Protection Fundamentals

Protection Fundamentals
By Craig Wester, John Levine 1 GE Consumer & Industrial Multilin

Outline Introductions Tools Discussion of future classes
Enervista Launchpad On – Line Store Demo Relays at ISO / Levine Discussion of future classes Protection Fundamentals ANSI number handout, Training CD’s 2 GE Consumer & Industrial Multilin

Introduction Speakers: Craig Wester – GE Multilin Regional Manager
John Levine – GE Multilin Account Manager 3 GE Consumer & Industrial Multilin

Objective We are here to help make your job easier. This is very informal and designed around ISO Applications. Please ask question. We are not here to “preach” to you. The knowledge base on GE Multilin Relays varies greatly at ISO. If you have a question, there is a good chance there are 3 or 4 other people that have the same question. Please ask it. 4 GE Consumer & Industrial Multilin

Tools 5 GE Consumer & Industrial Multilin

6 GE Consumer & Industrial Multilin

Demo Relays with Ethernet
Working with James McRoy and Dave Curtis SR 489 SR 750 G30 MIF II Training CD’s 8 GE Consumer & Industrial Multilin

Demo Relays at L-3 9 GE Consumer & Industrial Multilin

Future Classes GE Multilin Training will be the 2nd Friday of every month. We will cover: March – Basics, Enervista Launchpad, ANSI number and what they represent, Uploading, downloading, Training CD’s, etc. April – 489 Relay May – MIF II relay June Relay July - UR relay basic including Enervista Engineer August – UR F60 and F35 relays September – G30 and G60 including Transformer and Generator in same zone October – Communications and security November - Neutral Grounding Resistors December – Ct’s and PT’s 10 GE Consumer & Industrial Multilin

Desirable Protection Attributes
Reliability: System operate properly Security: Don’t trip when you shouldn’t Dependability: Trip when you should Selectivity: Trip the minimal amount to clear the fault or abnormal operating condition Speed: Usually the faster the better in terms of minimizing equipment damage and maintaining system integrity Simplicity: KISS Economics: Don’t break the bank 12 GE Consumer & Industrial Multilin

Art & Science of Protection
Selection of protective relays requires compromises: Maximum and Reliable protection at minimum equipment cost High Sensitivity to faults and insensitivity to maximum load currents High-speed fault clearance with correct selectivity Selectivity in isolating small faulty area Ability to operate correctly under all predictable power system conditions 13 GE Consumer & Industrial Multilin

Art & Science of Protection
Cost of protective relays should be balanced against risks involved if protection is not sufficient and not enough redundancy. Primary objectives is to have faulted zone’s primary protection operate first, but if there are protective relays failures, some form of backup protection is provided. Backup protection is local (if local primary protection fails to clear fault) and remote (if remote protection fails to operate to clear fault) 14 GE Consumer & Industrial Multilin

Primary Equipment & Components
Transformers - to step up or step down voltage level Breakers - to energize equipment and interrupt fault current to isolate faulted equipment Insulators - to insulate equipment from ground and other phases Isolators (switches) - to create a visible and permanent isolation of primary equipment for maintenance purposes and route power flow over certain buses. Bus - to allow multiple connections (feeders) to the same source of power (transformer). 15 GE Consumer & Industrial Multilin

Primary Equipment & Components
Grounding - to operate and maintain equipment safely Arrester - to protect primary equipment of sudden overvoltage (lightning strike). Switchgear – integrated components to switch, protect, meter and control power flow Reactors - to limit fault current (series) or compensate for charge current (shunt) VT and CT - to measure primary current and voltage and supply scaled down values to P&C, metering, SCADA, etc. Regulators - voltage, current, VAR, phase angle, etc. 16 GE Consumer & Industrial Multilin

Types of Protection Overcurrent
Uses current to determine magnitude of fault Simple May employ definite time or inverse time curves May be slow Selectivity at the cost of speed (coordination stacks) Inexpensive May use various polarizing voltages or ground current for directionality Communication aided schemes make more selective 17 GE Consumer & Industrial Multilin

Instantaneous Overcurrent Protection (IOC) & Definite Time Overcurrent
Relay closest to fault operates first Relays closer to source operate slower Time between operating for same current is called CTI (Clearing Time Interval) Distribution Substation 18 GE Consumer & Industrial Multilin

