directional protection seminar paper - razdelilna in...

23
Directional Protection Seminar paper - Razdelilna in industrijska omreˇ zja Armin Zaimovi´ c Faculty of Electrical Engineering, University of Ljubljana 5.May.2019 1

Upload: others

Post on 25-Apr-2020

13 views

Category:

Documents


1 download

TRANSCRIPT

Page 1: Directional Protection Seminar paper - Razdelilna in ...lrf.fe.Uni-lj.si/e_rio/Seminarji/SmernaZascita.pdf2 Construction of induction directional overcurrent relay The directional

Directional Protection

Seminar paper - Razdelilna in industrijska omrezja

Armin ZaimovicFaculty of Electrical Engineering, University of Ljubljana

5.May.2019

1

Page 2: Directional Protection Seminar paper - Razdelilna in ...lrf.fe.Uni-lj.si/e_rio/Seminarji/SmernaZascita.pdf2 Construction of induction directional overcurrent relay The directional

Contents

1 Introduction 41.1 Faults . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41.2 Network arrangements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

1.2.1 Radial . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41.2.2 Parallel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51.2.3 Ring . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61.2.4 Interconnected . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

2 Construction of induction directional overcurrent relay 7

3 Protection of network using directional protection 93.1 Protection of radial network . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

3.1.1 Earth fault directional protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113.1.2 Phase to phase fault protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

3.2 Protection of closed rings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

4 Working principle 14

5 Questions 215.1 How does directional protection determines flow of current in AC systems? . . . . . . . . . . 215.2 How do we measure residual current, and residual voltage? . . . . . . . . . . . . . . . . . . . 215.3 What is biggest disadvantage of directional protection? . . . . . . . . . . . . . . . . . . . . . . 21

6 Homework 22

2

Page 3: Directional Protection Seminar paper - Razdelilna in ...lrf.fe.Uni-lj.si/e_rio/Seminarji/SmernaZascita.pdf2 Construction of induction directional overcurrent relay The directional

List of Figures

1 Example of radial network . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52 Example of parallel network . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53 Example of ring network . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64 Constructional details of a typical Induction Type Directional Overcurrent Relay . . . . . . . 75 Electric cable as capacitor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96 Flow of capacitive current during a phase to earth fault . . . . . . . . . . . . . . . . . . . . . 107 Short circuit protection on a network with several sources . . . . . . . . . . . . . . . . . . . . 128 Protection of parallel connected lines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 139 Circuit diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1410 Current flows . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1411 Circuit diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1512 Determining the direction of current flow in an ac power circuit (resistive-inductive impedance

only) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1513 Selecting a characteristic angle to properly align the forward and reverse direction zones of

the directional overcurrent relay . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1614 Polarizing voltage of (a) phase ”A” and (b) phase ”C” for a current in phase ”A” and phase

”C” respectively . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1715 Directional protection tripping zones on (a) phase ”A” and (b) phase ”C” for a 45 degrees . . 1716 Range of phase angle values of current IA expected for a phase-to-ground fault on phase A. . 1817 Ranges of phase angle values of current IA expected for phase-to-phase faults between phase

A and phase B (left) and between phase A and phase C (right) . . . . . . . . . . . . . . . . . 1818 Range of phase angle values of current IA expected for faults on phase A . . . . . . . . . . . 1919 Setting the characteristic angle to 45◦ properly aligns the forward and reverse direction zones

of the directional overcurrent relay with the range of phase angle values expected for the faultcurrents. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

20 Current flows . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2121 Parallel feeders in single-end system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2222 Parallel feeders in single-end system - Solution . . . . . . . . . . . . . . . . . . . . . . . . . . 23

3

Page 4: Directional Protection Seminar paper - Razdelilna in ...lrf.fe.Uni-lj.si/e_rio/Seminarji/SmernaZascita.pdf2 Construction of induction directional overcurrent relay The directional

1 Introduction

Switchgear, cables, transformers, overhead lines and other electrical equipment require protection devices inorder to safeguard them during fault conditions. In addition, the rapid clearance of faults prevents touch andstep potentials on equipment from reaching levels which could endanger life. The function of protection isnot to prevent the fault itself but to take immediate action upon fault recognition. Protection devices detect,locate and initiate the removal of the faulted equipment from the power network in the minimum desirabletime. It is necessary for all protection relays, except those directly associated with the fault clearance, toremain inoperative during transient phenomena which may arise during faults, switching surges or otherdisturbances to the network. Protection schemes are designed on the basis of:

• safety

• reliability

• selectivity

While the earliest relays were electromechanical in construction, technological developments led to theintroduction of solid state or static relays using discrete devices such as transistors, resistors, capacitors, etc.Advent of micro-processors led to the development of microprocessor-based relays and this culminated withtoday’s state of the art system of numerical relaying where the measurement principles themselves changedfrom analogue to numerical.

