The Concept Of Residual Power
How did the question of "residual power "arise? It is essential to trace back to the drafting of the Basic Law, it was requested by some representatives of HKSAR that apart from the powers which have been explicitly stipulated and divided between the Central Authority and the HKSAR, there is "residual power" that should be enjoyed by Hong Kong.  Although the requirement was not adopted, after the implementation of the Basic Law, the question of the "residual power" aroused disputes again.
Some scholars in mainland argued that since the power of the Central Government is "original power", then what is the kind of powers included in "original power", which have not been authorized to HKSAR? 
The concept of "residual power" is derived from the federal system, and there's no specific term can be used to describe the powers that have not been delegated to Hong Kong included in the "original power" of Central Government.
Therefore, to discuss the controversy, it is essential to analyse the concept and doctrine of "residual power" as well as the division of powers between the Central Government and Hong Kong SAR, namely, the sovereignty and autonomy.
When the member states desire to form a new federal state, for the common benefit and goal, they usually give part of their power to the federal state, then they explicitly list the powers of the member states and the powers of the federation through a constitution, which is the legal ground to be based on when disputes occur. Then the powers which have not been listed are regarded as the "residual power". 
Take the United States as an example, the powers delegated to the federal states are explicitly stipulated, and the powers which have not been listed in the constitution are reserved to the member states.
However, in a single-system country, things are quiet different. According to professor Thong zhiwei, the fundamental difference between a federal state and a single-system state is such a standard, "that is to see whether the sovereignty is only enjoyed by the central government or shared with other local governments. In a single-system state, the sovereignty is concentrated in the central government while in a federation, the sovereignty is shared by the federation and the member states." 
In a single-system country, the powers enjoyed by the local governments are delegated by the central government. The sovereignty is concentrated in the central government. Moreover, the powers authorized to the local governments are explicitly listed. Powers which have not been listed yet are still in the hand of the central government, which can not be enjoyed by the local administrative areas. 
As to the state structure of China, some scholars argue that it is a single-system state with some "federal characteristics", while some other scholars can not agree with that.  Despite of many different views on the issue , one thing can be for sure is that the main structure of China is the single system.
According to what has been mentioned above, we can not simply use the term "residual power" to describe the powers included in the "original power" of the Central Government which have not been authorized to HKSAR.
Then what specific term can be used to explain that part of powers? It was defined by Zhang dinghuai and Mengdong as "retainable original power." 
The Central Government exercises complete sovereignty over the Hong Kong SAR, which is "entriely the same as the general relationship between the Central Authority and local authorities."  As was stipulated in the Joint Declaration, the government of United Kingdom will restore Hong Kong to PRC and the PRC government decided to resume the exercise of the sovereignty over Hong Kong.  The hand over only solves "the question of to whom the territory and sovereignty of Hong Kong belong."  The Basic Law is brought into effect to ensure the exercise of the sovereignty over the Hong Kong SAR and to make the principle of sovereignty more specific and universally binding.
As has been discussed before, the structure of PRC is single system. The sovereignty is in the hand of all the people and exercised by the highest organ of state power representing all the people. All the local governments, including Hong Kong SAR have no right to exercise the powers which belong to the sovereignty, such as foreign affairs and national defence, which are the main symbols of the national sovereignty.
Powers enjoyed by the Central Government & Restrictions on the Autonomy of HKSAR
According to Article 13(1) of the Basic Law, the CPG is responsible for the foreign affairs in relation to the Hong Kong SAR.  It was explicitly stipulated that the CPG enjoys the power to handle all the foreign affairs, for instance, "conducting diplomatic talks with foreign governments, concluding treaties or joining international organizations in the name of the State" 
Apart from handling all the foreign affairs, for the prosperity of HKSAR and considering the conditions of the Region, the Central Government also delegated powers to the Hong Kong SAR to "deal with external affairs in the economic and cultural fields."  , as is reflected in the Article 13(3)of the Basic Law.
Concerning the national defence, it is stipulated in the Article 14(1) of the Basic Law that the CPG is in charge of the defence of the HKSAR.  Defence encompasses external defence and internal defence, the defence here "only refers to external defence, not including public order within the HKSAR." as reflected in Article 14(2), the government of HKSAR in charge of the maintenance of public order in the Hong Kong SAR.
According to Article 2 of the Basic Law, the HKSAR is authorized by NPC to "exercise a high degree of autonomy...",  it seems that the Basic Law has cut out the sentence "except foreign and defence affairs which are the responsibilities of the CPG" in the Joint Declaration.
It was argued that the most significant reason to leave out the former sentence in the Joint Declaration is probably to eliminate the possible concern on the so-called "residual power".Once the former sentence has been cut out, the autonomy enjoyed by the HKSAR are clearly those powers explicitly stipulated in the Basic Law and no other powers. 
The autonomy of Hong Kong SAR is under the supervision of Beijing. To avoid Hong Kong SAR exceeding the scope of autonomy, it was stipulated in Article 17 of the Basic Law, if the NPCSC "considers that any law enacted by the legislature of the Regionis not in conformity with the provisions...regarding affairs within the responsibility of the Central Authoritie..., may return the law in question...any law returned...shall immediately be invalidated." 
Moreover, to restrict the the judicial power of Hong Kong SAR, it was stated in Article 19,"The courts of the Region shall obtain a certificate from the Chief Executive on questions of fact concerning acts of state such as defence and foreign affairs whenever such questions arise in the adjudication of cases." 
