6.1 Current Transformers (CTs): Ratio, Polarity, Excitation/Saturation, and Burden Testing

Current Transformers (CTs): Ratio, Polarity, Excitation/Saturation, and Burden Testing

Quick Summary: Current transformers (CTs) serve as the vital sensory interface between high-voltage power apparatus and secondary protective relays, revenue meters, and monitoring equipment. Field testing per IEEE C57.13 and NETA ATS/MTS Section 7.10 verifies ratio accuracy, polarity alignment, core excitation limits, and secondary loop burden to guarantee dependable fault detection and prevent protective relay misoperation.

In electrical power testing, an incorrectly wired, saturated, or improperly rated current transformer can cause catastrophic protection failures—either blinding a protective relay to an in-zone short circuit or causing false tripping during external through-faults. Commissioning and periodic maintenance technicians must master CT construction types, magnetic saturation physics, and standardized field testing protocols.

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1. Current Transformer Mechanical Constructions

Current transformers are classified by their physical construction and primary winding configuration:

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2. CT Polarity: Dot Convention and Field Verification

Instrument transformer polarity establishes the instantaneous relative direction of primary and secondary currents. By international standard (IEEE C57.13):

Standard Polarity Rule: When instantaneous primary current enters terminal H1, instantaneous secondary current exits terminal X1.

                     PRIMARY CURRENT (Ip)
                ----------------------------->
                        [H1]     [H2]
                      =================
                      |   CORE & SEC  |
                      =================
                        [X1]     [X2]
                <-----------------------------
                    SECONDARY CURRENT (Is)
                  (Leaves X1 toward relay)

Reversed polarity in differential protection schemes (ANSI 87), directional overcurrent relays (ANSI 67), or power/energy metering causes immediate false tripping or inverted power factor readings.

Field Polarity Test Methods

Method 1: DC Voltage Kick Test

The DC kick test is the standard field method for verifying polarity on standalone or unenergized CTs using an analog voltmeter (or galvanometer) and a low-voltage DC battery (6 V to 9 V):

1. Connect the positive terminal of the DC battery to primary terminal H1 through a momentary push-button switch, and the negative battery terminal to H2.
2. Connect the positive lead of a high-sensitivity analog DC voltmeter to secondary terminal X1, and the negative lead to X2.
3. On Switch Closure (Make): The meter pointer must deflect upscale (positive).
4. On Switch Opening (Break): The meter pointer must deflect downscale (negative).
5. If the meter deflects downscale upon closing the circuit, the CT polarity markings are reversed or internal leads are crossed.

+-----------------------------------------------------------------------------------------+
|                                 DC VOLTAGE KICK TEST                                    |
|                                                                                         |
|               [ + ]------------- [Push-Button] ------------- [ H1 ]                     |
|             6V Battery                                         |    (Primary)           |
|               [ - ]----------------------------------------- [ H2 ]                     |
|                                                                                         |
|                                      MAGNETIC CORE                                      |
|                                                                                         |
|               [ + ]----------------------------------------- [ X1 ]                     |
|             Analog D'Arsonval                                  |    (Secondary)         |
|             Meter (Upscale +)                                  |                        |
|               [ - ]----------------------------------------- [ X2 ]                     |
+-----------------------------------------------------------------------------------------+

Method 2: AC Voltage Comparison Method

Apply a low AC voltage (10 V to 120 V) across H1-H2. Jumper primary terminal H1 to secondary terminal X1. Measure voltage between H2 and X2 (V_H2-X2):

• Subtractive Polarity (Standard): V_H2-X2 = V_H1-H2 - V_X1-X2. The measured voltage is less than the applied primary voltage.
• Additive Polarity: V_H2-X2 = V_H1-H2 + V_X1-X2. The measured voltage is greater than the applied primary voltage.