1.1 Faults

All power system components are liable to faults involving anomalous current flow and insulation breakdownbetween conductors or between conductors and earth. The insulation material may vary from air, in thecase of a transmission line, to oil, SF6 or a vacuum, in the case of switchgear. The transmission anddistribution engineer is concerned with symmetrical faults involving all three phases with or without earth,and asymmetrical faults involving phase-to-phase and one or two phase-to-earth faults. In addition, interturn winding faults also occur in transformers and electrical machines.

1.2 Network arrangements

1.2.1 Radial

A simple radial feeder is shown in Fig. 1. The fault level is highest closest to the source and limited by theimpedances from source to fault location. Clearance of a fault near the source will result in loss of supplyto downstream loads. Protection selectivity must be such that a fault on farthest busbar won’t stop supplyto load nearer to source.

4

Page 5: Directional Protection Seminar paper - Razdelilna in ...lrf.fe.Uni-lj.si/e_rio/Seminarji/SmernaZascita.pdf2 Construction of induction directional overcurrent relay The directional

Figure 1: Example of radial network

1.2.2 Parallel

A parallel feeder arrangement is shown in Fig. 2. A fault on one parallel feeder should be cleared bysuitable protection such that it is quickly isolated from the supply. There should be no loss of supply viathe remaining healthy feeder to the load.

Figure 2: Example of parallel network

5

Page 6: Directional Protection Seminar paper - Razdelilna in ...lrf.fe.Uni-lj.si/e_rio/Seminarji/SmernaZascita.pdf2 Construction of induction directional overcurrent relay The directional

1.2.3 Ring

A ring feeder arrangement is shown in Fig. 3. Two routes exist for the power inflow to a faulted feeder ina closed ring system. It is therefore necessary for the protection devices only to isolate the faulted sectionand not disconnect the whole system from the source.

Figure 3: Example of ring network

1.2.4 Interconnected

This is a more complex arrangement of interconnected parallel and radial feeders, often with multiple powersource in-feeds. More sophisticated protection schemes are necessary in order selectively to disconnect onlythe faulted part of the system.

6

Page 7: Directional Protection Seminar paper - Razdelilna in ...lrf.fe.Uni-lj.si/e_rio/Seminarji/SmernaZascita.pdf2 Construction of induction directional overcurrent relay The directional

2 Construction of induction directional overcurrent relay

The directional power relay is unsuitable for use as a directional protective relay under short-circuit condi-tions. When a short-circuit occurs, the system voltage falls to a low value and there may be insufficient torquedeveloped in the relay to cause its operation. This difficulty is overcome in the Induction Type DirectionalOvercurrent Relay which is designed to be almost independent of system voltage and power factor

Figure 4: Constructional details of a typical Induction Type Directional Overcurrent Relay

Induction type directional overcurrent relay consists of two main parts:

• Directional element

• Non-directional element

1. Directional element: It is essentially a directional power relay which operates when power flows in aspecific direction. The potential coil of this element is connected through a potential transformer (P.T.) tothe system voltage. The current coil of the element is energized through a C.T. by the circuit current. Thiswinding is carried over the upper magnet of the non-directional element. The trip contacts (1 and 2) of thedirectional element are connected in series with the secondary circuit of the overcurrent element. Therefore,the latter element cannot start to operate until its secondary circuit is completed. In other words, the di-rectional element must operate first (i.e. contacts 1 and 2 should close) in order to operate the overcurrentelement.

2. Non-directional element: It is an overcurrent element similar in all respects to a non-directionalovercurrent relay. The spindle of the disc of this element carries a moving contact which closes the fixed

7

Page 8: Directional Protection Seminar paper - Razdelilna in ...lrf.fe.Uni-lj.si/e_rio/Seminarji/SmernaZascita.pdf2 Construction of induction directional overcurrent relay The directional

contacts (trip circuit contacts) after the operation of directional element. It may be noted that plug-settingbridge is also provided in the relay for current setting but has been omitted in the figure for clarity andsimplicity. The tappings are provided on the upper magnet of overcurrent element and are connected to thebridge.

