EPIC 2019: Multi-Megawatt Power Converters for Grid Applications and Power Quality Support by PITT

MULTI-MEGAWATT POWER CONVERTERS FOR GRID APPLICATIONS AND POWER QUALITY SUPPORT: STATE OF THE ART AND FUTURE Shyam Ramamurthy, PhD Christopher Lee

Electrical Distribution Division Mitsubishi Electric Power Products Electric Power Industry Conference October 28, 2019

AGENDA A. Power Converter For Grid Applications Review 1) Converter System Design Factors 2) Overview of Popular Topologies and Components 3) Role of Transformers 4) MW Converter Topology Advantages and Discussion B. Application Examples 1) PV Generation and Energy Storage 2) Voltage Support, Power Quality and Energy Support 3) Other Applications C. Proliferation Issues and Interactions 1) State of the Art 2) Grid Forming Applications and Black Start 3) Future Trends and Developments 2

BRIEF BACKGROUND •

Mitsubishi Electric Power Products, Inc. (MEPPI) is the US subsidiary of Mitsubishi Electric Corporation responsible for serving North American power systems

•

MEPPI’s Electrical Distribution Division (EDD) is leading the design, development, manufacture, and sale of distribution level Power Electronics from Warrendale, PA.

•

EDD Core Products: • 5kV-72kV Outdoor Free Standing Gas/ Vacuum Circuit Breakers • Distribution Power Electronics: VVC Systems Solutions

•

EDD Power Electronic Capabilities: – Grid Connected Converter Products and Systems: Design, Implementation, Test, and Verification – Medium and Low Voltage Lab and Design Space: 3,000ft2 Test Cell 3

SECTION A: POWER CONVERTERS REVIEW Evolution of Power Converters in the Distribution Grid Space

CONVERTER SYSTEM DESIGN FACTORS Power Quality (PQ) Controls

Converter

DC Combiners PV Source

AC Combiner Components and Switchgear

PV Source

Storage

• • •

Converters interface power from different dc sources and storage to the power system, Converters act as (customized for) power quality controls and participate in steady state and dynamic support beyond capability of the energy conversion converters, Performance specifications, system cost, availability, simple maintenance, minimizing spares quantity and type are the overall system performance factors 5

CONVERTER SYSTEM DESIGN FACTORS PV Plant ~12 MW, ~50 acres

MPPT Effects due to Panel State

Plant Power and Communication Loops Power Converter Skids

Factor

Discussion

Optimal Size of Converter

Considering PV plant layout, cabling costs, transformers and communications the maximum MW of power conversion at a single point is currently < 5 MW.

Shading, soiling and string mis-matches

Multiple MPPTs and need to find the global optimum- String or MW level

Divert energy within plant to storage

Curtail at plant interface, but capture the energy in local storage 6

CONVERTER SYSTEM DESIGN FACTORS Power System Interface AC_I

AC_G

Fuse SA

Power Converter PC_T

DC/DC DC_M

Fuse

DC Source

DC_G

AC_P

DC_P

Factor

Discussion

AC_I

Power System Voltage and Fault Level, Protection Requirements

AC_G

Power System Voltage to Ground, Operational Effects on Other Loads

AC_P

Protection and Isolation from AC Events

PC_T

Power Converter Topology, Plant Load Sharing

DC_M

Matching DC of Parallel Sources, Matching DC of Source to Power Converter, Load Sharing

DC_P

Protection and Isolation from DC Events

DC_G

DC Components Voltage to Ground 7

CONVERTER SYSTEM DESIGN FACTORS Power System Interface AC_I

AC_G

Fuse

Power Converter

PC_T

DC/DC DC_M

Fuse

DC Source

DC_G

AC_P

DC_P

Factor

Discussion

AC_I

Switchgear, fuses, fault levels evolved to use components that maximize volume and reduce component costs

AC_G

Effect on other loads in power system, interactions between converters

AC_P

Short-circuit levels, surge reduction

PC_T

Efficiency, semi-conductor technology feasibility, controls complexity, interface to dc sources

DC_M

Interface dc sources to a common dc bus -Less common, string 500 KW converters and 1 MW converters becoming more common to give the same advantages, Source technology evolving to match

DC_P

DC switch and fuse technology evolution limits feasible levels of power and energy

DC_G

PV restricted in voltage to ground, commercial batteries restricted in common mode voltage level tolerance and related currents 8

CONVERTER SYSTEM DESIGN FACTORS Factor

Discussion

Utility Plant Sizes

Currently range 5 MW to GW, spread over wide areas

Fault levels

High at plant voltage interface- needs to be controlled for practical converter design and arc flash

