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PROTECTIVE TRANSFORMERS

These are essential part of a protective system. The protective relays in AC power system are connected in the secondary circuit of a Current Transformer (CT0 or Potential Transformer (PT).

Figure represents a simple protection scheme using CT. It uses a circuit breaker, relay circuit (relay coil, trip coil) and a CT. 

Refer above figure, under normal working condition the normal load current flows through the line. The line itself acts as a primary windings for CT i.e. bar primary. And CT secondary is connected across relay coil. This coil also carries current and produces magnetic flux. This flux is not sufficient to close the relay contacts. So trip coil circuit opens and circuit breaker contacts remain closed. Refer Fig. When a fault occurs, say short circuit fault, current through the line (CT primary) increases to a large value. Hence, CT secondary current also increases same current flows through relay coil producing large flux, the relay contacts are now closed. The trip coil circuit is now complete, this causes opening of circuit breaker contacts.

Thus, faulty part is isolated from the bus bar. In most of the protective systems, CT is utilized for protection purpose.

The CT and PT can be classified into two groups.

1. Protective Transformers: Protective transformers are used for protection along with relays, trip coils, pilot wires etc.

2. Measuring transformers/instrument transformers: They are used for metering purpose i.e. for range extension for low range meters.

The students already have a know how about range extension using CT, PT in electrical measurements subject.

There is difference in design of protective transformers and instrument transformers. This is because the protective transformer has to carry fault current (heavy current) for some duration of time, on the other hand instrument transformer has to carry only current up to rated value only.

Requirements of CTs used for protection:

1.    It should have correct ratio even if the primary current is much greater than rated current of CT.

2.    The core should not saturate for high value of current.

3.    It should maintain the ratio error and phase angle error in tolerable limits. 

Cold rolled grain oriented silicon steel (3%) i.e. CRGO is used for core material of protective CT. This material has high permeability, high saturation level and small magnetizing current. For very high value of fault currents nickel-iron alloy material is used for core.

Some important terms related to protective CT are follows:

CT accuracy of 2% to 3% is essential for satisfactory working of distance protection scheme and differential protection scheme. For other protection schemes some higher percentage of error is permissible.

2. CT burden: It is defined as the load connected across CT secondary. It is expressed in VA (Volt Ampere). It is also expressed in terms of impedance at secondary rated current.

For protection CT, relay coil, meters, connecting leads are there on secondary side. So the total burden on CT will be that of relays + meters + connecting leads + secondary winding resistance.

Construction of Protection CT: Different forms, shapes of core are used.

(a) Ring type CT core: The core can have three types of shapes as shown in figure.The core is made up of stampings. The core material is nickel-iron alloy or CRG. This core uses bar primary Ample amount of insulation is to be provided between core and the secondary winding.

Continuously Wound Core: These are available in encapsulate form. Refer figure.Electrostatic spraying is done on core so that insulation is formed between core and secondary. Secondary winding is wound in toroidal form. Insulation tape is wrapped around it. Number of tapings can be taken out from secondary winding as shown.

CTs used in high voltage system: These are generally oil filled type. If operating voltage is above 400 kV, SF₆ (Sulphur hexa floride) gas insulation is used.

These CTs are installed in out door switchyard. Figure shows the two types of high voltage CTs. Dielectric oil is filled in the casing. Intermediate CTs: Sometimes the secondary current of main CT is not sufficient to operate the protective devices. In such cases, additional CT called intermediate CT is used. These are used for feeding additional current to protective devices so that proper operation takes place.

Protective PT: Potential Transformers are also known as voltage transformers. Similar to CT, these can be used for metering and protection.

Types of PT: Following two types of construction are used:

1.    Electromagnetic type,  2. Capacitor type.

Electromagnetic type voltage transformer: It is similar to conventional transformer with some additional features to minimize errors. It can be conveniently used up to 132kV. For lower voltages up to 3.2kV, dry type transformer with varnish impregnated and taped windings are quite satisfactory. For higher voltages oil immersed PTs are used. Recently PTs with windings impregnated and encapsulate in synthetic resins have been used because of this technique dry type PTs can be used up to 66 kV. For voltages above 60 kV, the several PTs are connected in cascade manner.

Capacitive Type PT/Capacitive Type Voltage Transformer (CVT):  At higher voltages, electromagnetic type PTs are very expensive so CVT are used. It consists of a capacitive voltage divider as shown in fig.

The Capacitors C₁ and C₂ are connected in series so the high voltage gets distributed across them. Thus, the voltage applied to primary (V_p) is (high voltage) – (voltage across C₁). To reduce phase angle error and ratio error in inductor L is connected as shown.

INDUCTION RELAYS

These relays work on principle of induction motor and use widely used for AC protection system.

An induction relay consists of a pivoted aluminum disc placed between two magnetic fields (the two magnetic fields have a phase difference). Torque is produced in the disc by the interaction of one magnetic field with currents induced in disc by the other.

Torque equation:

The two AC fluxes Φ₁ and  Φ₂  having phase difference a produce eddy currents i₂

and i₁ respectively. The current lag respective fluxes by 90⁰.