Operation: Under normal operating conditions, power flows in the normal direction in the circuit pro-tected by the relay. Therefore, Induction Type Directional Overcurrent Relay (upper element) does notoperate, thereby keeping the overcurrent element (lower element) unenergized. However, when a short-circuit occurs, there is a tendency for the current or power to flow in the reverse direction. Should thishappen, the disc of the upper element rotates to bridge the fixed contacts 1 and 2. This completes thecircuit for overcurrent element. The disc of this element rotates and the moving contact attached to it closesthe trip circuit. This operates the circuit breaker which isolates the faulty section. The two relay elementsare so arranged that final tripping of the current controlled by them is not made till the following conditionsare satisfied :

1. current flows in a direction such as to operate the directional element

2. current in the reverse direction exceeds the pre-set value

3. excessive current persists for a period corresponding to the time setting of overcurrent element.

8

Page 9: Directional Protection Seminar paper - Razdelilna in ...lrf.fe.Uni-lj.si/e_rio/Seminarji/SmernaZascita.pdf2 Construction of induction directional overcurrent relay The directional

3 Protection of network using directional protection

3.1 Protection of radial network

Capacitive currentAny live part forms a capacitor relative to earth (see fig. 5 ). This is particularly true for cables, for whichthe capacitance per kilometer is commonly of the order of several microfarads. It is also true for lines, butwith a capacitance of approximately 100 times lower in value.The capacitive effect of cables is such that supplying power to 50 km of cable under no-load conditions at20 kV, is the equivalent to connecting 3 MVAR of capacitors between the network and the earth!As long as the cable is supplied from a balanced three-phase voltage, the sum of the capacitive currentsof the three phases is almost zero. However, when there is a phase to earth fault in the network, one ofthe phase to earth voltages is lower than the others. The capacitive currents are no longer balanced and aresidual capacitive current is observed. The flow of currents is diagramatically represented in figure 6.

Figure 5: Electric cable as capacitor

9

Page 10: Directional Protection Seminar paper - Razdelilna in ...lrf.fe.Uni-lj.si/e_rio/Seminarji/SmernaZascita.pdf2 Construction of induction directional overcurrent relay The directional

Figure 6: Flow of capacitive current during a phase to earth fault

In order to incorporate protection equipment, it is necessary to calculate, for a given feeder, the maximumvalue of the residual capacitive current. This is the current that would be measured by a coil placed on thisfeeder when one phase is earthed upstream of it whilst the two others are at the network’s rated phase tophase voltage.It is generally called the feeder’s capacitive current. The value of this current is :

Ic = 3CωU

Where:

• C is capacitance of each phase relative to the earth

• U is phase to neutral voltage

• ω is the angular frequency

The choice of the neutral earthing connection arrangement is an important stage in designing an electricalnetwork. It is always a result of a compromise between several factors.A factor that is frequently favoured is the desire to reduce the fault current in order to improve the humansafety (by limiting the increase in potential of the fault earthing points), and of equipment (by limiting theenergy released through electrical short circuit arcing). We will see that by limiting the fault current we makeit more difficult to detect the fault and consequently it becomes essential to use an earth fault directionalprotection system. If the fault current is sufficiently weak, we no longer need to instantly cut off the supply,and this in turn enables a considerable improvement to be achieved in continuity of service. During afault, the capacitive current superposes itself on the current limited by the neutral earthing impedance.Consequently, in networks with large capacitive currents, the only way of obtaining a low fault current is tochoose an inductive earthing impedance whose current compensates for the capacitive current. When thisneutral point inductance is constantly adjusted to retain this balance (3LCω2 = 1), it is called a Petersencoil. In this case the fault current is theoretically zero.

