Power systems can be divided into primary systems and secondary systems, much like relay control circuits can be divided into main circuits and control circuits. In power systems, the key equipment that connects primary and secondary systems is the instrument transformer, which includes current transformers (CT) and voltage transformers (PT).
Instrument transformers are essentially special types of transformers. They are used to extend the range of instruments, relays, and other secondary devices, while also providing insulation from the primary circuit, ensuring the safety of both equipment and personnel. Specifically, current transformers (CT) convert high currents into low currents, and voltage transformers (PT) step down high voltages to lower voltages.
Since instrument transformers are a type of transformer, they operate on the same principles as transformers. To briefly explain how transformers work: they rely on electromagnetic induction to transfer electrical energy between circuits, changing voltage and current levels while maintaining the frequency of the system.
As shown in the figure above, the primary and secondary windings of the transformer are wound around the same iron core, meaning they are exposed to the same magnetic field. This magnetic field is generated by the primary current (assuming the transformer's secondary side is open-circuited). Since the current is alternating, the magnetic field is also alternating. In other words, both the primary and secondary windings are exposed to this alternating magnetic field, and as a result, they both generate induced voltages.
The number of turns for the primary and secondary windings are
and
, respectively. Assuming each turn of the winding produces an induced voltage
, the total induced voltage in the primary winding is
, and similarly, the total induced voltage in the secondary winding is
. Therefore, the ratio of the primary to secondary voltages equals the ratio of their turns, i.e.,
.
Additionally, when the secondary side of a transformer is loaded, a secondary current flows, which obviously generates a magnetic field. However, this magnetic field does not alter the original magnetic field within the core.
This is because as the secondary current increases, the primary current also increases. The magnetic field produced by the increased primary current exactly cancels out the magnetic field produced by the secondary current, thus maintaining the original magnetic field in the core unchanged. From this, we can see that the primary current changes in response to changes in the secondary current. Ignoring magnetizing current, we can derive the current ratio of the primary to secondary windings as
.
Having understood this, let's now explain why it's strictly forbidden to open-circuit the secondary of a current transformer (CT) or short-circuit the secondary of a voltage transformer (VT).
The function of a current transformer (CT) is to convert a high primary current into a lower secondary current, where the primary current can be tens or even hundreds of times the secondary current. During wiring, the primary winding is essentially the load circuit, passing directly through the transformer, which means the number of turns for the primary winding is
. Based on the relationship between current ratio and turns ratio, we can deduce that the number of turns in the secondary winding is tens or even hundreds of times that of the primary winding.
When a current transformer operates normally, its primary current is the load current, which is the operating current of the electrical equipment and cannot be altered manually. The secondary current changes in response to the primary current. In other words, differing from the previously discussed transformers, the primary current in a current transformer produces a strong original magnetic field (because the primary current is very large), while the magnetic field produced by the secondary current is used to counteract part of the magnetic field created by the primary current, resulting in a very small residual magnetic field within the core.
At this point, the secondary circuit is essentially in a short-circuit state, so even though the secondary winding has many turns, the secondary voltage remains very low.
If the secondary circuit of the current transformer is disconnected (open-circuited), the secondary current becomes zero, failing to produce a magnetic field. Clearly, the strong magnetic field generated by the primary current would then flow entirely through the core, causing core saturation, a dramatic increase in core loss, and subsequent overheating of the transformer. Moreover, since the number of turns in the secondary winding is many times greater than that in the primary winding, the secondary would induce a voltage much higher than normal, posing significant risks to both equipment and personnel.
On the other hand, voltage transformers are simpler; their operation is similar to that of a step-down transformer, converting high voltage to low voltage for measurement purposes, so the number of turns in the secondary winding is less than in the primary winding. The nominal secondary voltage for a measurement voltage transformer is typically 100V.
When a voltage transformer is operating normally, the primary winding is connected in parallel with the voltage to be measured, and the secondary winding is connected in parallel with the voltage windings of instruments, relays, etc., forming a circuit. These voltage windings have very high internal resistance. According to Ohm's Law, with a fixed voltage, the secondary current is very small, so the voltage transformer operates close to no-load conditions, as shown in the diagram above.
If a short circuit occurs in the secondary of the transformer, it means that the secondary resistance is very small, close to zero. Since the primary voltage remains constant, the voltage across the secondary winding also remains essentially unchanged. According to Ohm's Law, the secondary current will then surge dramatically, causing the windings to heat up, potentially damaging insulation, leading to high voltage entering the low-voltage circuit, thus endangering both personal safety and equipment.
