Battery energy storage power conditioning system

A method and apparatus for controlling a battery energy storage system of the type in which an inverter is coupled to convert direct current power from a DC source to a controlled frequency AC power suitable for supplementing utility power or for replacing utility power includes a control mechanism for operating the system and either a supplemental or replacement mode in parallel with a utility power system. The system favors frequency control over power control and maintains constant monitoring of frequency output with adjustment of power in response to any frequency shift of the inverter output. The system also includes apparatus for determining a power error signal based upon commanded power output in which the power error signal is applied as a phase shift control signal in the frequency control circuit for regulating the real component of power supplied by the inverter. In one form, the system is illustrated as multiple, parallel connected power control systems coupled to a common DC source for supplying a common AC output and each of the power control systems are regulated in response to the reactive component of power supplied therefrom so as to promote load sharing between the separate power control systems.

BACKGROUND OF THE INVENTION 
The present invention is directed to a battery energy storage system for 
supplementing utility power and, more particularly, to a control system 
for regulating power transfer from and to the battery energy storage 
system to allow the battery to be used in load leveling applications or to 
replace utility power in the event of utility power failure. 
There are numerous industrial, commercial and electric power utility 
applications in which it is desirable to provide battery energy storage 
systems with power capability varying from 0.5 to 40 megawatts and being 
capable of supplying such power for anywhere from about thirty minutes to 
about four hours. For example, a paper manufacturing facility or a steel 
rolling mill requires an orderly shutdown process in order to avoid major 
damage and loss of material. While the use of battery backup systems have 
been known for several years in smaller applications, it is only in recent 
years that attention has been focused on providing large battery backup 
systems capable of supplying power for operating industrial applications 
such as paper and steel rolling mills. Further, while supplying backup 
electrical power in the case of utility failure is critical to economic 
operation of some industrial facilities, it is also important to be able 
to provide battery power to accommodate load leveling at the industrial 
facility. Load leveling may be required in the event of voltage depression 
or in instances in which temporary excess power is demanded by the 
industrial application. Using load leveling to accommodate short-term, 
above average demand reduces the overall energy cost for the industrial 
application. 
Battery backup systems in the form of uninterruptible power supplies (UPS) 
are known in the art. In a conventional UPS application, typically 
designed for low power operation, all of the power to the load comes 
through the UPS so that the load has no direct connection to a power 
utility grid. Another form of application provides an isolated UPS that is 
charged from a separate circuit rather than directly from the utility and 
is isolated from the load until utility power fails. In either case, the 
basic hardware components of the battery energy storage system are a power 
conditioning system which provides bidirectional power conversion between 
the direct current (DC) battery system and the alternating current (AC) 
utility system, and a battery to supply the energy storage capacity. The 
typical power conditioning system includes a voltage source inverter 
designed to operate in an inversion mode when discharging the battery to 
supply the AC load or in a rectifying mode when the battery is being 
charged. Typically, high speed solid state electronic switches are 
operated in a pulse width modulation (PWM) mode to generate an AC voltage 
waveform with relatively little distortion. 
It is not believed that any of the prior systems have the capability for 
functioning in both a load leveling function and in a power backup system 
isolated from the utility. 
SUMMARY OF THE INVENTION 
Among the several objects of the present invention may be noted the 
provision of a method and apparatus incorporated in a power conditioning 
system for use with a battery energy storage system which provides an 
improved interface between the battery and a power distribution network; 
an improvement in power conditioning system regulator operation and 
structure for improved control or limit of power flow, terminal voltage 
and line current; and the provision of an improved method and apparatus 
incorporated in a power conditioning system which has the capability of 
operating in both a load leveling application synchronized to utility 
voltage and frequency and as a backup power source isolated from the 
utility power when utility power fails. In an illustrative embodiment, the 
invention comprises a battery backup control system for controlling a 
solid state inverter coupling a battery in parallel power transfer with 
the utility power system. The utility power system normally supplies AC 
electric power to a reactive load. The inverter includes a plurality of 
solid state, electronically controlled switches connected in circuit with 
at least one transformer with the transformer providing an interface 
between the battery backup system and the AC power system and the load. 
The solid state switches in the inverter are responsive to electronic 
control signals which gate the switches into and out of conduction so as 
to create a sine wave coupled through the transformer when the battery is 
used to supplement or backup the utility power. The control system 
comprises a gating logic circuit for generating the electronic control 
signals in response to a voltage command signal and a phase command 
signal, the voltage command signal establishing the conduction times of 
the controllable switches and the phase command signal establishing the 
output frequency and phase of the inverter output voltage. A voltage 
regulator is coupled for receiving a voltage error signal representative 
of any difference between a desired magnitude of voltage from the inverter 
and a measured magnitude of voltage at the inverter output. The voltage 
regulator is responsive to the voltage error signal for generating the 
voltage command signal with a magnitude and polarity sufficient to 
minimize the value of the voltage error signal. A power regulator monitors 
the frequency and phase of the inverter output voltage and compares the 
frequency and phase to a reference frequency for generating the phase 
command signal for maintaining the frequency and phase of the output 
voltage at the reference frequency. The power regulator includes a power 
processing circuit for receiving a power error signal representative of 
any difference between measured real power output of the inverter and a 
desired power signal and for generating a phase shift signal for summation 
with the phase command signal for regulating the real component of power 
supplied by the inverter. 