10

Page 11: Directional Protection Seminar paper - Razdelilna in ...lrf.fe.Uni-lj.si/e_rio/Seminarji/SmernaZascita.pdf2 Construction of induction directional overcurrent relay The directional

3.1.1 Earth fault directional protection

There are two types of ground faults: single line ground faults (SLG fault) and line-to-line ground fault(LLG fault). The ground fault is different from other types of short circuit because it has a zero sequenceof a current I0. Since in normal operations this current doesn’t exist, the relays for detecting ground fault

measure the component I0 = (Ia+Ib+Ic)3 and declaring the fault of I0 exceeds a threshold. However, in a

system with multiple sources or parallel paths, it is required that earth fault relays have directional unit.The reference phasor is sometimes called as polarizing quantity or residual quantity. Also, both voltage andcurrent polarizing signals are used with ground fault relaying. Before explaining these two polarizing signals,the term residual variable should be explained. For example, if the zero sequence variable Fh is defined as:

~Fh =1

3( ~F1 + ~F2 + ~F3)

Fr = ~F1 + ~F2 + ~F3

Where Fr is residual variable and it is three times greater than the zero sequence variable. The residualcurrent is either measured by three current transformers, one per phase, or by a coil (ring CT) around threephases: The usage of three current transformer has certain advantages:

• CT’s are generally dependable

• it is possible to measure high currents

But it also has certain disadvantages:

• saturation of the CT’s in the instance of a short circuit or when a transformer is switched on producesa false residual current

• in practise, the threshold cannot be set to under 10% of the CT’s rated current

Measuring using a ring CT:

• has the advantage of being very sensitive

• has the disadvantage of the coil low voltage insulated) being installed around a non-clad cable toinsulate it

Residual voltage is measured by three voltage transformers (VT), and usually these transformers have twosecondary windings: one is in the star connection and enables both phase to neutral and phase to phasevoltages to be measured and the other one is in delta connection, enabling the residual voltage to be measured.Also, one of the possible configurations is that main VT’s have one secondary winding that is in the starconnection, and grounded, while the set of auxiliary VT’s is used to measure the residual voltage.

3.1.2 Phase to phase fault protection

Directional phase protection equipment is used on a radial network for substations supplied simultaneouslyby several sources. In order to obtain good continuity of service, it is important that a fault affecting oneof the sources does not cause all the sources to trip. The required selectivity is achieved by installing phasedirectional protection equipment on the incomer of each of the sources. Figure 7 shows a typical layout forphase to phase fault protection equipment. In this figure, the arrow shows the direction of detection of eachdirectional phase protection equipment.

11

Page 12: Directional Protection Seminar paper - Razdelilna in ...lrf.fe.Uni-lj.si/e_rio/Seminarji/SmernaZascita.pdf2 Construction of induction directional overcurrent relay The directional

Figure 7: Short circuit protection on a network with several sources

Directional phase protection equipment is generally two phase. The time delays of the protection equip-ment are shown in Figure 7. The characteristic angles are set to take account of the chosen angle ofconnection. For an angle of connection of 90◦, the most universally used setting of the characteristic angleis 45◦.

It should be noted that if the generator set’s short circuit power is low compared with that of thenetwork, the directional protection equipment installed on the generator set’s incomer can be replaced bysimple overcurrent protection equipment with a threshold set to be both greater than the generator set’sshort circuit current and less than that of the network.

3.2 Protection of closed rings

In such networks, one on more rings are closed when in normal operation.The advantage of such a network structure is that is ensures excellent dependability of power to all consumerssituated on the ring ; it enables a faulty connection to be disconnected from the network without interruptingthe supply to the consumers.The disadvantage of this solution is its cost: it requires a circuit breaker to beinstalled at the end of each connection in addition to complex protection equipment.Two protection principles may be used:

1. differential protection

2. directional protection

The later functions if, on the ring, a single substation has one or more sources and also earths the neutral.In practice, the selectivity of directional protection equipment is ensured by logical selectivity systems.Compared with differential protection, which has the advantage of being quick, directional protection is lesscostly and easier to incorporate. Note that the detection of earthing faults is performed whatever the neutralpoint arrangement of the installation, whereas differential line protection equipment has limited sensitivity.

Parallel connected linesTwo parallel connected lines are the simplest and most frequently encountered example of a closed ring. Theprotection system must be designed in such a way that a fault on one line does not cause the other lineto trip. A typical protection system is shown in figure 8. In this figure, the arrow shows the direction ofdetection of each directional protection equipment. Directional phase protection equipment is of two phasetype. Its characteristic angle is set to take account of the chosen angle of connection (45◦ for an angle ofconnection of 90◦). The characteristic angle of directional earthing protection equipment is set accordingto the neutral point arrangement as explained in the previous paragraphs. The protection equipment timedelays are shown in the figure. Non directional protection equipment used on upstream substation feedersare time delayed so as to grade with the directional protection equipment of the downstream substation

