DESIGN OF REAL-TIME SYNCHRONIZATION
CONTROLLER IN ELECTRIC SHIP POWER
SYSTEMS
Technical Report
Submitted to:
The Office of Naval Research
Contract Number: N0014-08-1-0080
Submitted by:
Enrico Santi, Huaxi Zhang, Yucheng Zhang
December 2013
Approved for Public Release – Distribution Unlimited
Any opinions, findings, conclusions or recommendations expressed in this publication are those
of the author(s) and do not necessarily reflect the views of the Office of Naval Research.
MISSION STATEMENT
The Electric Ship Research and Development Consortium brings together in a single entity the
combined programs and resources of leading electric power research institutions to advance
near- to mid-term electric ship concepts. The consortium is supported through a grant from the
United States Office of Naval Research.
2000 Levy Avenue, Suite 140 | Tallahassee, FL 32310 | www.esrdc.com
TABLEOFCONTENTS
Executive Summary ........................................................................................................................ 2 1 Motivation ............................................................................................................................... 2 2 Control Scheme of Real-Time Synchronization Controller .................................................... 4 3 2.1 Frequency Control and Phase Control ............................................................................. 4 2.2 Design of Low-Pass Filters .............................................................................................. 7 Validation of Real-Time Synchronization Controller ............................................................. 8 3.1 Simulation Test of System Reconfiguration during Normal Operation ......................... 10 3.2 Simulation Test of System Reconfiguration under Fault Condition .............................. 13 4 Conclusion ............................................................................................................................. 15 5 Acknowledgements ............................................................................................................... 16 6 References ............................................................................................................................. 16 LISTOFFIGURES
Figure 1 Notional Baseline MVAC Ship Power System with a Three-Phase Bolted Fault ........... 3 Figure 2 Large Phase Difference between Subsystem Voltages Leading to Transient High-Peak
Current through CBs during System Reconfiguration Process....................................................... 3 Figure 3 4% Droop Function of Speed Regulator without RTSC (top) and with RTSC (bottom). 5 Figure 4 Structure of the RTSC Consists of Frequency Control and Phase Control ...................... 6 Figure 5 Voltage Signal Measured at the Port Bus of MVAC ESPS ............................................. 8 Figure 6 FFT Analysis of the Voltage Signal before Filtering ....................................................... 8 Figure 7 System start-up under Normal Conditions with RTSC Activated.................................. 10 Figure 8 System Reconfiguration Process during Normal Operation .......................................... 12 Figure 9 System Reconfiguration Process when a Three-Phase Bolted Fault Occurs ................. 15 i
Executive Summary In medium-voltage AC (MVAC) and high-frequency AC (HFAC) power distribution
systems for an Electric Ship, system reconfiguration may cause previously disconnected parts of
the electrical distribution system to be interconnected. This may lead to large transients, if the
overall system is not properly synchronized. This report proposes a Real-Time Synchronization
Controller (RTSC), which solves this problem by keeping all parts of the electrical distribution
system phase-synchronized at all times. The RTSC augments the conventional frequency droop
control for generators. Simulation results demonstrate the effectiveness of the approach in
reducing reconfiguration transients.
1 MOTIVATION An Electric Ship Power System (ESPS) is a micro-grid which is powered by multiple
synchronous generators. These generators are required to operate in either standalone or
interconnected modes depending on generator statuses, load requirements and other system
contingencies, such as faults. Demands for system reconfiguration may occur anytime, which
means that it must be possible to connect any generator to the grid reliably and successfully at
any time. Additionally, system reconfiguration should be smooth enough so that normal
operation of system loads is not interrupted. The ESPS is a megawatt level micro-grid which
could be partitioned in different ways according to different requirements such as load demands,
reliability and efficiency. Before system reconfiguration, the ESPS might be partitioned into two
or more subsystems. Each subsystem might be running at a different frequency depending on its
loading condition. Even assuming that two subsystems are operating at the same frequency, the
voltage at each subsystem might not be in phase with each other. Directly connecting these
unsynchronized generators/subsystems would lead to unacceptable transient oscillation in ESPS
and to other undesirable side effects.
