1. COMPARISON OF PRIMARY FREQUENCY CONTROL USING
TWO SMART LOAD TYPES
Diptargha Chakravorty, Zohaib Akhtar, Balarko Chaudhuri, Ron Hui
Control and Power Group, Department of Electrical and Electronic Engineering, Imperial College London
2016 IEEE PES General Meeting
RESEARCH SUMMARY
• Primary frequency control using smart loads with reactive compensation
(SLQ) has been shown in the past.
• Improvement in frequency regulation is shown here using smart loads with
back-to-back converter (SLBC).
• SLBC provides flexibility in terms of independent and wider control over
active and reactive power of smart load (SL).
• Frequency regulation with SLBCs is compared with SLQs through two case
studies- a) 4 generator, 2 area system b) 39 bus New England system.
• Unlike previous studies, detailed modelling is used for both
transmission system and distribution network down to medium voltage
level thereby avoiding inaccuracies associated with load aggregation and
spatial voltage variation.
CONCLUSION
• Fast short term power reserve (FSPR) from SLs will be crucial for future low
inertia system.
• For over-frequency event SLQ is less effective and performance deteriorates
as load pf approaches unity.
This research is funded by EPSRC, UK under the Autonomic Power Systems
(APS) grant (EP/I031650/1)
SMART LOAD
Smart load comprises of a part rated power
electronic compensator (Electric Spring) in
series with cluster of voltage dependent loads.
The injected series voltage (VESES) is
controlled to regulate the mains voltage (VC)
while allowing the voltage across the load
(VNC) and consequently its active power
consumption to vary, thereby, collectively
contributing towards system frequency
support.
A. Concept
Power
electronic
compensator
Voltage
dependent
load
VES
ES
VNC
Supply/mains
VCSmart
load
B. Configuration
SLQ SLBC
C
Power
inverter
VES ±90°
I0
Voltage
dependent
load
ZNCfNC
QES = VES×I
PES ≈ 0
VNC
VC
I0
Supply feeder
PNC
QNC QES
C
Converter #1
VES ES
Voltage
dependent
load
ZNCfNC
PES = VESIcosES
QES = VESIsinES
VNC
VC
Supply feeder
I0
I0
PNC PES
QNC QES
Converter #2
Vdc and Q(=0) control VES and ES control
PES ≈ VESIcosES
• Single converter (cheaper,
lower losses)
• Only Q support (VES angle
fixed at ±90°)
• Voltage OR Frequency
regulation
• Limited capability
• Two converters (expensive, higher
losses)
• Both P&Q support (VES & θES control)
• Voltage and/or Frequency regulation
• Wider capability
In both cases, smart load acts as a
controllable P,Q sink. For SLQ it’s inter-
dependent PQ control while for SLBC it’s
independent PQ control.C. Capability
• Capability depends on the type of load (voltage exponent), power factor (pf)
of load, converter rating, permissible load voltage (VNC) variation and
terminal (mains) voltage (VC).
• VNC limited to ±20% of nominal voltage across load (dotted lines).
• Additionally, converter rating limited to 20% of nominal rating of load (solid
lines).
SLQ
I. No +ΔPSL for unity pf
load.
II.Max +ΔPSL < Max
–ΔPSL for same
converter rating.
III. ΔQSL is dependent on
ΔPSL
SLBC
I. Wider capability in both
directions.
II. ΔQSL is independent of
ΔPSL.
III. For ↓pf capability↑ in
1st & 3rd quadrant.
TEST NETWORK
Modified 39 bus New England system
• 10 generator 39
bus system.
• Generators have
governors, AVRs,
PSSs.
• Spinning reserve
slightly higher
than largest
generator (830
MW).
• 19 loads
replaced by
multiple IEEE 69
bus distribution
networks
C. RoCoF and Converter rating
FREQUENCY REGULATION
• 30% penetration of non-
synchronous generation
(NSG) considered.
• Generator G8 disconnected
to create under frequency
event.
• Dynamic responses for
frequency and voltage
shown at distribution bus
14.
• Both SLQ and SLBC
improves frequency nadir.
• Voltage across load (VNC)
hence, power (PNC) reduces
to provide reserve for
frequency regulation.
A. Under-frequency event
B. Over-frequency event
• 30% penetration of non-
synchronous generation
(NSG) considered.
• Section of load
disconnected having
magnitude equal to G8.
• Dynamic response shows
SLBC achieves similar
regulation as in under-
frequency case.
• SLQ fails due to limited
capability (+ΔPSL axis).
• Voltage across load (VNC)
hence, power (PNC)
increases to support
frequency regulation.
• Both SLQ & SLBC improves RoCoF in under-frequency event. SLQ fails
in case of over-frequency event.
• Combined rating of SLBC converters comparable to SLQ for under-
frequency. SLQ rating increases significantly for over-frequency as they
operate near maximum +ΔPSL.