The battery control system further includes a regulator droop control 
responsive to the magnitude of reactive power for modifying the voltage 
error signal inversely with such magnitude in order to promote load 
sharing between parallel connected inverters coupled from the battery and 
AC load. The system also includes circuitry for limiting the magnitude of 
the voltage command signal between positive and negative limits and uses 
an inertial regulator for limiting the rate of change of the phase command 
signal. Still further, the power regulator includes apparatus for summing 
a first signal representative of the frequency of the inverter output 
voltage for the second signal representative of a desired output voltage 
frequency to produce a frequency error signal. A proportional plus 
integral regulator receives the frequency error signal and produces a 
frequency biasing signal corresponding to the frequency error. An 
amplifier receives the frequency error signal and provides a controlled 
amount of gain so that the signal can be summed with the frequency biasing 
signal to produce a power offset signal. The power offset signal is summed 
with a desired power signal to adjust inverter power output to a value 
sufficient to permit regulation of an inverter output frequency to a 
desired value. Adjustment of power may be necessary in order to assure 
that the frequency of the inverter remains constant. One reason for 
maintaining constant frequency is that the AC load generally includes 
inductive motors whose speed is related to power frequency.

DETAILED DESCRIPTION OF THE INVENTION 
Referring now to FIG. 1, there is shown a simplified one line schematic 
representation of a battery energy storage system. The battery is 
indicated at 10 and supplies direct current power through a power 
conditioning system 12 to a three-phase bus 14. While the description 
refers to a "battery" as an auxiliary power source, it is to be understood 
that other types of energy storage devices could be used such as, for 
example, capacitive systems or fuel cells. Furthermore, magnetic energy 
storage devices could be used but might require conversion circuitry to 
adapt their constant current characteristic to a constant voltage 
characteristic. The voltage on the three-phase bus is indicated by the 
vertical line E.sub.I. The three-phase bus 14 couples to primary windings 
18 of a power transformer 20. The secondary windings 22 of the power 
transformer are coupled to another three-phase bus 26. The bus 26 is 
isolated from a utility bus 28 by a three-phase breaker 30. A load 32 is 
coupled to the three-phase bus by a terminal bus indicated at V.sub.T. The 
three-phase lines 26 may include inductive and capacitive filtering as 
indicated by the LC filter 34 coupled to the terminal bus V.sub.T. For the 
purpose of this description, the voltage at the load will be considered to 
be the voltage V.sub.T at the terminal bus. It will be appreciated from 
this one line diagram that the battery 10 is essentially coupled in 
parallel power supply arrangement with the utility bus 28 so that in the 
event that power at the utility bus 28 is interrupted, power can be 
supplied from the battery 10 to the load 32. Further, the power supply 
from the utility bus 28 can be supplemented by power from the battery 10. 
This arrangement clearly differentiates from conventional UPS systems in 
which the power from utility bus 28 would pass through the power 
conditioning system 12, i.e., the utility bus, battery backup and load 
would be connected in a series circuit. Furthermore, the power control 
system 12 preferably includes a voltage source inverter designed to 
operate as either an inverter when discharging the battery or as a 
rectifier when the battery is being charged. More particularly, the power 
conditioning system 12 must be bidirectional to allow the battery to be 
charged from the utility power bus 28. 
For most operating conditions, the invention is generally achieved by 
making the power conditioning system regulator 12 operate so that the 
circuit has the apparent equivalence of a voltage source E.sub.I driving 
the reactance of the transformer 20 to produce output voltage V.sub.T. The 
phasor diagram of FIG. 2 illustrates the essential feature of the battery 
energy storage system operation. The generated voltage E.sub.I must be 
completely controllable within the current rating of the converter 
equipment, i.e., the AC current from the power conditioning system can be 
supplied at any phase angle relative to the terminal voltage V.sub.T. This 
permits the system to generate real and reactive power in all four 
quadrants. The battery energy storage system power generating capability 
is then limited only by the rating of the inverter and transformer within 
the power conditioning system and the available battery voltage. The 
active and reactive power controls are independent within the constraints 
of the inverter capacity. In FIG. 2, the capability curve is illustrated 
by the phantom line 36 with the center of the capability curve being 
defined by the end point of the terminal bus voltage V.sub.T. The voltage 
E.sub.I always lies on or within the dash line 36 with the phase angle of 
E.sub.I with respect to V.sub.T being determined by the transformer 
reactance voltage jX.sub.T I.sub.T where I.sub.T represents the net 
transformer current. The angular displacement between the voltage V.sub.T 
and the voltage E.sub.I is represented by the measurement .delta..sub.IT, 
where .delta..sub.IT represents a phase shift signal modifying the phase 
angle .theta.T . The maximum value of .delta..sub.IT is illustrated by the 
dotted line 37 extending from the origin 38 tangent to the capability 
curve 36. For purposes of operating the power conditioning system 12, an 
arbitrary phasor reference 40 is established with the displacement of the 
phasor E.sub.I being given by the angle .theta..sub.I and the displacement 
of the phasor V.sub.T being given by the angular displacement .theta.T or 
.theta.P.sub.LL. .theta.T is actually the measured angular displacement of 
the phasor V.sub.T while the value .theta..sub.PLL is the phase lock loop 
angle to be described in conjunction with the operation of the power 
conditioning system 12. 
