powernex associates inc

PowerNex Associates Inc.Power System Operation/Electricity Market Operation Overview

Module 3 Transmission and

Distribution

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ex Associates

Inc.


Module 3Transmission and

DistributionLearning Objectives:

To gain an understanding of the following:

Transmission system overview (4A) Components Types of transmission limits Major transmission limits in Ontario

Protection Control and Metering (4B)

Concepts of Special Protection Systems (4C)

Distribution (4D) How it’s different to transmission


Module 3ATransmission


Module 3ATransmission

What will be covered

Basic Transmission Components

Overview of Transmission System

Limits - where do they come from & why do we need them

Internal Key Interfaces and effects on Generation

Normal, High Risk and Emergency Operation

Ontario Interconnections

Transmission Impactive Outages


Transmission









Transmission The Single Line Diagram

The Single Line Diagram is a simple representation of the power system or a portion of it.

It shows the system as only one phase rather than three.

Its purpose is to show the power system with minimum

detail, ie an overview of what’s connected to what.


Transmission Basic Transmission Components

Single Line Diagram


Transmission

SF6 Circuit Breaker


Transmission

Air Blast Circuit Breaker


Transmission

Bulk oil circuit breaker


Transmission

Disconnect switch


Transmission

What happens when a disconnect switch is used instead of a circuit breaker?

Not pretty!


Transmission

A 3 phase Transformer


Transmission Power Transformer


Transmission

Transmission Lines in parallel, if lose one, flow is redistributed

Generally not so for Distribution lines (radial)

Parallel lines lower the impedance, the more parallel lines the higher the reliability and the lower the losses (I2R)

The higher the voltage, the greater the power carrying capacity (proportional to V squared)

Maximum power carrying ability at Surge Impedance Loading (when reactive inductance and reactive capacitance of line are equal and thus cancel each other, leaving only the resistance).


TransmissionTypical Power Grid System


Water Flow Analogy for Electricity Transmission System

Transmission


Transmission Electrical Equivalent to Water Analogy

Electrical Bus (Bucket)-Critical Voltage, Current, Frequency & Short Circuit Level

Generator (tap - water supply)-Provides Energy (MW), voltage support, &frequency support

Transmission Lines (Pipes)Impedance (Fixed Resistance to Flow)

Variable Loads (tap - water removal)Absorb MW and Voltage Support, Critical Voltage and Frequency Levels


Transmission

Typical Surge Impedance Loadings

500 Kv - 1,000 Mw 230 Kv – 200 Mw 115 Kv – 50 Mw

Ontario System made up of 500 Kv, 230 Kv, 115 Kv.

Distribution voltages < 50 Kv


Transmission

Double circuit EHV Transmission line


Transmission









Transmission Historical Note1965 Northeast Blackout

One 230 kV circuit at Beck (Q29BD) tripped

Four other circuits at Beck cascade trip within 2.7s

1700 MW power surge into New York causing a wide-spread blackout

NPCC formed to ensure utilities in the northeastern part of North America adopt practices to prevent another blackout


Transmission


Transmission

Three basic types of limits: Thermal

Voltage Decline/ Rise

Stability

Also Short Circuit limits


Lines designed to operate to specific “ground” clearance and maximum conductor temperature

Line clearance reduced as conductor temperature rises (sag)

Ground clearance decreases as Current flow increases

Ambient temperature rises

Wind velocity decreases

Sunlight increases

Safe Ground Clearance

Transmission

Thermal Limits


Transmission

Thermal Limits – Limited Time Ratings


Transmission

Thermal Limits also apply to equipment other than lines

For example: Transformers

Rated in MVA

Require sufficient cooling to dissipate heating

Hot spot and oil temp limits

As with lines, limited time ratings


Transmission

Voltage Limits

Must be able to sustain Voltage levels both pre and post contingency

Low or high voltages can cause equipment damage to Hydro One or Generator assets and also customer equipment

Under normal conditions, continuous voltages are to be maintained within predefined levels.

For example:

115 Kv voltage must be between 127* Kv and 113 Kv 230 Kv voltage must be between 250Kv* and 220 Kv 500 Kv must be between 550 Kv and 490 Kv

* In Northern Ontario 132 Kv and 260 Kv


Transmission

Voltage Limits

Transformer Voltages

Steady State Ratings, Maximum Acceptable Levels

110% of Input Winding Rating,

105% of Output Winding Rating at Full Load


Transmission

Stability Limits

These are the most complex limits

Instability can cause cascading outages

Affects generators, they go out of synchronism, “pole slipping”, “out of step” are terms used.

