer magnetization curve at which a 10% increase in voltage V requires a 50% increase in magnetization current Im.

For direct time overcurrent protection, if twice the setting current does cause saturation, the operation is ensured no matter how strong the fault. 

For IDMT overcurrent protections, saturation must not be reached for current values in the working part of the operation curve (a maximum of 20 times the setting current).

However, there are special types of CT that do not saturate. This are specific "wide band" current sensors. These sensors, most often without magnetic circuits and therefore not subject to saturation. Linked to an electronic device, their response is linear. These CTs are used and supplied with the digital technology protection units. They only require knowledge of the primary rated current.

Voltage Transformers

Voltage transformers reduced the high voltage across the power system into a value that can be connected to small protective relays. Normally, the secondary side of the VT used for protective relaying has a maximum voltage of 120 Volts. 

Voltage Transformers | Source: Indiamart

The voltage transformer have the following characteristics (IEC186). 
  • Electrical system frequency generally 50 or 60Hz, 
  • system's highest primary voltage (secondary voltage is standardized 100, 100/ , 110, 110/ Volts),
  • rated voltage factor 
  • VA power rating and accuracy class 
Note: 3-transformer assembly (requires 1 insulated high voltage terminal per transformer) 

A few additional key concepts for understanding voltage transformers:

  • Polarity - refers to the relative directions of the instantaneous voltages on the primary and secondary windings and is determined by the arrangement of the windings and the transformer connection terminals. Polarity is an instrument transformer is always subtractive, meaning that its primary terminals H1 and H2 and its secondary terminals X1 and X2 are arranged so that the voltage measured between a primary terminal and its adjacent secondary terminal will be the difference between the two voltages. Instrument transformer polarity is usually indicated with dot marks or plus/minus marks on the transformer and on drawings. Polarity is not important for simple voltage and current measurements, but it is crucial for power measurements and certain protective relay elements such as differential and directional. 

  • Burden - is simply the term for the load on an instrument transformer’s secondary winding. The burden consists of the relays or meters that are connected to the instrument transformer’s secondary winding. It also includes the impedance of the conductors between the secondary winding and the load. Voltage transformers are available with standard burden ratings that are designated with letter codes, as defined in IEEE standard C57.13. VTs usually have a maximum power rating of less than 500 Volt Amperes. 

  • Accuracy - there are two sources of error in instrument transformers: ratio error and phase error. Instrument transformers must be designed to ensure that even with these errors, secondary voltages and currents accurately represent primary voltages and currents. IEEE C57.13 establishes voltage transformer accuracy classes of 0.15, 0.3, 0.6, and 1.2, which applies when the VT secondary voltage is between 90% and 110% of rated voltage. Burden and accuracy are closely related to voltage transformers.

  • Protection Guide and Control
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