To better understand the present invention, reference is first made to FIG. 
3 which shows a basic control scheme for a simple uninterruptible power 
supply (UPS) in which the load is served entirely from the power control 
system with no connection between the power control system and the utility 
grid. The basic control comprises a pulse width modulation (PWM) gating 
logic circuit 42 which interfaces to the power converter/inverter 
switching devices of inverter 43. Preferably, the switching devices are 
solid state electronically controllable switching devices such as gate 
turnoff (GTO) devices. The PWM gating logic circuit 42 accepts a voltage 
magnitude command signal E.sub.I and a phase command signal .theta..sub.I 
and translates the signals E.sub.I and .theta..sub.I into gate pulses so 
that the fundamental component of the PWM wave created at the inverter 
output terminals has the desired voltage magnitude E.sub.I and phase 
.theta..sub.I. While the signals E.sub.I and .theta..sub.I represent the 
desired voltage and phase relationship for the AC output voltage of the 
inverter, it will be recognized that the signals are DC values 
representative of the desired outputs. However, the identifiers E.sub.I 
and .theta..sub.I are also used to indicate the actual output voltage 
magnitude E.sub.I and voltage phase angle .theta..sub.I. The voltage 
V.sub.T on bus 26 (the load voltage) is measured and is used to enhance 
the accuracy of the magnitude portion of the PWM output waveform. In this 
regard, a signal representative of the measured magnitude of voltage 
V.sub.T is supplied to a magnitude detector 44 whose output is 
proportional to the voltage V.sub.T and is supplied to a summing junction 
46. A second input of the summing junction 46 is a reference voltage 
V.sub.REF and the output of the summing junction is a voltage error signal 
V.sub.ER, i.e., the difference between the desired or selected magnitude 
of load or output voltage V.sub.REF and the measured output or load 
voltage V.sub.T. A voltage regulator 48 utilizes the error voltage 
V.sub.ER to adjust the magnitude of the internal reference voltage E.sub.I 
to maintain the terminal voltage at a desired set point. The voltage 
regulator may be constrained by voltage limits indicated by the input 
signal E.sub.LIMIT. This prevents the voltage regulator output from 
attempting to drive the reference voltage E.sub.I outside of a desired 
range of voltage. The angle .theta..sub.I is generated from a constant 
frequency setpoint of .omega..sub.SP which is applied to a ramp generator 
circuit 50. The ramp generator circuit 50 provides a repetitive ramp 
output signal for controlling the PWM gating logic circuit in which the 
ramp output varies uniformly from zero to 360 electrical degrees. The PWM 
gating logic signal although shown as a single line output actually 
comprises a plurality of parallel output signals supplied to the multiple 
stage PWM inverter 43. The PWM inverter 43 is operated to supply a pulsed 
output signal to segments of a multi-stage power transformer. The details 
of the PWM inverter 43 and the power transformer are given in Vol. 26 of 
the January/February 1990 issue of the IEEE/IAS Transactions at page 63, 
et seq. in an article entitled "Ten Megawatt GTO Converter For Battery 
Peaking Service" authored by Loren H. Walker, the disclosure of which is 
hereby incorporated by reference. 
As shown in the above mentioned IEEE Transaction article, the power 
inverter 43 actually comprises three identical power units feeding nine 
single phase transformers. The inverter operates as an eighteen pulse, 
stepped wave, bidirectional, voltage source GTO converter. Each GTO in the 
inverter is paralleled by a reverse diode to give the converter the 
capability of handling power flow in both directions. In the embodiment 
described in the above mentioned article, the converter is constructed as 
three identical six-pulse inverter cabinets with each cabinet being 
designated as a power conversion module. Each cabinet contains two, 
three-phase bridges of GTO's including a leading three-phase bridge and a 
retarded three-phase bridge. Each GTO is gated with a 60 Hz squarewave, 
180.degree. conduction signal with the GTO gating within the three-phase 
bridges being displaced 120.degree. in a conventional manner. Within each 
array of leading and retarded bridges, one GTO is gated on every 20 
degrees to provide the stepped voltage output. The transformer primaries 
are connected between corresponding points on the two three-phase bridges 
with the secondaries of the transformers being connected in a zig-zag 
connection to form an eighteen pulse stepped wave output voltage, i.e., an 
output voltage having eighteen pulses per 360 electrical degrees. The 
turns ratios of the transformers are selected to obtain the desired 
waveform in the line to line voltage. The detailed description of the 
inverter, the voltage output and the operation of the inverter is given in 
greater detail in the IEEE Transaction paper. 
