Topic 7: Pilot Protection

Topic 7: Pilot Protection

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EE483EE483--Power System ProtectionPower System Protection

Dr. E. A. Dr. E. A. FeilatFeilatElectrical Engineering DepartmentElectrical Engineering DepartmentSchool of EngineeringSchool of EngineeringThe University of JordanThe University of Jordan

Introduction

Alternating-current lines are commonly classified by function, which is related to voltage level. Although there are no utility- wide standards, typical classifications are as follows:

Distribution (0.4 to 34.5 kV). Circuits transmitting power to the final users.

Sub transmission (13.8 to 138 kV). Circuits transmitting power to distribution substations and to bulk loads.

Transmission (69 to 765 kV). Circuits transmitting power between major substations or interconnecting systems, and to wholesale outlets. Transmission lines are further divided into:

• Low Voltage (LV):≤

1 kV, 1kV < (MV) ≤

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• High-voltage (HV): 69 to 230 kV

• Extra-high-voltage (EHV): 345 to 765 kV

• Ultra-high-voltage (UHV): greater than 765 kV

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Transmission lines Protection must be compatible with the Protection of all the equipment they connect.

Requires co-ordination of settings, operation times, etc.

Directionality associated with the design of the power system

A radial system can have fault current flowing only one direction

A network can have fault current flowing in either direction

Length of the line has direct effect on the setting of the relays

Relays are applied to protect a given line segment and back up adjacent line segments

Difficult to distinguish between a fault at the end of a line and the beginning of the next

Voltage class must be also considered when applying a relay system

The higher voltage levels would have more complex relay systems3

Introduction

Most faults experienced in a power system occur on the lines connecting generating sources with usage points. Just as these circuits vary widely in their characteristics, configurations, length, and relative importance, so do their Protection and techniques.

There are several protective techniques commonly used for line Protection:

Instantaneous over current

Time over current

Step time over current

Inverse time distance

Directional instantaneous and / or time over current

Zone distance

Pilot relaying4

Protection Type Selection

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Line Classification

Radial Lines or Feeders

Only one positive-sequence source.

Possible zero-sequence contribution to ground faults from both ends.

Distribution lines without synchronous motor load.

Looped Lines

Positive-sequence sources at both ends.

Trip both ends.

Transmission and some distribution lines

Transmission Line ProtectionHow Do We Protect Transmission Lines?

Overcurrent (50, 51, 50N, 51N)

Directional Overcurrent (67, 67N)

Distance (Impedance) (21, 21N)

Pilot Wire Protection

Line Current Differential (87)

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Transmission Line Protection

Overcurrent Protection

Non-Directional

Relay responds to overcurrent condition

Instantaneous (IOC) device #50

No intentional time delay

Time Overcurrent (TOC) device #51

Various curve types, including

inverse,

very inverse,

extremely inverse

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Protection Types of Distribution and Transmission lines

Over current Protection:

the simplest, most economical protection type

has found widespread use in distribution utility and industrial systems

limited to radial lines

the addition of directionality extends the application of over current Protection to looped lines.

Distance Protection:

used in many transmission lines.

to increase operating speed, a communications channel can be used to exchange information between directional or distance elements.

this type of arrangement is directional comparison pilot Protection.

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Protection Types

Differential Protection:

can be applied to transmission lines over a communications channel. This arrangement offers the best Protection.

Historically, the current-balance principle served to protect parallel transmission lines. This principle involved comparing the magnitudes of the currents of both lines. A fault at one of the lines created a difference between these currents.

A variant of the differential principle is the phase comparison principle, in which we compare the phase angles of the currents at both line ends.

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Pilot wire differential Protection

Protection of Cables and Short Lines

Pilot protection schemes use communication channels to send information from the local relay terminal to the remote relay terminal, thereby allowing high-speed tripping for faults occurring within 100% of the protected line.

Pilot wire protection: Pilot protection in which a metallic circuit is used for the communications channel between relays at the circuit terminals.

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BU

S

BU

S

Communication Communication ChannelChannel

Local Local RelayRelay

Remote Remote RelayRelay

Distance Protection

• For the radial system, disregarding the influence of load, the fault current in each phase is balanced and is equal to the phase current measured by the relays at the substation.

This current depends on the following parameters:• System voltage• Line impedance• Distance to the fault• Thevenin impedance equivalent to the system “behind” the

substation bus

The Thevenin impedance depends on the conditions of the system, such as the topology and system loading.

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Distance Protection

This relay is called impedance or “under-impedance” relay because the relay design is such that the relay operates for an impedance condition. The relay measures or “sees” a given impedance, equal to the ratio of the applied sinusoidal voltage and the applied sinusoidal current.