The effects of synchronization or lack thereof on transient stability of Medium Voltage
AC (MVAC) ESPS is now illustrated through a simulation, which shows the detrimental effects
that occur when the generators/subsystems are not synchronized before they are connected to a
larger network. Figure 1 shows the structure of notional MVAC ESPS [1, 2]. In this testing
scenario, a three-phase bolted fault is set to occur in the middle of Port Bus at t = 1 s. Before the
fault occurs, the system is operating in split mode: Zone 1, Radar and Port Bus Propulsion Motor
are fed from Port Bus which is powered by Main Turbine-Generator 2 (MTG2) and Auxiliary
Turbine-Generator 2 (ATG2); Zones 2, 3, 4, High Power Pulsed Load and Starboard Bus
Propulsion Motor are fed from Starboard Bus which is powered by MTG1 and ATG1 generators.
Each MTG or ATG is composed of one gas turbine and one synchronous generator [1]. After the
fault occurs, the circuit breakers (CBs) on both sides of the fault location are tripped off in order
to isolate the fault and the CBs on the Bow and Stern sides are required to close in order to
ensure that adequate power be delivered to each load in the system. For example, after fault
isolation, auxiliary generator ATG2 would not have enough power capability to supply Port Bus
Propulsion Motor. Figure 2 (a) shows the large transient high-peak current through CBs on Stern
Bus Tie – note that the transient current is ten times the steady-state current. This current is due
to the large phase difference between the voltages at Starboard Bus and Port Bus, as shown in
Figure 2 (b). The Port Bus voltage is measured at the location of the three-phase bolted fault. The
adverse impact of this transient current includes: high machine winding stresses, large rotor iron
currents, pulsating torques, and mechanical resonances that pose risk of potential damage to
2
machines. Additionally, the large transient currents may trip off CBs and cause the
reconfiguration process to fail.
Figure 1 Notional Baseline MVAC Ship Power System with a Three‐Phase Bolted Fault 40
20
0
-20
-40
0
1
2
3
4
5
6
Time (s)
7
8
9
10
11
(a) Large Transient Current (in kA) through CBs on Stern Bus (b) 180⁰ Phase Difference between Voltages at Port Bus and Starboard Bus Figure 2 Large Phase Difference between Subsystem Voltages Leading to Transient High‐Peak Current through CBs during System Reconfiguration Process 3
From the example described above, it can be learned that phase difference between bus
voltages poses hazard to the successful and reliable system reconfiguration. Frequency difference
between generators in different subsystems due to loading condition also adversely affects the
system reconfiguration process. When subsystems are connected with frequency difference
present, generators in one subsystem will oscillate against those in other subsystems, which
results in longer time for the whole ESPS to reach steady state. If the frequency difference is too
large, it may even cause transient stability issues which may eventually lead to system
reconfiguration failure.
Therefore, generators in each subsystem must be synchronized both in frequency and
phase before they are connected into a single network. Before connecting subsystems, the
differences in frequency and phase should be minimized. When these synchronization criteria are
satisfied, subsystems can be successfully connected to a large network by closing CBs.
The Real-Time Synchronization Controller (RTSC) described in this report is able to
enforce synchronization of generators at any time when the ESPS is in service. It is highly
critical to the success of system reconfiguration. The requirement of flexibility, stability and
reliability for the reconfiguration process can be addressed as well by implementing the RTSC.
Note that this controller is designed based on MVAC ESPS and has been tested on the Notional
Baseline MVAC ESPS model. However, with its parameters properly tuned, the RTSC could be
implemented in the High Frequency AC (HFAC) system as well.
2 CONTROL SCHEME OF REAL‐TIME SYNCHRONIZATION CONTROLLER The Real-Time Synchronization Controller (RTSC) is designed to be a part of the
supervisory control for generators. Its goal is to maintain all generators/subsystems in the ESPS
synchronized at all times during operation, even when there is no physical connection or power
flow between them, so that they can be successfully connected together at any time. The RTSC is
based on droop control in frequency and consists of two control loops: 1) frequency control
(FC); and 2) phase control (PC). Also, to prevent jitter caused by harmonics in voltage
measurement and to avoid interference between frequency control and phase control, low-pass
filters are utilized in the RTSC.