While an inverter-driven power transformer arrangement has been initially 
implemented, it will be recognized that the transformer could be replaced 
by a filter reactor if the inverter voltage matches line or utility 
voltage. Such matching may occur if the voltage source is regulated, for 
example, by using a series chopper or other control means to regulate 
voltage. Thus, the interface between the inverter and utility or load 
system may be a coupling means other than the transformer illustrated in 
the IEEE Transaction paper. 
FIG. 4 illustrates a basic control for a battery energy storage system 
which is connected to a utility system and is always operated in a utility 
connected mode. The basic control is similar to the control illustrated in 
FIG. 3 except that synchronizing functions have been provided to 
synchronize the inverter output waveform to the power utility waveform, 
both in phase and frequency. The synchronizing function is provided by 
detecting the phase of the measured voltage output from the power inverter 
in a phase detector 56. The output signal .theta.T from phase detector 56 
represents the phase of the voltage V.sub.T which is the voltage supplied 
by the utility. The phase signal is supplied to a phase lock loop (PLL) 
regulator 58 of a type well known in the art which generates a phase lock 
loop frequency output signal .omega.PLL. The signal .omega.PLL is supplied 
to the angle ramp generator 50 in place of the previously supplied signal 
.omega..sub.SP. The ramp generated by angle ramp generator circuit 50 
becomes the phase lock loop feedback signal .theta..sub.PLL supplied to 
the PLL regulator 58. The phase lock loop circuit including the PLL 
regulator and angle ramp 50 is a conventional type of phase regulator well 
known in the art and provides the synchronizing function to control the 
phase of the inverter terminal voltage E.sub.I. Referring back to FIG. 2, 
it will be noted that the angle .theta..sub.1 is the angle between the 
arbitrary reference and the voltage E.sub.I. The signal .theta.T from the 
phase detector 56 is the angle between the reference and the terminal bus 
voltage V.sub.T. As long as the system is operating in steady state, the 
angle .theta..sub.T and the angle .theta..sub.PLL will be the same angle. 
Since the control of FIG. 4 is intended to be operated in conjunction with 
utility power, there is no separate independent frequency reference signal 
supplied to the phase lock loop. 
The phase signal provides a handle for controlling the amount of real power 
supplied by the power inverter 43. More particularly, the amount of 
reactive power versus the amount of real power coupled through the 
transformer 20 (or 54) can be adjusted by controlling the angle 
.theta..sub.I. Referring again to FIG. 2, the value .theta..sub.I 
represents the displacement of the voltage phasor E.sub.I from the 
arbitrary reference indicated by line 40. The offset between the voltage 
phasor V.sub.T and the voltage phasor E.sub.I is the transformer reactance 
voltage. This angle determines the amount of real power that is forced to 
flow through the transformer reactance whether being moved into the 
utility system or into the battery system. The phase shift signal 
.delta..sub.IT is used to adjust the value of .theta..sub.I to vary this 
angle and thereby to control the amount of real power flowing through the 
transformer. The signal .delta..sub.IT is developed by a simple power 
regulator 64 operating as an integrator on a power error signal P.sub.ER 
and is summed with .theta..sub.PLL at summer 65 to produce .theta..sub.I. 
The power error signal is generated by the difference between an actual 
measured real power component P.sub.B and a power reference P.sub.REF 
developed at summing junction 66, where the power reference signal 
P.sub.REF represents the desired power output of the inverter. A 
.delta.LIMIT signal supplied to the power regulator 64 controls the limits 
by which the value of .delta.IT can be varied to control the angle between 
V.sub.T and E.sub.I. 
Turning now to FIG. 5, there is shown a functional block diagram of a 
control system in accordance with the present invention which can achieve 
the desired transition functions between the connected and isolated modes 
of operation described with regard to FIGS. 3 and 4. It will be noted that 
among the improvements over the prior art are the use of measured terminal 
bus frequency .omega.P.sub.LL to bias the commanded power, the use of an 
inertial type of regulator to generate the angle .delta.IT and the 
configuration of the control to modulate the normal utility connected 
power. This arrangement eliminates any need for structural changes when 
switching between connected and isolated modes of operation. The other 
structural differences between the system of FIG. 4 and that of FIG. 5 is 
in the use of measured reactive power Q.sub.B as an input signal to a 
modified voltage regulator 67, the provision of a frequency reference 
signal .omega..sub.REF to an inertial power regulator 68 (replacing power 
regulator 64) along with the coupling of the signal .omega..sub.PLL from 
the phase lock loop to the power regulator 68. The signal .omega..sub.REF 
represents a desired frequency of the output voltage generated by the 
inverter and would typically be representative of a frequency of 60 Hz for 
U.S. use. The signal .omega..sub.PLL during stable operation represents 
the actual output voltage frequency. The particular function implemented 
by the addition of the signals .omega..sub.REF and Q.sub.B and the 
changing of the power regulator to an inertial type of power regulator is 
described in more detail with respect to FIGS. 6 and 7. 