Induction Relays:

                                                        Induction Relays

 

                                         Disc type                            Cup type

1.    Shaded pole station

2.    Watt hour meter type

 Shaded Pole Structure:

                                                

The general arrangement of shaded pole structure is as shown in fig. It consists of a pivoted Al disc free to rotate in the air gap of electromagnet. One half of each pole of magnet is surrounded by a copper shading ring. The copper shading ring, splits the exciting current into two out of phase components. A shaded pole flux lags behind the un-shaded pole flux normally by 40⁰ to 50⁰. Torque is produced on the disc by interaction of one magnetic field with currents induced due to other magnetic field.

E_u          =        emf induced by un-shaded portion of pole

E_s          =        emf induced by shaded portion of pole

θ_u          =        flux in un-shaded pole

θ_s          =        flux in shaded pole

I_u           =        current in un-shaded part

I_s           =        current in shaded part.

Watt-hour meter type Arrangement is the same as used in watt-hour meters. A pivoted Al disc rotates freely between poles of two electromagnets. Upper magnet carries primary and secondary winding. Primary winding carries current I₁ i.e. relay current. Secondary is connected to winding of lower magnet.

IDMT Relays: Current setting: The method of adjusting pickup value is known as current setting. The operating coil of relay is provided with several tapings as shown. By changing the tapings torque on disc and hence time of operation of relay is changed.

The value of each tap are expressed in percentage F.L. rating of CT.

Therefore, pickup current = rated sec. current of CT x current setting.

Suppose relay is connected to sec. of CT. of ratio 400/5 and taping of 125% is selected then pickup value = 5 x 1.25 = 6.25A

                                       Fault current in relay coil

Plug setting multiplier = ---------------------------------

                                              Pickup current

                                                      Fault current in relay coil

Fault current in relay coil = --------------------------------------------------

                                            Rated CT sec. current x current setting

e.g. if relay is connected to 400/5 Amp. CT set at 150% with a fault current of 2400 Amp then plug setting multiplier is calculated as under

Pickup value = 5 x 1.5 = 7.5

Fault current in relay coil i.e. at sec. of CT = 2400 x 5/400 =  30 A

Hence  P.S.M. = 30/7.5 = 4

Time setting multiplier A relay is provided with a control for adjusting the time of operation.

The actual operating time is obtained from P.S.M/time curve is 3 sec (see the curve), then actual operating time = 3 x 0.1 = 0.3 sec.

Some Important Terms:

Relay Timing: An important characteristic of a relay is its time of operation. By time of operation is meant the length of time from the instant the actuating element is energized to the instant when the relay contacts are closed.

For controlling this time some mechanical accessories need to be provided.

1. Instantaneous relay; When the current in the relay coil exceeds the minimum calibrated value of current, relay contacts are instantly closed. This time is less than 0.1 second.

2. Inverse time relay; In this type, the operating time is approximately inversely proportional to the magnitude of actuating quantity. This is achieved by the provision of a drag magnet or oil dash pot or time limit fuses.

3. Definite time lag relay; In this type there is a definite time elapsed between the instant of pickup and the closing of the relay contacts.

Time/P.S.M. Curve:

Figure shows the curve between time of operation and plug setting multiplier of a typical relay. The horizontal scale is marked in terms of plug-setting multiplier and represents the number of times the relay current is in excess of the current setting. The vertical scale is marked in terms of the time required for relay operation. If the P.S.M. is 10, then the time of operation (from the curve) is 3 seconds. The actual time of operation is obtained by multiplying this time by the time-setting multiplier.

It is evident from fig that for lower values of over current, time of operation varies inversely with the current but as the current approaches 20 times full-load value, the operating time of relay tends to become constant. This feature is necessary in order to ensure discrimination on very heavy fault currents flowing through sound feeders.

Calculation of Relay Operating Time: In order to calculate the actual relay operating time, the following things must be known:

(a)  Time/P.S.M. curve.                      (b) Current setting

(c) Time setting                                   (d) Fault current

(e) Current transformer ratio

The procedure for calculating the actual relay operating time is as follows:

1.    Convert he fault current into the relay coil current by using the current transformer ratio.

2.    Express the relay current as a multiple of current setting i.e. calculate the P.S.M.

3.    From the time/P.S.M. curve of he relay, read off the time of operation for the calculated P.S.M.

4.    Determine the actual time of operation by multiplying the above time of the relay by time-setting multiplier in use.

Example: Determine the time of operation of a 5 ampere, 3 second over current relay having a current setting of 125% and a time multiplier of 0.6 connected to supply circuit through a 400/5 current transformer when the circuit carries a fault current of 4000 A. Use the curve  as shown in fig.

Solution: Rated secondary current of CT = 5 A

                                    Pickup current                    = 4 x 1.25 = 6.25 A

Fault current in relay coil,                           = 4000 x 5/400 = 50A

Hence Plug-setting multiplier (P.S.M.)       = 50/6.25 = 8

Corresponding to the plug-setting multiplier of 8 the time of operation is 3.5 seconds.

