Question

A 6-pulse HVDC rectifier produces which harmonics in AC line current?

• A. 5th, 7th, 11th, 13th ...
• B. 6th, 12th, 18th ...
• C. 3rd, 9th, 15th ...
• D. Only 1st

Hint:

To quickly find AC-side harmonics for any converter, use the formula \(h = kp \pm 1\).
To find DC-side voltage ripple harmonics, use the formula \(h_{DC} = kp\).
For a 12-pulse converter, the AC side harmonics are \(11\text{th}, 13\text{th}, 23\text{rd}, 25\text{th}, \dots\) which significantly reduces filtering requirements.

Answer 1

The Correct Option is A

Step 1: Understanding the Question:
The question asks about the harmonic components present in the AC line current of a 6-pulse High Voltage Direct Current (HVDC) rectifier converter.
Rectifiers behave as non-linear loads, introducing harmonic currents into the AC supply system.

Step 2: Key Formula or Approach:
For a \(p\)-pulse converter, the harmonic orders (\(h\)) generated in the AC side line current are given by the relation:
\[ h = k \cdot p \pm 1 \] where:
\(p\) is the pulse number of the converter, and
\(k\) is any positive integer (\(k = 1, 2, 3, \dots\)).

Step 3: Detailed Explanation:

• For a 6-pulse rectifier, the pulse number \(p = 6\).

• Substituting different values of \(k\) into the formula \(h = 6k \pm 1\):

• For \(k = 1\):
\[ h = 6(1) \pm 1 \implies h = 5, 7 \]
• For \(k = 2\):
\[ h = 6(2) \pm 1 \implies h = 11, 13 \]
• For \(k = 3\):
\[ h = 6(3) \pm 1 \implies h = 17, 19 \]
• Thus, the AC line current contains harmonics of the order of \(5\), \(7\), \(11\), \(13\), \(17\), \(19\), etc.

• Note that triplen harmonics (multiples of 3) are absent in balanced three-phase systems with star-delta or ungrounded star configurations, and even harmonics are absent due to half-wave symmetry.

Step 4: Final Answer:
The harmonics produced in the AC line current of a 6-pulse converter are the 5th, 7th, 11th, 13th, etc.