3. Turns Ratio and Voltage Ratio Testing (IEEE C57.13 / NETA ATS 7.10.1)

Current transformer ratio verification ensures that the secondary current faithfully replicates primary current scaled by the nominal turns ratio (N_p / N_s):

$\text{Turns Ratio (TR)} = N_s / N_p = I_{primary} / I_{secondary}$

Testing Methodologies

1. Current Injection Method (Primary Injection):

   • Inject a known high current (e.g., 100 A on a 600:5 CT) into primary terminals H1-H2 using a high-current test set.
   • Measure secondary current at X1-X2 using a calibrated precision ammeter.
   • Calculate true ratio: Ratio = I_injected / I_measured.
   • Evaluation: Excellent for commissioning because it tests the entire primary bus, CT core, secondary wiring, and relay input simultaneously; however, equipment is bulky.

2. Voltage Ratio Method (Secondary Voltage Injection):

   • Apply an accurate AC test voltage (V_s) below the saturation knee-point across secondary terminals X1-X2.
   • Measure the resulting induced open-circuit voltage (V_p) across primary terminals H1-H2 using a high-impedance digital voltmeter.
   • Calculate ratio: Ratio = V_s / V_p = N_s / N_p.
   • Evaluation: Highly portable, extremely precise, and standard practice for multi-ratio bushing CTs (MRBCTs) across all tap combinations (X1-X2, X2-X3, X1-X5, etc.).

Acceptance Criteria (NETA ATS Table 100.9 / IEEE C57.13):

• Revenue Metering CTs: Ratio error must not exceed ±0.3% or ±0.5% (depending on accuracy class) at rated current.
• Relay Protection CTs: Ratio error must not exceed ±1.2% at rated current and ±10% at 20 times rated current (100 A secondary).
• Multi-ratio CT tap connections must match engineered single-line drawing schedules exactly.

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4. CT Excitation, Saturation, and Knee-Point Determination

When primary fault current increases, the CT magnetic core must develop higher secondary voltage (V_s = I_s × Z_burden) to drive current through the connected secondary loop. As magnetic flux density (B) approaches the core saturation limit (B_sat ≈ 1.6 to 1.9 Tesla for silicon steel), the core requires disproportionately large magnetizing current (I_e). The secondary current drops, causing severe waveform distortion and ratio error.

Secondary
Voltage (Vs)
    ^
    |                             / Saturated Region (Core Saturated)
    |                            /
    |                           / <--- Knee-Point Voltage (Vk)
    |                          /
    |                         /  Linear / Operating Region
    |                        /   (Low Excitation Current)
    |                       /
    |                      /
    +----------------------------------------------------> Excitation Current (Ie)

Field Excitation Test Procedure:

1. Isolate the CT primary circuit and ensure primary terminals are open and clear.
2. Connect variable AC test voltage supply to secondary terminals X1-X2 with a precision voltmeter and ammeter (or automated CT analyzer).
3. Apply voltage in progressive steps, recording secondary voltage (V_s) and magnetizing current (I_e) from initial low excitation through the saturation knee-point.
4. Plot data on log-log coordinate graph paper.

Knee-Point Voltage (V_k) Definitions:

• IEEE C57.13 Standard (45° Tangent Method): The knee-point is the point on the log-log excitation curve where the tangent to the curve makes a 45° angle with the horizontal axis.
• IEC 60044-1 / 61869-2 Standard (10% / 50% Rule): The knee-point is defined as the voltage level at which a 10% increase in secondary test voltage causes a 50% increase in excitation current (I_e).

