Measuring current transformers in circuits for relay protection and automation

The power equipment of electrical substations is organizationally divided into two types of devices:

1. power circuits through which all the power of the transported energy is transmitted;

2. secondary devices that allow you to control the processes taking place in the primary loop and control them.

Power equipment is located in open areas or in closed switchgear, and secondary equipment is located on relay panels, in special cabinets or separate cells.

The intermediate connection, which performs the function of transmitting information between the power unit and the measuring, management, protection and control bodies, are measuring transformers. Like all such devices, they have two sides with different voltage values:

1. high voltage, which corresponds to the parameters of the first loop;

2.low voltage, allowing to reduce the risk of impact of energy equipment on service personnel and the cost of materials for the creation of control and monitoring devices.

The adjective "measurement" reflects the purpose of these electrical devices, as they very accurately simulate all the processes taking place on the power equipment and are divided into transformers:

1. current (CT);

2. voltage (VT).

They work according to the general physical principles of transformation, but have different designs and methods of inclusion in the primary circuit.

How current transformers are made and work

Principles of operation and devices

In design measuring current transformer the conversion of the vector values ​​of currents of large values ​​flowing in the primary circuit into proportionally reduced in magnitude, and in the same way the directions of the vectors in the secondary circuits are determined.

Magnetic circuit device

Structurally, current transformers, like any other transformer, consist of two insulated windings located around a common magnetic circuit. It is made with laminated metal plates that are melted using special types of electrical steels. This is done to reduce the magnetic resistance along the path of magnetic fluxes circulating in a closed loop around the coils and to reduce losses through eddy currents.

A current transformer for relay protection and automation schemes can have not one magnetic core, but two, differing in the number of plates and the total volume of iron used. This is done to create two types of coils that can work reliably when:

1. Nominal working conditions;

2.or at significant overloads caused by short-circuit currents.

The first design is used to make measurements, and the second is used to connect protections that turn off emerging abnormal modes.

Arrangement of coils and connecting terminals

The windings of current transformers, designed and manufactured for permanent operation in the circuit of the electrical installation, meet the requirements for the safe passage of current and its thermal effect. Therefore, they are made of copper, steel or aluminum with a cross-sectional area that excludes increased heating.

Since the primary current is always greater than the secondary, the winding for it stands out significantly in size, as shown in the photo below for the right transformer.

The left and middle structures have no power at all. Instead, an opening is provided in the housing through which a power supply wire or fixed bus passes. Such models are used, as a rule, in electrical installations up to 1000 volts.

On the terminals of the transformer windings there is always a fixed fixture for connecting busbars and connecting wires using bolts and screw clamps. This is one of the critical places where the electrical contact can be broken, which can cause damage or disrupt the accurate operation of the measurement system. The quality of its clamping in the primary and secondary circuits is always paid attention to during operational checks.

Current transformer terminals are marked at the factory during manufacture and are marked:

• L1 and L2 for the input and output of the primary current;

• I1 and I2 — secondary.

These indices mean the winding direction of the turns relative to each other and affect the correct connection of the power and simulated circuits, the characteristic of the distribution of current vectors along the circuit. They are paid attention to during the initial installation of transformers or replacement of defective devices, and are even examined by various methods of electrical checks both before the assembly of the devices and after installation.

The number of turns in the primary circuit W1 and secondary W2 is not the same, but very different. High voltage current transformers usually have only one straight bus across the magnetic circuit which acts as the supply winding. The secondary winding has a larger number of turns, which affects the transformation ratio. For ease of use, it is written as a fractional expression of the nominal values ​​of the currents in the two windings.

For example, the entry 600/5 on the nameplate of the box means that the transformer is intended to be connected to high-voltage equipment with a rated current of 600 amperes, and only 5 will be transformed in the secondary circuit.

Each measuring current transformer is connected to its own phase of the primary network. The number of secondary windings for relay protection and automation devices is usually increased for separate use in current circuit cores for:

• Measuring tools;

• general protection;

• tire and tire protection.

This method eliminates the influence of less critical circuits on more significant ones, simplifies their maintenance and testing on working equipment at operating voltage.

For the purpose of marking the terminals of such secondary windings, the designation 1I1, 1I2, 1I3 is used for the beginning and 2I1, 2I2, 2I3 for the ends.

Isolation device

Each current transformer model is designed to operate with a certain amount of high voltage on the primary winding. The insulation layer located between the windings and the housing must withstand the potential of the power network of its class for a long time.

On the outside of the insulation of high-voltage current transformers, depending on the purpose, the following can be used:

• porcelain tablecloth;

• compacted epoxy resins;

• some types of plastics.

The same materials can be supplemented with transformer paper or oil to insulate the internal wire crossings on the windings and eliminate turn-to-turn faults.

Accuracy class TT

Ideally, a transformer should theoretically operate accurately without introducing errors. In real structures, however, energy is lost to internally heat the wires, overcome magnetic resistance, and form eddy currents.