Plant Voltage Interface

>10 to 34.5 KV or higher, converter still needs a transformer at interface

DC Voltage to Ground

PV Panels, batteries and DC sources need low voltage with respect to ground

De-coupling

Multiple switching converters need de-coupling

Availability

Smaller units for more redundancy

Maintenance

More trained personnel at low voltage and ratings- easier as fault and voltage level reduces 9

MULTI-MEGAWATT CONVERTER defined as: Power Converter applied in energy conversion plants and power quality applications In the power range 2- 5 MW, possibly in 0.5- 1 MW sections due to the factors of: • • • • •

System cabling costs Matching performance of parallel strings Having sections dedicated for storage Reduce the size of generation loss in case of failure Isolation and maintenance, reducing fault levels- ac and dc

POPULAR TOPOLOGY REVIEW VDC

a b c

TWO - LEVEL CONVERTER Most common 3 phase converter used for Motor Drives, PV, Energy Storage, and Power Quality Advantage:  Typically low voltage, and minimal semiconductor voltage stress, no balancing of series or floating capacitors  Easily connected to different DC sources  Lower AC voltage results in lower voltage to ground complications  Common DC Input allows for simpler fusing Disadvantages:  Lower AC and DC bus voltages, typically requiring step up transformer on output, and limit DC Source voltages  High conduction losses for higher power 11

POPULAR TOPOLOGY REVIEW NEUTRAL POINT CLAMPED (NPC) CONVERTER

Q1A

C1 D1A

VDC

D2A

Q1B D1B

Q2A

D2B

Q3A

Q4A

Q1C D1C

Q2B

D2C

Q3B

Q4B

Q2C

Q3C

Q4C

Most common 3 level converter used for PV, Motor Drive, and Energy Storage Applications Advantage:  3 or more level converter reduces harmonics and dV/dt stress for comparable voltages  Higher level count without significantly more switching device complexity  Allows for higher DC Bus and AC Output voltages  Common DC Input allows for simpler fusing  Extendable to higher N level counts for higher AC voltages Disadvantages:  Series Capacitors requires balancing  Uneven Power Devices loss profile limits switching and complicates cooling structure  Higher voltage to ground depending on midpoint or bus referencing to ground 12

POPULAR TOPOLOGY REVIEW Q1A

Q2A

Q5A

VDC

Q1B

Q2B

Q2C

Q5B Q6A

Q1C

Q5C Q6B

Q6C

Q3A

Q3B

Q3C

Q4A

Q4B

Q4C

NEUTRAL POINT PILOTED (NPP) CONVERTER Active neutral point converter used for motor drives Advantage:  Allows for higher switching frequencies compared to similar NPC converter  More balanced device losses compared to NPC  Extendable to higher AC voltages by using additional devices in series  Common DC Input allows for simpler fusing Disadvantages:  Higher conduction loss on devices  Enhanced complexity for a given N level due to increased number of devices  Typically applied at higher voltages resulting in higher voltage to ground stress  High voltage to ground if – DC bus is reference to ground 13

POPULAR TOPOLOGY REVIEW C1

VDC

Q1A

Q1B

Q1C

Q2A

Q2B

Q2C

Q3A

Q3B

Q3C

Q4A

Q4B

Q4C

FLOATING CAPACITOR (FC) CONVERTER Multilevel converter using floating capacitors otherwise known as capacitor clamped topology Advantage:  Additional output levels without adding clamp devices  Easier setup for back to back setup due to minimal midpoint connections  Common DC Input allows for simpler fusing Disadvantages:  Increased switching complexity due constant balancing of floating capacitors  Increased size and DC bus complexity due to sizing of floating capacitors  Floating capacitors result in increased DC component count imposing voltage to ground 14

POPULAR TOPOLOGY REVIEW CASCADED H BRIDGE (CHB) CONVERTER a

Q3 C1

Cascaded cell converter allows for wye or delta connection Advantage:  Use of common H-Bridge “Power Block” allows for extremely high voltages and power levels  High number allows for lower filtering and better power quality  Inherent ability to introduce redundancy at cell level  Minimize use of transformer in application Disadvantages:  Requires control complexity to balance cell voltages  Difficult to connect real power sources due to voltage isolation issues with floating cells  High voltage to ground isolation is required at the cell level  Not cost effective at low voltage/power 15