Distribution Substation
(TOC) Coordination Relay closest to fault operates first Relays closer to source operate slower Time between operating for same current is called CTI Distribution Substation 19 GE Consumer & Industrial Multilin

Time Overcurrent Protection (TOC)
Selection of the curves uses what is termed as a “ time multiplier” or “time dial” to effectively shift the curve up or down on the time axis Operate region lies above selected curve, while no-operate region lies below it Inverse curves can approximate fuse curve shapes 20 GE Consumer & Industrial Multilin

Time Overcurrent Protection (51, 51N, 51G)
Multiples of pick-up 21 GE Consumer & Industrial Multilin

Classic Directional Overcurrent Scheme for Looped System Protection

Types of Protection Differential current in = current out Simple
Very fast Very defined clearing area Expensive Practical distance limitations Line differential systems overcome this using digital communications 23 GE Consumer & Industrial Multilin

Differential Note CT polarity dots
This is a through-current representation Perfect waveforms, no saturation 24 GE Consumer & Industrial Multilin

Differential Note CT polarity dots
This is an internal fault representation Perfect waveforms, no saturation 25 GE Consumer & Industrial Multilin

Types of Protection Voltage
Uses voltage to infer fault or abnormal condition May employ definite time or inverse time curves May also be used for undervoltage load shedding Simple May be slow Selectivity at the cost of speed (coordination stacks) Inexpensive 26 GE Consumer & Industrial Multilin

Types of Protection Frequency
Uses frequency of voltage to detect power balance condition May employ definite time or inverse time curves Used for load shedding & machinery under/overspeed protection Simple May be slow Selectivity at the cost of speed can be expensive 27 GE Consumer & Industrial Multilin

Types of Protection Power
Uses voltage and current to determine power flow magnitude and direction Typically definite time Complex May be slow Accuracy important for many applications Can be expensive 28 GE Consumer & Industrial Multilin

Types of Protection Distance (Impedance)
Uses voltage and current to determine impedance of fault Set on impedance [R-X] plane Uses definite time Impedance related to distance from relay Complicated Fast Somewhat defined clearing area with reasonable accuracy Expensive Communication aided schemes make more selective 29 GE Consumer & Industrial Multilin

Impedance Relay in Zone 1 operates first
X Z L Relay in Zone 1 operates first Time between Zones is called CTI R Source A B 21 T 1 2 Z 30 GE Consumer & Industrial Multilin

Impedance: POTT Scheme
POTT will trip only faulted line section RO elements are 21; 21G or 67N 31 GE Consumer & Industrial Multilin

Power vs. Protection Engineer: Views of the World
180 Opposites! 32 GE Consumer & Industrial Multilin

Typical Bulk Power System
Generation-typically at 4-20kV Transmission-typically at kV Receives power from transmission system and transforms into subtransmission level Subtransmission-typically at kV Receives power from subtransmission system and transforms into primary feeder voltage Distribution network-typically kV Low voltage (service)-typically V 33 GE Consumer & Industrial Multilin

Protection Zones Generator or Generator-Transformer Units Transformers
Buses Lines (transmission and distribution) Utilization equipment (motors, static loads, etc.) Capacitor or reactor (when separately protected) Unit Generator-Tx zone Bus zone Line zone Transformer zone Generator ~ XFMR Bus Line Motor Motor zone 34 GE Consumer & Industrial Multilin

Zone Overlap Overlap is accomplished by the locations of CTs, the key source for protective relays. In some cases a fault might involve a CT or a circuit breaker itself, which means it can not be cleared until adjacent breakers (local or remote) are opened. Zone A Zone B Relay Zone A Relay Zone B CTs are located at both sides of CB-fault between CTs is cleared from both remote sides Zone A Zone B Relay Zone A Relay Zone B CTs are located at one side of CB-fault between CTs is sensed by both relays, remote right side operate only. 35 GE Consumer & Industrial Multilin

Electrical – Mechanical Parameter Comparisons

Effects of Capacitive & Inductive Loads on Current

Motor Model and Starting Curves

What Info is Required to Apply Protection
One-line diagram of the system or area involved Impedances and connections of power equipment, system frequency, voltage level and phase sequence Existing schemes Operating procedures and practices affecting protection Importance of protection required and maximum allowed clearance times System fault studies Maximum load and system swing limits CTs and VTs locations, connections and ratios Future expansion expectance Any special considerations for application. 40 GE Consumer & Industrial Multilin

