CT Saturation in Substations

Introduction: A Real Incident That Changed Everything

One night, a porcupine entered our substation trench. It cut the CT secondary wires. The wires stayed open for hours. Then the CT exploded. (CT saturation)

This incident taught me about CT saturation. It’s not just theory. It’s about real safety and protecting equipment.

What is CT saturation?

A current transformer (CT) is like a step-down device. It reduces high currents to safe levels. But every CT has limits.

CT saturation happens when the core reaches its max. Think of it like a sponge. First, a sponge absorbs water. Then it gets full. After that, it can’t absorb more.

A CT core works the same way. It handles magnetic flux up to a limit. Beyond that, it saturates. Then the CT stops working right.

Normal CT Operation vs Saturation

Normal operation:

• Primary current flows through the conductor
• Then the CT transforms it to secondary current
• So the ratio stays accurate (like 200/5)
• Next, relays and meters get correct readings

During saturation:

• First, the core reaches its limit
• Then the CT can’t keep the right ratio
• So secondary current gets wrong
• Finally, protection relays may fail

Learn more about how CTs work in our guide on CT polarity testing

Main Causes of CT Saturation

1. Too Much Primary Current

This is the most common cause. During faults, currents can be 10-20 times normal.

Example: Your CT is rated for 200A. Then a short circuit creates 3000 A. So the CT core gets overwhelmed. Saturation happens fast.

High fault currents create huge magnetic flux. But the core can’t handle it. Understanding fault current levels helps you pick the right CT.

2. High Secondary Burden

Burden means the load on the CT secondary. This includes:

• Relay coils
• Meters
• Cable resistance
• Connections

More burden needs more voltage. This creates more flux. So saturation risk goes up.

Long cables add resistance. Also, many relays add load. Each one pushes the CT toward saturation.

3. DC Offset in Fault Currents

When a fault occurs, current has two parts:

• AC component (alternates normally)
• DC component (doesn’t alternate)

The DC part is dangerous. It pushes the core to one limit. But it doesn’t reverse like AC does.

This happens in the first fault cycles. Also, it’s worst at voltage zero crossing.

4. Leftover Magnetism

After a fault clears, some magnetism stays. The core remembers the last event.

Here’s the problem: The next fault starts with a partly magnetized core. So saturation happens faster. It also happens at lower currents.

5. Wrong CT Ratio

Small ratios saturate easier than large ratios.

Example:

• A 50/5 CT saturates faster than a 2000/5 CT
• This is because fewer turns mean higher flux
• Then higher flux means earlier saturation

6. Open Secondary Circuit—The Porcupine Story

Now back to our porcupine incident. This shows a special case.

What happened:

1. First, porcupine cut the CT wires
2. But primary current kept flowing
3. So secondary circuit was open
4. Still, CT tried to keep working
5. Then voltage spiked to thousands of volts
6. Next, core went into deep saturation
7. Soon, insulation failed
8. Finally, CT exploded

Key lesson: Never leave CT secondaries open. Always short them when you disconnect. An open CT is very dangerous.

Real Case: Overloading in Pakistan

This happened on an 11 kV feeder in Pakistan. The company wanted more load on the line.

Setup:

• CT ratio: 200/5
• Old relay type
• Relay setting: Maximum (200%)
• Feeder max: 200A

What went wrong: They overloaded the feeder on purpose. So current went to 250-300A. They thought the relay would protect them.

What happened:

Step 1: First, CT started saturating above 200A

• The CT was rated for 200 A only
• So at 250A, the core couldn’t cope
• Then saturation began

Step 2: Next, secondary current got wrong

• Normally, 250A should give 6.25A secondary
• But CT only made 4-5A
• Also, the wave got clipped
• So peaks were cut off

Step 3: Meanwhile, relay didn’t see enough current

• The relay needed 10A to trip
• But it only saw 4-5A
• So relay never reached pickup
• Thus no trip happened

Step 4: Finally, failure occurred

• Primary current stayed at 250-300A
• Meanwhile, CT stayed hot and saturated
• Then insulation broke down
• Next, arcing started
• So CT and trolley exploded

This shows why relay settings matter. But settings alone can’t fix a saturated CT.

Effects of CT Saturation

1. Protection Fails

This is the biggest danger. Relays need accurate CT output.

Saturated CTs give wrong readings. So relays may:

• Fail to trip during faults
• Trip wrong during normal operation
• Act unpredictably
• Create confusion

2. Equipment Damage

Our cases showed clear patterns:

• First, CTs can overheat
• Then insulation can fail
• Next, explosions can happen
• Finally, nearby equipment gets damaged

3. Safety Risks

Also, saturation creates dangers:

• High voltages on circuits
• Risk of shock
• Fire from heat
• Danger during maintenance

4. System Problems

When protection fails, issues spread:

• First, faults aren’t cleared fast
• Then damage spreads
• Next, system becomes unstable
• Finally, blackouts can happen

In differential protection, CT saturation is critical.

How to Prevent CT Saturation

1. Pick the Right CT

Choose the right ratio:

• First, calculate max fault current
• Then add 20% safety margin
• Finally, select CT ratio

Example: If max load is 200A and fault is 3000A:

• Don’t use 200/5 CT
• Instead, use 400/5 or higher
• This gives headroom

Also pick a proper class:

• Class 5P for protection
• Class X for differential
• TPY class for high accuracy

2. Reduce Secondary Burden

Cut cable length:

• First, keep relays close to CTs
• Also, use thick cables
• Remember, every meter adds burden

Limit devices:

• Use only needed meters
• Remove unused equipment
• Each device adds load

3. Set Relays Right

Set relays based on real needs:

• Don’t use max settings
• Calculate based on actual loads
• Also think about CT limits

From the Pakistan case: Max settings needed more current. So this made it worse.

4. Test and Maintain

Do annual CT tests:

• Check ratio accuracy
• Measure insulation
• Test for leftover flux
• Also check burden

Check connections:

• Keep connections tight
• Clean terminals often
• Look for rust

5. Use Modern Features

New relays have protection built in:

• Saturation detection
• Adaptive settings
• DC offset fix

Also, digital relays handle saturation better than old types.

6. Never Open CT Secondaries

Critical rule:

• Always short CT before disconnect
• Use shorting links
• Keep secondary closed during work
• Also train all staff

Our porcupine incident proved this.

Quick Checklist for Engineers

Before you start:

• First, check that the CT ratio matches the load.
• Then calculate burden
• Next, check all connections
• Also confirm relay settings
• Test CT ratio
• Check for open circuits
• Finally, review fault calculations

During operation:

• Watch load levels
• Don’t exceed CT rating
• Also watch for odd relay behavior
• Check CT temperature

After any fault:

• First, test for leftover flux
• Then check CT accuracy
• Also look for damage
• Finally, review how protection worked

Conclusion

CT saturation is serious. It causes protection to fail. Also, it damages equipment and creates danger.

The porcupine case taught us about open circuits. Then the Pakistan case showed what happens when you push CTs too hard.