EE394J10 DG Grid Interconnection

7/30/2019 EE394J10 DG Grid Interconnection

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1 Alexis Kwasinski, 2012

EE 394J10 Distributed Technologies

Grid-Microgrids

Interconnection


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Motivation

Reasons for connecting a microgrid to a main grid:

Availability: Highly available power grids may act as an additional

source for micro-grids. Operations/stability:

Direct connection of ac microgrids to a large power grid

facilitates stable operation but only if the power grid acts as a

stiff source to the microgrid.

When using renewable energy sources, a grid connection mayallow reducing the need for energy storage in the microgrid.

If not all loads in a microgrid are critical, a grid connection may

allow to reduce the investment in local generation.

Economics:

Microgrids are typically planned with extra capacity with respect

to the local load. This extra power capacity can be injected back

into the grid in order to obtain some economic benefit.

Grid interconnection allows to reduce fuel operational costs by

using the grid at night when electricity costs are low.


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Definitions

Point of common coupling (PCC): it is the point in the electric circuit

where a microgrid is connected to a main grid.


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Standards

There are several standards specifying various aspects grid

interconnection of a local power generation source. Arguably the most

important one is IEEE 1547.

IEEE 1547 has several parts: Main body

IEEE Standard 1547.1 IEEE Standard Conformance Test Procedures for Equipment

Interconnecting Distributed Resources with Electric Power Systems.

EEE Standard 1547.2 IEEE Application Guide for IEEE Std 1547, IEEE Standard for

Interconnecting Distributed Resources with Electric Power Systems.

IEEE Standard 1547.3 IEEE Guide for Monitoring, Information Exchange, and Control of

Distributed Resources Interconnected with Electric Power Systems.

IEEE Standard 1547.4 IEEE Guide for Design, Operation, and Integration of Distributed

Resource Island Systems with Electric Power Systems.

IEEE Standard 1547.5 has not been issued, yet. Its intended scope is to address issues

when interconnecting electric power sources of more than 10 MVA to the power grid.IEEE Standard 1547.6 IEEE Recommended Practice for Interconnecting Distributed

Resources with Electric Power Systems Distribution Secondary Networks.

IEEE Standard 1547.8 has not been issued, yet. Its intended scope is to provide

supplemental support for implementation methods for expanded use of the previous

standards, for example when addressing issues with high penetration of residential PV

systems.


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Standards

Main provisions from IEEE 1547: The micro-grid must not actively regulate the voltage at the PCC.

The grounding approach chosen for the local area power and energy system (LAPES) must

not create overvoltages that exceed the ratings of the equipment connected to the main grid or

must not affect ground fault protection coordination in the main grid.

The distributed resources in the LAPES must be able to parallel with the main grid without

causing voltage fluctuations at the PCC greater than 5% of the prevailing voltage level of the

Area electric power system (EPS) at the PCC and flicker must be within acceptable ranges.

The LAPES must not energized the main grid when the main grid is not energized.

Each distributed resource (DR) unit of 250 kVA or more or DR aggregate of 250 kVA or more

at a single PCC shall have provisions for monitoring its connection status, real power output,

reactive power output, and voltage at the point of DR connection.

A visible-break isolation device must be located between the main grid and a DR unit only

when required by the main grid provider practices.

The interconnection system must meet applicable surge and EMI standards.


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Standards

Main provisions from IEEE 1547: When a fault occurs in the main grid circuit to which a LAPES is connected, then the micro-

grid local power generation units must stop to power this circuit before reclosure from the maingrid happens.

The interconnection system must be able to measure relevant indicated voltages and

frequencies at the PCC or the point of connection of DR and disconnect within a given allowed

time all local power generating units in the micro-grid when these measured voltages or

frequencies fall within a range specified in a table in this standard. For example, when

voltages fall below 50 % of the base voltage, the LAPES must disconnect its DR within 0.16

seconds (one 60 Hz cycle). The time extends to 2 seconds for voltages between 50 and 88 %

of the base voltage. Disconnection must occur within 1 second if measured voltages are

between 110 and 120 % of the base voltage and within 0.16 seconds if the voltage exceeds

120 % of the base voltage. For frequency measurements, any DR of 30 kW or less must

disconnect 0.16 seconds if the measured frequency is above 60.5 Hz or below 59.3 Hz. The

same disconnect time applies for DR of more than 30 kW when the frequency exceeds 60.5Hz, but for the lower range at these power levels disconnect within 0.16 seconds must occur if

the frequency falls below 57 Hz, whereas disconnection is adjustable between 0.16 and 300

Hz if the frequency falls between 59.8 and 57 Hz.

Reconnection of a LAPES to a main grid may occur at least 5 minutes after voltages and

frequency fall within indicated required ranges.