2.1
Frequency Control and Phase Control The function of frequency control is to force all generators to operate at rated system
frequency (e.g. 60 Hz in MVAC ESPS and 240 Hz in High Frequency AC ESPS). FC is
implemented by adding a variable offset dynamically to the speed regulator reference in each
generator. In order for generators to operate reliably in parallel and share generated power in
proportion to their respective power rating, speed regulators of generators are set in droop mode,
so that the generator frequency has a constant proportional relationship to the turbine’s output
mechanical power, as shown in Figure 3 (top). The droop setting is 4% in MVAC ESPS. As a
result of droop control, the generator frequency may drop up to 4% below nominal as a function
of load. To remedy this, the RTSC frequency control introduces an offset ∆
in the droop
function, so that in steady state the per-unit frequency is exactly one, as shown in Figure 3
(bottom).
4
Figure 3 4% Droop Function of Speed Regulator without RTSC (top) and with RTSC (bottom) The offset of frequency control for the ith Generator is calculated by the RTSC control as
in Eq. 1.
∆
∗∑
∑
(1)
_
where, ∆
is the offset of frequency control for the ith Generator, is the droop setting
of the speed regulator in the ith Generator (e.g., 4% as in Fig. 3),
_ is the rated active
power of the jth Generator,
is the instantaneous total output power of the jth Turbine
Engine.
represents the relationship between the Generators i and j:
1 if the two
0 if the two generators are located in
generators are located in the same subsystem;
different subsystems; is the total number of generators in the ESPS. The ratio in the right hand
side of Eq. 1 represents the percentage loading of the generators in the subsystem containing
Generator with respect to their combined rated power.
When implementing the RTSC in an ESPS, one subsystem is chosen as the reference
subsystem and the other subsystems are set as the target subsystems. Note that different
subsystems are not connected to each other, i.e., they are islanded. The criterion to choose the
reference subsystem is that it should have the largest power rating among all subsystems. As the
control is implemented in software, the reference subsystem can be chosen dynamically
according to system operating state. The complete structure of the RTSC is depicted in Figure 4.
5
Figure 4 Structure of the RTSC Consists of Frequency Control and Phase Control In the figure only two subsystems are shown for simplicity: a reference subsystem,
consisting of Generators 1 – m, and a target subsystem, consisting of Generators m+1 – n.
Additional subsystems would be shown as separate target subsystems. Both the reference and the
target subsystem have frequency control, but only the target subsystem has phase control. The
goal of frequency control is to ensure that all subsystems operate at the same frequency, whereas
the goal of phase control is to keep voltages of the target subsystem(s) and the reference
subsystem in phase. Frequency control calculates for each generator the offset of frequency
control ∆
as a function of real power generated using Eq. 1. In the phase control block,
three-phase voltage measurements are taken at the generator terminals in the target subsystem
and the reference subsystem. The harmonics resulting from non-linear load current are
eliminated by Low-Pass Filter I (LPF-I). In the “Phase Calculation” blocks in Figure 4, using
Eq. (2) the three-phase voltage measurements are transformed to the static
axis frame and
,
is calculated.
the phase, , of the space vector
6
√
(2)
√
2
,
where , , are the three-phase measured voltages;
is the voltage portion on static axis;
is the voltage portion on static axis; is the phase of space vector
,
.
The phase difference, ∆ , between the target subsystem and the reference subsystem, calculated
as
∆
,
goes into the “Signal Processing” block and its value is constrained from [-2π, 2π] to [-π, π]
using phase wrap. The offset of phase control is produced by the proportional (P) “Gain”
controller in Figure 4. According to synchronization criterion, the slip frequency between two
subsystems must be lower than 0.1 Hz for 60 Hz power system. So the gain of P controller, , in
the phase control can be chosen according to Eq. (3).
. ∗
(3)
As a result of the RTSC control of Figure 4, in steady state all subsystems operate at
the nominal frequency (p.u. frequency of one) and the voltages of all target subsystems are
in phase with the reference subsystem.
2.2
Design of Low‐Pass Filters There are two low-pass filters (LPF_I and LPF_II) in the RTSC, as shown in Figure 4.
