user4562: The TimeControl formats I've found on the web split the TimeControl in time periods separated by a special character, for instance:
G/90 (one period with 90min to end the game)
G/90+30 (one period with 90min to end the game and 30sec of increment)
40/90, SD/30+30 (two periods, 90min to play 40 moves followed by 30 min to end the game with 30sec of increment)
Now, in the last example the increment is related to the second period (the sudden death).
According to http://www.fide.com/fide/handbook.html?id=39&view=category for all major FIDE events the TimeControl is 90 minutes for the first 40 moves followed by 30 minutes for the rest of the game with an addition of 30 seconds per move starting from move one. Here the increment starts from move 1. 
How can I describe that in a formal notation?
Is 40/90+30, SD/30+30 the correct way?
PGN standard has a different approach (the separator character is ":" and the increment is allowed only in the sudden death period, which is simply expressed in seconds):
40/5400:1800+30
but it seems the increment is not allowed for the first period (in case of a multi-period time conrol). 
How can I express the FIDE TimeControl in the PGN tag as well?
I tryed to download PGN games of FIDE events but I couldn't find any of them with TimeControl tag.

user3005: From what I have found out from studying the PGN Specification, I think that the PGN spec allows to have the following:

5400/40+30:1800+30

Start with 90 minutes
for the first 40 moves,
add then another 30 minutes for the rest of the game.
In both periods, use 30 seconds increment.



So it would be possible to have different increments (for whatever reason) in your tournament.
I have downloaded the DGT clock manuals (2010 and 3000), here are some quotes from it:

2010: Bonus. In Bonus settings players receive an additional amount of time for each move (typically 2-3
seconds bonus time per move in blitz, 10 seconds bonus time in rapid, and 30 seconds in classical
chess). The bonus time is added before each move from the start of the game. Playing with bonus
time per move is also called playing with increments or playing with Fischer Bonus after the 11 th World Chess Champion Bobby Fischer who championed the system. Playing with increments has become the standard in chess. Options 10-14 have a Bonus setting in the last period only (and Time settings in preceding periods). Options 15-21 have Bonus settings in all periods. Note that according to FIDE rules the clock can no longer be operated in Bonus settings when one of the players runs out of time in the last period. The clock then freezes and a blinking flag will be shown.


3000: Fischer Bonus
This method was named after chess world champion Bobby Fischer: “Fischer Bonus” called FISCH in this manual. In this method a certain amount of time is added after each move. If players use less time per move than the set bonus time, then the time on the clock will be higher after the player has
finished the move than at the start of the move. ...


Period transitions with different bonus or delay times 
The DGT 3000 makes it possible to program subsequent periods with different bonus or delay times.

My conclusion from that is, that it should be possible to have different (or no) increment in different periods, and the only way to specify that is to have the increment in all periods repeated.

user4418: 
but it seems the increment is not allowed for the first period

As far as I can see from the standard it is allowed in the first period only if there is only one period. Specifically it is allowed only in the last period.

•The fifth TimeControl field kind is used for an "incremental" control
  period. It should only be used for the last descriptor in a
  TimeControl tag value and is usually the only descriptor in the value.
  The format consists of two positive integers separated by a plus sign
  ("+") character. The first integer gives the minimum number of seconds
  allocated for the period and the second integer gives the number of
  extra seconds added after each move is made. So, an incremental time
  control of 90 minutes plus one extra minute per move would be given by
  "4500+60" in the TimeControl tag value

The only suggestion is the standard one regarding these questions about PGN which is to put the information in a comment field. The standard is old and no longer maintained as far as I know.