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Standards

Main provisions from IEEE 1547: Reconnection of a LAPES to a main grid may occur at least 5 minutes after voltages and

frequency fall within indicated required ranges.

A microgrid must not inject dc current greater than 0.5% of the full rated output current at

the PCC.

Harmonic current injection by the LAPES into the main grid measured at the PCC must not

exceed certain levels both in total and for given harmonic order ranges. The total demand

distortion must not be more than 5 % of the local main grid maximum load current integrated

demand (15 or 30 minutes) without the DR unit, or the DR unit rated current capacity,

whatever is greater. Base of this same base current, harmonic content for harmonics with an

odd order below 11 must not exceed 4 %. If the odd harmonic order is between 11 and 17 the

limit is 2 %, whereas this limit falls to 1.5 % for odd harmonics with an order between 17 and

23 and 0.6 % for odd harmonics with an order between 23 and 35. For odd harmonics with an

order above 35, the harmonic content with respect to the indicated current must not exceed0.3 %. For even harmonics their content limits are a quarter of those indicated for the odd

harmonic orders.



Standards

Other important provisions from IEEE 1547.6 about network

protections (NP) on the grids side:

The presence of DR should not:

- cause any NP to exceed its fault-interrupting capability.

- cause any NP to operate more frequently than prior to DR operation.

- prevent or delay the NP from opening for faults on the network feeders.

- delay or prevent NP closure.

- require the NP settings to be adjusted except by consent of the area EPSoperator.

- cause an islanding condition within part of a grid network.



Interconnection methods and technologies

Interconnection methods:

Directly through switchgear

Power electronic interfaces Static switches

Directly through circuit breakers:

Relatively simple and inexpensive

Slow (3 to 6 cycles to achieve a complete disconnection). Since electrical characteristics on both sides of the circuit breakers

must be the same, then, electrical characteristics on the micro-grid

side are dependent on the grid characteristics. For example, use of

a circuit breaker implicitly limits the micro-grid to have, at least

partially, an ac power distribution system in order to match the grids

electrical characteristics.

Power flow through the PCC cannot be controlled




Directly through circuit breakers:

Example of one of such systems:

Use of static switches:

Usually based on SCRs in antiparallel configuration to allow

bidirectional power flow





They are costlier and more complex than using circuit breakers.

Usually, conventional circuit breakers are still used to provide away to achieve full galvanic isolation. A Bypass switch is also added

for maintenance reasons.





They allow for many open/close operations

They act much faster than conventional circuit breakers (in theorder of half a cycle to a cycle). Sometimes IGBTs are used instead

of SCR because IGBTs tend to be faster than SCRs and their

current is inherently limited.

Still power flow cannot be controlled.

There are some conduction losses in the devices.




Power electronic interfaces:

It is the costlier option but it is also the most flexible one.

Allow for power distribution architecture characteristics on bothsides of the PCC to be completely different.

Both real and reactive power flow can be controlled.

Reaction times to connection or disconnection commands are

similar to those provided by static switches, although in the case of a

power electronic circuit, it response also depends on its dynamicperformance, given by its controller, topology, and internal energy

storage components characteristics.

Still, in many cases, a circuit breaker will still be required at the

grid-side terminal of the power electronic interface with a LAPES in

order to provide a way to physically disconnect the micro-grid from

the grid.

Also, similarly to static switches, the presence of a power electronic

circuit will lead to some power losses not found in the approach

using mechanical interfaces.



Grid-connected inverter control

Consider the following configuration in which it is assumed that we can

control both real and reactive power can be controlled at the inverter

(this is not always the case).




With a small voltage drop in the grid impedance (so the voltage at the

PCC, va is fixed and cannot be regulated by the inverter:

The inverter impedance depends on the inverter output filter

parameters and the inverter controller.

In most typical applications, for low frequencies the inverter impedanceis mostly resistive. Hence,

22 2

cos sinG INV INV INV G INV INV

INV INV

V V R X V RP

R X

2

2 2

cos sinG INV INV INV G INV

INV

INV INV

V V X R V X Q

R X

2,

cosG INV GINV R

INV

V V VP

R

,

sinG INV INV R

INV

V VQ

R




If the inverter is controlled as in most PV grid-tied inverters so their

power factor is close to 1, then

Hence,

If due to the output filter or the controller parameters the inverter

presents an inductive equivalent series impedance, then

,0 0

INV RQ

,

G INV G

INV R

INV

V V VP

R

,

sinG INV

INV X

INV

V VP

X

2,

cosG INV G

INV X

INV

V V VQ

X




Condition for unity power factor

Consider the following triangle based on the above condition:

Then

and

, 0 cosINV X INV GQ V V

INVV

GV

xV

2 2

sinINV GX

INV INV

V VV

V V

2 2

,

G INV G

INV X

INV

V V VP

X



Islanding

In IEEE Standard 1547.4 an intentional island is said to be the result

of intentional events for which the time and duration of the plannedisland are agreed upon by all parties involved.