+-----------------------------------------------------------------------------------------+
|                       TYPICAL LOG-LOG EXCITATION TEST DATA                             |
|                              (600:5 C400 Protection CT)                                |
|                                                                                         |
|   Test Point    Secondary Voltage (Vs)    Excitation Current (Ie)     Operational State |
|   -----------------------------------------------------------------------------------   |
|       1                 25 V                      0.008 A             Linear Zone       |
|       2                 50 V                      0.015 A             Linear Zone       |
|       3                100 V                      0.028 A             Linear Zone       |
|       4                200 V                      0.055 A             Linear Zone       |
|       5                360 V                      0.120 A             Approaching Knee  |
|       6 (Vk)           410 V                      0.210 A             IEEE 45° Knee Pt  |
|       7                440 V                      0.480 A             Saturation Zone   |
|       8                460 V                      1.150 A             Deep Saturation   |
+-----------------------------------------------------------------------------------------+

Core Remanence and Demagnetization Procedures

When an asymmetric fault is interrupted or when DC current is applied (such as during DC winding resistance or polarity testing), residual magnetic flux (remanence) remains trapped in the CT core. Remanent flux can consume up to 80% of core flux capacity, causing premature saturation on subsequent faults.

• Demagnetization Protocol:
  1. Apply secondary AC test voltage and smoothly raise it past the saturation knee-point.
  2. Slowly, continuously, and smoothly decrease the applied AC voltage down to exact zero without abrupt switching.
  3. Alternatively, modern automated test sets apply low-frequency decaying bipolar DC voltage pulses to drive residual flux to zero.

5. ANSI C-Class Accuracy Ratings & Burden Calculations

IEEE C57.13 classifies relaying current transformers using a standardized alphanumeric designation:

Designation Format: C or T followed by secondary terminal voltage (e.g., C100, C200, C400, C800)

• "C" Classification (Calculated): Indicates that the ratio error can be calculated mathematically because leakage flux in the core window is negligible (standard for toroidal window and bushing CTs with fully distributed secondary windings).
• "T" Classification (Tested): Indicates that leakage flux is significant (e.g., wound primary CTs), requiring physical testing to determine performance.
• Voltage Number (e.g., 400 in C400): The secondary terminal voltage the CT can deliver at 20 times rated secondary current (20 × 5 A = 100 A) to a standard burden without exceeding 10% ratio error.

$\text{Rated Secondary Terminal Voltage } (V_T) = 100\text{ A} \times Z_{burden}$

Total Secondary Burden Equation:

$Z_{total} = R_{internal\_CT} + R_{lead\_wire} + Z_{relay\_input} + Z_{auxiliary}$

For a phase-to-ground fault on a 4-wire Wye circuit, lead resistance accounts for round-trip distance (2 × R_one_way).

+-----------------------------------------------------------------------------------------+
|                         STEP-BY-STEP C-CLASS BURDEN CALCULATION                         |
|                                                                                         |
|   GIVEN:                                                                                |
|   - CT Rating: 1200:5 C400 (Internal winding resistance R_ct = 0.35 Ω)                  |
|   - Cable Run: 240 ft of #10 AWG copper (approx. 1.0 Ohm/1,000 ft one-way; the CT   |
|     secondary loop is 480 ft of conductor, so R_leads = 0.48 Ohm)                       |
|   - Microprocessor Relay Burden: 0.05 Ω                                                 |
|   - Maximum Available Fault Current: 18,000 A Primary                                   |
|                                                                                         |
|   CALCULATIONS:                                                                         |
|   1. Secondary Fault Current (Is):                                                      |
|      Is = 18,000 A / (1200 / 5) = 18,000 / 240 = 75 A Secondary                         |
|                                                                                         |
|   2. Total Connected Burden (Z_total):                                                  |
|      Z_total = R_ct + R_leads + Z_relay = 0.35 Ω + 0.48 Ω + 0.05 Ω = 0.88 Ω             |
|                                                                                         |
|   3. Required Terminal Voltage at Fault Current:                                        |
|      V_req = Is × (R_leads + Z_relay) = 75 A × (0.48 Ω + 0.05 Ω) = 75 A × 0.53 Ω        |
|      V_req = 39.75 V                                                                    |
|                                                                                         |
|   4. Maximum Capability Check (at 20x Nominal = 100 A):                                 |
|      V_max_developed = 100 A × 0.88 Ω = 88.0 V                                          |
|                                                                                         |
|   CONCLUSION:                                                                           |
|   Since 88.0 V is well below the C400 rating (400 V), the CT will operate entirely in   |
|   its linear magnetic region without saturation during an 18 kA primary fault.          |
+-----------------------------------------------------------------------------------------+