Turning now to FIG. 6, there is shown an expanded block diagram of the 
power regulator with the frequency reference signal .omega..sub.REF and 
the phase lock loop frequency signal .omega..sub.PLL being combined to 
generate the frequency error signal E.sub..omega.. As can be seen, the 
.omega..sub.PLL signal which represents the actual frequency of the 
inverter output is subtracted from the .omega..sub.REF signal in a summing 
junction 70 to generate the E.omega. error signal. The E.omega. error 
signal is provided to a frequency bias circuit comprising a first control 
loop including a conventional proportional plus integral regulator 72 and 
a deadband circuit 74. The deadband circuit provides some range of 
variation of the frequency error signal, for example, approximately 1/2 Hz 
without any change of output signal. This limits response due to natural 
fluctuations of the power system frequency. The proportional plus integral 
regulator 72 converts the error signal to a conventional bias signal which 
is applied to a summing junction 76. A second loop includes a proportional 
droop circuit 78 which may be an amplifier with a fixed gain that receives 
the E.sub..omega. error signal and provides an immediate compensation 
signal to the summing junction 76, the compensation signal being added to 
the output signal from the proportional plus integral regulator 72. The 
output of the summing junction 76 is a power offset signal which is 
coupled to a summing junction 80 whose other input is the power reference 
signal P.sub.REF. Accordingly, the frequency offset signal from summing 
junction 76 serves to modify the power reference signal. The purpose of 
such modification is to adjust the power reference signal as a function of 
frequency shifts. More particularly, the intent of the system is to 
attempt to hold the system output frequency constant so that if there is 
an error between the output frequency and the reference frequency, the 
power reference signal is adjusted to compensate for the frequency error. 
Still further, the power system to which the inverter is coupled may 
include reactive loads such as alternating current induction and 
synchronous motors whose speed is directly related to the frequency of the 
inverter output signal. If additional power is supplied from the inverter, 
the machines will tend to accelerate while a reduction in power will cause 
the frequency to drop due to the inductive reaction of the machines as 
they begin to slow down. Accordingly, the frequency bias circuit provides 
an important function in enabling control of the torque output of the 
machines coupled to the inverter output. 
The improved power regulator 68 also introduces an inertial regulator 84 
which modifies the power error signal to simulate the inertia of 
synchronous machines. More particularly, the inertial regulator 84 
prevents sudden frequency changes or power changes which can cause 
transient torques to be generated by the motors coupled to the inverter 
output if sudden changes in the inverter output are experienced. The 
inertial regulator comprises a conventional electronic circuit having the 
characteristics of an integrator in that its output signal gradually 
increases in response to an increase in the input signal. 
If the power reference signal is modified by the frequency bias circuit, 
the resultant signal identified as P.sub.ORD is developed at an output 
terminal of the summation circuit 80 and applied to a summation circuit 82 
where the commanded power or ordered power is compared to the measured 
output power P.sub.B of the system. Note here that the signal P.sub.B 
represents the real power developed at the output of the inverter. The 
output signal from the summation circuit 82 represents the power error 
signal which is applied to the inertial regulator 84. The signal developed 
by the inertial regulator as described above represents the desired 
frequency .omega..sub.1 of the internal voltage E.sub.1 and, if the 
frequency is properly tracking, will be the same as the frequency 
.omega.P.sub.LL. In this regard, the signal .omega..sub.I developed at the 
output of the inertial regulator 84 is summed in a summing junction 86 
with the .omega..sub.PLL signal. Any difference between the phase lock 
loop frequency and the signal .omega..sub.1 results in an error signal 
which is applied to an integrator 88 to develop the .delta..sub.IT signal 
described with regard to FIG. 5. The integrator 88 is a conventional type 
of integrator whose output signal .delta..sub.IT is an angle offset which 
can be summed with the output signal from the phase lock loop described in 
FIG. 5 to generate the output signal .theta..sub.1. It will be recognized 
that the .omega..sub.PLL signal is taken from the phase lock loop as shown 
in FIG. 5 and therefore represents the actual frequency of the inverter 
output signal. In the event that the utility breaker opens suddenly, the 
.omega..sub.PLL signal will represent the actual frequency of the voltage 
being generated by the inverter 43 and the power regulator will cause the 
power output of the inverter to be adjusted as a function of the variation 
in output frequency. The integrator 85 in the inertial regulator becomes 
important to limit any attempted frequency change in the control system. 