Stability usually a problem on a system with long transmission lines

If the receiving end voltage “angle” lags the sending end voltage “angle” by 90 or more degrees, then unstable


Transmission Stability


Transmission Summary of Security Criteria

Voltage Levels, meet customer and equipment voltage limits

Stability, acceptable damping

Element Loading, operate to appropriate thermal rating of equipment

Short Circuit, breakers have capability of clearing worst short circuit condition


Transmission “Bad Things happen”

What bad things can happen?

Virtually anything, but must be practical and reasonable.

Following 1965 Blackout NPCC developed a practical list of “bad things”

All members of NPCC must operate their systems by being able to recover from events on this list without having adverse effects on the systems of other members.

This costs money due to congestion. But the costs of a cascading blackout far outweigh the congestion costs.


Transmission Recognized “Bad Things” (Contingencies) - NPCC

Criteria Permanent 3-Phase fault (worst kind of fault) – with normal fault clearing

Simultaneous permanent phase to ground faults on adjacent circuits (same tower) – with normal fault clearing

Permanent phase to ground fault on any generator, circuit, transformer or bus section - with delayed fault clearing (Breaker Failure)

Loss of any element without a fault

Permanent phase to ground fault on circuit breaker - with normal fault clearing

Failure of a circuit breaker, associated with a Special Protection Scheme, to operate


Transmission How are limits developed?

Limits are developed using computerised simulation studies.

A data base representing all of the power system components, their electrical characteristics and their connectivity has been developed and is constantly being updated when new equipment is added to the system.

This data base also includes data on interconnected systems (electrically it’s all one big system)

Application software is used by engineers to run fault simulation studies and test the operation of the system following a particular fault.

From the results of these studies operational security limits are produced.



Thermal Limits Fairly simple

Off line load flows predict the change in flows on remaining elements post contingency, ie distribution factors

In real time these distribution factors are used to predict change in loading on the remaining elements following a contingency

Thus the operator can determine pre-contingency loading on other lines



Voltage Limits

Developed by off line simulation studies

Use NPCC criteria to “throw” faults at the system

Calculate the post contingency effects on voltage

Are post contingency voltages within limits? If OK move on to next simulation. If not OK then reduce pre contingency loadings (by redispatch) in the simulation until can meet post contingency voltage criteria.

This then becomes the Operating Security Limit.

Pre-contingency loadings not to exceed these.



Stability Limits

Developed by off line simulation studies, simulating the system dynamics

Use NPCC criteria to “throw” faults at the system

Calculate the post contingency effects on stability

Is the system stable pre contingency, during the contingency and post contingency? If OK move on to next simulation. If not OK then redispatch system to reduce pre contingency loadings in the simulation until post contingency stability achieved.

This loading limit becomes the Operating Security Limit.

Pre-contingency loadings must not exceed these.


Transmission How are Limits Developed?

Some notes re simulation studies performed:

time consuming

only study a limited number of contingencies and system conditions (eg winter peak, summer minimum, summer peak)

study conditions set up for most severe contingency usually at peak power transfers

successful study results reduced (nominally10%) to provide acceptable margins

results simplified for easier computer monitoring


Transmission Operating Security Limits

System needs to be secure

Pre-contingency

Post-contingency

Stable During Contingency (Transient Stability-non faulted generators not removed from system, acceptable equipment operation during/immediately after fault clearing, non cascading outages)


Transmission Internal Ontario Key Interfaces

Because of the dynamic nature of the power system and its multiple parallel paths, limits generally are not expressed in terms of individual line loadings (other than some thermal limits)

Rather, limits are expressed in terms of interface flows and are called Operating Security Limits.

An interface is defined as a group of Transmission lines and the limit is expressed as the sum of the flows on this group of lines.


Transmission Interface Limit Characteristics

‘Base’ limit

All transmission facilities are in-service

Directional

Certain outages result in a penalty in MW

Some limits simple constants;

others more complex, and have multiple parameters including other limits!


Transmission Historical Flows


Transmission

The End


Module 3BProtection, Control and

Metering


Protection, Control and Metering


Introduction to protection,

how and why.

Introduction to control

Intoduction to metering,

revenue metering, operational metering, telemetering



All Elements of a power system must be protected from faults

All Power system elements must be monitored (status, loading, etc). Much of the power system is operated (switching, hydro unit loading) under remote control.

All generator output and all customer load must be metered (revenue grade metering)

Loading on lines, transformers etc must also be metered (non revenue grade metering)



To provide protection, control and metering, must constantly take real time measurements of system conditions

Current, voltage and frequency measurements are the basis of all protection and metering.

Current and voltage provided with the help of Instrument Transformers

Not to be confused with Power Transformers


Protection, Control and MeteringInstrument Transformers

On a 500kv line the voltage from line to ground is ~ 290,000 volts and current can be in the hundreds of amps.

