powerstream - ms protection philosophy

8/9/2019 PowerStream - MS Protection Philosophy

1/11Filename: MS Protection Philosophy Page 1 of 11

Discussion Paper

Title: Substation Protection PhilosophyPrepared By: D. Fairchild / G. Allen Status: FinalNumber: DP025 Date: Dec 7, 2009

REVISION NOTES

April 12, 2006 – 44 kV single phasing protection was adopted by Barrie Hydro.November 24, 2009 – Adapted for PowerStream Municipal Substation (MS) protections.

1.0 Preface

The purpose of this discussion paper is to outline PowerStream’s substation protectionphilosophy. Feeder, transformer and bus protections will be identified along with settingguidelines.

General coordination goals are discussed and taken into consideration in the protectionsettings. A common protection philosophy is applied to all stations. However, theapplication of the philosophy will vary depending on station size and voltage.

2.0 Introduction

PowerStream currently owns and operates over 50 municipal distribution substations.These Stations are supplied from 44 kV subtransmission circuits and 28kV distributioncircuits. They step the voltage down to one of three distribution levels. Each stationtypically consists of 3 to 4 feeders, supplying a combination of three phase and single-phase loads. Typically these loads are protected with fuses at their point of connectionto the feeder.

Distribution line protection of radial feeders is performed using overcurrent (50, 51, 50N,51N) elements. Current only, is used to detect faults on the feeders. The basic principlelies in the fact that short-circuit currents are generally larger than load currents. In mostcases, there is a separation between the load-current region and the fault current region.It is then possible to set the overcurrent relays to operate in the fault current region.

However depending on the electrical characteristics of the system, fault current canapproach load currents (for example at the end of a long heavily loaded feeder). Caremust be taken to ensure that this is avoided. Short circuit studies are completed in orderto determine maximum and minimum fault current levels for a given substation and itsfeeders. Refer to discussion paper “DP022 – Municipal Station Available Fault Levels.”These values are used to select relay pickup settings, which in turn establishes zonecoverage and reach characteristics of the feeder protections.



3.0 Protection Design Principals and Protection Settings Guidelines

A description of the Municipal Substation protection systems and associated protectionsetting guidelines is described in the following sections.

3.1 Feeder Protection Design Principals

The basic principles are used in designing a distribution protection system, are asfollows:

1. The first is to treat all faults initially as temporary.2. Second, lockout should only occur when it has been determined that a fault is

permanent.3. Third, the smallest possible portion of line should be removed from service in

the case of a fault.4. Fourth clear the fault as quickly as possible to minimize hazard to the public,

damage to equipment and to minimize the impact on power quality.

At the time of writing, PowerStream has implemented two feeder protectionphilosophies, as follows:

Fuse saving, typically applied on rural feeders in PowerStream North – A fuse savingprotection scheme allows the feeder breaker to clear non-permanent faults on the entirefeeder with out blowing sectionalizing fuses. In the event of a permanent fault beyond afuse, the fuse will blow to isolate the faulty section. This philosophy is currently used onboth the overhead and underground system.

A fuse saving feeder protection will operate, as follows:

• Transient fault: – On the feeder, before the first fuse - The feeder will trip on 50a and

lockout. – On the feeder, beyond the first fuse - The feeder will trip on 50b and

reclose. – On the load side of a lateral fuse - The feeder will trip on 50b and reclose.

• Permanent fault: – On the feeder, before the first fuse - The feeder will trip on 50a and

lockout. – On the feeder beyond the first fuse - The feeder protection will:

• trip on 50b and reclose, then• trip on 51 and lockout.

– On the load side of a lateral fuse - The feeder protection will:• trip on 50b and reclose, then

• The fuse will clear the fault.

Trip Saving, typically applied on Urban feeders in PowerStream South – A trip savingprotection scheme allows the feeder breaker to clear transient and permanent faults onthe feeder. Faults on the load side of lateral fuses are cleared by the associated lateralfuse.

A trip saving feeder protection will operate, as follows:Transient fault:



• On the feeder, before the first fuse - The feeder will trip on 50a andlockout.

• On the feeder, after the first fuse - The feeder will trip on 50b andreclose.

• On the load side of a lateral fuse - The fuse will blow and clear thefault.

Permanent fault:• On the feeder, before the first fuse - The feeder will trip on 50a and

lockout.

• on the feeder, after the first fuse - The feeder protection will:• trip on 50b and reclose, then

• trip on 51 and lockout.• on the load side of a lateral fuse - The fuse will blow and clear the

fault.