POPULAR TOPOLOGY REVIEW MODULAR MULTI-LEVEL CONVERTER (MMC) Multilevel converter known in HV-DC and motor drive applications Advantage:  Use of common building block like CHB converter  Can achieve high voltage levels using single dc bus  Inherent ability to introduce redundancy at cell level  Ability to connect dc energy sources on common dc bus on multi level system Disadvantages:  Voltage leg imbalances result in circulating current, requiring phase reactors  High cell count required for equivalent voltage levels in comparable converters  Single DC bus connection results in high voltage or DC:DC converter to operate  Floating H-Bridge cells result in high voltages to ground  Not cost effective at low voltage/power

VDC

ROLE OF TRANSFORMERS MAIN ROLE IS VOLTAGE STEP-UP AND MATCHING TO POWER SYSTEM Advantages:  Reduce fault levels at power converter  Additional impedance that can be considered in the filtering  POI moves to transformer primary  Increased de-coupling between power converter stations  Isolate common mode from system and between converters  De-couple surges Disadvantages:  Increase in losses  Additional protection monitoring  Oil handling  Space requirements

2 Level

NPC

NPP

Floating Cap

CHB

MMC

MW CONVERTER TOPOLOGY SYSTEM FACTORS COMPARISON Higher Power System Voltage transformer-less

○

◔

◑

Cost (2-5MW)

●

◑

◕

DC Terminals Voltage to Ground

●

◕

○

Efficiency (2-5MW)

◕

●

◕

◑

DC Fault Isolation

●

◕

○

AC Grid Decoupling Transformer-less

○

◔

Redundancy

◑

●

Plant DC+AC Cabling Cost

◑

●

◔

Plant Electrician Training

●

◕

◑

◔

Overall

●

◕

SYSTEM FACTORS Ranking

○ Worst ◔ Minor

◑ Medium ◕ Strong

● Best

Comparison chart outlining the difference topologies with respect to system factors for Grid Applications based on unity weighting of system factors

SECTION B: APPLICATION EXAMPLES

PV GENERATION AND STORAGE Fast PQ Control Here

Converter

DC Combiners PV Source

PV Source Plant Controls

• • •

• •

AC Combiner Components and Switchgear

Storage

Power Generation from PV, Energy Arbitrage using Storage Spare capacity on plant converters used for VAR controls Frequency and Voltage Support using (likely not co-located) power converters (based on converter side measurements) with slow adjustments by plant controls for stability issues and steady state error correction PV and Storage Coordinated and AC_I constraints honored by plant controls Slow reactive power and power factor loops (via plant controller and plant interface metering) 20

PV GENERATION AND STORAGE Fast PQ Control Here

Converter

DC Combiners PV Source

PV Source Plant Controls

AC Combiner Components and Switchgear

Typical Converter Features • DC Pre-charge • AC Pre-charge if Night vars enabled • MPPT * • Response to DC Transients* • P-Q Capability

Storage

• MODBUS TCP Communications • Plant Controls command constraints, voltage and power factor • DNP3 standard at Plant controls *(Manufacturer specific IP) 21

POWER QUALITY CONTROLS (D-STATCOM) Converter Power Quality Controls

DC Combiners PV Source

PV Source Fast PQ Control Here

Plant Controls

AC Combiner Components and Switchgear

Storage

• Fast VAR Controls at power system interface, provides responsive 1- cycle voltage correction, 2-cycle VAR and power factor correction with tighter regulation than using converter side voltage • Coordinated voltage controls eliminate stability issues • Corrects voltage/current unbalances and harmonic currents at plant interface • Mitigation of transient over-voltages at the power system interface 22

POWER QUALITY CONTROLS (D-STATCOM) Converter Power Quality Controls

DC Combiners PV Source

PV Source Fast PQ Control Here

Plant Controls

AC Combiner Components and Switchgear

Typical PQ Converter Features • DC and/or AC Pre-charge • Fast Coordinated voltage, Q control Power factor loops standard • MODBUS TCP & DNP3 Communications • Plant Controls can command constraints, voltage, power factor, compensation levels

Storage

Hardware options for: • Fast voltage measurements* • Power factor measurement* • Load current measurement* • High harmonic compensation* • 100% negative sequence current * *(Manufacturer specific IP and capabilities) 23

OTHER POWER QUALITY APPLICATIONS Harmonic Mitigation

Provides load harmonic current components and cleans supply current waveform 1

1.5 Line Current

Compensated Current

0.8 1 0.6

0.4 0.5 0.2

Current

-0.2 -0.5 -0.4

-0.6 -1 -0.8

-1

-1.5 0

0.002

0.004

0.006

0.008

0.01

0.012

0.014

0.016

0.018

0.002

0.004

0.006

0.008

0.01

0.012

0.014

0.016

0.018

time(s)

