Transformer CT Selection — Protection vs. Metering Class, Saturation & Knee-Point Voltage

Introduction

A current transformer is the eyes of the protection and metering system — but a poorly selected CT can blind the relay precisely when it is needed most. During a heavy through-fault, a protection CT that saturates produces a distorted secondary current that the differential relay interprets as an internal fault, causing a false trip. During normal loading, a metering CT without proper accuracy may under-report energy consumption by 2–3%, compounding into significant revenue errors. This article explains CT selection for power transformer applications based on IEC 61869-2, covering accuracy classes, knee-point voltage, secondary burden, and common pitfalls.

1. CT Accuracy Classes: Protection vs. Metering

1.1 The Distinction

CT Type	Purpose	Accuracy Class	Key Characteristic
Metering CT	Revenue / energy metering	0.2, 0.5, 0.2S, 0.5S	Accurate from 1% to 120% of rated current; must saturate above to protect meters
Protection CT	Overcurrent, differential	5P, 10P, PX	Accurate at high multiples of rated current (up to 10–40× In); must not saturate
Combined CT	Both metering and protection	e.g., 0.5/5P20	Dual-core; secondary windings on separate cores

1.2 Metering CT Classes

Class	Current Error (±%) at 100% In	Phase Error (±minutes)	Security Factor
0.2S	0.2	10	FS5 or FS10
0.5S	0.5	30	FS5 or FS10
0.2	0.2 (at 100–120% In)	10	FS5
0.5	0.5 (at 100–120% In)	30	FS5

The "S" designation (e.g., 0.2S) means the CT maintains accuracy down to 1% of rated current — essential for revenue metering where load varies from near-zero to full load.

1.3 Instrument Security Factor (FS)

The FS rating limits the secondary current during a fault so that connected meters are not damaged:

FS5: At 5× I_n, the composite error is >10% (CT begins saturating)
FS10: At 10× I_n, saturation begins

For metering CTs, lower FS is better (more protection for meters). For protection CTs, higher accuracy limit factor (ALF) is better.

1.4 Protection CT Classes

Class	Composite Error at ALF	Phase Error at ALF	Typical ALF
5P	±1% ratio, ±60 min	±1% composite	10, 15, 20, 30
10P	±3% ratio	±3% composite	10, 15, 20, 30
PX (Class X)	Specified by Vk, Ie, Rct	—	—

Example: 5P20 means the CT maintains ±1% ratio error up to 20× rated primary current. This is the most common protection class for transformer differential protection.

2. CT Ratio Selection

2.1 Primary Current Rating

The CT primary current I_pn should be ≥ the maximum continuous load current but not more than 2× the nominal load current (to maintain metering accuracy at low loads):

1.0 × I_load ≤ I_pn ≤ 2.0 × I_load

For a transformer HV side:

I_load = S / (√3 × U_HV)

Example: 20 MVA, 110/11 kV
I_HV = 20,000 / (√3 × 110) = 105.0 A
→ Select CT ratio: 150/1 A or 150/5 A

For the LV side:

I_LV = 20,000 / (√3 × 11) = 1049.7 A
→ Select CT ratio: 1200/1 A or 1200/5 A

2.2 Secondary Current Rating

Secondary	Advantages	Disadvantages
1 A	Lower cable voltage drop (1/25 of 5 A burden); longer distances possible	More susceptible to open-circuit voltage
5 A	Less noise-sensitive; legacy standard	Higher cable losses (I²R); limited run length

Rule of thumb: Use 1 A CTs for cable runs exceeding 50 m. Use 5 A CTs for switchgear-mounted relays with short connections.

3. Burden and Lead Length

3.1 Burden Calculation

The total burden on a CT consists of:

P_total = P_relay + P_lead + P_connections

Where:

• P_relay = relay burden (typically 0.05–1.0 VA per phase for modern digital relays)
• P_lead = I_s² × R_lead (typically the dominant component)
• P_connections = ~0.1 VA (bolted terminal loss)

3.2 Lead Resistance Calculation

For a CT secondary cable of length L (one-way, so 2L round-trip):

R_lead = ρ × (2L) / A

For copper (ρ = 0.01786 Ω·mm²/m):

Cable CSA (mm²)	R (Ω/m)	For L=100 m (2L=200 m)
2.5	0.00714	1.43 Ω
4.0	0.00447	0.89 Ω
6.0	0.00298	0.60 Ω

For a 1 A CT at L = 200 m round-trip, 2.5 mm² cable:

P_lead = 1² × 1.43 = 1.43 VA

For a 5 A CT:

P_lead = 5² × 1.43 = 35.75 VA

This is why 5 A CTs are problematic for long cable runs — the lead burden alone exceeds many CT ratings.

3.3 CT Rated Burden

Select a CT with rated burden ≥ P_total:

CT_burden ≥ (I_sn² × R_ct) + P_total

Where R_ct = CT secondary winding resistance.

4. Knee-Point Voltage and Saturation

4.1 Definition

The knee-point voltage V_k is the voltage at which a 10% increase in voltage causes a 50% increase in magnetizing current. This is the onset of saturation.

4.2 V_k Requirement for Protection

For overcurrent protection (5P, 10P):

V_k ≥ ALF × I_sn × (R_ct + R_burden)

For a 5P20, 5 A CT with R_ct = 0.3 Ω, R_burden = 1.0 Ω:

V_k ≥ 20 × 5 × (0.3 + 1.0) = 20 × 5 × 1.3 = 130 V

For transformer differential protection (87T), the requirement is more stringent because CT saturation during an external through-fault must not cause spurious differential current:

V_k ≥ 2 × IF_max × (R_ct + 2R_l + R_relay)

Where I_{f_max} is the maximum through-fault current in secondary amps.