Because of this, at least a little, but the transformation process is disturbed, which affects the accuracy of reproduction in the scale of the primary current vectors from their secondary values ​​with deviations in the orientation in space. All current transformers have a certain measurement error, which is normalized as a percentage of the ratio of the absolute error to the nominal value in amplitude and angle.

Accuracy class current transformers are expressed by the numerical values ​​«0.2», «0.5», «1», «3», «5», «10».

Class 0.2 transformers work for critical laboratory measurements.Class 0.5 is intended for accurate measurement of currents used by level 1 meters for commercial purposes.

Current measurements for the operation of the relays and control accounts of the 2nd level are carried out in class 1. The actuation coils of the drives are connected to the current transformers of the 10th accuracy class. They work exactly in the short-circuit mode of the primary network.

TT switching circuits

In the power industry, three- or four-wire power lines are mainly used. In order to control the currents passing through them, various schemes are used to connect measuring transformers.

1. Electrical equipment

The photo shows a variant of measuring the currents of a three-wire power circuit of 10 kilovolts using two current transformers.

Here it can be seen that the A and C primary phase connection busbars are bolted to the terminals of the current transformers and the secondary circuits are hidden behind a fence and led from a separate cable harness into a protective tube which is routed to the relay compartment for connection of circuits to the terminal blocks.

The same installation principle applies in other schemes. high voltage equipmentas shown in the picture for 110 kV network.

Here the enclosures of the instrument transformers are mounted at height using a grounded reinforced concrete platform, which is required by safety regulations. The connection of the primary windings to the supply wires is done in a cut, and all the secondary circuits are brought out in a nearby box with a terminal junction.

The cable connections of the secondary current circuits are protected from accidental external mechanical impact by metal covers and concrete plates.

2.Secondary windings

As noted above, the output conductors of current transformers are brought together for operation with measuring devices or protective devices. This affects the assembly of the circuit.

If it is necessary to control the load current in each phase using ammeters, then the classic connection option is used - a full star circuit.

In this case, each device shows the current value of its phase, taking into account the angle between them. The use of automatic recorders in this mode most conveniently allows you to display the shape of sinusoids and build vector diagrams of load distribution based on them.

Often, on outgoing feeders 6 ÷ 10 kV, in order to save, not three, but two measuring current transformers are installed, without using one phase B. This case is shown in the photo above. Allows you to plug ammeters into an incomplete star circuit.

Due to the redistribution of the currents of the additional device, it turns out that the vector sum of phases A and C is displayed, which is oppositely directed to the vector of phase B in the symmetrical load mode of the network.

The case of switching on two measuring current transformers for monitoring the line current with a relay is shown in the photo below.

The scheme allows full control of balanced load and three-phase short circuits. When a two-phase short circuit occurs, especially AB or BC, the sensitivity of such a filter is greatly underestimated.

A common scheme for monitoring zero-sequence currents is created by connecting measuring current transformers in a full star circuit and the winding of a control relay to a combined neutral wire.

The current flowing through the coil is created by adding the three phase vectors. In symmetrical mode, it is balanced, and during the occurrence of single-phase or two-phase short circuits, the unbalance component is released in the relay.

Performance characteristics of measuring current transformers and their secondary circuits

Operational switching

During the operation of the current transformer, a balance of magnetic fluxes is created, formed by currents in the primary and secondary windings. As a result, they are balanced in magnitude, directed oppositely and compensate for the influence of the generated EMF in closed circuits.

If the primary winding is open, current will stop flowing through it and all secondary circuits will simply be disconnected. But the secondary circuit cannot be opened when the current passes through the primary, otherwise, under the action of the magnetic flux in the secondary winding, an electromotive force is generated, which is not spent on the current flow in a closed loop with low resistance, but is used in Standby mode.

This leads to the appearance of a high potential of the open contacts, which reaches several kilovolts and is able to break the insulation of the secondary circuits, disrupt the operation of the equipment and cause electrical injuries to the service personnel.

For this reason, all switching in the secondary circuits of current transformers is carried out according to a strictly defined technology and always under the supervision of supervisors, without interrupting the current circuits. To do this, use:

• special types of terminal blocks that allow you to install an additional short circuit for the duration of the interruption of the section taken out of service;

• testing current blocks with short jumpers;

• special key design.

Recorders for emergency processes

Measuring devices are divided according to the type of fixing parameters for:

• nominal working conditions;

• the occurrence of overcurrent in the system.

The sensitive elements of the recording devices directly proportionally perceive the incoming signal and also display it. If the current value is entered at their input with distortion, then this error will be introduced into the readings.

For this reason, devices designed to measure emergency currents, rather than nominal ones, are connected to the core of the protection of a current transformer, and not to measurements.

Read about the device and principles of operation of measuring voltage transformers here: Measuring voltage transformers in circuits for relay protection and automation