They strongly influence the performance of the RTSC and must be designed carefully. Both
filters are implemented as first-order low-pass filters in the RTSC. Their cut-off frequency is
determined by the load condition and other parameters such as the inertia constants of generators
and the time delay of turbine engines.
Design of LPF_I: Due to the nonlinear loads like the diode-rectifier in ESPS, the
measured voltage contains 2nd and high-order harmonic components. To help design LPF_I,
Fourier analysis is performed to determine the filter cut-off frequency. Figure 5 shows the
voltage signal measured at the Port Bus of the MVAC system without RTSC, and its FastFourier Transform (FFT) is shown in Figure 6. As these figures show, in addition to the 60 Hz
fundamental component, some high-order harmonics are present. Based on the result of FFT, the
cut-off frequency of LPF_I is set to 100 Hz for the RTSC in the MVAC ESPS.
7
Voltage (V)
5000
0
-5000
21
21.05
Time (s)
21.1
Figure 5 Voltage Signal Measured at the Port Bus of MVAC ESPS Figure 6 FFT Analysis of the Voltage Signal before Filtering Design of LPF_II: The measured instantaneous power signal at the input of the frequency
control block relates to the droop setting, the delay of speed regulator, inertia constant of
generator, and loading condition. To avoid interference between the frequency control and the
phase control, the cut-off frequency of LPF_II can be determined by Eq. (4).
(4)
where
is the cutoff frequency of LPF-II,
is the inertia constant of generator,
is
the delay in speed regulator and turbine engine combined. The inertia constants of the generators
in MVAC ESPS are 4 s and 3.5 s for main and auxiliary generators, respectively. Since the
delay is less than 1 s, the cut-off frequency is chosen as 0.2 Hz.
3 VALIDATION OF REAL‐TIME SYNCHRONIZATION CONTROLLER The effectiveness of the proposed RTSC is verified via simulation tests based on the
waveform-level notional baseline MVAC ESPS [1]. The MVAC ESPS has two subsystems, the
Port Bus and Starboard Bus as shown in Figure 1. The details of the MVAC ESPS including
each subsystem structure and parameters are documented in [1]. The only modification with
respect to the MVAC ESPS described in [1] is that various loads that are connected to each bus
are replaced with a lumped load for each bus. The lumped load on Port Bus is 30 MW, 15 MVar
and the one on Starboard Bus is 18 MW, 9 MVar. The cut-off frequency for LPF-I and LPF-II
are 100 Hz and 0.2 Hz, respectively.
8
When the system operates in steady state, the two subsystems operate in standalone
mode. When certain system contingencies occur, such as a step change in power demand or a
fault, system reconfiguration will be needed. The CBs at Stern and Bow sides in Figure 1 will
close and the two subsystems will combine to form a ring bus. Power generation from four
generators will be shared via the ring bus and the contribution from each generator is
proportional to its power rating. To demonstrate the necessity of the RTSC in such system
contingencies, two scenarios are tested: a) system reconfiguration during normal operation; and
b) system reconfiguration under fault condition. In both testing scenarios, system transient
performance will be compared for two cases: with and without the RTSC activated.
The system start-up process is now described. Figure 7 shows the frequency profile and
the phase difference between voltages at Port Bus and Starboard Bus for the case with the RTSC
activated (frequency control activated at 150 s and phase control activated at 180 s). The ESPS is
operating in split mode and each subsystem reaches steady state before 150 s. Since frequency
control is not active yet, the droop characteristic of Figure 3 (top) causes the frequencies of the
two buses to be less than 1.0 p.u. and different from each other, due to different loading
conditions. After the frequency control is activated at 150 s, the frequency of two subsystems
return back to 1.0 p.u. in approximately 10 s and the phase difference between them takes a nonzero value that does not change any longer after that. After the phase control is activated at 180 s,
the phase difference between subsystems becomes zero in about 10 s with minor oscillations in
the frequency profile.
(a) Frequency of Port and Starboard Subsystems in Per Unit 9
4
3
2
1
0
-1
-2
-3
-4
140
150
160
170
180
190
200
210
220
Time (s)
(b) Phase Difference between Port and Starboard Subsystems in Radian Figure 7 System start‐up under Normal Conditions with RTSC Activated 3.1
Simulation Test of System Reconfiguration during Normal Operation During normal operation, responding to the requirements of energy efficiency and
generation power redundancy, the Port and Starboard Buses may need to be connected together
to form a ring bus, so that the total load can be shared among all four generators. The transient
dynamics during the reconfiguration process (frequency and voltage waveforms) are shown in
Figure 8 for two cases: RTSC disabled and RTSC enabled. The system reconfiguration occurs at
219.6 s.
