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Pressure Release Valve in Transformer

A Pressure Release Valve (PRV) is a transformer protection device that automatically releases excessive internal pressure from an oil-filled transformer tank.

⚡ Why is it needed?

During a serious internal transformer fault, such as an internal short circuit or arcing, the fault can rapidly heat transformer oil and produce gas. This can cause a sudden increase in tank pressure.

Without pressure relief, excessive pressure may damage or rupture the transformer tank.

🔧 Working Principle

Normal condition → Fault occurs → Oil/gas pressure rises → Set pressure reached → PRV opens → Pressure released → PRV closes/reset

🛡️ Main Functions

Protects the transformer tank from overpressure

Releases rapidly generated gas/oil pressure

Reduces the risk of tank rupture

Provides a mechanical backup to electrical protection systems

Some models provide an alarm/trip microswitch when the valve operates

📍 Typical Location

The PRV is generally mounted on the transformer tank cover or upper tank wall, depending on the transformer design.

🔥 Important Difference

PRV: Protects mainly against rapid/excessive internal pressure.

Buchholz Relay: Detects gas accumulation and oil movement in conservator-type transformers.

Pressure/Vacuum Relief Device: May handle both positive and negative tank pressure, depending on its design.

Simple concept:

> Internal fault → pressure rises rapidly → PRV opens → excess pressure escapes → transformer tank is protected.

How to Replace a Transformer Bushing

Replacing a transformer bushing is a critical maintenance procedure in a power substation. A transformer bushing provides electrical insulation and allows a high-voltage conductor to pass safely through the grounded transformer tank.
A damaged, cracked, leaking, or electrically failed bushing must be replaced carefully to prevent insulation failure, oil contamination, and transformer damage.

1️⃣ Tools and materials required
Replacement bushing of the correct voltage, current, and mechanical ratings
Suitable gaskets and sealing materials
Torque wrench and appropriate hand tools
Lifting equipment, if required
Transformer oil handling and filtration equipment
Insulating oil sampling equipment
Cleaning materials and lint-free cloths
Electrical testing instruments
Approved PPE and safety equipment

2️⃣ Step-by-step bushing replacement procedure

Step 1️⃣ : Isolate and make the transformer safe

🔹Disconnect the transformer from all possible sources, including HV, LV and auxiliary supplies.

🔹Apply lockout/tagout (LOTO).

🔹Verify absence of voltage using approved equipment.

🔹Discharge and apply protective earths as required by the approved safety procedure.

🔹Follow the site-specific safety and switching instructions.

Step 2️⃣ : Prepare the transformer

🔹Check the transformer oil level and bushing arrangement.

🔹Identify the bushing type and internal connection.

🔹Lower the oil level below the bushing mounting opening if required by the design.

🔹Use a clean, dry and controlled work area to prevent moisture and dust from entering the transformer.

Step 3️⃣ : Disconnect the old bushing

🔹Remove the external conductor connections.

🔹Remove the terminal fittings and accessories as specified.

🔹Access and disconnect the internal lead or draw-lead connection according to the manufacturer's instructions.

🔹Support the bushing properly before releasing its mounting hardware.

Step 4️⃣ : Remove the defective bushing

🔹Loosen the mounting bolts in the specified sequence.

🔹Carefully lift the bushing vertically using approved lifting points.

🔹Avoid damaging the mounting fl**ge, internal leads, or tank opening.

🔹Inspect the mounting surface and internal connection for damage or contamination

Step 5️⃣ : Install the new bushing

🔹Verify the replacement bushing's ratings and compatibility.

🔹Install the correct new gasket and check the sealing surfaces.

🔹Carefully lower the new bushing into position.

🔹Reconnect the internal lead according to the manufacturer's procedure.

🔹Tighten mounting bolts in the specified sequence and to the specified torque.

Step 6️⃣ : Refill oil and check sealing

🔹 Restore the oil level using compatible, properly processed transformer oil.

🔹Follow the manufacturer's vacuum filling or oil processing requirements where applicable.

🔹Allow trapped air to escape using the designated procedure.

🔹Check the fl**ge, seals, and connections for oil leakage.

Step 7️⃣ : Perform electrical and mechanical tests

🔹Measure insulation resistance where applicable.

🔹Perform capacitance and power-factor (tan delta) tests for applicable condenser bushings.

🔹Verify electrical connections, clearances, oil level, and sealing.

🔹Carry out any additional tests required by the manufacturer and maintenance plan.

Step 8️⃣ : Commission the transformer

🔹Remove temporary protective earths only under the approved switching procedure.

🔹 Ensure all personnel and tools are clear.

🔹Restore the transformer according to the approved energization sequence.