C37.2: Device Numbers Partial listing 41 GE Consumer & Industrial
Multilin

One Line Diagram Non-dimensioned diagram showing how pieces of electrical equipment are connected Simplification of actual system Equipment is shown as boxes, circles and other simple graphic symbols Symbols should follow ANSI or IEC conventions 42 GE Consumer & Industrial Multilin

1-Line Symbols [1] 43 GE Consumer & Industrial Multilin

1-Line [1] 47 GE Consumer & Industrial Multilin

1-Line [2]

3-Line 49 GE Consumer & Industrial Multilin

Diagram Comparison 50 GE Consumer & Industrial Multilin

C37.2: Standard Reference Position
1) These may be speed, voltage, current, load, or similar adjusting devices comprising rheostats, springs, levers, or other components for the purpose. 2) These electrically operated devices are of the nonlatched-in type, whose contact position is dependent only upon the degree of energization of the operating, restraining, or holding coil or coils that may or may not be suitable for continuous energization. The de- energized position of the device is that with all coils de-energized 3) The energizing influences for these devices are considered to be, respectively, rising temperature, rising level, increasing flow, rising speed, increasing vibration, and increasing pressure. 4.5.3) In the case of latched-in or hand-reset relays, which operate from protective devices to perform the shutdown of a piece of equipment and hold it out of service, the contacts should preferably be shown in the normal, nonlockout position 51 GE Consumer & Industrial Multilin

CB Trip Circuit (Simplified)
O1 trip handle, pr1 and pr2 electromechnincal with phase and ground, s1 seal in ts1 is the seal in coil 52 GE Consumer & Industrial Multilin

Showing Contacts NOT in Standard Reference Condition Some people show the contact state changed like this 53 GE Consumer & Industrial Multilin

Showing Contacts NOT in Standard Reference Condition Better practice, do not change the contact style, but rather use marks like these to indicate non-standard reference position 54 GE Consumer & Industrial Multilin

Lock Out Relay 55 GE Consumer & Industrial Multilin

CB Coil Circuit Monitoring: T with CB Closed; C with CB Opened

CB Coil Circuit Monitoring: Both T&C Regardless of CB state

Current Transformers Current transformers are used to step primary system currents to values usable by relays, meters, SCADA, transducers, etc. CT ratios are expressed as primary to secondary; 2000:5, 1200:5, 600:5, 300:5 A 2000:5 CT has a “CTR” of 400 Current Turns Ratio (CTR) 58 GE Consumer & Industrial Multilin

Standard IEEE CT Relay Accuracy
IEEE relay class is defined in terms of the voltage a CT can deliver at 20 times the nominal current rating without exceeding a 10% composite ratio error. For example, a relay class of C100 on a 1200:5 CT means that the CT can develop 100 volts at 24,000 primary amps (1200*20) without exceeding a 10% ratio error. Maximum burden = 1 ohm. 100 V = 20 * 5 * (1ohm) 200 V = 20 * 5 * (2 ohms) 400 V = 20 * 5 * (4 ohms) 800 V = 20 * 5 * (8 ohms) 59 GE Consumer & Industrial Multilin

Excitation Curve 60 GE Consumer & Industrial Multilin

Standard IEEE CT Burdens (5 Amp) (Per IEEE Std. C57.13-1993)

Current into the Dot, Out of the Dot Current out of the dot, in to the dot

Voltage Transformers VP VS
Voltage (potential) transformers are used to isolate and step down and accurately reproduce the scaled voltage for the protective device or relay VT ratios are typically expressed as primary to secondary; 14400:120, 7200:120 A 4160:120 VT has a “VTR” of 34.66 VP VS Relay 63 GE Consumer & Industrial Multilin

Typical CT/VT Circuits
Courtesy of Blackburn, Protective Relay: Principles and Applications 64 GE Consumer & Industrial Multilin

CT/VT Circuit vs. Casing Ground
Case Secondary Circuit Case ground made at IT location Secondary circuit ground made at first point of use 65 GE Consumer & Industrial Multilin

Equipment Grounding Prevents shock exposure of personnel
Provides current carrying capability for the ground-fault current Grounding includes design and construction of substation ground mat and CT and VT safety grounding 66 GE Consumer & Industrial Multilin