Use instrument transformers to get proportional volts and amps which can be handled by relays, meters etc

These are called Voltage Transformers (VTs) and Current Transformers (CTs).

Sometimes VTs are referred to PTs (Potential transformers), they’re synonymous

VTs are connected between the line and ground and have a turns ratio such that a secondary voltage of 120v represents rated primary voltage.

CTs are connected in series with the line and have various turns ratios that can be selected. For example 1000:5,1600:5 etc



Voltage Transformer



Instrument Transformers

Examples

On a 500 kV line

500 Kv is equivalent to 120 volts 1000 amps is equivalent to 5 amps

On a 230 Kv line

230 Kv is equivalent to 120 volts 1000 amps is equivalent to 5 amps



Relays

Relays are sensing devices which operate when the monitored quantities reach certain thresholds.

They then send a signal to operate a device such as a circuit breaker

Examples later


Protection, Control and MeteringMeters

Read monitored quantities

Voltage Current Power (calculated in the meter) KVA (calculated in meter) KVAR (calculated in Meter) Frequency

Display and/or store instantaneous quantities as well as integrated quantities

Instantaneous Current Voltage Frequency

Integrated Power Mw Reactive Power Mvar



Operational Metering Used for

Operating Statistics Loading trends All generators, transformers, lines have this ~ +/- 3 % accuracy

Revenue Metering Used for

Billing mainly but can also be used for above purposes Pre market only included customers, interties but not generators Now all generators (aggregates) have revenue meters High accuracy ~ +/- 0.3%, expensive Expensive because of CT and VT accuracy, especially High

Voltage


Protection, Control and MeteringTelemetering

To provide real time metered quantities at a central location eg IESO, OPG, HydroOne Grid Control Centre etc

Meter outputs sent to Remote Terminal Unit (RTU) and from there to central location via communication channel.



Metered data from Revenue Meters and from Operational Meters are supplied to various applications used by generators, transmitters and IESO

Confidentiality of information



All Elements of a power system must be protected from faults

Must eliminate all sources of infeed to the fault

Must be accomplished with high speed (within 2 to 3 cycles, 1 cycle = 1/60 of a second)

Must not isolate more equipment than necessary



Fault Clearing Devices Must be capable of Interrupting fault level (short circuit concerns)

Capable of Clearing only Faulted Zone (Zone tight relaying)

Fast Acting to Protect Equipment and Limit Cascading Faults

Must Interrupt all sources of fault infeed



Fault Clearing Devices Normally

accomplished with

- combination of protective relaying scheme, 3 phase circuit breaker and high speed communication media to send trip signals to remote terminals



Example of a transformer protection.

Differential Protection

If I1 + I2 > 0 then a trip signal is sent to the circuit breaker to remove transformer from service



Fault Clearing Device – Concerns

Fault Clearing Device Failures Mitigated by:

Breaker failure schemes Duplicated protections on all major Transmission

elements (A and B protections, expensive!) Duplicate communication equipment



Example of differential protection on a transmission line



Examples of protections on the power system Generators Overspeed (mechanical) Reverse Power Thermal Stator Ground Inadvertent synchronisation Overcurrent

Transformers Differential Overcurrent Gas accumulation

Busbar Differential Overcurrent

Transmission Line Impedance Differential



A ground fault on the low voltage side of this substation creates an arcing fault.

Unfortunately, protection hardware fails to open the high

voltage side.



Protection panel in a relay building



IESO has no direct physical control of power system facilities (switching, unit loading etc). It only give orders!

However some switching is done automatically, such as fault switching, generation rejection, capacitor and reactor switching, transformer tap changes and such like.

But by and large all physical control of power system elements is performed by the transmitters and the generators.

Examples: synchronising and desynchronising generating units, switching at transformer stations, isolation of equipment for maintenance



Demarcation point between Hydro One and Generator facilities generally at the high voltage disconnect switch. Generator owns the switch.

All upstream facilities in the switchyard and beyond are owned and operated by Hydro One.

There are exceptions



Transmitter All control of switching on the HV system now takes

place remotely from Hydro One’s Grid Control centre in Barrie.

Can remotely control all switching on 500 Kv, 230 Kv and 115 Kv systems. Controls some 44 Kv where parallel paths exist (non radial)

Controlled by SCADA system (System Control and Data Acquisition)


Protection, Control and Metering Generator

Operators must be able to monitor and control equipment remotely

In staffed locations, eg Fossil stations, generator and plant control systems may be hard wired to the control room

As most hydroelectric stations are now operated remote from the site, digital control systems are normally used



Data, metered quantities, status of equipment (open or closed) is fed from device to a Remote Terminal Unit (RTU)

From RTU data is digitised and transferred to a local Central Processing Unit (PU), basically a computer.