The advantages of the application of a distance relay in comparison to that of an overcurrent relay are:

Greater instantaneous trip coverage

Greater sensitivity (overcurrent relays have to be set above twice load current)

Easier setting calculation and coordination

Fixed zone of Protection, relatively independent of system changes, requiring less setting maintenance

Higher independence of load

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Distance Protection

Impedance Representation:

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Distance Protection

Impedance Representation:

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Distance Protection

Need for Directionality

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Distance Protection

Need for Directionality

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Distance Protection

MHO Relay (Directional Impedance relay)

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Distance Protection

• This example shows the calculations involved in the determination of a simple impedance relay setting.

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Distance Protection

• This example shows the calculations involved in the determination of a simple impedance relay setting referred to the secondary circuit.

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Distance Relay Timing and Coordination

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Three Zone Distance Protection

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Distance Protection

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Distance Protection

This form of polygonal impedance characteristic is provided with forward reach and resistive reach settings that are independently adjustable. It therefore provides better resistive coverage than any mho-type characteristic for short lines.

This is especially true for earth fault impedance measurement, where the arc resistances and fault resistance to earth contribute to the highest values of fault resistance. To avoid excessive errors in the zone reach accuracy, it is common to impose a maximum resistive reach in terms of the zone impedance reach.

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Distance Protection

Under-Reach - Effect of Remote Infeed• A distance relay is said to under-reach when the impedance presented to it

is apparently greater than the impedance to the fault.

Percentage under-reach is defined as:

ZR = intended relay reach (relay reach setting)ZF = effective reach

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Distance ProtectionUnder-Reach - Effect of Remote InfeedThe main cause of underreaching is the effect of fault current infeed at remote

busbars. This is best illustrated by an example.

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Distance ProtectionUnder-Reach - Effect of Remote InfeedThe main cause of underreaching is the effect of fault current infeed at remote busbars.

This is best illustrated by an example.

The relay at A will not measure theCorrect impedance for a fault on line section ZC due to current infeed IB .

Consider a relay setting of ZA +ZC .For a fault at point f, the relay is presented with an Impedance:

So for relay balance:Therefore the effective reach is:

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Distance Protection

Under-Reach - Effect of Remote Infeed

• It is clear from the Equations that the relay will underreach.

• It is relatively easy to compensate for this by increasing the reach setting of the relay, but care has to be taken. Should there be a possibility of the remote infeed being reduced or zero, the relay will then reach further than intended.

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Distance Protection

Over-Reach• A distance relay is said to over-reach when the apparent

impedance presented to it is less than the impedance to the fault.

Percentage over-reach is defined by the equation:

ZR = intended relay reach (relay reach setting)ZF = effective reach

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Distance Protection

Over-Reach (Effect of Out Feed Problem)

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Three-Step Distance Protection

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Pilot Protection

Pilot Protection (or teleProtection) is a generic name for the design of different transmission line Protection alternatives that use a communications channel.

• The most important advantage of pilot Protection is the provision of high speed tripping (permission) or to prevent high speed tripping (blocking) at all terminals for faults anywhere on the line.

• Without pilot Protection, high-speed tripping for all terminals will only occur for faults that are within the area where the zone 1 elements overlap.

• Pilot protection allows over-reaching zones of protection to ensure full protection of the line as well as high speed tripping.

Pilot Protection is typically applied to transmission lines with nominal voltage levels of 115 kV and greater.

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Pilot Protection

Directional Comparison ( V, I )Phase Comparison (I)

Differential (I)

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Pilot Protection

It is an adaptation of the principles of differential relaying that prevents the use of control cable between terminals.

Pilot refers to a communication channel between the ends of a line to provide instantaneous clearing over 100% of the line.

The communication channels generally used are:

Power line carrier,

Microwave

Communication cable

Fiber optics.

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Applications of Unit ProtectionBalanced Voltage Systems are used on feeder systems where CT’s

are away from one another with independent relays at both ends

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Fault fed from Both ends

• An internal fault fed from A and B increases the current in primary winding (11) and (11a) with a corresponding current reversal in (11a)

• This results in the induced voltage in (12a) adding to that in (12) - producing an operating torque in both discs - tripping both ends

A B

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Differential Protection of Transmission Lines

The ideal way of protecting any piece of power system equipment is to compare the current entering that piece of equipment, with the current leaving it.

Under normal healthy conditions the two are equal. If the two currents are not equal, then a fault must exist. This is the principle of “Differential Protection”, which is commonly used in TX Protection.

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The current differential relay is a unit protection intended for overhead lines and underground cables

CommunicationsChannel

Exchange of logic information on relay status

RL

Relays RelaysT

R

R

T

LI RI

Channel of Communication

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Direct optical fibre link – up to about 2 – 20 km, depending on the type of optical fibre. Used mainly for cables

For cables only

The data is digitised before being sent over an optical fibre. The comparison is on a per phase basis.

Power Line Carrier

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Pilot Protection

For comparison purposes, Pilot Protection can be divided into two groups,

• Directional comparison systems• Current-only systems.1. Directional comparison Protection uses the channel to exchange

information on the status of directional or distance elements at both terminals. If both elements operate, there is an internal fault.

