Current Transducer with true RMS voltage output
Any RMS converter that meets your accuracy spec and that has an averaging period of 200 mS will meet your requirement. See below. 

In the datasheet here note that it says "averaging time constant: 200 milliseconds.
 This means that 


it calculates the RMS value of the AC signal for 200 milliseconds and outputs it as a DC RMS value. 
it then calculates the RMS value for another 200 mS  period and outputs it. 


IF the periods are sequential and non overlapping the output changes every 200 MS or less.

BUT if this was a digital system they may sample every mS, take 200 samples at t= 1mS to t = 200 mS, then calculate the RMS value for the 200 samples and output it.
 One mS later they may take a sample for t=201 mS and throw away sample for t=1 MS and calculate the RMS value for periot 1 - 201 and output it.

Note that If a repeating (periodic) waveform signal with period of 200 mS or a factor of 200 mS (100, 50, 40, 25, 20, 10, 5, 4, 2, 1, ... mS) THEN the output value would be steady DC.

Note that if the input signal was a repeating waveform with period NOT equal to 200 mS or a factor thereof then the output will fluctuate and the variatiins will be meaningless for practical purposes.