The basic coordination strategy first established coordination pairs starting at the loadand moving towards the source. The maximum load current is then determined. Thepickup of all overcurrent devices can then be set, based on short circuit values,maximum load current and equipment ratings.

Typically protections of underground feeders do not incorporate a reclosing scheme, inthat underground faults are nearly always permanent. It is recommended that feederswhich are 80% or more underground not be permitted to reclose. For the most partPowerStream underground feeders have significant sections of overhead line andtherefore the aforementioned would only apply to a select few feeders on the system.

3.1.1 Feeder Phase Overcurrent Protection

Feeder phase protections are intended to operate for phase-to-phase and three phasefaults.

The pickup of a phase overcurrent relay, used to protect a primary distribution feeder,must be greater than the maximum load that will be served. A fault to load ratio (FTLR)of 4:1 (3:1 minimum) is generally used. The fault current magnitude, which a phase relaymeasures, decreases as the distance to the fault increases. For very long feederswhere the FTLR is less than 3:1 a phase distance relay can be used.

A Time Current Curve (TCC) chart for a typical feeder phase protection, employing the“trip saving” philosophy is shown below.



A discussion of the settings for each of the timed and instantaneous elements follows.

Phase Timed Overcurrent Protection (51)

The phase timed overcurrent protection (51) is designed to provide coordinatedprotection with other downstream devices.

Pick-up Setting

The pickup of the phase timed overcurrent protection must be greater than themaximum load that will be served. The feeder load current should not exceed50% of the pickup value.

The pickup and time dial must also be set low enough to detect and clear end-of-line faults (E.O.L) in a reasonable time. The feeder phase timed overcurrentprotection is designed to protect the entire length of the feeder. This includeslaterals and sub-laterals. Multiplying the E.O.L. 3-phase fault current by 2

Typical Feeder Phase Protection

0.01

0.10

1.00

10.00

100.00

100 1000 10000 100000

Current (A @ 27.6kV)

T i m e ( s )

High Set

Instantaneous

Low Set

Definite Time

U3: US Very Inver se

Time Overcurr ent



provides a margin for fault resistance. For three phase faults this equates toapproximately twice the total impedance and 1.73 times the total impedance forphase-to-phase faults (Source: SEL Prot-403).

The pick-up setting also needs to be less than 0.8 of bus backup pick-up setting.

Time Dial Setting & Inverse Timed Overcurrent Curve Type

It is recommended that end-of-line faults be cleared in 2.5 seconds maximumunder normal conditions and 3.5 seconds maximum under contingencyconditions. Normal conditions will be defined by the system configuration with allof its normally open points in their open state. Contingency conditions will bedefined by the system configuration other than being in its normal state.

Guidelines for desired timed element operating times are, as follows:• 0.3 Seconds slower than slowest tapped fuses on feeder• Clear minimum feeder end fault within 1.5 seconds

These operating times are achieved by selecting the appropriate time dial settingand inverse timed overcurrent curve type.

A graph showing the five standard curve types is shown below.

The Extremely Inverse curve is used in most instances, because, as compared toother curves, provides the fastest clearance of faults at higher current values.The extremely inverse characteristic closely approximates fuse curves, to whichthe feeder overcurrent relay most often must coordinate to obtain selective fault



clearance. The extremely inverse characteristic is also the preferableovercurrent characteristic for feeder “cold load pickup”. The longer operatingtime at low values of overload current, inherent with the extremely inversecharacteristic, can prevent unnecessary operations.

Once the desired curve has been selected; the time dial setting is calculated using the

formulas shown above.

A feature called “electromechanical reset timing” is implemented on all feeder relays atstations with primary fusing. This feature emulates the reset characteristics of aninduction disc on an electromechanical relay. Basically the feature causes the feederbreaker to trip faster should it re-close into a fault. In the event that a feeder’s low setprotections are blocked, it is possible that two timed trips could occur. The preheatingeffect of the power fuse reduces its time-curve, which could result in a coordination



problem. Electromechanical reset provides an added coordination margin between thefeeder and power fuse. This feature was implemented based on past experience.

Phase High Set Instantaneous Protection (50a)

Phase High Set Instantaneous Protection (50a) is set for fast clearance of close-in faults,

as well as protecting a portion of the feeder emanating from the station.

In order to avoid locking out a feeder for faults located downstream of a fused cutout, thephase high set instantaneous element must be set greater than the maximum availablefault current at the first fused device.

The 50a Pick up setting is:

• Greater than the Ø-Ø fault current available at closest tapped fuse.

• Low enough to detect a fault on the station feeder egress cables.