12

Page 13: Directional Protection Seminar paper - Razdelilna in ...lrf.fe.Uni-lj.si/e_rio/Seminarji/SmernaZascita.pdf2 Construction of induction directional overcurrent relay The directional

incomers. In the instance of a short circuit on one of the lines, the current is divided according to theimpedance in each circuit : part flows directly from the upstream substation in the faulty line, the restpasses through the downstream substation.The protection equipment is activated in the following order :

1. A1, D1 and D2 detect the fault

2. A1 trips (time delay: 0.1 s)

3. D2 resets before its time delay has elapsed

4. D1 trips (time delay: 0.4 s)

When a short circuit occurs near to the upstream substation’s busbars, the proportion of current pass-ing through the downstream substation is very low, less than the directional phase protection thresholdvalue.This is the case when the position < x > of the fault is between 0 and twice the ratio of Is

Isc(between

the directional protection relay’s threshold and the short circuit current). In this case, the faulty line feeder’sovercurrent protection equipment (D1) trips first (time delay: 0.4 s) with A1 tripping next. The total timeto elimination of the fault is therefore prolonged. This disadvantage can be overcome by installing a secondovercurrent relay on feeders D1 and D2 with a high threshold (tripping for a Isc corresponding to less than90% of the length of the line) with a time delay of 0.1 s.

Figure 8: Protection of parallel connected lines

13

Page 14: Directional Protection Seminar paper - Razdelilna in ...lrf.fe.Uni-lj.si/e_rio/Seminarji/SmernaZascita.pdf2 Construction of induction directional overcurrent relay The directional

4 Working principle

Current in any conductor of an electric circuit can flow from left to right or from right to left. This iscommonly referred to as the direction of current flow. In dc power circuits, the polarity of the currentindicates the direction of current flow. In ac power circuits, however, the polarity of the current alternatesconstantly. Consequently, polarity cannot be used to determine the direction of current flow.

In an ac power circuit, one determines the direction of current flow from the phase shift between thevoltage E and the current I at any given point of the circuit, as shown in Fig. 9.

When the current flows from left to right in the circuit, the absolute value of the phase shift between thevoltage and current is 90 degrees or less (Figure 10a), the exact value of the phase shift being dependenton the value of the circuit impedance Z. This direction is generally considered as the forward direction. Onthe other hand, when the current flows from right to left in the circuit, the absolute value of the phase shiftbetween the voltage and current is 90 degrees or more (Figure 10b), the exact value of the phase shift beingdependent on the value of the circuit impedance Z. This direction is generally considered as the reversedirection.

Figure 9: Circuit diagram

(a) Current flows in forward direction (b) Current flows in reverse direction

Figure 10: Current flows

In most power networks, the impedance is of resistive-inductive nature. In this case, the expected rangeof phase angle values of the current is reduced to 90 degrees for each direction of current flow, as shown in

14

Page 15: Directional Protection Seminar paper - Razdelilna in ...lrf.fe.Uni-lj.si/e_rio/Seminarji/SmernaZascita.pdf2 Construction of induction directional overcurrent relay The directional

Fig. 11 and Fig. 12.

Figure 11: Circuit diagram

(a) Current flows in forward direction (b) Current flows in reverse direction

Figure 12: Determining the direction of current flow in an ac power circuit (resistive-inductive impedanceonly)

The operation of the directional overcurrent relay along with its main parameters is explained below:

• A directional overcurrent relay consists of overcurrent relay plus a directional element that determinesthe direction of current flow. These two units operate jointly for a predetermined current magnitudeand direction. The relay is activated only for current flow to a fault in one direction and when thecurrent is higher than the maximum current for which relay doesn’t operate.

• A directional overcurrent relay can monitor line current on two phases, on that way measuring twocurrents and two voltages. With this kind of operation, any phase-to-phase fault can be detected. Inthe case of monitoring line current on all three phases, the relay measures three currents and threevoltages. This allows detection of any phase-to-phase fault as well as any phase-to-ground fault.

• In electromechanical and static directional overcurrent relays, the reference voltage for phase A isgenerally voltage EBC (VBC), and not phase voltage EA (VA) as it could be expected. This is becausevoltage EBC isn’t affected when a ground fault occurs on phase A, thereby providing a more stablereference voltage. It is analogous for the phases B and C. This will be explained in more details furtherin the paper. This logic could not be used for modern directional overcurrent relays (digital andnumerical units) because these relays generally use phase voltage EA as the reference voltage for phase

15

Page 16: Directional Protection Seminar paper - Razdelilna in ...lrf.fe.Uni-lj.si/e_rio/Seminarji/SmernaZascita.pdf2 Construction of induction directional overcurrent relay The directional

A and other techniques to obtain a reference voltage that is even more stable (these techniques arebeyond the scope of this seminar paper, and because of that modern relays won’t be further explained).However, there is still a reason to study electromechanical and static directional relays because some oftheir principles and settings are used in modern directional overcurrent relays even if a phase-to-phasevoltage is used as the reference voltage instead of a phase voltage.