4562: <p>The TimeControl formats I've found on the web split the TimeControl in time periods separated by a special character, for instance:</p>

<p>G/90 (one period with 90min to end the game)</p>

<p>G/90+30 (one period with 90min to end the game and 30sec of increment)</p>

<p>40/90, SD/30+30 (two periods, 90min to play 40 moves followed by 30 min to end the game with 30sec of increment)</p>

<p>Now, in the last example the increment is related to the second period (the sudden death).</p>

<p>According to <a href="http://www.fide.com/fide/handbook.html?id=39&amp;view=category" rel="nofollow">http://www.fide.com/fide/handbook.html?id=39&amp;view=category</a> for all major FIDE events the TimeControl is 90 minutes for the first 40 moves followed by 30 minutes for the rest of the game with an addition of 30 seconds per move starting from move one. Here the increment starts from move 1. </p>

<p>How can I describe that in a formal notation?
Is 40/90+30, SD/30+30 the correct way?</p>

<p>PGN standard has a different approach (the separator character is ":" and the increment is allowed only in the sudden death period, which is simply expressed in seconds):</p>

<p>40/5400:1800+30</p>

<p>but it seems the increment is not allowed for the first period (in case of a multi-period time conrol). </p>

<p>How can I express the FIDE TimeControl in the PGN tag as well?
I tryed to download PGN games of FIDE events but I couldn't find any of them with TimeControl tag.</p>
 3005: <p>From what I have found out from studying the <a href="https://github.com/mliebelt/pgn-spec-commented/blob/main/pgn-specification.md#961-tag-timecontrol" rel="nofollow noreferrer">PGN Specification</a>, I think that the PGN spec allows to have the following:</p>
<ul>
<li><code>5400/40+30:1800+30</code>
<ul>
<li>Start with 90 minutes</li>
<li>for the first 40 moves,</li>
<li>add then another 30 minutes for the rest of the game.</li>
<li>In both periods, use 30 seconds increment.</li>
</ul>
</li>
</ul>
<p>So it would be possible to have different increments (for whatever reason) in your tournament.</p>
<p>I have downloaded the DGT clock manuals (2010 and 3000), here are some quotes from it:</p>
<blockquote>
<p>2010: Bonus. In Bonus settings players receive an additional amount of time for each move (typically 2-3
seconds bonus time per move in blitz, 10 seconds bonus time in rapid, and 30 seconds in classical
chess). The bonus time is added before each move from the start of the game. Playing with bonus
time per move is also called playing with increments or playing with Fischer Bonus after the 11 th World Chess Champion Bobby Fischer who championed the system. Playing with increments has become the standard in chess. Options 10-14 have a Bonus setting in the last period only (and Time settings in preceding periods). Options 15-21 have Bonus settings in all periods. Note that according to FIDE rules the clock can no longer be operated in Bonus settings when one of the players runs out of time in the last period. The clock then freezes and a blinking flag will be shown.</p>
</blockquote>
<blockquote>
<p>3000: Fischer Bonus
This method was named after chess world champion Bobby Fischer: “Fischer Bonus” called FISCH in this manual. In this method a certain amount of time is added after each move. If players use less time per move than the set bonus time, then the time on the clock will be higher after the player has
finished the move than at the start of the move. ...</p>
</blockquote>
<blockquote>
<p>Period transitions with different bonus or delay times <br/>
The DGT 3000 makes it possible to program subsequent periods with different bonus or delay times.</p>
</blockquote>
<p>My conclusion from that is, that it should be possible to have different (or no) increment in different periods, and the only way to specify that is to have the increment in all periods repeated.</p>
 4418: <blockquote>
  <p>but it seems the increment is not allowed for the first period</p>
</blockquote>

<p>As far as I can see from the <a href="http://www.saremba.de/chessgml/standards/pgn/pgn-complete.htm#c9.6.1" rel="nofollow">standard</a> it is allowed in the first period only if there is only one period. Specifically it is allowed only in the last period.</p>

<blockquote>
  <p>•The fifth TimeControl field kind is used for an "incremental" control
  period. It should only be used for the last descriptor in a
  TimeControl tag value and is usually the only descriptor in the value.
  The format consists of two positive integers separated by a plus sign
  ("+") character. The first integer gives the minimum number of seconds
  allocated for the period and the second integer gives the number of
  extra seconds added after each move is made. So, an incremental time
  control of 90 minutes plus one extra minute per move would be given by
  "4500+60" in the TimeControl tag value</p>
</blockquote>

<p>The only suggestion is the standard one regarding these questions about PGN which is to put the information in a comment field. The standard is old and no longer maintained as far as I know.</p>