The high set instantaneous protection shall cause the feeder breaker to lockout in the

event the element is asserted. This protection is never to be block in any protectionscheme.

Phase Low Set Instantaneous Protect ion (50b)

The phase low set protection (50b) is Intended for transient faults, this protection shouldbe blocked after a reclose, to allow coordinated time tripping. It should be set to providecoverage of the entire feeder.

The 50b Pick up setting is:

• Equal or greater than 2 times maximum load• Less than 0.5 feeder end Ø-Ø fault level

A Delay Setting, to implement the trip saving philosophy, is added to the 50b element topermit tapped fuses to clear first for Ø-Ø faults.

Low Set Block (Field Switching)

As a result of PowerStream’s sensitive ground protection, and most recently theintroduction of the phase low set instantaneous protection, there is a need to block theseelements during switching operations. Any circuit unbalance above the ground elementsminimum pick-up is seen as a fault. Typically an unbalance can be caused from singlephase switching of large load or energizing a large number of single-phase transformersat once.

The low set instantaneous protections must be blocked remotely in order to avoidtripping the breaker. Operating Policy and Procedure C0713 addresses this issue.

3.1.2 Feeder Ground Overcurrent Protection

Ground overcurrent relays can be set to be more sensitive than phase overcurrent relaysbecause their pickup can be set without regard to maximum loading. The maximum



imbalance in the load is the only consideration to determine the pickup. The increasesensitivity does however come at cost. The protection is so sensitive that trips can occurduring single phase switching and other temporary unbalances. PowerStream hasattempted to address this with a unique blocking scheme, discussed later.

The majority of system faults, as high as 80 to 90 percent, involve ground in one form or

another. The exception to this is three-phase and phase-to-phase faults, which are rareoccurrences. Clearly it makes good sense to employ ground protections on thedistribution system since ground fault currents almost always pose additional risk topublic safety and equipment damage.

Guidelines for feeder ground overcurrent protection settings are shown below:

Feeder Ground High Set Instantaneous Overcurrent (50Na)

A ground high set instantaneous overcurrent element is used to quickly clear temporaryground faults.

Guideline for Ground High Set Instantaneous Overcurrent (50Na) – Pick up setting > Ø-G fault current available at closest tapped fuse

Feeder Ground Low Set Definite Time Overcurrent (50Nb) – Prior to Reclose

The ground low set definite time overcurrent element is used to clear temporary groundfaults

Guideline for Ground Low Set Instantaneous Overcurrent (50Nb) – Pick up setting:

• Equal or greater than 2 times maximum load unbalance current• Less than 0.5 feeder end Ø-G fault level

– Delay Setting – permit tapped fuses to clear first for Ø-G fault-Trip Saving

Feeder Ground Timed (51N)

Guidelines for Timed Overcurrent (51N) settings, are as follows: – Pick-up Setting:

• Equal or greater than 2 times maximum load unbalance current• Less than 0.5 feeder end Ø-G fault level• Less than 0.8 ground bus backup pick-up

– Time Dial Setting:• 0.3 Seconds slower than slowest tapped fuses on feeder• Clear minimum feeder end fault within 1.5 seconds

Reclose (79)

All PowerStream feeders are permitted to perform a single shot reclose attempt.Feeders that are predominately under ground (80% or more) will not attempt a reclose.



Under ground faults are generally permanent in nature and as such a reclose attempt isnot warranted (the feeder will trip on timed over current protection).

In some stations the reclose maybe blocked locally or remotely. These two operationsare independent of each other, meaning that if the reclose is blocked remotely it can notbe unblocked locally and vise-versa.

Cold Load Pickup

The cold load pickup feature is designed to elevate the phase overcurrent pickup of thefeeder relay upon loss of load diversity.

Cold load mode is automatically enabled after the feeder breaker has been open for apredetermined period of time, on some stations it may also be invoked remotely throughOperator control via the SCADA system. The cold load mode will remain in effect for apredetermined period of time after the feeder breaker has been closed.

If cold load is being picked up other than from closing the feeder breaker (i.e. field

switching), the low set instantaneous elements must be blocked. Currently this blockingfeature can only be set remotely from the control room SCADA system.

Feeder Protection Logic

The feeder protection logic is shown in Appendix ‘A’

3.2 Transformer Protection

The majority of power transformers are protected with high side fuses. The larger 20MVA power transformers on the 44/13.8 kV system are protected with high side circuit

breakers or circuit switchers.

The transformer protection is designed primarily to protect the transformer fromdamaging fault currents. A certain degree of overload protection is also provided withhigh side fusing. However, this is the primary job of the transformer backup protection.