Current Imbalance

Provides load imbalanced current components and balances supply current waveform 1

1 Phase A Phase A

0.8

Phase B

Phase C

0.6 0.6 0.4 0.4 0.2 0.2 0

Voltage

-0.2 -0.2 -0.4 -0.4 -0.6 -0.6 -0.8 -0.8 -1 -1

0 0

0.005

0.01

0.015

0.02

0.025

0.03

0.035

0.005

0.01

0.015

0.02

0.025

0.03

0.035

time(s)

OTHER POWER QUALITY APPLICATIONS 1

1 Gen 1

0.8

Compensated Votlage

Gen 2

0.8

Gen 2 Gen 1

0.6

0.4

0.2

Voltage

Grid Bridging

0.6

-0.2

-0.4

-0.6

-0.8

-1

-1 0

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.01

0.02

0.04

0.03

time(s)

0.05

0.06

0.07

time(s)

Watt injection as well as interim cycle replacement during generation loss

1 Line Voltage

0.8

0.6

0.4

0.2

Line Voltage

Voltage

Line Bridging

Compensated Voltage

0.8

-0.2

-0.4

-0.6

-0.8

-1

-1 0

0.005

0.01

0.015

0.02

0.025

time(s)

0.03

0.035

0.04

0.045

0.05

0.005

0.01

0.015

0.02

0.025

0.03

0.035

0.04

0.045

0.05

time(s)

Interim cycle replacement during line loss

SECTION C: PROLIFERATION ISSUES AND INTERACTIONS

PROLIFERATION ISSUES- STATE OF THE ART Factor

Discussion

Converters

Majority operate in grid following (GFL) and virtual current source mode. Need strong grid for stable operation.

Inertia

System inertia is reducing as plants are retired leading to more PLL dynamic interactions- more visible currently in microgrids

Mode of operation

Fault current participation is increasingly required

Modeling

Accurate modeling of ride-through curves and cessation response, implementation of real and reactive power droop controls and responses

PROLIFERATION ISSUES- GRID FORMING (GFM) â&#x20AC;˘ More converters will need to participate in grid forming to provide a stable grid â&#x20AC;˘ Converters will need to participate in black-start GFL

GFM

VSC, always in Virtual Current Source Mode

VSC, mostly in Voltage Source Mode- enters other modes when reaching its limits

Slight overload capabilities

Will need high overloads

Use strong grid for synchronization

Widely separated converter plants, synchronized by fast communication infrastructure or autonomous (?)

Dominantly PLL driven controls

Frequency source and PLL blended controls New Controls developments needed!

PROLIFERATION ISSUES- GRID FORMING (GFM) Power Converter 1

Power Converter 2

ZFilter1

ZFilter2

ZLine1

ZLine2

RLoad

ZFilterN- Physical internal filter of power converter ZLineN- External Impedance between Power Converter and Load

â&#x20AC;¢ Parallel Converters in grid forming mode feeding loads, for example, during black start.

PROLIFERATION ISSUES- GRID FORMING SOA (GFM)

â&#x20AC;¢

State of the Art Controls results in overcurrent and voltage sags during PLL interactions (Freq oscillation 1.5 to 2 secs and Phase jump 2.25 to 2.3 secs), Simple voltage fold-back for current limiting. 30

PROLIFERATION ISSUES- GRID FORMING (GFM) Power Converter 1 Controller 1 Zv1

Power Converter 2 Controller 2 Zv2

ZFilter1

ZFilter2

ZLine1

ZLine2

ZvN- Virtual Impedance realized in controls ZFilterN- Physical internal filter of power converter ZLineN- External Impedance between Power Converter and Load

â&#x20AC;˘ Parallel Converters with Virtual Impedances (50 X physical filter) in grid forming mode feeding loads, for example, during black start. â&#x20AC;˘ Controllers in the power converter adjust for the virtual impedance and the effects of the impedance influence the network, but the physical voltage levels realized by the converter remain unchanged. 31

PROLIFERATION ISSUES- GRID FORMING WITH VZ(GFM)

â&#x20AC;¢ Virtual Impedance realized by controls decouples the current from transients and minimizes voltage sags

PROLIFERATION ISSUES- GRID FORMING (GFM)

â&#x20AC;¢ Illustrates that internal calculated internal modulation index is higher than 1 for the virtual impedance converter 33

SUMMARY • Multi-MW Converters dominate the market in Renewable and Power Quality applications due to system factors discussed • The evolving bulk power system requirements need grid forming support from power converters • Major development emphasis needed in power converter controls

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