There are several reasons why intentional island operation of a micro-

grid may occur, but a common one is a preemptive disconnection from

the grid in anticipation of a power outage on the main grid side caused

by an event that can be anticipated, such as an incoming hurricane orstorm, or wildfires. The advantage of this intentional islanding operation

instead of waiting for the outage in the main grid to occur in order to

switch the LAPES to operate in islanding mode is that an intentional

islanding allows for a controlled transition that prevents potential failures

or quality issues in the micro-grid. Two phases can be distinguished in islanded operation:

transition from grid connected to island operation

operation isolated from the grid.



Islanding

During the transition into island operation it is important that:

voltage disturbances are quickly dampened and that protection

schemes both inside the LAPES and in the grid are not affected.

When the transition is completed it is important that

the micro-grid has sufficient local power generation and energy

storage in order to ensure that loads are powered with the agreed

quality level. For example, in ac micro-grids it is important that

distributed resources are able to provide real and reactive power to

the specified load range. This is particularly important in order to

avoid loss of stability if there are large motors in the LAPES that

require significant amounts of reactive power during startup Also for ac micro-grids, their control systems must be able to

regulate both voltage and frequency within acceptable ranges. In dc

micro-grids, neither frequency regulation nor reactive power

generation are issues to consider.



Islanding

Eventually, it can be anticipated that the micro-grid would be

connected to the main grid again. Grid connection of dc micro-grids or

ac micro-grids with a power electronics interface with the main gridtends to be simpler than the case of ac micro-grids connected to the

main grid through circuit breakers, contactors, or static switches

because in the dc micro-grid and the ac micro-grid with a power

electronics interface cases reconnection control resides only in this

power electronic interface. That is, the controller in this power electronic

interface would controlled in order to realize on its grid side some

voltage waveform so its amplitude, frequency and phase angle are

within specified limits to allow reconnection.

In the other ac micro-grid casesthose directly connected to the maingrid though mechanical switchgear or static switchesreconnection is

more complicated because there is no possibility of directly controlling

the voltage waveforms at the PCC. In this case, ensuring that the

voltage, frequency and phase angle are within acceptable limits depend

on how the LAPES distributed resources are controlled.



Islanding

According to IEEE Standard 1547.4 these approaches can be

distinguished in this case of ac micro-grids in order to achieve a

successful reconnection: Active synchronization: In this approach, the LAPES controller matches the voltage

signal on the PCC micro-grid side to those of the PCC on the grid side immediately

before closing the islanding devices, such as a circuit breaker. Implementation of this

approach requires measuring these three voltage signal parametersamplitude,

frequency and phase angleon both sides of the PCC. A communications channel inorder to exchange information between the micro-grid and the main grid is also

necessary. This need for sensing and communications may lead to a higher failure rate

as the sensing and communications subsystems may become a single point of failure.

Passive synchronization: In this approach a device is used to monitor the voltage at

both sides of the PCC and allows the LAPES to connect to the main grid only when the

voltage signal on the LAPES side is within some given required range of the main gridanalogous voltage parameters. Like the active synchronization approach, passive

synchronization requires sensing and communications, leading to the same potential

reliability concerns. In addition, this method may be slower than active synchronization.

Open transition: This approach is more basic than the other two, because the method

involves connecting both ends of the PCC after interrupting disconnecting the LAPES

load. Once the micro-grid is connected to the grid then this load is brought back online.


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22 l i i ki

Islanding

According to IEEE Standard 1547.4, un-intentional islanding

operations are inadvertent events that are typically initiated by loss of

area EPS or equipment failure, and the DR island system may beautomatically sectionalized from the area EPS by protective equipment.

Once the island has been established, the same considerations that wereconsidered for the intentional island condition applies to the un-intentional island.

Contrary to the case of intentional islanding, during an un-intentional island it is

not possible to prepare the LAPES for such transition, such as verifying that there issufficient local generation to sustain a stable operation powering all loads. Hence,

in case it is expected that local generation capacity may be insufficient to sustain

the load during un-intentional islands, black start functions or standby generators

with transfer switches have to be allocated within the LAPES.

Once the issue in the main grid that led to loss of service to the micro-grid feederis solved, it may be of interest to reconnect soon the main grid to the micro-grid.

However, such connection cannot occur until the voltage and frequency of the grid

are stable and within acceptable ranges. In order to ensure meeting such

requirement, a delay of up to five minutes may be provided between the time power

is restored at the PCC from the main grid and the time reconnection to the LAPES

is established.

EE394J10 DG Grid Interconnection

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