The transformer protection must coordinate with both upstream and downstreamprotections. Refer to discussion paper “DP021 – Municipal Station Transformer Fusing”for the proper selection of the power fuse.

Combined Primary Transformer/Bus Protection

Stations with either a high side circuit breaker or circuit switcher are equipped withdifferential protection. Designed to detect internal transformer faults or secondary busfaults, the energy into the transformer is compared to the energy flowing out of thesecondary bus. If this energy is greater than a predetermined set point, the differentialrelay is operated, which trips off the entire station. Currently this protection isimplemented on Barrie’s 20 MVA, 13.8 kV stations and MS415/419.



Transformer backup phase overcurrent protection at Barrie 13.8 kV stations withdifferential relays coordinate with the transformer damage curve. Current inputs comefrom 44 kV current transformers.

Transformer Backup Protection

The transformer backup protection is designed to provide overload protection for thetransformer. In the case of a high side circuit breaker or circuit switcher the transformerprotection and transformer backup protection are one in the same. The main differencebeing the location of the protection CT’s. The bus backup relay is also provided to clearfaults on feeders where they are not cleared by the feeder protections.

Backup Phase Time Overcurrent (51B-T)

The recommended pickup setting of the phase time overcurrent element is 1.2 times theemergency transformer rating and 0.5 times the minimum three phase fault. The timedial is set to coordinate with both the feeder phase time overcurrent element and thetransformer power fuse. The maximum available fault current is considered when

coordinating with the feeder protection. The maximum phase to phase available faultcurrent is considered when coordinating with the power fuse.

Note: The 51B-T protection is intended to protect the transformer from fault conditions. Itis not intended to protect the transformer from accidental overloading.

Transformer Rapid Pressure Relay

Power transformers equipped with a rapid pressure relay are designed to isolate thetransformer in the event of an internal fault. The thermal and mechanical stress resultingfrom such a fault creates a rapid rise in pressure internal to the tank. This pressure is

detected by the relay, which in turn drives an output contact tripping the entire station. Inthe case of MS331, which is equipped with a rapid pressure relay, there exists noprimary device to automatically isolate the transformer.

Breaker Failure

In the rare event that a breaker should fail to clear a detected fault, the breaker failurerelay will issue a command to a lockout relay, tripping off the entire station. Howeversome stations are not equipped with an upstream device to trip and rely on the primaryfuse. In this case the breaker failure feature is used strictly for annunciation purposes.

3.3 Bus Protection

Municipal Substations do not typically have dedicated bus protections. In some casesthere is no bus protection and in other cases the bus protection is combined with thetransformer protection (please section 3.3). This document will discuss bus protection ina general sense and should not be considered to be a completed coverage of MS busprotections.



Bus Differential (87)

Where there is a dedicated bus protection, the bus differential protection will be theprimary bus protection. The bus protection will trip all breakers attached to the bus.

Backup Phase Time Overcurrent (51B-Bus)

The bus phase overcurrent pickup is set greater than 1.2 times the maximum emergencyloading of the station; and preferably equal to 0.5 times the minimum three-phase busfault.

The timing is normally set to trip in 1.2 seconds under a maximum three phase bus fault.The time dial setting should provide, at a minimum, a 0.4 second co-ordination intervalbetween the bus and feeder protections.

Backup Ground Time Overcurrent (51NB-Bus)

The bus ground timed overcurrent element pick-up should be set above the combined

neutral unbalance permissible at the transformer neutral. The setting should also be lessthan 0.5 times the minimum line-to-ground bus fault current.

The timing is normally set to trip in 1.2 seconds under a maximum three phase bus fault.The time dial setting should provide, at a minimum, a 0.4 second co-ordination intervalbetween the bus and feeder protections.

3.4 44 kV Single Phasing Protection

Based on Schweitzer discussion paper AG97-11, PowerStream implemented thedetection of single phasing. Typically such a condition occurs when a single power fuse

operates or there is a loss of a 44 kV supply phase. The resulting condition causesunbalanced voltages to be applied to the transformer bank and its connected load. Thisis an undesirable power quality condition, particularly to three phase customers withinduction motors.

The recommendation to implement this protection was adopted April 12, 2006. In theevent a single phasing condition is detected the substation is disconnected from thepower system after 10 seconds. Currently the 86 relay is used to trip the feederbreakers. The relay must be manually reset before the breakers can be remotelyclosed. It is recommended that the tripping scheme be automated such that power canbe restored remotely.

powerstream - ms protection philosophy

Documents