David Smith, Sales Support Manager, Product Solar Group,
Grid Export Limitation
Segen Seminar
What is Network and Export control ?
When the potential amount of locally generated electricity has reached a level
when the DNO cannot accept any more without breaching its licence conditions
& ESQCR*, a constraint may be needed.
The solutions are;
o Upgrade the network to accommodate the requirement.
o Impose an export constraint to limit the export of energy generated
*ESQCR - Electricity Safety Quality & Continuity Regulations (Principal stakeholders – HSE, DECC,
DBIS, OFGEM)
Key aspects ;
•
•
•
DNO is licensed and legally bound to provide phase voltage to customers at 230V +/- 10% (ie
between 207 & 253V) and within frequency range
Demand customers reduce voltage with increased demand
Prosumers can increase grid voltage with increased generation
(G83/2 and G59/3 is 262V to allow 3% volt rise between Point of connection and inverter)
© ABB
17 April 2015
| Slide 2
When is export limiting network reinforcement
required?
The DNO may propose an export constraint or network reinforcement for 2
principal reasons when you apply to connect a generator
o The voltage rise may put the network out of ESQCR compliance
o The thermal capacity of the network has been reached.
•
•
The delivering of active power ( current and
voltage in phase) into the grid generates a
voltage drop in the grid-line which increases
the voltage in the connection point of the
plant (Vinv). This is caused by grid line
resistivity (RL).
RL
Vgrid
Iinv
XL
PFV
PDC
Vinv
If the inverter’s current lags the voltage with
a certain phase angle (inductive behaviour),
there is reduction of Vinv voltage.
NB - If the network fault level has been reached, export constraint will not resolve
the issue!!
© ABB
| Slide 3
UK Requirement for export limitation
(Commercial/LV)
• They must be fail safe. In other words they must limit export if the export
limiter fails or it loses its power supply.
• The system must not rely on wireless coms (e.g. Bluetooth, Wifi etc.)
between the various component parts of the export limiter system (e.g. the
sensors, the export limiter and the inverters).
• The system must meet all relevant power quality requirements (e.g. G5/4 for
harmonics, P28 for flicker, P29 for unbalance).
• The system must operate within 1s
• They system commissioning tests shall normally be witnessed by DNO
• Note, if the system is not fail-safe by design then an additional reverse
power relay must be included to disconnect the generation if the export goes
above an agreed limit.
© ABB
| Slide 4
Export limiting control
“Measure, calculate and control”
PV Panels
Inverter(s)
Point of Connection
Measurement
G59
Loads
Calculate and control
© ABB
17 April 2015
| Slide 5
Grid
Export limiting control
“Measure, calculate and control”
Measurement - Total active Power (kW) in the export line
needs to be measured and monitored.
Four Quadrant (Bi-directional) power energy meters
•
•
•
2 x relay ouputs
MID compliant option
Modbus RS485 option
Provides calculation
reference
•
© ABB
17 April 2015
| Slide 6
Measurement class CTs
Export limiting control
“Measure, calculate and control”
PV Panels
Inverter(s)
Point of connection
Measurement
G59
Loads
Calculate and control
© ABB
17 April 2015
| Slide 7
Grid
Products to achieve export limiting control
Introducing PMU
Calculate and Control -
with information of Total active Power (kW) and sum of each
inverters output active power, export limiting device can instruct inverters to reduce active power.
© ABB
17 April 2015
| Slide 8
Explanation of MPPT tracking
© ABB
17 April 2015
| Slide 9
Products to achieve export limiting control
“Measure, calculate and control”
Calculate and Control -
with information of Total active Power (kW) and sum of each
inverters output active power, export limiting device can instruct inverters to reduce active power
Shaded area demonstrates
limitation mode and
available power
Inverter power limitation
mode to keep exported power
lower than required.
Power
2nd threshold band
1st threshold band
100% inverter power
consumed by load
Time




The METER measures the power injected to the grid and 2 power thresholds level are selected on the METER.
When the thresholds are reached the METER closes a single digital output connected to the export limiter (PMU)
Upon recieving signal, export limiter (PMU) reduces the inverter(s) output power via RS485 to set thresholds,
The power limitation /threshold level is dependant upon % of export power allowed – ie adjusted according to plant needs
© ABB
17 April 2015
| Slide 10
Measure, calculate and control
Simplified schematic
PV Panels
3 PH + N
(Voltage
reference
Inverter(s)
x 3 CT
RS485 LINE
Loads
Input select = 7
Power
management
unit - PMU
Digital signals from meter
K4
K1/K2
GND
© ABB
| Slide 11
Q2+
Q1+
Q1- / Q2-
Grid
Products to achieve export limiting control
“backup protection and failsafe”
PV Panels
Inverter(s)
Point of connection
Measurement
G59
Loads
Calculate and control
© ABB
17 April 2015
| Slide 12
Grid
Measure, calculate and control
Failsafe - Simplified schematic
3 PH + N
Voltage
reference
3 PH + N
Voltage
reference
Loads
G59/3 relay
CM-UFD.M33
Connected to G59 relay
remote trip line (auto reclose)
© ABB
| Slide 13
x 3 CT
Reverse power relay
Grid
Measure, calculate and control
Failsafe - Simplified schematic – small scheme
3 PH + N
Voltage
reference
x 3 CT
x
Example ;
•
Undervoltage trip option
•
Optionally motorized
(auto reclose)
Reverse power relay
Connected to MCCB
with undervoltage trip
© ABB
| Slide 14
Grid
Measure, calculate and control
Failsafe - Simplified schematic – single inverter scheme
3 PH + N
Voltage
reference
x
For example, TRIO has the power limitation
set to 90% (maximum permitted power
injection) meter relay output to power off the
inverter if >90% of the max power is injected
into the grid.
Additional Meter output connected to inverters inhibit line
© ABB
| Slide 15
x 3 CT
Grid
Measure, calculate and control
Introducing PLC – larger scheme
3 PH + N
Voltage
reference
XT series
Breakers
Grid
x 3 CT
Trip and
reclose
B23-312-100
MID approved
Meter
Loads
G59/3 relay
CM-UFD.M33
RS485
Trip signal
Inverter monitoring and Control – MODBUS RS485
© ABB
| Slide 16
AC500 PLC Range
Summary of architectures
Solution
Inverter
Shedding
Method
Contactor on output of
each inverter
Pro
Con
o
o
o
Low cost
Inverter agnostic
Simple to understand –
DNOs happy
o
o
Can loose significant power
generation.
Additional “wear and tear” through
increased switching of inverters
internal protection relays – impact
inverters life.
PV system
shedding
Disconnection of PV
output
o
As above
o
None if used in correct application –
ie “ Xmas day scenario” – IE 365 day
onsite load
Power
Diverters
Active power used in
onsite load
o
Some devices DNO
approved
No loss of active
generation
o
High cost especially for 3 phase
systems.
Flicker compliant?
Proof of sufficient load to absorb
energy
Relatively low cost.
No impact on inverter life
and warranty.
Grid control ready
o
o
Intelligent
Inverter
Control +
failsafe
© ABB
17 April 2015
| Slide 17
Control of inverters
active power + failsafe
o
o
o
o
o
o
o
Slight loss of generation but not
significant if designed well.
Inverter specific.
DNO satisfaction.
Inverters are already smart grid ready……..
Inverter has the capabiity to
contribute to grid stability
and control in order to
increase grid quality
Frequency control
Capability to select Active
power reduction and Reactive
power regulation
Voltage Control
= Smart grid ready
Power Factor Management
Industry accepts inverters as part of the solution
© ABB
17 April 2015
| Slide 18