A Novel Droop Method for Converter

...

of module #[pic 115] and [pic 116] is the droop gain. Because the outputs of all modules are tied together, output voltages of all modules are equal. As can be seen in Fig. 1 the output current of each module is determined by the difference between the output voltage set-point of individual modules and by the droop gain [pic 117] [4].

As can be seen in Fig. 1, if the gain [pic 118] is high, the current sharing accuracy improves but the output voltage variation range is increased. Therefore a trade-off must be made between the load sharing and the output voltage regulation performance when designing the control circuit.

0885–8993/02$17.00 © 2002 IEEE

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26 IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 17, NO. 1, JANUARY 2002

[pic 119]

Fig. 1. Load regulation characteristic of the droop method.

Fig. 3. Output voltage variation according to the current sharing error.

[pic 120]

Fig. 2. Maximum output voltage variation of the droop method.

As can be seen in Fig. 2, if the output voltages of all modules

are

at no load, the output voltage vari-

ation is maximum.

is the nominal output voltage and

[pic 121][pic 122][pic 123][pic 124][pic 125][pic 126][pic 127][pic 128][pic 129][pic 130][pic 131][pic 132][pic 133][pic 134][pic 135][pic 136][pic 137][pic 138][pic 139][pic 140][pic 141][pic 142][pic 143][pic 144][pic 145][pic 146][pic 147]

[pic 148][pic 149][pic 150][pic 151][pic 152][pic 153] is the output voltage set-point accuracy. The max-imum output voltage variation is determined by

(2)

[pic 154][pic 155][pic 156][pic 157][pic 158][pic 159][pic 160][pic 161][pic 162][pic 163][pic 164][pic 165][pic 166][pic 167][pic 168][pic 169][pic 170][pic 171][pic 172][pic 173][pic 174][pic 175][pic 176][pic 177][pic 178][pic 179][pic 180]

The droop gain, [pic 181] is determined using (3) by the maximum output voltage variation specification

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Fig. 3 shows the output voltage variation curve according to the current sharing error. To limit the output current sharing error between modules to 5% of the rated current of each

module,

must

be twenty one times

.

If

is 1% of

, then

is 21% of

to limit the current sharing error to 5% of the rated

[pic 182][pic 183][pic 184][pic 185][pic 186][pic 187][pic 188][pic 189][pic 190][pic 191][pic 192][pic 193][pic 194][pic 195][pic 196][pic 197][pic 198][pic 199][pic 200][pic 201][pic 202][pic 203][pic 204][pic 205][pic 206][pic 207][pic 208][pic 209][pic 210][pic 211][pic 212][pic 213][pic 214][pic 215][pic 216][pic 217][pic 218][pic 219][pic 220][pic 221][pic 222][pic 223][pic 224][pic 225]

current of each module. This value cannot be accepted by a circuit designer, so the current sharing error cannot be improved by the [pic 226][pic 227][pic 228][pic 229] specification.

III. PROPOSED DROOP METHOD

To improve the current sharing accuracy and the voltage reg-ulation of the conventional droop method, a novel droop method is proposed. In this scheme, the peak output current of each

module,

is stored and compared with the output cur-

rent set value,

. If

is less than

, then the

[pic 230][pic 231][pic 232][pic 233][pic 234][pic 235][pic 236][pic 237][pic 238][pic 239][pic 240][pic 241][pic 242][pic 243][pic 244][pic 245][pic 246][pic 247][pic 248][pic 249][pic 250][pic 251][pic 252][pic 253][pic 254][pic 255][pic 256][pic 257][pic 258][pic 259][pic 260][pic 261]

output voltage of each module follows (6) like the conventional droop method. However, when [pic 262][pic 263][pic 264][pic 265] exceeds [pic 266][pic 267][pic 268][pic 269], the output voltage follows (7). As can be seen in (7), the difference

between[pic 270][pic 271][pic 272][pic 273][pic 274][pic 275][pic 276]

and

is multiplied by a gain,and

lowers the output voltage.

is designed to be greater than

[pic 277][pic 278][pic 279][pic 280][pic 281][pic 282][pic 283][pic 284][pic 285][pic 286][pic 287][pic 288][pic 289][pic 290][pic 291][pic 292][pic 293][pic 294][pic 295][pic 296][pic 297][pic 298]

so that the current sharing error is reduced

when[pic 299]

[pic 300][pic 301][pic 302][pic 303][pic 304][pic 305][pic 306][pic 307][pic 308][pic 309][pic 310][pic 311][pic 312][pic 313][pic 314][pic 315][pic 316][pic 317][pic 318][pic 319][pic 320][pic 321][pic 322][pic 323][pic 324][pic 325][pic 326][pic 327][pic 328][pic 329][pic 330][pic 331][pic 332][pic 333][pic 334][pic 335][pic 336]

(6)

[pic 337][pic 338][pic 339][pic 340][pic 341][pic 342][pic 343][pic 344][pic 345][pic 346][pic 347][pic 348][pic 349][pic 350][pic 351][pic 352][pic 353][pic 354][pic 355][pic 356][pic 357][pic 358][pic 359][pic 360][pic 361][pic 362][pic 363][pic 364][pic 365][pic 366][pic 367][pic 368][pic 369][pic 370][pic 371][pic 372][pic 373][pic 374][pic 375][pic 376][pic 377][pic 378][pic 379][pic 380][pic 381]

Then the maximum output current sharing error [pic 382][pic 383][pic 384][pic 385] determined by

[pic

...