🔹Monitor voltage, current, oil leakage, unusual noise, and temperature after energization.

3️⃣ Important precautions

Critical safety points

▪️Never replace a bushing on an energized transformer.

▪️Prevent moisture, dust, and foreign objects from entering the transformer.

▪️Do not reuse damaged gaskets.

▪️Never apply arbitrary tightening torque. Follow the exact manufacturer's specifications.

▪️Follow the correct vacuum, oil filling, and air-venting procedure for the specific bushing design.

▪️Do not energize the transformer until required inspection and test results have been reviewed and approved.

The procedure varies significantly between conventional porcelain bushings, condenser-type bushings (OIP, RIP or RIS), and plug-in or draw-lead designs.

4️⃣ Testing after replacement

Test | Purpose

▪️Insulation resistance | Checks insulation condition

▪️Capacitance test | Checks condenser bushing capacitance

▪️Tan delta / power factor | Evaluates dielectric losses

▪️Contact resistance | Checks electrical connection quality, where applicable

▪️Oil leak inspection | Checks sealing integrity

▪️Oil quality tests | Checks oil condition when required

Testing requirements depend on the bushing type and the transformer maintenance specification.

One important question: Are you replacing a 33 kV, 132 kV, or 220 kV power transformer bushing, and is it an oil-filled condenser bushing or a conventional porcelain bushing? The exact replacement sequence differs considerably between them.

Transformer TTR Test

TTR = Transformer Turns Ratio Test.
It is a routine transformer test used to verify that the transformer’s turns ratio and tap-changer operation match the nameplate/design values.

🔹 Basic Principle

The transformer turns ratio is:

➡️ TTR= N HV ÷ N LV = V HV ÷ V LV

Where:

N = Number of turns

V = Voltage

HV = High Voltage winding

LV = Low Voltage winding

🔹 Why is TTR Test performed?

1. Verify the transformer turns ratio

2. Detect shorted or open turns

3. Check tap-changer operation

4. Identify incorrect winding connections

5. Detect winding problems after repair or transportation

6. Compare measured ratio with the nameplate ratio

7. Help identify winding or connection abnormalities between phases

🔹 Typical Test Procedure

1. Isolate the transformer

De-energize and properly isolate it.

Apply required safety/earthing procedures.

2. Connect the TTR tester

Connect the tester to the HV and LV terminals according to the manufacturer's connection diagram.

3. Select the test voltage/connection

Enter the transformer nameplate information and winding configuration.

4. Test each phase

Measure the ratio for all phases and, for transformers with taps, at each required tap position.

5. Compare results Compare measured TTR with the calculated/nameplate ratio.

🔹 Example

Suppose a transformer is:

11 kV / 415 V

Nominal ratio:

➡️ TTR= 11000 ÷ 415 =26.51

So the measured ratio should be close to the expected ratio, allowing for the applicable standard and manufacturer's specified tolerance.

🔹 What can an abnormal TTR indicate?

Result Possible indication

Ratio significantly different Wrong connection or winding problem
One phase differs from others Phase-specific winding/connection issue
Ratio changes unexpectedly between taps Tap-changer/contact problem
Large deviation from nameplate Possible winding damage or incorrect data

Important: The acceptable deviation should be judged according to the applicable standard, transformer manufacturer requirements, and test instrument method—not from a single universal percentage.

Instrumentation Industrial Standard Codes & Colour System | Power Plant Instrumentation

Transformer tap Changer Set-up in High Voltage Side

A transformer tap changer is usually installed on the high-voltage (HV) winding for several practical reasons:

Why is the tap changer on the HV side?

1. Lower current

For the same transformer power:

🔹 I = S ÷ √3V

Lower current means the tap changer contacts can be smaller and easier to design.

2. Smaller switching contacts

Changing taps involves switching between winding sections.

On the HV side, the lower current reduces arcing and contact wear, especially important for an OLTC.

3. Finer voltage adjustment

The HV winding generally has more turns.

A small change in the number of HV turns produces a relatively small percentage change in voltage, allowing accurate regulation.

4. Better control of LV output voltage

The tap changer changes the effective turns ratio:

🔹 VH ÷ VL= NH ÷ NL

5. Economic and practical design

Because the HV current is lower, the tap-changing mechanism generally requires less current-handling capability than it would on the LV side.

Simple example

For a 10 MVA, 33/11 kV transformer:

HV current ≈ 175 A

LV current ≈ 525 A

So placing the tap changer on the 33 kV side means it switches approximately 175 A rather than 525 A.

Important: Not every transformer has the tap changer on the HV side. The actual location depends on transformer design, voltage level, regulation requirements, and whether it uses OLTC or off-circuit tap changing.