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6. Secondary Burden Testing & The Lethal Open-Circuit Hazard

Field Burden Measurement Protocol

1. Disconnect CT secondary leads at the CT terminal block.
2. Using an external secondary injection test set, inject rated secondary current (5.0 A) into the secondary loop conductors toward the protective relay.
3. Measure the voltage drop (V_loop) across the injection terminals.
4. Calculate measured secondary burden: Z_burden = V_loop / 5.0 A.
5. Compare against maximum allowable burden values from the protection coordination study.

The Open-Circuit Secondary Hazard

An energized current transformer operates as a primary-driven constant current source. Under normal closed-circuit conditions, secondary ampere-turns (N_s × I_s) almost completely cancel primary ampere-turns (N_p × I_p), leaving only a tiny net magnetizing flux (<1%) in the core.

If the secondary circuit is opened while primary current flows:

1. Secondary counter-magnetomotive force drops instantly to zero (N_s × I_s = 0).
2. The entire primary current becomes 100% unconstrained magnetizing current.
3. The magnetic core is driven into extreme saturation in fractions of an AC half-cycle.
4. At every AC current zero-crossing, the core rapidly flips from negative to positive saturation flux density (+Φ_max to -Φ_max).
5. Per Faraday's Law (e = -N_s × dΦ/dt), the near-instantaneous change in flux generates extremely sharp voltage spikes reaching 2,000 V to 10,000+ V peak across the open secondary terminals.

+-----------------------------------------------------------------------------------------+
|                       OPEN-CIRCUIT CT SECONDARY WAVEFORM                                |
|                                                                                         |
|   Primary Flux (Φ)                                                                      |
|        +Bsat --------+                  +--------+                                      |
|                      |                  |        |                                      |
|                      |                  |        |                                      |
|        -Bsat         +------------------+        +------------------                    |
|                                                                                         |
|   Induced Secondary Voltage Spikes (e = -Ns dΦ/dt)                                      |
|             +kV  ^        |                  ^        |                                 |
|                  | Peak   |                  | Peak   |                                 |
|                  | Spike  |                  | Spike  |                                 |
|        0 V ------+--------+------------------+--------+-------------                    |
|                           | -kV                       | -kV                             |
|                           v                           v                                 |
+-----------------------------------------------------------------------------------------+

Life Safety & Equipment Protection Mandates:

• Lethal Electric Shock: Induced voltage spikes will cause fatal electrocution and catastrophic flashover.
• Destructive Core Heating: Severe eddy currents and hysteresis losses rapidly destroy winding insulation, permanently damaging the CT.
• Shorting Switch Operation: Technicians must verify that CT shorting blocks/switches (e.g., States sliding links or test switches) are firmly closed before disconnecting any secondary wiring, relays, or test plugs.

A voltage test alone does not make a CT circuit safe. A CT secondary is a current-driven circuit. While the shorting link is closed the loop carries the full reflected secondary current at only a few volts, so a conventional absence-of-voltage test reads essentially zero and the circuit looks de-energized — right up to the moment a technician opens the loop and the CT drives it to thousands of volts. The current edition of NFPA 70E (2027) added requirements in Article 120 for additional testing where an absence-of-voltage test by itself does not demonstrate that equipment is de-energized, and it names absence of current in current-driven circuits as the example. In practice: confirm the shorting device is actually shorting (measure current on the loop, or verify the link mechanically), never rely on a voltmeter alone, and treat every CT secondary as energized while primary current can flow.

• Safety Grounding: Every CT secondary circuit must be grounded at exactly one physical point (typically at the first relay panel or switchgear marshalling cabinet). Multiple grounds create parallel circulating ground loops that corrupt relay metering accuracy.