Instrument Transformers--Industrial Electrical Power Systems

Purpose

The voltage transformers and current transformers continuously measure the voltage and current of an electrical system and are responsible to give feedback signals to the relays to enable them to detect abnormal conditions. The values of actual currents in modern distribution systems varies from a few amperes in households, small industrial/commercial houses, etc. to thousands of amperes in power-intensive plants, national grids, etc., which also depend on the operating voltages. Similarly, the voltages in electrical systems vary from few hundreds of volts to many kilo volts. However, it’s impossible to have monitoring relays designed and manufactured for each and every distribution system and to match the innumerable voltages and currents being present. Hence the voltage transformers and current transformers are used which enable same types of relays to be used in all types of distribution systems ensuring the selection and cost of relays to be within manageable ranges.

The main tasks of instrument transformers are:

• To transform currents or voltages from usually a high value to a value easy to handle for relays and instruments.

• To insulate the relays, metering and instruments from the primary high-voltage system.

• To provide possibilities of standardizing the relays and instruments, etc. to a few rated currents and voltages.

Instrument transformers are special versions of transformers in respect of measurement of current and voltages. The theories for instrument transformers are the same as those for transformers in general.

Basic theory of operation

The transformer is one of the high efficient devices in electrical distribution systems, which are used to convert the generated voltages to convenient voltages for the purpose of transmission and consumption. A transformer comprises of two windings viz., primary and secondary coupled through a common magnetic core.

When the primary winding is connected to a source and the secondary circuit is left open, the transformer acts as an inductor with minimum current being drawn from the source. At the same time, a voltage will be produced in the secondary open-circuit winding due to the magnetic coupling. When a load is connected across the secondary terminals, the current will start flowing in the secondary, which will be decided by the load impedance and the open-circuit secondary voltage. A proportionate current is drawn in the primary winding depending upon the turns ratio between primary and secondary. This principle of transformer operation is used in transfer of voltage and current in a circuit to the required values for the purpose of standardization.

A voltage transformer is an open-circuited transformer whose primary winding is connected across the main electrical system voltage being monitored. A convenient proportionate voltage is generated in the secondary for monitoring. The most common voltage produced by voltage transformers is 100-120 V (as per local country standards) for primary voltages from 380 V to 800 kV or more.

However, the current transformer is having its primary winding directly connected in series with the main circuit carrying the full operating current of the system. An equivalent current is produced in its secondary, which is made to flow through the relay coil to get the equivalent measure of the main system current. The standard currents are invariably 1A and 5A universally.

Voltage transformers

There are basically, two types of voltage transformers used for protection equipment.

1. Electromagnetic type (commonly referred to as a VT)

2. Capacitor type (referred to as a CVT).

The electromagnetic type is a step down transformer whose primary (HV) and secondary ( LV) windings are connected as below. HV--------- LV

++++ Electromagnetic type--The number of turns in a winding is directly proportional to the open-circuit voltage being measured or produced across it. The above diagram is a single-phase VT. In the three-phase system it’s necessary to use three VTs at one per phase and they being connected in star or delta depending on the method of connection of the main power source being monitored. This type of electromagnetic transformers are used in voltage circuits up to 110/132 kV. For still higher voltages, it’s common to adopt the second type namely the capacitor voltage transformer (CVT). ++++ gives the basic connection adopted in this type. Here the primary portion consists of capacitors connected in series to split the primary voltage to convenient values.

The magnetic voltage transformer is similar to a power transformer and differs only so far as a different emphasis is placed on cooling, insulating and mechanical aspects. The primary winding has larger number of turns and is connected across the line voltage; either phase-to-phase or phase-to-neutral. The secondary has lesser turns however, the volts per turn on both primary and secondary remains same.

The capacitor VT is more commonly used on extra high-voltage (EHV) networks. The capacitors also allow the injection of a high-frequency signals onto the power line conductors to provide end-to-end communications between substations for distance relays, telemetry/supervisory and voice communications. Hence, in EHV national grid networks of utilities, the CVTs are commonly used for both protection and communication purposes.

++++ Capacitor-type VT

It should be remembered that these voltage transformers are also used for measuring purposes. It’s possible to have one common primary winding and two or more secondary windings in one unit. The voltage transformers having this kind of arrangement are referred to as two core or three core VT depending on the number of secondary windings.

Vector diagram

The vector diagram for a single-phase voltage transformer is as follows. The primary parameters are suffixed with p while the secondary parameters have suffix s. It’s to be noted that the vector diagram for a three-phase connection will be identical, except for the phase shift introduced in each phase in relation to the other phases. The capacity of a voltage transformer is normally represented in VA rating, which indicates the maximum load that can be connected across its secondary. The other common name for this VA rating is 'burden'. Output burdens of 500 VA per phase are common.

Accuracy of voltage transformers

The voltage transformers shall be capable to produce secondary voltages, which are proportionate to the primary voltages over the full range of input voltage expected in a system. Voltage transformers for protection are required to maintain reasonably good accuracy over a large range of voltage from 0 to 173% of normal.

However, the close accuracy is more relevant for metering purposes, while for protection purposes the margin of accuracy can be comparatively less. Permissible errors vary depending on the burden and purpose of use and typical values as per IEC are as follows.