System Grounding Limits overvoltages
Limits difference in electric potential through local area conducting objects Several methods Ungrounded Reactance Coil Grounded High Z Grounded Low Z Grounded Solidly Grounded 67 GE Consumer & Industrial Multilin

System Grounding Ungrounded: There is no intentional ground applied to the system-however it’s grounded through natural capacitance. Found in kV systems. Reactance Grounded: Total system capacitance is cancelled by equal inductance. This decreases the current at the fault and limits voltage across the arc at the fault to decrease damage. X0 <= 10 * X1 68 GE Consumer & Industrial Multilin

System Grounding High Resistance Grounded: Limits ground fault current to 10A-20A. Used to limit transient overvoltages due to arcing ground faults. R0 <= X0C/3, X0C is capacitive zero sequence reactance Low Resistance Grounded: To limit current to A R0 >= 2X0 69 GE Consumer & Industrial Multilin

System Grounding Solidly Grounded: There is a connection of transformer or generator neutral directly to station ground. Effectively Grounded: R0 <= X1, X0 <= 3X1, where R is the system fault resistance 70 GE Consumer & Industrial Multilin

Grounding Differences….Why?
Solidly Grounded Much ground current (damage) No neutral voltage shift Line-ground insulation Limits step potential issues Faulted area will clear Inexpensive relaying 71 GE Consumer & Industrial Multilin

“Somewhat” Grounded Manage ground current (manage damage) Some neutral voltage shift Faulted area will clear More expensive than solid, less expensive then ungrounded 72 GE Consumer & Industrial Multilin

Ungrounded Very little ground current (less damage) Big neutral voltage shift Must insulate line-to-line voltage May run system while trying to find ground fault Relay more difficult/costly to detect and locate ground faults If you get a second ground fault on adjacent phase, watch out! 73 GE Consumer & Industrial Multilin

System Grounding Influences Ground Fault Detection Methods
Low/No Z 74 GE Consumer & Industrial Multilin

System Grounding Influences Ground Fault Detection Methods
Med/High Z 75 GE Consumer & Industrial Multilin

Medium/High Resistance Ground Low/No Resistance Ground
Basic Current Connections: How System is Grounded Determines How Ground Fault is Detected Medium/High Resistance Ground Low/No Resistance Ground 76 GE Consumer & Industrial Multilin

Substation Types Single Supply Multiple Supply
Mobile Substations for emergencies Types are defined by number of transformers, buses, breakers to provide adequate service for application 77 GE Consumer & Industrial Multilin

Industrial Substation Arrangements
(Typical) 78 GE Consumer & Industrial Multilin

Industrial Substation Arrangements
(Typical) 79 GE Consumer & Industrial Multilin

Utility Substation Arrangements
(Typical) Single Bus, 1 Tx, Dual supply 2-sections Bus with HS Tie-Breaker, 2 Tx, Dual Supply Single Bus, 2 Tx, Dual Supply 80 GE Consumer & Industrial Multilin

(Typical) Bus 1 Bus 2 Breaker-and-a-half –allows reduction of equipment cost by using 3 breakers for each 2 circuits. For load transfer and operation is simple, but relaying is complex as middle breaker is responsible to both circuits Ring bus –advantage that one breaker per circuit. Also each outgoing circuit (Tx) has 2 sources of supply. Any breaker can be taken from service without disrupting others. 81 GE Consumer & Industrial Multilin

(Typical) Main bus Aux. bus Bus 1 Bus 2 Tie breaker Main Reserve Transfer Double Bus: Upper Main and Transfer, bottom Double Main bus Main-Reserved and Transfer Bus: Allows maintenance of any bus and any breaker 82 GE Consumer & Industrial Multilin

Switchgear Defined Assemblies containing electrical switching, protection, metering and management devices Used in three-phase, high-power industrial, commercial and utility applications Covers a variety of actual uses, including motor control, distribution panels and outdoor switchyards The term "switchgear" is plural, even when referring to a single switchgear assembly (never say, "switchgears") May be a described in terms of use: "the generator switchgear" "the stamping line switchgear" 83 GE Consumer & Industrial Multilin

