CT,VT,CVT

Connections of a CT and a PT to supply, load and relay.

CT, VT, CVTThe 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

CTs and VTs are the eyes and ears of the protective system. They extract information from the power system and form an important link between the high-voltage high-current power system and the low-voltage low-current protective system. CTs and VTs have two important roles to play, namely:1 Electrically isolate the protective relays from the high-voltage power system for, the purpose of safety of the operating personnel.2 Step down the current and voltage to standard values of 1 A, 5 A, 110 V so that the design of relays can be standardized irrespective of the actual primary voltage, and current.

Current Tranformer• There are two types of current transformers:• 1. Wound primary type• 2. Bar primary type.

Bar primary Wound primary

• The wound primary is used for the smaller currents, but it can only be applied on low fault level installations due to thermal limitations as well as structural requirements due to high magnetic forces.

• For currents greater than 100 A, the bar primary type is used • Protection CTs are most frequently of the bar primary, toroidal core with

evenly distributed secondary winding type construction. In such CTs, the secondary winding can be conveniently put inside the high voltage bushings.

Current transformer• The basis of all transformers is that:Amp-turns on the Primary = Amp-turns on the

secondarye.g. 100 A × 1 turn = 1 A × 100 turns• The primary current contains two components:• An exciting current, which magnetizes the core

and supplies the eddy current and hysteresis losses, etc.

• A remaining primary current component, which is available for transformation to secondary current in the inverse ratio of turns.

• The exciting current is not being transformed and is therefore the cause of transformer errors.

• The amount of exciting current drawn by a CT depends upon the core material and the amount of flux that must be developed in the core to satisfy the output requirements of the CT. that is, to develop sufficient driving voltage required, pushing the secondary current through its connected load or burden. This can be explained vectorally as shown in figure

Vector diagram for a current transformer

The transition from the unsaturated to the saturated region of the open-circuit excitation characteristic makes a CT not to produce equivalent primary current beyond certain point. This transition is defined by ‘knee-point’ voltage in a CT, which decides its accurate working range.

Equivalent circuit of CT

Phasor diagram of CT

CT errors

Phase angle error

• The conventional CT core saturates during transient causing delay in high speed relay operation

• Secondary resistance• The secondary resistance of a CT is an important factor, as the CT

has to develop enough voltage to push the secondary current through its own internal resistance as well as the connected external burden. This should always be kept as low as possible.

• Burden :circuit connected to secondary winding. Expressed in volt-ampere at rated secondary current at rated power factor.

CT specification• A current transformer is normally specified in terms of:• A rated burden at rated current• An accuracy class• An upper limit beyond which accuracy is not guaranteed (known as

the accuracy limit factor, ALF), which is more vital in case of protection CTs.

• In terms of the specification a current transformer would, for example, be briefly referred to as 15 VA 5P20 if it were a protection CT or 15 VA Class 0.5 if it is a metering CT. The meanings of these figures are as below:

Open circuits of CTs• Current transformers generally work at a low

flux density. Core is then made of very good metal to give small magnetizing current.

• On open-circuit, secondary impedance now becomes infinite and the core saturates. This induces a very high voltage in the primary up to approximately system volts and the corresponding volts in the secondary will depend on the number of turns , multiplying up by the ratio (i.e. volts/turn no. of turns).

• Since CT normally has much more turns in secondary compared to the primary, the voltage generated on the open-circuited CT will be much more than the system volts,leading to flashovers.

• HENCE AS A SAFETY PRECAUTION, NEVER OPEN-CIRCUIT A CURRENT TRANSFORMER ON LOAD!!!

Voltage (potential)Transformer VT or PT• Voltage transformers are much like power

transformers operating on very light load.• There are basically, two types of voltage

transformers used for protection equipment.• 1. Conventional two-winding, electromagnetic

type (commonly referred to as a VT)• 2 Capacitive voltage divider 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

• 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 is 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 upto 110/132 kV.

Capacitive voltage transformer (CVT)

• Capacitive voltage transformer consists of a capacitive voltage divider as shown in Figure A.6(a).

• However, a simple capacitor divider cannot be used as a CVT because the output voltage will depend on the current drawn by the burden.

• Hence, a tuning coil is used so that it resonates with the equivalent capacitance seen looking into the capacitor potential divider.

• This makes the output voltage, a constant fraction of line voltage and independent of the current drawn.

• In EHV national grid networks of utilities, the CVTs are commonly used for both protection and communication purposes.

• Difficulty : Ferro resonance

CT Testing