Can the secondary sides of current transformers be connected in parallel?

The following explains in detail why parallel connection is prohibited and the correct approach.

Why parallel connection is absolutely prohibited?

A current transformer is essentially a current source. Its original design is that during normal operation, its secondary side is close to a short-circuit state. Connecting it in parallel disrupts this operating state, leading to the following serious consequences.

Generates extremely high circulating current, burning out the transformer

Even if two CTs have the same turns ratio, there will inevitably be slight differences in their characteristics (such as excitation characteristics, residual magnetism, and internal resistance).

When their secondary sides are connected in parallel, these slight differences form a closed loop between the secondary circuits of the two CTs, generating a large circulating current.

This circulating current can far exceed the rated current of the CT secondary side (usually 5A or 1A), causing severe overheating, insulation damage, and ultimately burning out the CT. 2. Complete Loss of Measurement and Protection Functions

When CTs are connected in parallel, the current flowing through the load is no longer the vector sum of the two secondary currents. Instead, it is "swamped" and distorted by the internal circulating current.

Ammeters and protective relays connected in the loop will not be able to obtain accurate current information, resulting in completely inaccurate measurement readings. Protective devices (such as overcurrent relays) may malfunction or fail to operate, causing widespread power outages or equipment damage.

Core Saturation and High Voltage Generation

Circulating currents can cause severe saturation of the CT core. When the core is saturated, the excitation current increases dramatically, potentially inducing extremely high voltages on the secondary side, threatening personal safety and equipment insulation.

To put it simply, this is like connecting two batteries (especially those with slightly different voltages) directly, positive to positive and negative to negative. This will generate a huge short-circuit current within the batteries, rapidly damaging them. The principle behind paralleling CTs is similar.

Correct Application

Since parallel connection is not possible, what should be done when increasing current capacity or meeting special wiring requirements is needed?

1. When increasing capacity is required (for example, driving multiple devices)

2. Correct Approach: Series Connection

Connect the current coils of multiple devices (such as ammeters and relays) in series within the secondary circuit of a single CT.

Advantages: All devices receive the same secondary current, ensuring accurate measurements and preventing circulating currents.

Wiring Diagram: CT secondary output → Current coil of device A → Current coil of device B → ... → Return to CT secondary

3. When current addition is required (for example, sum current protection), Correct Approach: Perform software synthesis within the protection device. In modern digital protection devices, the secondary current signals from different CTs are typically connected to different input channels. The protection device's internal program then performs vector addition to calculate the sum current. Traditional Practice: Use Dedicated Auxiliary Current Transformers

In special applications such as relay protection, if two currents need to be summed, a specially designed auxiliary current transformer with a specialized magnetic circuit and winding design should be used to combine the currents, rather than simply connecting the secondary sides of two main CTs in parallel.

Golden Rule: The secondary side of the current transformer must always be reliably grounded during operation (for safety reasons) and must never be open-circuited. Furthermore, paralleling the secondary sides of different CTs is strictly prohibited.

LVZW-35 current transformer

LVZW-35 current transformer produced by Shaanxi West Power Tongzhong Electrical Co., Ltd. is a sensor used to measure large current in 35kV system. It is mainly used in substations, power systems and electrical equipment. The transformer adopts magnetic core type, which is characterized by high linearity, strong anti-interference ability, small size and simple structure. It is a current transformer with very high cost performance.

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