It will be recognized that the settings of the deadband 74, the gain at 
the proportional droop block 78 are selected to coordinate with the 
variations of the power system to which the inverter is connected and also 
with the loads to which the inverter is to supply power. Furthermore, the 
system can be adapted to modify the settings of the deadband, proportional 
droop and the inertial regulator in an adaptive manner such as when the 
status of the utility breaker is changed, either to connect the utility to 
the system or to disconnect the utility from the load system. 
When the battery energy storage system is connected to the utility, the 
frequency bias circuit and the inertial power regulator force the system 
to operate as though the battery load were another synchronous machine 
connected to the utility power. This permits protection circuit and system 
operation procedures to be virtually the same as standard utility 
practice. The ability to dynamically adjust parameters for inertia, 
damping and frequency bias permits the battery energy storage system to 
provide a beneficial impact on the overall power system when connected. 
Turning now to FIG. 7, there is shown an expanded block diagram of the 
voltage regulator 48 of FIG. 5. In this voltage regulator, there is 
provided a new gain control block 90 identified as a regulator droop 
correction. The block 90 is connected to receive a feedback signal Q.sub.B 
representing the measured reactive power produced by the inverter 43. The 
output of block 90 is applied to a summing junction 92 where it is 
combined with the measured voltage magnitude signal V.sub.T and a voltage 
reference signal V.sub.REF. The voltage error signal from the block 92 now 
represents not only the difference between the voltage reference signal 
and the voltage magnitude signal but also includes a factor relating to 
the reactive power produced by the system. The voltage error signal 
V.sub.ER is applied to integral regulator 94 to produce the voltage 
command signal E.sub.I. The magnitude of the output signal from the 
regulator 94 is limited by a signal E.sub.LIMITs to preselected minimum 
and maximum voltage values E.sub.Imin and E.sub.Imax. It will be noted 
that the signal from the regulator droop block 90 is subtracted from the 
effective voltage error signal in block 92 so that if the system connected 
to the inverter 43 begins to draw too much reactive power, a lower voltage 
reference is supplied to force the value of E.sub.I to decrease. The 
purpose of this function is to promote load sharing of reactive power with 
other power conditioning systems connected in parallel to the common DC 
source 10. If the value of E.sub.I attempts to increase above the 
reference value of E.sub.T, this will force the reactive component Q.sub.B 
to increase since the coupling transformer 20 is reactive and any voltage 
magnitude change will cause more effect on the reactive component of 
output power than the real component. Note that the angle control as 
illustrated in FIG. 2 is more effective in controlling real power while 
voltage magnitude control is more effective in controlling the magnitude 
of reactive power. 
More particularly, if one were to assume an increase in the reference 
voltage signal V.sub.REF, the result would be to force the internal 
voltage E.sub.I to increase and cause generation of more reactive power 
which will cause the value of Q.sub.B to increase. The feedback of this 
signal through the regulator droop block 90 will result in a decrease in 
the voltage error signal by subtraction in summing block 92 to thereby 
limit the amount of reactive power flowing. If the system is connected to 
a stiff power system, the magnitude signal V.sub.T would not change 
rapidly and would cause E.sub.I to continue to rise to force V.sub.T to 
increase thereby causing E.sub.I to ramp up rapidly. By introducing the 
Q.sub.B droop, the ramp up of the internal signal E.sub.I is restricted. 
This function promotes sharing of power between multiple power 
conditioning systems if the transformers 20 in each of the systems have 
slightly different characteristics. The advantage is the ability to use 
less expensive unmatched transformers while still promoting load sharing. 
Without this function, a battery energy storage system with a smaller 
transformer impedance will try to carry more reactive load causing more 
current to flow which might lead to failure of the switching electronics. 
By introducing the reactive power feedback signal Q.sub.B, the E.sub.I 
value for that particular unit can be caused to be better balanced with 
the reactive power supplied by other units. While the derivation of the 
real and reactive parts of the output power is not specifically shown in 
FIG. 7, it will be appreciated that such function is well known in the 
art. In particular, it is common to measure terminal volts and phase angle 
at the inverter output, measure the output current and its phase angle, 
convert these measured values to power and then compute real and reactive 
parts by simply computing the sine of the power function to generate a 
signal representative of the reactive power and to obtain the cosine of 
the power function to generate a signal representative of the real 
component power. 
Referring now to FIG. 8, there is shown a simplified block diagram of one 
method for obtaining the voltage and angle limits used in the block 
diagrams of FIGS. 6 and 7. Before proceeding with a description of the 
operation of the functional block diagram of FIG. 8, it is necessary to 
first understand some basic concepts upon which the limits set in FIG. 8 
are determined. As will be appreciated from the above description of the 
operation of the inventive system, a fast power control may be detrimental 
to overall power system's stability, particularly when the battery energy 
storage system is the primary source of power. Accordingly, some of the 
functions implemented in the present invention are designed to restrict 
the speed at which the control system responds. The overall concept is 
based upon maintaining an internal voltage phasor E.sub.I within the 
current capability of the inverters connected to the battery energy 
storage system. Referring again to FIG. 2, the boundary 36 represents the 
maximum allowable current from the battery energy storage system in terms 
of the voltage drop across the impedance X.sub.T of the transformer 20. 
