Hide shipping rates when free shipping is available

/**
 * Hide shipping rates when free shipping is available.
 * Updated to support WooCommerce 2.6 Shipping Zones.
 *
 * @param array $rates Array of rates found for the package.
 * @return array
 */
function my_hide_shipping_when_free_is_available( $rates ) {
    $free = array();
    $free_available = false;
    foreach ( $rates as $rate_id => $rate ) {
        if ( 'free_shipping' === $rate->method_id ) {
            $free_available = true;
//            $free[ $rate_id ] = $rate;
            break;
        }
    }
    foreach ( $rates as $rate_key => $rate ){
        if ( $free_available && 6 === $rate->instance_id ){
            unset( $rates[$rate_key] );
        }
    }
    // this will return only free shipping method
    return $rates;
}
add_filter( 'woocommerce_package_rates', 'my_hide_shipping_when_free_is_available', 100 );