4.3 PX Class (Class X) Specification

PX CTs are specified directly by physical parameters rather than accuracy classes:

Parameter	Symbol	Example
Turns ratio	—	1200/1
Knee-point voltage	Vk	≥ 300 V
Excitation current at Vk	Ie	≤ 30 mA
Secondary winding resistance	Rct	≤ 4.0 Ω

PX CTs are used where precise saturation-free performance is required, notably transformer differential and busbar protection.

5. CT Saturation and Its Consequences

5.1 Causes of Saturation

• DC offset in fault current: The asymmetrical component of a fault current drives the CT core into saturation much faster than the symmetrical component. The X/R ratio of the system determines the DC time constant.
• Remanence: Previous fault or DC injection leaves the core magnetized. The next fault may saturate the CT at a much lower current.
• Undersized CT: CT rated for normal load but inadequate for fault-level currents.

5.2 Differential Protection Implications

During an external fault, the CTs on the HV and LV sides of the transformer should produce identical secondary currents (after ratio and phase compensation). If one CT saturates and the other does not, the differential relay sees a spurious difference current and may trip. This is the single most common cause of false differential trips.

Mitigation:

• Ensure V_k meets the 2× margin requirement
• Use CTs with anti-remanence air gaps (Class PR, TPY) for critical applications
• Apply relay algorithms with saturation detection (differential restraint)

6. CT Selection Checklist for Transformers

Step	Action	Reference
1	Calculate rated primary currents (HV and LV)	Transformer nameplate
2	Select CT ratio: Ipn ≥ 1.0× and ≤ 2.0× Iload	IEC 61869-2
3	Choose secondary rating: 1 A or 5 A	Based on lead length
4	Determine protection class: 5P20 or PX	IEC 61869-2
5	Determine metering class: 0.2S or 0.5S	IEC 61869-2
6	Calculate total burden (relay + leads + connections)	Burden diagram
7	Select CT burden rating ≥ calculated	Manufacturer catalog
8	Verify Vk for protection core	Magnetization curve
9	Check cable lead length and CSA against burden	Cable schedule

FAQ

Q: Can I use a single CT core for both protection and metering?

Technically yes (combined CT with two secondaries on one core), but it is poor practice for revenue metering. If the protection core saturates, it draws magnetizing current that affects the metering accuracy. Always use separate cores (and separate CTs if space permits) for protection and metering. For transformer differential protection, independent CTs on the HV and LV sides are mandatory.

Q: What happens if the CT secondary circuit is opened while the primary is energized?

The CT attempts to maintain the secondary current by driving the secondary voltage to dangerously high levels — potentially 5–20 kV at the open terminals. This can (1) puncture the CT insulation, (2) injure personnel, and (3) permanently magnetize the core (remanence). Always short-circuit CT secondary terminals before disconnecting loads. Modern test switches with make-before-break contacts prevent open-circuiting.

Q: How do I select CTs for transformer differential protection with different ratios on HV and LV sides?

The differential relay performs ratio matching internally. Select CT ratios that approximate the transformer ratio. For a 20 MVA, 110/11 kV transformer (ratio 10:1), use 150/1 on HV and 1200/1 on LV. The relay applies a ratio correction factor: 150/1200 = 0.125. Modern numerical relays handle this automatically; electromechanical relays require interposing CTs.

Q: What is the difference between Class P, PR, and TPY CTs?

Class P (protection) has a closed iron core with no air gap — prone to remanence. Class PR (protection, remanence-limited) has small air gaps that reduce remanence to ≤10%, making it suitable for auto-reclosing applications. Class TPY (transient performance, gap) has larger air gaps for near-zero remanence and is specified for the most demanding applications (EHV line protection, generator differential). TPY CTs are larger and more expensive than P CTs of the same ratio.

Q: How do I verify CT polarity after installation?

Inject a DC current pulse (e.g., from a 9 V battery) on the primary side and observe the secondary current pulse with an analog millivoltmeter. If the meter deflects positive when the battery positive is connected to P1 (primary polarity mark), the secondary terminal corresponding to S1 is the correct polarity terminal. Reverse polarity causes differential relays to trip on inrush and should be corrected immediately.

Q: What is the multiratio CT and when should I use it?

Multiratio CTs have tapped secondary windings that provide multiple ratios (e.g., 600/300/200/1 A from a 600/1 winding with taps). They offer flexibility for future load growth. The downside is that lower-tap ratios reduce the effective accuracy because the full winding is not used. For transformer applications where the load is predictable, a single-ratio CT with optimized accuracy is preferred.

References & Standards

Document	Title	Relevance
IEC 61869-2	Additional requirements for current transformers	CT accuracy classes, testing
IEC 60044-1	Instrument transformers — Current transformers	Legacy standard (superseded but still referenced)
IEC 60044-6	Requirements for protective current transformers	Transient performance (TPY, TPZ classes)
IEEE C57.13	Standard requirements for instrument transformers	IEEE equivalent for CT specification
IEC 60255-187	Protection relays — Functional standard for differential	Differential relay CT requirements

*Du Fu, ZY POWER Production Engineer — The CT may be small, but its job is to faithfully reproduce the truth at any current magnitude.*