So Actual relay operating time                    = 3.5 x Time setting

                                                                     = 3.5 x 0.5 = 21 seconds.

Induction Type Over Current Relays (Non-Directional) Construction: The relay works on induction principle. Actuating quantity is current (from CT secondary).  The primary winding is connected to secondary of CT in the line to be protected. A tapped (1……..7). The tapings are connected to plug setting bridge so that number of active turns on relay can be changed and desired current setting can be obtained. Secondary winding is connected to lower magnet by series connection. Due to induction principle emf is induced in secondary due to current in primary. The torque exerted on the disc is due to interaction of eddy currents in the disc and flux produced by upper and lower magnet. The two fluxes have phase displacement. The spindle carries moving contact which bridges the two fixed contacts which are connected to trip circuit. The moving contacts move through a preset angle. By adjusting this angle (From 0⁰ to 360⁰) travel of moving contact can be adjusted and hence time setting is also possible. Restraining torque is provided with the help of spring.

Operation: Torque is produced on Al disc due to interaction of eddy current in the disc and two fluxes produced by upper and lower magnet. Under normal working condition restraining torque is greater than driving torque which is produced by relay coil current. Si Al disc doesn’t rotate. If current exceeds preset value, driving torque is greater than restraining torque. And disc rotates through preset angle causing bridging of trip circuit contacts. The trip circuit operation causing the opening of CB, fault is cleared.

Induction Type Directional Power Relay: This relay operates when power in the circuit flows in specific direction. Construction: Upper magnet carries potential coil energized by PT secondary and lower magnet carries current coil energized by secondary of CT. The current coil is provided with number of tapings (not shown for simplicity of figure.) which is connected to plug setting bridge so that any desired current setting is obtained. The restraining torque is produced by spiral spring. Spindle carries moving contact which bridges the two fixed contacts when disc is rotated through preset angle. By adjusting this angle, travel of moving disc is adjusted and any desired time setting can be given to the relay.      

Operation: Ø is produced due to current in potential coil lags V by 90⁰.  Ø₂ produced by current coil is in phase with I. The interaction of Ø₁ and Ø₂ with eddy current in the disc, produce driving torque.    The direction of driving torque depends upon direction of power flow. When power in circuit is in normal direction, driving torque and restraining torque (due to spring) help each other to turn away moving contacts from fixed contacts. So relay is inoperative. If reverse power flow is there and driving torque is large enough then disc rotates in the reverse direction and moving contacts close the trip circuit. Thus C.B. operates and fault is cleared.

The directional power relay is unsuitable to  used under short circuit condition., Because when S.C. occurs voltage is very very low so driving torque is very less. This difficulty is overcome in directional over current relay which is designed such that operation is independent of voltage and pf.

Induction Type Directional Over current Relay (or Earth Fault Relay) Construction:  It consists of two elements –directional, non-directional.

Directional element: This is similar to directional power relay. This operates when power flow is in specific direction. The potential coil is connected to P.T. secondary. The current coil is connected to C.T. secondary. This winding is also wound to central limb of upper magnet of non-directional element and back to C.T. as shown. The trip contacts 1 and 2 are connected in series with secondary circuit of non-directional element. So if 1 and 2 are shorted by moving contact then only secondary circuit of non-directional element will be completed i.e. directional element operates first and then non-directional element.

Non-directional element: Similar to non-directional over current relay circuit. Plug setting bridge is also provided for current setting (not shown in above diagram for simplicity)/ the tapings are provided on upper magnet (primary). Spindle carries moving contacts.

Operation: Under normal working conditions power flows in normal direction. So upper element (directional)does not operate. There fore, lower element can not operate. When short circuit occurs there is a tendency of current to flow in the reverse direction. There fore, element operates closing the contacts 1 and 2 and thus completing circuit to over current relay. This operates the C.B. because trip circuit is closed and fault is cleared.

Induction Cup Type Relays: Relay has 2-4 or more electromagnets energized by relay coil. There is stationary iron core and hollow metallic cylindrical cup which is free to rotate in the gap between iron core and poles of electromagnet. The eddy currents are induced in metallic cup and these currents interact with the flux produced by other magnets and hence torque is produced.

Modern induction cup relay consists of four or more poles, a control spring and moving contacts are mounted on the arm attached to spindle of cup.

The relay responses for V and I. It has inverse current time characteristic. Operating time is of the order of 0.01 sec.

These are used for distance protection. The characteristic depends on type of structure used.

Distance or impedance relays.

In impedance relay torque produced by V is in opposite directed to that produced by I. The relay operates if V/I ratio is less then predetermined value. Under normal working conditions impedance is Z₁. If fault F₁ occurs impedance

Z = V/I = Voltage at substation/Fault current.

So   Z < Z_L  so relay operates.

Type of Distance Relays:

Definite Distance Relay: Operates instantaneously for faults up to a predetermined distance from relay.

The armatures of two magnets are mechanically coupled to the beam.

The torque produced by two electromagnets is in opposite direction.