Limits are imposed on the voltage magnitude V.sub.T to force E.sub.I to 
operate within this boundary 36. The current I.sub.T is defined with 
respect to the terminal voltage V.sub.T and is equal to the values I.sub.W 
+JI.sub.V, where I.sub.W is the real component of current and JI.sub.V is 
the imaginary component of current through the transformer impedance. As 
discussed with regard to FIG. 7, one of the functions of the power 
regulator 68 is to force the real component of current I.sub.W to be the 
same for each battery energy storage system connected to the load since 
each system will receive a common frequency and power reference signal. 
The reactive component of current I.sub.V is controlled by the voltage 
regulator and balancing of the reactive current requires some droop 
introduced by the regulator droop 90 in the voltage command signal. 
Before turning to the details of the computation of each of the particular 
values used in controlling the battery energy storage system, reference is 
first made to the simplified block diagram of FIG. 9 which illustrates an 
overview of a conventional battery energy storage system. The system 
includes a DC bus which provides a voltage E.sub.DC from a plurality of 
batteries and connected to the bus. Typically, the system includes a 
plurality of batteries arranged in individual strings with each string 
comprising in excess of a thousand cells in series and each string being 
connected in parallel to the battery bus. A typical battery bus voltage 
may run in the range of 1750 to 2860 volts DC. Connected to the battery 
bus are a plurality of paralleled power conditioning systems (PCS) such as 
the systems 12 of FIG. 1 with each of the PCS 12s feeding a corresponding 
one of the transformers 20 connected to the terminal bus V.sub.T, labeled 
as the AC bus in FIG. 9. A station control 96 provides for operator input 
to set the voltage and power outputs of each of the PCS's 12. The nominal 
steady-state current for each PCS 12 in per unit values is given by the 
relationship 
##EQU1## 
where P.sub.ord is the ordered power as set forth in FIG. 6 and V.sub.ord 
is the voltage reference indicated at V.sub.REF in FIG. 7. The value of 
X.sub.droop is the amount of droop created by the functional regulator 
droop block 90 in FIG. 7. Note that the station 96 may provide control 
signals to gradually adjust the values of P.sub.ord and V.sub.ord to 
maintain system level requirements for the AC bus. As previously 
mentioned, power control and internal angle of the PCS 12 are based on 
making the PCS operate with a characteristic similar to a synchronous 
machine. For example, when connected to a power grid with the utility 
fully operational, voltage is maintained according to the local area needs 
established by the station level control and frequency and phase are 
adjusted to maintain a scheduled power flow to the AC bus. When separated 
from the grid or during a system startup isolated from the grid, the PCS 
12 will establish the frequency and phase of the voltage. In either event, 
the behavior of the PCS 12 can be made superior to a rotating synchronous 
machine since the inertia, droop and damping can be set and dynamically 
adjusted to suit the needs of the overall power system or load. 
The PCS control is based on measuring the real power flow into the external 
AC system at the transformer high side, i.e., on the AC bus side of the 
transformer 20. The measured power identified as P.sub.meas is then 
compared with an ordered value P.sub.ord and processed through the 
regulator of FIG. 6. The regulator of FIG. 6 is structured as a 
representation of the inertia (2h) and damping (d) effects of a 
synchronous machine as indicated by the equations in the inertial 
regulator 84. Non-windup limits (F.sub.min, F.sub.max) are used on 
frequency to prevent too great an excursion during a transient. 
The output of the power regulator .omega..sub.I is the internal frequency 
of the PCS 12. This frequency signal .omega..sub.I is compared against the 
measured system frequency .omega..sub.PLL calculated by the phase lock 
loop. The difference frequency is integrated to calculate the angle of the 
internal voltage relative to the system, the angle being indicated as 
.delta..sub.IT. The non-windup limits+.delta.ITmax and -.delta.ITmax are 
applied to the integrator 88 to limit the magnitude of the .delta.IT 
function. 
Referring again to the voltage regulator, the magnitude of the internal 
voltage phasor E.sub.I is determined by the voltage regulator of FIG. 7. 
The control loop uses the integrating block 94 with non-windup limits 
E.sub.Imax and E.sub.Imin which are dynamically adjusted to maintain 
inverter reactive current within the capability of the inverter bridge. 
Voltage error is calculated as a difference between the voltage command 
V.sub.REF and measured terminal voltage V.sub.T. Terminal voltage V.sub.T 
is measured on the high side of the transformer 20. The droop component 
from block 90 is calculated as the product of the current I.sub.T in 
quadrature with the terminal voltage and a droop reactance X.sub.droop. 