++++ Vector diagram of a voltage transformer

== +--+ Accuracy class, voltage transformers ===

Range Limits of Errors

Burden Voltage Ratio Phase Displacement Standard Class % % % Min Application

IEC 186 0.1 25-100 80-120 0.1 5 Laboratory

Precision metering, revenue metering

0.5 25-100 80-120 0.5 20 Standard revenue metering

1.0 25-100 80-120 1.0 40 Industrial grade meters 3.0

Instruments Protection--Protection

*Vf = Voltage factor

=====

The accuracy is not a major cost-deciding factor for a voltage transformer due to the high efficiency of the transformers, which normally ensures that there is no major voltage drop in the secondary leads. Thus, it’s common to select voltage transformers based on the loads (choosing appropriate rated burden). The question of accuracy of VT's used in protection circuits can be ignored and is generally neglected in practice.

Connection of voltage transformers

Electromagnetic voltage transformers may be connected inter-phase or between phase and ground. However, capacitor voltage transformers can only be connected phase-to- ground.

Voltage transformers are commonly used in three-phase groups, generally in star-star configuration. Typical connection: With this arrangement, the secondary voltages provide a complete replica of the primary voltages as shown below and any voltage (phase-to-phase or phase-to- ground) may be selected for monitoring at the secondary.

Primary, Secondary

++++ Voltage transformers connected in star-star configuration

++++ Vector diagram for VTs in star-star configuration

Connection to obtain the residual voltage

It’s common to detect ground faults in a three-phase system using the displacement that occurs in the neutral voltage when ground faults take place. The residual voltage (neutral displacement voltage, polarizing voltage) for ground fault relays can be obtained from a VT between neutral and ground, for instance at a power transformer neutral. It can also be obtained from a three-phase set of VTs, which have their primary winding connected phase to ground and one of the secondary windings connected in a broken delta. ++++ the measuring principle for the broken delta connecting during an ground fault in a high-impedance grounded (or un grounded) and an effectively grounded power system respectively.

++++ Connection to source residual voltage --- Primary -- Secondary -- Residual voltage

From the figure below it can be seen that a solid close-up ground fault produces an output voltage of Ursd = 3 × Usn in a high-impedance grounded system and Ursd = U2n in an effectively grounded system. Therefore a VT secondary normal voltage of:

… which is often used in high-impedance grounded systems and U2n = 110 V in effectively grounded systems. A residual voltage of 110 V is obtained in both the cases. VTs with two secondary windings, one for connection in Y and the other in broken delta can then have the ratio:

...for high-impedance and effective grounded systems respectively. Other nominal voltages than 110 V e.g. 100 V or 120 V are also used depending on national standards and practice.

Ferro-resonance in magnetic voltage transformer

When the ferro-resonance in a CVT is an internal oscillation between the capacitor and the magnetic IVT, the ferro-resonance in a magnetic voltage transformer is an oscillation between the magnetic voltage transformer and the network. The oscillation can only occur in a network having an insulated neutral. An oscillation can occur between the network's capacitance to ground and the non-linear inductance in the magnetic voltage transformer. The oscillation can be triggered by a sudden change in the network voltage.

High-impedance grounded power system--Solid ground fault--Fault free -- Residual voltage (neutral displacement voltage) from an open delta circuit

It’s difficult to give a general figure of a possible risk of ferro-resonance. It depends on the design of the transformer. We can roughly calculate that there will be a risk of resonance when the zero sequence capacitance expressed in S km trans mission line …

U_n = System voltage in kV. The corresponding value for cable is:

Damping of ferro-resonance

The magnetic voltage transformer will be protected from ferro-resonance oscillation by connecting a resistor across the open delta point in the three-phase secondary winding. A typical value is 50-60 ?, 200 W.

++++ Damping of ferro-resonance

Protection of voltage transformers

It’s possible to protect a voltage transformer from secondary short-circuit by incorporating fuses in the secondary circuits. Short-circuit on the secondary winding gives only a few amperes in the primary winding and is not sufficient to rupture a high-voltage fuse. Hence high-voltage fuses on the primary side don’t protect the transformers, they protect only the network in case of any short-circuit on the primary side.

Voltage drop in voltage transformers

The voltage drop in the secondary circuit is of importance. The voltage drop in the secondary fuses and long connection wires can change the accuracy of the measurement. It’s especially important for revenue metering windings of high accuracy (class 0.2, 0.3). The total voltage drops in this circuit must not be more than 0.1%.

Typical values of resistance in fuses: 6 A 0.048 10 A 0.024 16 A 0.0076 25 A 0.0042 A 6-10 A is a typical value for safe rupture of the fuses.

The voltage drop in the leads from the VT to the associated equipment must be considered as this, in practice, can be alarming mainly in case of measuring circuits. This is the one that separates the metering circuits (with low burden) from protective circuits (with higher burdens).

++++ The accuracy of a voltage transformer is guaranteed at the secondary terminals -- 100m 2.5mm^2 =0.72? 100m 4mm^2=0.72? 170VA relay burden; 30VA metering burden

Secondary grounding of voltage transformers

To prevent secondary circuits from reaching dangerous potential, the circuits should be grounded. Grounding should be made at only one point of a VT secondary circuit or galvanically interconnected circuits. A VT with the primary connected phase-to- ground shall have the secondary grounded at terminal n. A VT with the primary winding connected across two-phases, shall have that secondary terminal grounded which has a voltage lagging the other terminal by 120°. Windings not under use shall also be grounded.

++++ VTs connected between phases

++++ shows the methods of connection in a three-phase system with primary connected in star and secondary connected in two different ways viz., star and broken delta. Alternatively, it’s often a common practice to ground the white phase as shown. This practice stems from metering where the two wattmeter method requires two CTs and two line voltages. With this arrangement the red and blue phases now at line potential to the white and it saves the expense and bother of running a neutral conductor throughout the panels.

++++ A set of VTs with one Y-connected and one broken delta secondary circuit

++++ VT secondary grounded on white phase