Figure 8 (a) ~ (c) depict the reconfiguration process without RTSC activated. At 219.6 s,
the phase difference between the two subsystems is 180°, which is the worst case for connecting
two subsystems. After the CBs on both Stern and Bow sides are closed, the current through CBs
reaches a peak value of 55.9 kA with transient ac rms component of 21 kA. The starboard bus
voltage drops to 240 V peak (96% deviation from rated voltage of 4160 V) and it takes 2.0 s to
recover. The maximum transient deviation in frequency reaches 0.024 p.u. (1.46 Hz) and it takes
about 2.5 s for the four generators to reach a steady state condition in which they operate
synchronously. Due to the large transient current through the CBs, the CBs might trip off, which
would result in reconfiguration failure.
Figure 8 (d) ~ (f) show the reconfiguration process with RTSC activated prior to it (as
shown in Figure 7). Neither large transient current through CBs nor significant voltage sag
appears. The maximum transient deviation in frequency is only 0.006 Hz. The reconfiguration is
almost transient-free. The current through the CBs on the Stern Bus increases steadily until the
reconfigured system reaches steady state in 4.0 s. Thus, the transient stability of reconfiguration
process during normal operation is greatly improved by implementing the RTSC.
10
phase a
phase b
phase c
60
40
20
0
-20
-40
-60
219.4
219.6
219.8
220
220.2
220.4
220.6
Time (s)
(a) Three‐Phase Currents (in kA) through CBs on Stern Side without RTSC Activated 8000
6000
4000
2000
0
-2000
-4000
-6000
-8000
218
219
220
221
222
223
224
225
Time (s)
(b) Voltage (in Volt) on Starboard Bus without RTSC Activated 1.02
MTG1
MTG2
ATG1
ATG2
1.01
1
0.99
0.98
0.97
0.96
0.95
0.94
0.93
0.92
218
219
220
221
222
223
224
225
226
227
Time (s)
(c) Frequency (in Per Unit) of Generators without RTSC Activated 11
228
1
phase a
phase b
phase c
0.8
0.6
0.4
0.2
0
-0.2
-0.4
-0.6
-0.8
-1
219.4
219.6
219.8
220
220.2
220.4
220.6
Time (s)
(d) Three‐Phase Currents (in kA) through CBs on Stern Side with RTSC Activated 8000
6000
4000
2000
0
-2000
-4000
-6000
-8000
218
219
220
221
222
223
224
225
Time (s)
(e) Voltage (in Volt) on Starboard Bus with RTSC Activated 1.0005
MTG1
MTG2
ATG1
ATG2
1.0004
1.0003
1.0002
1.0001
1
0.9999
0.9998
0.9997
0.9996
0.9995
218
219
220
221
222
223
224
225
226
227
Time (s)
(f) Frequency (in Per Unit) of Generators with RTSC Activated Figure 8 System Reconfiguration Process during Normal Operation 12
228
3.2
Simulation Test of System Reconfiguration under Fault Condition In this section, the system reconfiguration process following a three-phase bolted fault is
tested. At 219.6 s, a three-phase bolted fault occurs in the middle of Port Bus, which is the most
severe fault to this system, as shown in Figure 1. Considering a typical fault clearance time of 3.5
cycles (equivalent to 0.058 s, 0.5 cycle of fault detection plus 3 cycles of fault clearance by
tripping off CBs), the fault is cleared at 219.658 s and the CBs at Stern and Bow sides close
instantaneously.
From Figure 9 (a) – (c) it is found that without RTSC activated the current reaches a peak
value of 60.1 kA with transient ac rms component of 23.4 kA. The voltage on Starboard Bus
drops to 127 V peak (97.8% deviation from rated voltage) and it takes 2.0 s to recover. The
maximum transient deviation in frequency reaches 0.023 p.u. (equivalent to 1.39 Hz) and it takes
about 2.5 s for the four generators to reach a steady state condition in which they operate
synchronously. The results are similar to those from test of reconfiguration during normal
operation without RTSC activated.