• The reference voltage may be rotated to properly align the forward and reverse direction zones of thedirectional overcurrent relay. This rotation angle is referred to as the characteristic angle. Fig. 11.shows the forward and reverse direction zones for two values of characteristic angle when line voltageEBC is used as the reference voltage.

Figure 13: Selecting a characteristic angle to properly align the forward and reverse direction zones of thedirectional overcurrent relay

As it is already mentioned, in order to detect the current direction, the phase displacement betweenvoltage and short circuit current must be determined. When is cos(ϕ) ≥ 0 (−π

2 ≤ ϕ ≤ π2 ) then the active

power seen by the directional relay is positive. The active power seen by the directional relay is negative inthe situation when is cos(ϕ) ≤ 0 (π2 ≤ ϕ ≤ 3π

2 ).The displacement ϕ can be determined by comparing the phase current with a polarizing voltage as a

reference. This explanation will be done for electromechanical and static relays, which means that if thecurrent is in the phase A, the polarizing voltage will be EBC (VBC), which is perpendicular to the current Iaat ϕ = 0 (Figure 12a). Analogously, the polarizing voltage will be VAB if the current is in the phase C, andthe voltage will be perpendicular to the current at ϕ = 0 (Figure 12b). The angle between phase currentand chosen polarizing voltage is called connection angle and has the value of 90 degrees when ϕ = 0.

16

Page 17: Directional Protection Seminar paper - Razdelilna in ...lrf.fe.Uni-lj.si/e_rio/Seminarji/SmernaZascita.pdf2 Construction of induction directional overcurrent relay The directional

Figure 14: Polarizing voltage of (a) phase ”A” and (b) phase ”C” for a current in phase ”A” and phase ”C”respectively

The tripping zone is a half plane defined by characteristic angle θ. This angle is the angle betweenpolarizing vector and a line perpendicular to the boundary line (Fig. 15). The usual values for this angleare: 30 degrees, 45 degrees or 60 degrees. Because of its importance, this angle will be more briefly explainedin the next chapter.

Figure 15: Directional protection tripping zones on (a) phase ”A” and (b) phase ”C” for a 45 degrees

It can be seen that phase current Ia is in the tripping zone when θ − π2 < ψ1 < θ + π

2 and for the otherhalf plane is in the nontripping zone. The similar conclusion could be made for current Ic. This current isin the tripping zone when θ− π

2 < ψ3 < θ+ π2 and is in the nontripping zone for all other angles. Angles ψ1

and ψ3 correspond to ϕ1 and ϕ3, respectively (ϕi = ψi + 90)

17

Page 18: Directional Protection Seminar paper - Razdelilna in ...lrf.fe.Uni-lj.si/e_rio/Seminarji/SmernaZascita.pdf2 Construction of induction directional overcurrent relay The directional

The value (45◦) of the characteristic angle used in directional overcurrent protectionThe range of phase angle values of current expected for faults on phase A includes the ranges of phase anglevalues for a phase-to-ground fault, a phase-to-phase fault between phase A and phase B, and a phase-to-phasefault between phase A and phase C. These three cases are treated separately below to ease understanding.The circuit impedance considered is of resistive-inductive nature, as this is representative of actual circuitimpedances. When a phase-to-ground fault occurs on phase A, the range of phase angle values of currentIA that is expected is illustrated in Figure 16.

Figure 16: Range of phase angle values of current IA expected for a phase-to-ground fault on phase A.

The ranges of phase angle values of current IA expected for phase-to-phase faults between phase A andphase B and between phase A and phase C are shown in Figure 17.

Figure 17: Ranges of phase angle values of current IA expected for phase-to-phase faults between phase Aand phase B (left) and between phase A and phase C (right)

The superposition of the ranges of phase angle values of current IA involved in Figure 16 and Figure 17is presented in Figure 18. The total range covers 150◦.

