The best time to invest in high-quality dividend stocks is when market fear and uncertainty bludgeon share prices to irrationally low levels.

Right now, the combination of a market correction and the oil crash has left many great midstream MLPs such as Holly Energy Partners (NYSE:HEP) in such a position. Let me show you three reasons this little known MLP is such a great long-term buy at today's price and deserves a spot in your diversified income portfolio. 

Historically high-yield

HEP Chart
HEP data by YCharts

Holly Energy Partners has had a rough year, though not nearly as bad as most MLPs. That said, its current yield of 7.5% is not only over three times that of the S&P 500's 2.2%, but it's also the highest it's been in five years. 

HEP Dividend Yield (TTM) Chart
HEP Dividend Yield (TTM) data by YCharts

Then again, yield is just one of three factors dividend investors need to pay attention to, with dividend security and future growth potential being as important, if not more so. After all, an overly generous yield could be a market indicator that an MLP is in trouble financially and about to cut its payout, which could send unit prices crashing and result in a massive loss of investor capital. Luckily for Holly Energy Partners' investors, that is not the case here. 

Strong business model means a highly secure distribution and ...
Holly Energy Partners' business model is an income investor's best friend, with 100% of revenue coming from long-term fixed-fee contracts and over 80% of revenue secured by minimum volume commitments, or MVCs, among the highest in the midstream industry. 

Source: Holly Energy Partners investor presentation.

This tollbooth business creates stable, recurring cash flow that is nearly immune to volatile energy prices and has allowed Holly Energy Partners to rack up an enviable track record of 43 consecutive quarters of distribution growth, no matter what oil prices are doing at the time. 

Source: Holly Energy Partners investor presentation.

The key to Holly Energy Partners' super-reliable payout growth is management's conservative, long-term focus on distribution security. 

For example, Holly Energy Partners protects its yield with a three- and six-month distribution coverage ratio, or DCR, of 1.48 and 1.47, respectively. 

The rule of thumb for MLPs is that a DCR of between 1 and 1.1 is sustainable, while anything above 1.1 allows for secure long-term payout growth. Holly Energy Partners' DCR of nearly 1.5 means that five-year analyst distribution growth projections of 6.9% CAGR are both reasonable and supported by historical evidence.

Better yet, over the past year Holly Energy Partners' DCR has only improved because in the past six months the DCF per unit grew a solid 9.3% year over year, while the quarterly payout was increased by a smaller, though still generous 6%.  

 ... continued good growth prospects ahead

HEP Dividend Chart
HEP Dividend data by YCharts

To continue its impressive track record of secure and generous payout growth -- which has resulted in long-term market-crushing total returns -- Holly Energy Partners is pursuing a three-pronged strategy of organic growth projects, third-party asset acquisition, and asset dropdowns from its sponsor and general partner, HollyFrontier Corporation (NYSE:HFC)

HollyFrontier Corp. has a 37% equity stake in Holly Energy Partners, which, combined with its incentive distribution rights, or IDRs, gives it strong motivation to construct or acquire new midstream assets -- of which Holly Energy Partners has the right of first refusal to buy via dropdown acquisitions -- to help its MLP grow its distribution. 

One example of such a dropdown is HollyFrontier's Naphtha hydrogen fractionation unit, which the corporation will sell to Holly Energy Partners for $62 million and that is expected to generate $6.9 million in annual EBITDA -- this means the deal will pay for itself in nine years while providing yet another stable cash flow generator over time. 

As part of this dropdown, HollyFrontier Corp plans to sign a 15-year tolling contract with its MLP, which will contain a MVC that ensures the deal will be immediately accretive to Holly Energy Partners' investors.

This deal is just one portion of an eight-part $400 million-to-$450 million growth plan, which should see Holly Energy Partners' EBITDA grow 42% -- 12.4% CAGR -- between 2014 and 2017, and help management achieve its 8% distribution growth rate target over that time period.  

Bottom line: Wall Street is being shortsighted about Holly Energy Partners 
Holly Energy Partners' current yield shows the market's short-sightedness. With a bank vault-like secure distribution, generated by a superb business model that is immune from commodity prices, this MLP should not be trading at a 7.5% yield. 

Add in a probable 7% to 8% or so long-term distribution growth rate, and it should be clear to all income investors that Holly Energy Partners truly represents an exceptional dividend growth opportunity that is likely to continue to generate market-beating total returns for years to come.