From there to a master CPU at the control centre where data is processed and presented to the operator on his/her screen.

This is a two way street, Operator receives data and Operator can send instructions (eg load a unit, desynchronise a unit, increase or decrease VArs etc)

Susceptible to failures, some depend on a third party communication path,

Alternative is to send agent to site to perform manual operations.


Protection Control and Metering


Protection, Control and Metering All Nuclear units controlled locally

All Fossil units controlled locally

All NW hydroelectric units controlled remotely from Thunder Bay (NWCC)

All NE hydroelectric units controlled remotely from Porcupine (NECC)

All Ottawa River and Madawaska River units controlled remotely from Chenaux

Saunders units controlled locally

All Beck and Decew units controlled from Beck



The End


Module 3CSpecial Protection Systems

(SPS)


Special Protection Systems


What they are

Why they are needed

How they work


Special Protection Systems (Virtual Transmission)

Used to expand operating security limits, post contingency, therefore can operate to higher limit pre- contingency

Fast acting, are triggered by the contingency

Generation rejection (used the most)

Load rejection

Capacitor switching (provides reactive compensation to raise voltage levels and reduce phase angle between voltage and current)

can be switched automatically as part of special protection scheme

Reactor switching (provides reactive compensation to lower voltage levels and can also be used to control short circuit levels)

often used pre-contingency to lower system voltages, switched out-of-service post contingency


Special Protection Systems Generation Rejection

Increase Power Transfer Capability of limiting components of Grid,

Stop Gap measure originally but continues in use

Improve Generation Resource availability



A 230 kV shunt capacitor bank

Is switched in to control low voltage and out to control high voltage

Does not necessarily have to part of an SPS, can be normal daily operation


Special Protection Systems Some Generation Rejection

Examples Beauharnois/Saunders Chenaux/Mountain Chute G/R scheme Stewartville G/R Darlington G/R Lambton G/R Lower Notch G/R Otto Holden G/R (Run Back)

Bruce Power G/R



P502X/D501P/L20D/L21S G/R & LR

NE 115 kV L/R and G/R Scheme

Abitibi Canyon G/R Scheme

Lower Notch G/R Scheme

Moose River Basin G/R for loss of ExV or XxE



The End


Module #3DDistribution


Distribution Interface with Transmission

System 500 kV

230 Kv

27.6 kV

Distribution lines

Distribution lines


DistributionInterface with the transmission system

Distribution voltages generally assumed to be < 115 kV

Various distribution voltages:

66 kV subtransmission – not common in Ontario

44 kV subtransmission – common in rural Ontario

27.6 kV subtransmission – very common in Ontario, especially in urban areas

13.8 kV

4 kV


Distribution

Transmission lines are connected in parallel, if one is lost then flow is redistributed automatically

Generally not so for Distribution lines (radial), especially in rural areas.

When there is a permanent fault then switching has to occur to reroute power flow, will result in an outage (hopefully short)

Most faults (>90%) are transient and protection will open the feeder breaker, followed by an automatic reclosure. If the fault was transient (eg caused by a lightning strike) then customer will only see a momentary flicker.


Distribution Transmission Parallel, Distribution

Radial500 kV

230 Kv

27.6 kV

Distribution lines

Distribution lines


Distribution Typical Hydro One interface between Transmission and

Distribution

To various customer loads

To various customer loads

27.6 kV radial distribution

feeders.

Each transformer is capable of supplying

the entire station

Normally open

Double circuit 230 kV parallel

transmission lines

Normally open

Busbar 1 Busbar 2


Distribution


Distribution

Distribution equipment is less complicated

Maintenance of receiving end voltage is important; capacitor switching used extensively

Protection systems are simpler

Overcurrent, over/under voltage the main protections

Reliability of Distribution system is “local”, not a NERC or NPCC issue, does not affect the interties


Distribution

Some large industrial Customers are fed directly from 230 kV to a step down transformer.

No intermediate “subtransmission”


Distribution Structures for different

voltages

500 kV 230 kV 115 kV 44 kV 13.8 kV 4 kV


Distribution

Subtransmission and distribution lines can be on the same structure

4 kV

44 kV


Distribution

Three phase supply to a commercial facility


Distribution

LDC’s and LSE’s

LDC – Local Distribution Company

LSE – Load Serving Entity

In NYISO LSE’s are responsible for ensuring long term supply for LDC’s in their area. This is supposed to ensure an adequate supply.

In Ontario we have the Ontario Power Authority, no LSE’s (yet)


Distribution

The End

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