18

Page 19: Directional Protection Seminar paper - Razdelilna in ...lrf.fe.Uni-lj.si/e_rio/Seminarji/SmernaZascita.pdf2 Construction of induction directional overcurrent relay The directional

Similarly, the total range of phase angle values of the current is also 150◦ for faults on phase B or phase C.

Figure 18: Range of phase angle values of current IA expected for faults on phase A

Setting the characteristic angle to 45◦ properly aligns the forward and reverse direction zones of thedirectional overcurrent relay with the vectors of fault current expected for phase A, as illustrated in Figure19. Notice that the forward direction zone encloses every expected vector of fault current for phase A witha safety margin of 15◦ on each side of the expected range of phase angle values of current. This ensuresoptimal operation of the directional overcurrent relay.

19

Page 20: Directional Protection Seminar paper - Razdelilna in ...lrf.fe.Uni-lj.si/e_rio/Seminarji/SmernaZascita.pdf2 Construction of induction directional overcurrent relay The directional

Figure 19: Setting the characteristic angle to 45◦ properly aligns the forward and reverse direction zones ofthe directional overcurrent relay with the range of phase angle values expected for the fault currents.

20

Page 21: Directional Protection Seminar paper - Razdelilna in ...lrf.fe.Uni-lj.si/e_rio/Seminarji/SmernaZascita.pdf2 Construction of induction directional overcurrent relay The directional

5 Questions

5.1 How does directional protection determines flow of current in AC systems?

Since in AC systems polarity cannot be used to determine flow of current, as we would do in DC systems.In AC systems, one determines the direction of current flow from the phase shift between the voltage E andcurrent I at any given point of system.

(a) Current flows in forward direction (b) Current flows in reverse direction

Figure 20: Current flows

5.2 How do we measure residual current, and residual voltage?

Residual current is either measured using three current transformer, one per phase, or by coil (ring CT)around three phases.Residual voltage is measured by three voltage transformer (VT), and usually theses transformers have twosecondary winding: one is in the star connection, enabling phase to phase and phase to neutral voltage tobe measured. And other is delta formation, enabling residual voltage to be measured.

5.3 What is biggest disadvantage of directional protection?

Beside of price, one of the biggest disadvantage of directional protection is that relays usually have time-delayunit. In big, meshed grid, time-delay for certain relays could be higher than the value that is allowed, andbecause of this some other protection should be used.

21

Page 22: Directional Protection Seminar paper - Razdelilna in ...lrf.fe.Uni-lj.si/e_rio/Seminarji/SmernaZascita.pdf2 Construction of induction directional overcurrent relay The directional

6 Homework

One example is for single-end fed system of parallel feeders. Example of fault is given in Figure 20 below.

Figure 21: Parallel feeders in single-end system

1,2,3,4 are relays, which can be directional or non-directional.Fault is marked with red arrow, and let’s assume that this is line to ground fault. During line to groundfault we know that impedance of whole system drops at values in range of few dozens of ohms, which issignificantly lower than it should be during normal operation. Since the impedance is lower, using Kirchofflaw we can calculate that current jumps to higher values. Higher current values means that lines will startto heat up, and that could lead to unwanted scenarios for whole system, since we will end up losing ourlines, equipment etc. That’s why we need to use some kind of protection, to ensure no unwanted scenariowill happen.a)Explain what would happen if we would use non-directional protection.b)Solve situation using directional protection and time relaysBy looking at network shown in Figure 21 we can see that current in normal operation flows from the sourceto the load through both lines. In scenario given here fault occurs on one line, but since electricity distributesitself in dependence of impedance that means that almost all of electricity goes through the fault.

22

Page 23: Directional Protection Seminar paper - Razdelilna in ...lrf.fe.Uni-lj.si/e_rio/Seminarji/SmernaZascita.pdf2 Construction of induction directional overcurrent relay The directional

a) Let’s take a look what will happen if non-directional protection was used:So the fault current goes through R1, R2 and through R3,R4. If non-directional IEDs(Intelligent electronicdevices) are used all relays will trip, since they only measure current, and not it’s direction. By doing so,they will isolate loads (P1, P2, P3) from the source of energy. Which is something that we don’t want tohappen.

b)By introducing directional protection as shown in Figure 22 below we see that R2 will trip since cur-rent flows in direction which will trip the relay. Since R2 reacts faster than the overcurrent relay it willisolate fault, and by doing so we can continue to supply power to the consumers.

Figure 22: Parallel feeders in single-end system - Solution

23