Understanding Transformer Types and Winding Requirements

This question asks about specific requirements for the secondary winding in different types of transformers. Transformers are essential electrical devices used to change voltage levels in AC circuits. They work based on the principle of electromagnetic induction, typically having a primary winding and a secondary winding around a core.

Which Transformer Needs a Closed Secondary Winding?

Let's look at the main types of transformers mentioned in the options and understand their functions and typical operating conditions:

• Distribution Transformer: These are used to step down voltage from transmission lines to levels suitable for residential and commercial use (e.g., from a few kV to 240V/120V). The secondary winding supplies power to loads, so it is connected to the distribution network and is effectively 'closed' through the connected loads. However, it's not a requirement that it must be closed at all times; it can be open (no load connected).
• Power Transformer: These are typically used in transmission networks to step up or step down voltage over long distances (e.g., from generator voltage to transmission voltage or between different transmission voltage levels). Similar to distribution transformers, their secondary windings supply power to the network and are closed through the interconnected system. But, like distribution transformers, the requirement is not that it must always be closed; it operates based on system load.
• Current Transformer (CT): This is a type of 'instrument transformer' used to measure AC current. A CT steps down a large primary current to a smaller, measurable secondary current, typically 1A or 5A. The primary winding is connected in series with the circuit whose current is to be measured. The secondary winding is connected to a low-impedance device like an ammeter or a relay coil.
• Potential Transformer (PT) / Voltage Transformer (VT): This is another type of instrument transformer used to measure AC voltage. A PT steps down a large primary voltage to a smaller, measurable secondary voltage, typically 100V or 110V. The primary winding is connected in parallel with the circuit whose voltage is to be measured. The secondary winding is connected to a high-impedance device like a voltmeter or relay voltage coil. Its secondary is usually connected to an instrument and is thus closed under normal operation, but it doesn't face the same severe hazard as a CT if left open.

Why a Current Transformer's Secondary Must Be Closed

The critical difference lies in how a Current Transformer (CT) behaves when its secondary winding is open-circuited. A CT operates with its primary current determined by the external circuit load, not by the secondary load. When the secondary winding is open, the impedance in the secondary circuit becomes extremely high. According to Faraday's law, the changing magnetic flux in the core induces a voltage in the secondary winding. In a CT with the secondary closed, the secondary current flows, creating a demagnetizing flux that opposes the flux created by the primary current. This limits the total flux in the core.

However, if the secondary is open, there is no secondary current to oppose the primary flux. The core flux increases dramatically to a very high level. This leads to:

• Very High Voltage: The induced voltage across the open secondary terminals becomes dangerously high ($V = -N_s \frac{d\Phi}{dt}$), potentially reaching thousands of volts. This poses a severe safety hazard to personnel and can damage connected instruments or the CT's insulation.
• Core Saturation and Damage: The excessive flux can drive the core into deep saturation, leading to inaccurate measurements if it is later used. Sustained high flux and resulting heat can also permanently damage the core or insulation.

Therefore, it is absolutely crucial that the secondary winding of a current transformer is always kept closed, either through a connected instrument or a shorting link, whenever current is flowing in the primary circuit.

Let's summarize the behavior of the secondary windings for the given transformer types:

Based on this analysis, the Current Transformer is the type where the secondary winding must always be kept closed to prevent dangerous overvoltage and damage.

Revision Table: Key Transformer Facts

Additional Information on Current Transformers and Safety

Safety is paramount when working with current transformers. If a CT's secondary circuit needs to be disconnected from its instrument (e.g., for maintenance or replacement) while the primary circuit is energized, the secondary terminals must first be short-circuited using a shorting link or switch. This ensures that the demagnetizing flux is maintained, keeping the core flux and secondary voltage at safe levels. Only after shorting can the instrument be safely disconnected. After reconnecting the instrument, the shorting link is then removed.

The relationship between primary and secondary currents in an ideal CT is given by the turns ratio:

\( \frac{I_p}{I_s} = \frac{N_s}{N_p} \)

Where \( I_p \) is the primary current, \( I_s \) is the secondary current, \( N_p \) is the number of turns in the primary winding, and \( N_s \) is the number of turns in the secondary winding.

This relationship holds true when the secondary is loaded (closed circuit). In an open-circuit condition, this simple relationship breaks down due to the high core flux and saturation.