2. If one of the elements operates and the other restrains, the fault is outside the protected line.

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Directional comparison Pilot Protection schemes are designed around sending one bit of data across the teleProtection channel at very high speed.

In some schemes, this one bit tells the other end that it has permission to trip (permissive).

In other schemes, the bit represents a signal to tell the other end not to trip (block).

There are many variations but the most prevalent Intertripping Schemes are the following:

Direct Underreaching Transfer Trip (DUTT)

Permissive Overreaching Transfer Trip (POTT)

Permissive Underreaching Transfer Trip (PUTT)

Directional Comparison Blocking (DCB)

Directional Comparison Unblocking (DCUB)

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Pilot Protection (Directional comparison)

1. Direct Underreaching Transfer Trip (DUTT)• When the zone 1 unit of the relays operate, they initiate a signal that is sent

along the communications link to trigger an immediate tripping at the other end of the line.

• The scheme is simple and has the advantage of being extremely fast; however, it has the disadvantage that it may set off undesirable circuit breaker tripping if there is any maloperation of the communication equipment.

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Bus

Line

Bus

Zone 1

Zone 1

2. Permissive Underreaching Transfer Trip (PUTT)• This scheme is similar to the DUTT scheme, but differs in that

the zone 2 unit at the receiving end has to detect the fault as well before the trip signal is initiated.

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Bus

Line

Bus

Zone 1

Zone 2

Zone 2

Zone 1

To protect end ofline

3. Permissive Overreaching Transfer Trip (POTT)

At the minimum, a POTT scheme requires a forward overreaching element at each end of the line. This is typically provided by a Zone 2 element set to reach about 120%-150% of the line length. If each relay sees the fault in the forward direction, then the fault can be determined to be internal to the protected line.

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Bus

Line

Bus

Zone 1

Zone 2

Zone 2

Zone 1

Local Local RelayRelay Remote Remote RelayRelay

Remote Remote Relay FWD Relay FWD II GNDGNDLocal Relay Local Relay –– Z2Z2

ZONE 2ZONE 2

Local Relay Local Relay FWD IFWD IGNDGND

TRIPTRIP

Remote Relay Remote Relay –– Z2Z2

POTT TX

ZONE 2ZONE 2

POTT RX

CommunicatioCommunicatio n Channeln Channel

POTT Scheme

Presenter

Presentation Notes

In the POTT scheme, the Remote relay speeds up the tripping of an end zone fault by sending a permission to Trip key from the Remote relay to the local relay under 2 circumstances. ----The first reason that the remote relay will send a Permissive key is when the it detects a fault occurring within it’s over-reaching zone 2. This is where the expression “over-reaching” comes from in the term Permissive Over-reaching Transfer Trip. ---The second reason that the remote relay will send a permissive key is when it detects that ground directional overcurrent is flowing in it’s forward direction if this feature is enabled and configured. -----Therefore either Negative sequence directional overcurrent Forward element or the Neutral directional overcurrent Forward element, if configured, will send a POTT key to the Local relay, if both are configured,----- as well as when the over-reaching zone 2 pickup flag turns ON The Local relay POTT logic will only cause the breaker to trip ---if it gets the POTT key from the remote relay in the form of a receive AND, -----the local relay has detected a fault within it’s zone 2 area of protection OR, -----it detects that ground directional current is flowing in it’s Forward direction if this function is configured ----Therefore either the local relay’s Forward Negative sequence directional overcurrent element or the Forward Neutral directional overcurrent element as well as the picking up of a Zone 2 fault, will cause the POTT scheme to trip the breaker if it receives a permissive key from the remote relay.

4. Directional Comparison Blocking (DCB)

In a directional comparison blocking scheme, each line terminal has reverse looking elements (Zone 3) and forward overreaching elements (Zone 2). The relay will send a block signal to the remote end if it sees the fault in the reverse direction.

Relay detection of a fault in the reverse direction indicates that the fault is outside of the protected zone. The logic allows the relay to trip if it sees the fault in the forward direction and does not receive a blocking signal from the remote end.

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5. Directional Comparison Un Blocking (DCUB)• Same Basic Logic as POTT Scheme

Allows tripping for short time when channel fails

May Overtrip for External Fault

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Phase-comparison and current-differential systems only use current information. The figure depicts a schematic diagram of current-only systems.

Phase-comparison systems compare the phase of the currents at the line terminals. For internal faults, these currents are in phase. For external faults, the currents are approximately 180o out of phase

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Pilot Protection (Current based comparison)

OO

Pilot wires

SUBSTATION 1 SUBSTATION 2

O: Operating unitsFault current

Internal fault = Current flow through the operating units= Trip

Load current

Load current = No current through the operating units = No trip

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Pilot Protection (Differential)

The differential current is not exactly zero for external faults.

The most common causes of false differential current in transmission line differential relays are the following:

Line charging current

Tapped load

Channel time-delay compensation errors

Current transformer saturation

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Pilot Protection (Differential)

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Topic 7: Pilot Protection