The internal voltage phasor E.sub.I is scaled by the measured battery 
voltage (E.sub.DCF) to generate the proper voltage ratio for the firing 
pattern generator block 42. 
The dynamic voltage limits with respect to the current capability of the 
PCS 12 are calculated from the magnitude of a voltage phasor which must be 
added or subtracted from the internal voltage E.sub.I to reach the 
circular boundary defined at 36 in FIG. 2. One method of performing this 
calculation is to define a phasor which is parallel with the terminal 
voltage phasor V.sub.T. One can then calculate the intersection of the 
corresponding parallel line with the voltage circle 36 defined by the 
maximum value of current. This process requires calculation of an 
intermediate variable I.sub.Vmax from the equation (where I.sub.Vmax is 
limited to positive values): 
##EQU2## 
where I.sub.max is the maximum current capability of the PCS 12 as defined 
by the circle 36, I.sub.W is the measured real component of current 
I.sub.T and I.sub.Vmax represents the maximum available reactive current 
capability of the PCS. Voltage limits can then be calculated from this 
information and the measured terminal voltage to determine the minimum and 
maximum internal voltage magnitude at the boundary with the line 36. The 
voltage limits are also compared against the factor K.sub.AC E.sub.DCf 
which defines the maximum AC voltage capability of the inverter based on 
the measured DC voltage. The voltage limits are adjusted to be less than 
or equal to the factor K.sub.AC E.sub.DCf. This is necessary to prevent 
wind-up in regulator 88 if the battery voltage is low during a discharge. 
The equations for minimum and maximum voltage limits are, respectively: 
##EQU3## 
FIG. 9 illustrates real power limiting achieved by clamping the voltage 
angle phase shift signal .delta..sub.IT as a function of the real part of 
measured current I.sub.W. This clamping function is coordinated with the 
voltage limit calculation to limit the real power component of the current 
while maintaining the reactive component to within the capability defined 
by the maximum current I.sub.max. The values of .delta..sub.ITmax and 
.delta..sub.ITmin are determined by the angle limit regulator of FIG. 9. 
The response of the limit regulator 108 is set by adjusting its integral 
gain K.sub.W. The real component of current limit I.sub.Wmax is made equal 
to a fixed percentage value of I.sub.max. A value less than 100% (but 
typically greater than about 80%) is used to limit the real component of 
current I.sub.W to less than I.sub.max. During normal operation the 
regulator is clamped at .delta..sub.max. The fixed value used for 
.delta..sub.max is based on the maximum angle at nominal per unit voltage 
(VT=1) in which: .delta..sub.max =sin.sup.-1 (X.sub.T .multidot.I.sub.max) 
Returning now to FIG. 8, the measured phase currents are applied to an 
overload limit algorithm block 98 which performs the calculations 
described above to derive the values I.sub.Vmax and I.sub.Wmax. I.sub.Vmax 
is applied to a voltage limit regulator 100 which also receives the 
measured terminal voltage V.sub.T, measured battery voltage E.sub.DCf, 
real component of current I.sub.W and reactive component of current 
I.sub.V. The voltage limit regulator uses these values as described above 
to calculate the values E.sub.Imax and E.sub.Imin. Similarly, the angle 
limit regulator block 102 receives the values of I.sub.Wmax and I.sub.W 
and uses those values to calculate the angle limits .delta.I.sub.max and 
.delta.I.sub.min. FIG. 10 is an expanded illustration of block 102 showing 
use of an integrator 108 with limits .smallcircle. and .delta..sub.max for 
calculating .delta.I.sub.max from the difference between I.sub.Wmax and 
I.sub.W. The value of .delta.I.sub.max is inverted at block 110 to produce 
.delta.I.sub.min. The system also includes a battery voltage limit 
regulator 104 which is responsive to signals representing the difference 
between the maximum allowable battery voltage V.sub.DCmax and measured DC 
voltage V.sub.DCmeas and a signal representative of the difference between 
the minimum battery voltage V.sub.DCmin and V.sub.DCmeas to provide the 
signals .delta..sub.Vmax and .delta..sub.Vmin. The angle signals are 
applied to the limit selector block 106 which also receives the 
.delta.I.sub.max and .delta.I.sub.min signals. The output of the limit 
selector block 106 are the .delta.ITmax and .delta.ITmin signals. 
What has been described is an improved power control system for use in 
connecting a battery energy storage system to an AC power bus for 
supplementing power from a utility bus or replacing such power in the 
event of utility failure. The invention provides a method for holding 
system frequency constant by adjusting the power out of the system so as 
to control loads such as AC motors. Furthermore, the system promotes load 
sharing between parallel connected power conditioning systems by limiting 
reactive power developed by each of the power conditioning systems. 
While the invention has been described in what is presently considered to 
be a preferred embodiment, many variations and modifications will become 
apparent to those skilled in the art. Accordingly, it is intended that the 
invention not be limited to the specific illustrative embodiment but be 
interpreted within the full spirit and scope of the appended claims.