1. Basic Concept of Transformer Voltage Regulation

In normal operation, a transformer's primary power supply and secondary load change with system variations, causing fluctuations in the secondary-side voltage. To stabilize the secondary voltage and ensure power supply quality, voltage regulation devices are used.

The voltage ratio of a transformer is determined by the turns ratio of its windings. By adjusting the number of turns in the windings connected, the output voltage can be changed. To achieve this, transformers are designed with multiple taps on the windings, which are connected to voltage regulation devices. The switching of these taps is carried out using a tap changer, which alters the number of turns involved in the circuit, thereby adjusting the transformer's output voltage.

2. Types of Transformer Voltage Regulation Devices

Voltage regulation devices can be divided into two categories: No-load tap changers and On-load tap changers.

No-load Tap Changer: Comprises a no-load tap changer switch and manual operating mechanism. Tap switching is done manually when the transformer is not energized.

On-load Tap Changer (OLTC): Comprises an on-load tap changer switch and can be operated manually, or remotely via electrical control, without disconnecting the transformer from the load. This allows for voltage adjustments while the transformer is under load.

3. Basic Principle of On-load Tap Changer (OLTC)

An OLTC allows the transformer to switch taps while under load, which must meet two key conditions:

1.The current must not be interrupted during the tap switching process.

2.There must be no short circuits between taps during switching.

To achieve this, the OLTC temporarily bridges two taps while switching, ensuring current continuity. A transition impedance (resistance or reactance) is introduced to limit the circulating current between the two taps, preventing short circuits. This process enables a seamless transition between different taps.

The OLTC circuit comprises three parts:

Transition circuit

Tap selection circuit

Voltage regulation circuit

4. Structure of On-load Tap Changer

An OLTC consists of two main components:

• Tap Selector: Connects adjacent taps without carrying load current directly. It acts like a no-load tap changer.
• Switching Mechanism: Completes the actual load switching by transitioning between taps while avoiding short circuits or interruption.

Key components:

• Transition Resistors: Installed to ensure current continuity during tap switching.
• Moving Contacts: Mounted on insulated plates, responsible for making and breaking connections as taps are changed.

OLTCs come in various types:

• Linear Regulation: Simple tap connections.
• Coarse-Fine Regulation: Expands voltage regulation range by combining coarse and fine adjustment windings.
• Reversible Regulation: Can reverse polarity to increase or decrease voltage.

5. What is a Transformer Voltage Regulation Device?

Transformer voltage regulation devices enable adjustments to the transformer’s output voltage through tap changers. The two main types are:

1.No-load Tap Changer: Voltage adjustment is performed when the transformer is de-energized.

2.On-load Tap Changer (OLTC): Allows voltage regulation without interrupting the power supply, essential for transformers where voltage adjustments are frequent and precise regulation is required.

The difference lies in the operation:

• No-load tap changersinterrupt the circuit momentarily during tap switching, meaning they can’t be used under load.
• OLTCsoperate without breaking the load current by using transition resistors, enabling tap switching under load without causing damage.

Although OLTCs are more expensive and larger than no-load tap changers, they are crucial in systems where voltage precision and stability are critical.

Advantages of OLTC:

• Improves Voltage Compliance: Helps maintain voltage near rated values, reducing power loss.
• Enhances Reactive Power Compensation: Allows for adjustments that improve power factor and prevent overcompensation, reducing losses in capacitors and transformers.

6. OLTC Internal Operation

The OLTC's main components (switching mechanism and tap selector) work together to adjust voltage. The OLTC transitions between taps by passing the load current through transition resistors, avoiding arc formation during switching. The tap selector alternates between tap positions without breaking the circuit, ensuring a smooth transition.

7. Restrictions on OLTC Operation

OLTCs should not be operated under certain conditions:

1.Overload Operation: Exceeding rated current may damage the tap changer contacts.

2.Alarm Triggers: Malfunctions in the OLTC system (such as gas relay activation) must be addressed before operation.

3.Insufficient Insulation Oil: If the oil level is too low, the device may be unsafe to operate.

4.Exceeded Switching Frequency: OLTCs have limits on the number of operations.

5.System Abnormalities: Any detected malfunction in the OLTC must be resolved before further use.