In short:

> HV side = higher voltage + lower current + more winding turns → easier, more economical, and finer tap regulation.

SFRA Test Performed on a Transformer?

SFRA (Sweep Frequency Response Analysis) is a diagnostic test used to detect mechanical and electrical problems inside a power transformer without opening or dismantling it.

1️⃣ What is the purpose of the SFRA test?

🔹 The main purpose is to check the mechanical integrity of transformer windings and core by measuring their frequency response.

🔹 It helps identify internal changes that may not be detected by ordinary electrical tests.

2️⃣ What faults can SFRA detect?

🔹 Winding deformation: Axial displacement, radial deformation, winding buckling or movement.

🔹Winding movement or shorted turns: Changes in winding geometry and electrical characteristics caused by faults.

🔹Core and clamping problems: Core displacement, changes in core connections or clamping conditions.

🔹Mechanical damage after faults:
Detects possible internal structural changes following severe short circuits or transportation.

💡 Note: SFRA indicates changes in the transformer's frequency response; it does not independently confirm every specific fault.

3️⃣ How does the SFRA test work?

1) A low-voltage signal with a range of frequencies is injected into a transformer winding.

2) The instrument measures the response signal over the selected frequency range.

3) The analyzer plots the response as a frequency-response curve.

4) The curve is compared with a previous fingerprint or a suitable reference measurement.

A significant deviation in the response may indicate a change in the transformer’s internal condition.

4️⃣ SFRA test principle

🔹The transformer behaves like a complex network of resistance, inductance and capacitance.

🔹The measured response depends on its internal electrical and mechanical arrangement.

▪️H(f) = V response (f) ÷ Vinput (f)

Where:

🔹H(f) = frequency response

🔹V input = injected signal

🔹V response = measured output signal

🔹f = frequency

Changes in winding geometry or core structure can change the response curve.

5️⃣When should SFRA be performed?

Situation | Purpose

🔹Factory testing | Establish a baseline fingerprint

🔹Before and after transportation | Check for mechanical damage

🔹After external short-circuit faults | Check winding movement

🔹During commissioning | Verify condition before energization

🔹Periodic maintenance | Compare against previous results

📌 Important: SFRA is a comparative diagnostic test. Reliable interpretation requires consistent test connections, tap position, grounding and measurement conditions.

🔹 It is commonly used alongside winding resistance, turns ratio (TTR), insulation resistance and other transformer diagnostic tests.

Why Is the Starting Current High in a DC Motor?

A DC motor draws a very high current at starting because the back EMF is zero when the motor is stationary, and the armature has very low resistance.

Let's understand the working principle step by step.

1️⃣ What happens when a DC motor starts?

When a DC supply is applied to a stationary DC motor:

* The armature starts rotating from zero speed.

* At zero speed, the motor generates no back EMF.

* The armature winding resistance is very low.

* Therefore, a large current flows through the armature.

This high current can damage the armature winding, commutator, brushes, and connected supply system if not controlled.

2️⃣ Starting current formula

The armature current of a DC motor is:

▪️ Ia = V - Eb ÷ Ra

Where:

Symbol | Meaning
Ia | Armature current (A)
V | Applied DC voltage (V)
Eb | Back EMF (V)
Ra | Armature resistance (Ω) |

At starting:

N=0 ⇨ Eb=0

Therefore:

I start = V ÷ Ra

Because is very small, the starting current becomes very large.

3️⃣ Practical example

Example: DC shunt motor

Supply voltage = 220 V

Armature resistance = 0.5 Ω

Starting current:

I start = 220 - 0 ÷ 0.5

Starting current = 440 A

If the motor's rated armature current is 40 A, the starting current in this simplified example is 11 times the rated current.

4️⃣ How does the current decrease after starting?

As the motor accelerates, its rotational speed increases, producing back EMF.

Eb = P Phi ZN ÷ 60A

Back EMF is proportional to motor speed and flux.

As speed increases:

🔹 Back EMF increases.

🔹 Effective voltage across the armature resistance decreases.

🔹 Armature current decreases toward its normal operating value.

The motor eventually reaches a steady operating speed where developed torque balances the load torque and losses.

5️⃣ How is high starting current controlled?

1️⃣ DC motor starter

A series starting resistance limits the initial current. The resistance is gradually removed as the motor accelerates.

2️⃣ Electronic DC drive

Controls armature voltage or current electronically to provide a controlled acceleration and limit starting current.

Key takeaway: A DC motor has zero back EMF at standstill. Since the armature resistance is very low, applying full rated voltage directly can cause excessive starting current. A suitable starter or current-limited drive is used to protect the motor.