Switchgear Examples

Switchgear: MetalClad vs. Metal-Enclosed
Metal-clad switchgear (C ) Breakers or switches must be draw-out design Breakers must be electrically operated, with anti-pump feature All bus must be insulated Completely enclosed on all side and top with grounded metal Breaker, bus and cable compartments isolated by metal barriers, with no intentional openings Automatic shutters over primary breaker stabs. Metal-enclosed switchgear Bus not insulated Breakers or switches not required to be draw-out No compartment barriering required 85 GE Consumer & Industrial Multilin

Switchgear Basics [1] All Switchgear has a metal enclosure
Metalclad construction requires 11 gauge steel between sections and main compartments Prevents contact with live circuits and propagation of ionized gases in the unlikely event of an internal fault. Enclosures are also rated as weather-tight for outdoor use Metalclad gear will include shutters to ensure that powered buses are covered at all times, even when a circuit breaker is removed. 86 GE Consumer & Industrial Multilin

Switchgear Basics [2] Devices such as circuit breakers or fused switches provide protection against short circuits and ground faults Interrupting devices (other than fuses) are non-automatic. They require control signals instructing them to open or close. Monitoring and control circuitry work together with the switching and interrupting devices to turn circuits on and off, and guard circuits from degradation or fluctuations in power supply that could affect or damage equipment Routine metering functions include operating amperes and voltage, watts, kilowatt hours, frequency, power factor. 87 GE Consumer & Industrial Multilin

Switchgear Basics [3] Power to switchgear is connected via Cables or Bus Duct The main internal bus carries power between elements within the switchgear Power within the switchgear moves from compartment to compartment on horizontal bus, and within compartments on vertical bus Instrument Transformers (CTs & PTs) are used to step down current and voltage from the primary circuits or use in lower-energy monitoring and control circuitry. 88 GE Consumer & Industrial Multilin

Air Magnetic Breakers 89 GE Consumer & Industrial Multilin

SF6 and Vacuum Breakers 90 GE Consumer & Industrial Multilin

A Good Day in System Protection……
CTs and VTs bring electrical info to relays Relays sense current and voltage and declare fault Relays send signals through control circuits to circuit breakers Circuit breaker(s) correctly trip What Could Go Wrong Here???? 91 GE Consumer & Industrial Multilin

A Bad Day in System Protection……
CTs or VTs are shorted, opened, or their wiring is Relays do not declare fault due to setting errors, faulty relay, CT saturation Control wires cut or batteries dead so no signal is sent from relay to circuit breaker Circuit breakers do not have power, burnt trip coil or otherwise fail to trip Protection Systems Typically are Designed for N-1 92 GE Consumer & Industrial Multilin

Protection Performance Statistics
Correct and desired: 92.2% Correct but undesired: 5.3% Incorrect: 2.1% Fail to trip: 0.4% 93 GE Consumer & Industrial Multilin

Contribution to Faults

Fault Types (Shunt) 95 GE Consumer & Industrial Multilin

Short Circuit Calculation Fault Types – Single Phase to Ground

Short Circuit Calculations Fault Types – Line to Line

Short Circuit Calculations Fault Types – Three Phase

AC & DC Current Components of Fault Current

Variation of current with time during a fault
Figure 2.4 Variation of current with time during a fault 100 GE Consumer & Industrial Multilin

Variation of generator reactance during a fault
Fig. 2.5 Variation of generator reactance with time during a fault 101 GE Consumer & Industrial Multilin

Useful Conversions 102 GE Consumer & Industrial Multilin

Per Unit System Establish two base quantities:
Standard practice is to define Base power – 3 phase Base voltage – line to line Other quantities derived with basic power equations 103 GE Consumer & Industrial Multilin

Per Unit Basics 104 GE Consumer & Industrial Multilin

Short Circuit Calculations Per Unit System
Per Unit Value = Actual Quantity Base Quantity Vpu = Vactual Vbase Ipu = Iactual Ibase Zpu = Zactual Zbase 105 GE Consumer & Industrial Multilin

Short Circuit Calculations Per Unit System

Short Circuit Calculations Per Unit System – Base Conversion
Zpu = Zactual Zbase Zbase = kV 2base MVAbase Zpu2 = MVAbase kV 2base2 Zpu1 = MVAbase1 kV 2base1 X Zactual X Zactual  Zpu2 =Zpu1 x kV 2base1 x MVAbase kV 2base2 MVAbase1 107 GE Consumer & Industrial Multilin