With RTSC activated, the transient responses are improved dramatically, as shown in
Figure 9 (d) – (f). The transient current is reduced to a peak value of 13 kA with transient ac rms
component of 3.8 kA. The voltage drops to 4968 V peak (15.5% deviation from rated voltage)
and the system recovers in as short as 0.4 s. The maximum transient deviation in frequency is
0.0065 p.u. (equivalent to 0.39 Hz) and it takes about 1.0 s for the four generators to reach
synchronism.
With RTSC control, comparing the results for normal reconfiguration (Section 3.1)
versus the results for fault condition, the transients under fault condition are somewhat larger, but
still limited to a small and acceptable level. In conclusion, the transient stability of ESPS during
reconfiguration under different system contingencies is greatly enhanced by embedding the
RTSC into generator control.
phase a
phase b
phase c
60
40
20
0
-20
-40
-60
219.4
219.6
219.8
220
220.2
220.4
220.6
Time (s)
(a) Three‐Phase Currents (in kA) through CBs on Stern Side without RTSC Activated 13
8000
6000
4000
2000
0
-2000
-4000
-6000
-8000
218
219
220
221
222
223
224
225
Time (s)
(b) Voltage (in Volt) on Starboard Bus without RTSC Activated 1.02
MTG1
MTG2
ATG1
ATG2
1.01
1
0.99
0.98
0.97
0.96
0.95
0.94
0.93
0.92
218
219
220
221
222
223
224
225
226
227
228
Time (s)
(c) Frequency (in Per Unit) of Generators without RTSC Activated phase a
phase b
phase c
60
40
20
0
-20
-40
-60
219.4
219.6
219.8
220
220.2
220.4
220.6
Time (s)
(d) Three‐Phase Currents (in kA) through CBs on Stern Side with RTSC Activated 14
8000
( )
6000
4000
2000
g
0
-2000
-4000
-6000
-8000
218
219
220
221
222
223
224
225
Time (s)
(e) Voltage (in Volt) on Starboard Bus with RTSC Activated 1.02
MTG1
MTG2
ATG1
ATG2
1.015
1.01
1.005
1
0.995
0.99
0.985
0.98
218
219
220
221
222
223
224
225
226
227
228
Time (s)
(f) Frequency (in Per Unit) of Generators with RTSC Activated Figure 9 System Reconfiguration Process when a Three‐Phase Bolted Fault Occurs 4 CONCLUSION A novel real-time synchronization controller for ESPS is described in this report. The
controller is applicable to both MVAC and HFAC systems. Its goal is to keep multiple
generators in synchronization at all times regardless of connectivity, which ensures successful
and reliable system reconfiguration whenever it is needed. The RTSC consists of two parts,
frequency control and phase control, which are described in the report. Low-pass filters are tuned
to avoid interference between these two controls. To verify the effectiveness of the proposed
RTSC, two scenarios reflecting different system contingencies are simulated based on MVAC
Notional Baseline System. All test results demonstrate that the transient current level, voltage
sag, and frequency deviation are all limited within an acceptable level to deliver a safe
reconfiguration process.
In conclusion, the transient stability of ESPS during reconfiguration process is greatly
enhanced by applying the RTSC to the control system. Due to the high requirement for flexibility
and reliability in ESPS, it is suggested that the RTSC be implemented in practice.
15
5 ACKNOWLEDGEMENTS 6 REFERENCES [1] James Langston, Mischa Steurer, Jonathan Crider, Scott Sudhoff, Yonggon Lee, Edwin Zivi, Roger Dougal,
Yucheng Zhang, Robert Hebner, Abdelhamid Ouroua, “Waveform-Level Time-Domain Simulation
Comparison Study of Three Shipboard Power System Architectures”, 2012 International Simulation MultiConference, Genoa, Italy, July 2012.
[2] Norbert Doerry, “Next Generation Integrated Power System: NGIPS Technology Development Roadmap”,
[Online]. Available:
http://www.dtic.mil/cgi-bin/GetTRDoc?AD=ADA519753
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