Concerning the power of interpretation of the Basic Law, restrictions have been set down in Article 158, it is stated that if the courts of Hong Kong SAR need to interpret the provisions "concerning affairs which are the responsibility of the CPG or concerning the relationship between CPG and the Region", shall seek an interpretation from NPCSC 
Furthermore,the Central Government holds the power to appoint the Chief Executive and the principal officials in Hong Kong SAR. The question of whether the appointment is "substantial" or "in name only" once arises and caused controversy. If regard the appointment as "in name only", then the appointment by the Central Government is only a form and there's no substantial effect. If the appointment is substantial, that is to say, the Central Government has the right to refuse to appoint the CE elected by the Hong Kong SAR.  Some scholars argue that for the high degree of autonomy the appointment should be in name only, while others hold the view that the appointment must be substantial to stick to the principle of sovereignty.  According to Xu zijian, the appointment is substantial in accordance with the provisions shown in the Basic law. The most essential thing is that, "the application of the power to appoint the CE and the principal officials does not interfere directly with the executive power of Hong Kong SAR, but through the appointment , the Central Government restrict the executive power of Hong Kong SAR in an indirect way." 
Autonomous Powers Enjoyed by HKSAR
The scope of the autonomy enjoyed by the Hong Kong SAR is explicitly stipulated in Article 2 of the Basic Law. The Region is granted by the NPC to exercise the executive power, legislative power, judicial power and final adjudication. 
It was stipulated in Article 17 of the Basic Law that the Hong Kong SAR enjoys the legislative power.  As the policy "one country,two systems" is applied in Hong Kong, it may establish its own legal system and laws, and "generally the national laws are not to be applied there.  Thus, the legislative power granted to it is the "real power" and it is greater than the power delegated to the autonomous regions to enact regulations. 
It is provided by the Article 16 of the Basic Law that, Hong Kong enjoys executive power and manage the administrative affairs on its own. The provision here is a general statement but not a specific version because it is difficult to list the administrative affairs completely and accurately.  The administrative affairs which fall within the responsibility of the Hong Kong SAR are covered specifically in Chapter V and Chapter VI of the Basic Law.
Independent Judicial Power , Including That of Final Adjudication
According to Article 19(1) of the Basic Law, it is stated that Hong Kong is authorized to have independent judicial power, and that of final adjudication.  The independent judicial power is prescribed in Article 85 of the Basic Law as "free from any interference." 
In any state of the world, the power of final adjudication is enjoyed by the supreme court of the country, probably the court of a local region, as HKSAR , was authorized such kind of power is the "only one of its kind".  As Hong Kong SAR is delegated such kind of power ,its judicial power will also be improved greatly. 
The Power to Handle External Affairs Independently
The external Affairs are covered specifically and clearly in the Chapter VII of the Basic Law. For instance, the representatives of the Government of the HKSAR may participate in negotiations which directly affect the region as the members of the delegations of the CPG.  By using the name of "Hong Kong,China" the Hong Kong SAR may maintain and develop relations with foreign states or regions in some particular fields. 
Understanding Article 20
What is worth mentioning, it is stipulated in the Article 20 of the Basic Law that the Hong Kong SAR may exercise other powers which are delegated by the NPC, the NPCSC, or the CPG.  How to understand the "other powers" ?
Firstly, it has been specifically explained in the general principle of the Basic Law that the powers owned by the Hong Kong SAR are all authorized by the Central Government, the one who enjoys the original powers is without doubt, the Central Government. The purpose to grant powers to the Hong Kong SAR is all for its high degree of autonomy and to realize that the Region is administered efficiently. And obviously, the concept of "residual power" is not suitable to be applied here.
Moreover, obviously, It is the "reasonable concession"of the CPG that realize the enjoyment of the autonomy of the Region. The provision is not to ensure the power of the Region to receive the other powers authorized by the CPG. 
Furthermore, according to the background of the drafting of the Basic Law and the position placed of the Article 20 in the Basic Law, it is "a promise to make the people of Hong Kong feel eased."  That is to say, for the need to administer the Region more efficiently , it is possible for the CPG to further grant powers to Hong Kong.
According to Professor Xu chongde, the limitation of authorizing power to Hong Kong SAR is that "how much power is granted, then how much the Region enjoys." 
After the hand over, the Hong Kong SAR once had administered power which has not been delegated to it yet. For instance, to license the land for building the Disneyland ,the time limit extended to 100-year, which exceeded the 50 year stipulated in the Basic Law. It indeed had exercised exceeding power, but as seen from the attitude of the Central Government, no dissent was shown toward the building of the Disneyland. Although it may be consistent with the prosperity of Hong Kong, however, it may be better for the Hong Kong to seek for authorization from CPG before excercising such power. 
Conclusion and Prospect
The doctrine of "residual power", which runs counter to the principles and core value of the Basic Law, is inapplicable in the Hong Kong SAR. To reflect the features of the original power in a single-system country, the concept of "retainable original power" put forward by Zhangdinghuai and Mengdong may be more suitable to describe those powers which are in the hand of the Central Government and not granted to Hong Kong. 
During the implementation of the Basic Law and the theory of "One country, two system", it is inevitable that continuing controversy occurred about the division of powers between Central Government and Hong Kong as well as the balance between sovereignty and autonomy.
In the future, to solve the disputes, it is significant to enhance the communication between the CPG and HKSAR, to form a tradition and probably to amend the relevant provisions, in accordance with the current social background and the actual circumstances.