Information for Short Circuit, Load Flow and Voltage Studies
To perform the above studies, information is needed on the electrical apparatus and sources to the system under consideration 108 GE Consumer & Industrial Multilin

Utility Information kV MVA short circuit Voltage and voltage variation
Harmonic and flicker requirements 110 GE Consumer & Industrial Multilin

Generator Information
Rated kV Rate MVA, MW Xs; synchronous reactance X’d; transient reactance X’’d; subtransient reactance 111 GE Consumer & Industrial Multilin

Motor Drive kV Rated HP or KW Type
Sync or Induction Subtransient or locked rotor current Is it regenerative Harmonic spectrum 112 GE Consumer & Industrial Multilin

Transformers Reactors Rated primary and secondary kV
Rated MVA (OA, FA, FOA) Winding connections (Wye, Delta) Impedance and MVA base of impedance Reactors Rated kV Ohms 113 GE Consumer & Industrial Multilin

Cables and Transmission Lines
For rough calculations, some can be neglected Length of conductor Impedance at given length Size of conductor Spacing of overhead conductors Rated voltage Type of conduit Number of conductors or number per phase 114 GE Consumer & Industrial Multilin

ANSI 1-Line 115 GE Consumer & Industrial Multilin

IEC 1-Line 116 GE Consumer & Industrial Multilin

Short Circuit Study [1] 117 GE Consumer & Industrial Multilin

A Study of a Fault……. 120 GE Consumer & Industrial Multilin

Fault Interruption and Arcing

Arc Flash Hazard 122 GE Consumer & Industrial Multilin

Arc Flash Mitigation: Problem Description
An electric arc flash can occur if a conductive object gets too close to a high-amp current source or by equipment failure (ex., while opening or closing disconnects, racking out) The arc can heat the air to temperatures as high as 35,000 F, and vaporize metal in equipment The arc flash can cause severe skin burns by direct heat exposure and by igniting clothing The heating of the air and vaporization of metal creates a pressure wave (arc blast) that can damage hearing and cause memory loss (from concussion) and other injuries. Flying metal parts are also a hazard. 123 GE Consumer & Industrial Multilin

Methods to Reduce Arc Flash Hazard
Arc flash energy may be expressed in I2t terms, so you can decrease the I or decrease the t to lessen the energy Protective relays can help lessen the t by optimizing sensitivity and decreasing clearing time Protective Relay Techniques Other means can lessen the I by limiting fault current “Non-Protective Relay Techniques” 124 GE Consumer & Industrial Multilin

Non-Protective Relaying Methods of Reducing Arc Flash Hazard
System design modifications increase power transformer impedance Addition of phase reactors Faster operating breakers Splitting of buses Current limiting fuses (provides partial protection only for a limited current range) Electronic current limiters (these devices sense overcurrent and interrupt very high currents with replaceable conductor links (explosive charge) Arc-resistant switchgear (this really doesn't reduce arc flash energy; it deflects the energy away from personnel) Optical arc flash protection via fiber sensors Optical arc flash protection via lens sensors 125 GE Consumer & Industrial Multilin

Protective Relaying Methods of Reducing Arc Flash Hazard
Bus differential protection (this reduces the arc flash energy by reducing the clearing time Zone interlock schemes where bus relay selectively is allowed to trip or block depending on location of faults as identified from feeder relays Temporary setting changes to reduce clearing time during maintenance Sacrifices coordination FlexCurve for improved coordination opportunities Employ 51VC/VR on feeders fed from small generation to improve sensitivity and coordination Employ UV light detectors with current disturbance detectors for selective gear tripping 126 GE Consumer & Industrial Multilin

Fuses vs. Relayed Breakers

Arc Flash Hazards 128 GE Consumer & Industrial Multilin

Arc Pressure Wave 129 GE Consumer & Industrial Multilin

Arc Flash Warning Example [1]

Copy of this presentation are at:

QUESTIONS? 134 GE Consumer & Industrial Multilin

Protection Fundamentals

Similar presentations

Presentation on theme: "Protection Fundamentals"— Presentation transcript:

Similar presentations

About project

Feedback

Log in

Auth with social network:

Protection Fundamentals

Similar presentations

Presentation on theme: "Protection Fundamentals"— Presentation transcript:

Similar presentations

About project

Feedback