Generator Assessment of Hydro Power Station Adequacy after Reconstruction from Classical to HIS SF6 Substation

Received Dec 9, 2016 Revised Mar 12, 2017 Accepted Mar 26, 2017 Reliability analysis of substations and generator assessment of power plant stations are very important elements in a design and maintenance process. This paper presents a generator adequacy assessment of a classical “H” scheme for an open conventional substation, which is often used, and a new HIS High Integrated Switchgear with SF6 gas isolation. Generator adequacy indices of both types of classical and HIS switchgear were compared and the results showed a high level of reliability and availability of the HIS presented substation. The input data were the annual reports of Croatian TSO-Transmission System Operator (HOPS) and statistics of operation events of Croatian National Electricity (HEP Inc.). For the HIS substation, the input reliability data were used from relevant international literature since only few of HIS substations are installed in Croatia. The generator is modelled with a three-state Markov state space model and Monte Carlo simulations were used for the generator assessment analysis. Adequacy indices LOLP and EDNS were obtained using DIgSILENT software. Keyword:


INTRODUCTION
During the analysis of the composite transmission system network reliability, it is generally not enough to view the switchgears solely as network junction points, and thereby omit the influence of individual switchgear components on the reliability of the system as a whole. Every switchgear configuration has a significant impact on the occurrence of power line outages and consequently on the entire system reliability [1], [2]. This especially concerns the multiple power line failures which considerably depend on switchgear failures [3], [4]. The paper analyzes switchgear reliability using Mont Carlo, the loss of one consumer and generator adequacy assessment using Monte Carlo simulation [5]. The basic definitions related to modelling failures from a switchgear are as follows: a. Switchgear components: transformers, circuit breakers, disconnectors and busbars; surge arresters, current and load measuring transformers are usually omitted. The protection system is generally not taken into the account; b. Switchgear availability: the probability of presence of a corresponding electrical connection between switchgear busbars from a HV to LV side; c. Passive failure: unavailability is reduced to the faulty component (it does not cause a triping of protection). It should be noted that the definitions of active and passive failures vary from an author to author; f. Maintenance: a planned activity aiming at improving the condition of a component. It can be postponed if required by switchgear conditions. Generator adequacy assessment is a part of dependability evaluation (reliability, availability, maintainability, and safety) [6]. The analysis is performed for the events of the first level of coincidence which is a forced failure or switching-off of one component. The paper deals with the generator adequacy assessment connected on a substation using a computer software. The load in a substation is modelled with a load duration curve which is then modelled with different load states considered in the generator assessment analysis using Monte Carlo simulation [7][8]. The paper includes the generator assessment analysis using a sequential Monte Carlo simulation module in DIgSILENT Power factory software [9].

MARKOV STATE SPACE MODEL FOR BUSBARS AND TRANSFORMERS WITHIN THE SWITCHGEAR
Busbars and transformers, as renewable components, with regard to reliability of supply, can have two different statesthey are either available or unavailable. A two-states component model is the most oftenly used model since it gives the best description of the continuous operation of a component. It is presented in Figure 1.
Frequency of being in a state 1 and 2 is defined as follows:

MODEL OF A SYSTEM WITH TWO COMPONENTS
The model presented in Figure 2 shows the state of two different components -each component can be either ready to operate or not ready to operate (a coincidence of a transformer and busbar failure). Since it had been shown earlier that the expressions: present stationary availability, that is, unavailability of one component, the stationary probabilities of the states in the case of two-components system can be expressed as follows: According to the model of a system with two different components, the frequency of an individual state can be determined either as a product of the state probability and the sum of intensity of abandoning that identical state, or as a product of the sum of intensity of entering the state and the probability of a state that is being abandoned. Frequencies are equal regardless of being observed from a perspective of exits or from a perspective of entrances. According to that, frequencies of states are as follows:

MODEL OF COMPONENT WITH MAINTENANCE
Switchgear components count as renewable components and their maintenance (planned repair) is carried out periodically. Maintenance of switchgear components increases their reliability and availability because the tendency of growth of failure intensity function is being reduced and maintained at a sufficiently low constant value. Figure 3 shows the Markov state space model of a component with maintenance. It is presumed that the planned repair of a component will not be performed when a component is not functional, and upon finishing the repair, a component is again ready for the operation. Stationary system solutions, i.e. stationary probabilities of the states are: Based on the earlier considerations, it follows that state "1"denotes component availability, state "2" denotes a failure-induced component unavailability, and state "3" component unavailability due to repair: Frequencies of failure and repair states are:

FAILURE COINCIDENCE MODEL WITH PLANNED MAINTENANCE
Preventive maintenance is conducted in order to keep the frequency of component failures at the lowest possible level. However, when these coincide with failures of other components in the system, the number of system failures may be increased. Thus, if possible, preventive maintenance and repairs should be conducted when they would not have negative effects on the system. It is generally considered that once the repair has been initiated, it has to be finished. Figure 4 shows a case of failure coincidence with the planned maintenance. If the possibility of a transition from state "3" into state "4" is removed, then the request that the repair cannot be initiated in state "3" is accepted, i.e. during the failure state of the other component. However, if state "4" does not represent the system failure state, that transition is allowed and the failure and repair processes are independent. The probabilities of being in certain states in case the transition from state IJECE Vol. 7, No. 2, April 2017 : 729-740 733 "3" into state "4" is allowed, since state "4" does not represent the system failure state are as follows: However, if state "4" also means system failure, the transition from state "3" into state "4" is not allowed, which means that the frequency of repair of the first component in the third and fourth system equation has a zero value ( ). In that case, the values are:  Since the most common frequencies of the component repair and maintenance are significantly higher than the respective frequencies of entering into those states, i.e. the multiple products of very small values can be disregarded, the near solution values (10) are: (11) The frequency of the system failure, (failure and repair coincidence state) which also means the failure of the system, is: The mean time of failure coincidence with the planned maintenance and repair is: (13) Figure 5 shows the model of the component with the derated state (reduced capacity). It is assumed that the transition of the component to the derated state from the forced outage is not allowed because it is believed that during its stay in the forced outage, the repair is performed and that component is ready for the operation with the rated capacity.

GENERATOR MODEL WITH A DERATED STATE
The mean time of generators states:

DESCRIPTION OF A HYDRO POWER PLANT SUBSTATION 7.1. Basic Characteristics of the Previous Switchgear
It is placed at the level of 183,5 m in the narrow area between the engine room facility and building entrance at the end of diversion channel [12]. The switchgear is in "H" scheme with five feeders, two block transformer feeders, each equipped with a circuit breaker, disconnectors, current measuring transformers and surge arresters, two outgoing feeders equipped with outgoing disconnectors only, section feeder with a diagonally placed disconnector for the connection between blocks, i.e. transmission lines. It is dimensioned for short circuit power of 3500 MVA. The switchgear is connected to the Nedeljanec substation by 110 kV double transmission line (Al/Fe 240/40 mm 2 ). Considering that outgoing feeders do not have circuit breakers installed, distance protection in a hydro power plant works at the switching off of the circuit breaker in a block transformer feeder and the circuit breaker in the outgoing feeder of Nedeljanec substation which in this way represents distant busbars of this switchgear. Switchgear equipment is nearly 30 years old, except for the new SF 6 circuit breakers produced by "ABB" and installed four years ago in a place of the old pneumatic circuit breakers 3P 123 type, produced by "Končar d.d". The whole equipment is installed on the iron stands made from steel pipes and profile, coated with anticorrosive paint, fixed to a concrete base with anchor bolts. The connection of the devices is done by wires (Al/Fe 240/40 mm 2 ), i.e. AlMg  70 mm pipes. A busbar system is carried out by wires attached above the devices by using double tension insulator strings with seven glass insulators, type U 160 BS, on each of them. Busbars are attached to "T" portals made from welded iron profiles. The classical 110 kV switchgear is designed for the outdoor use and shown in Figure 6.

Description of the new SF6 High Integrated Switchgear
The new metal enclosed SF 6 gas insulated switchgear, high integrated for the outdoor installation (HIS) at the new location will be installed on the concrete laminated foundation. The switchgear will be constructed for the outdoor installation along with the appropriate treatment of the external surfaces, which enables the installation in open spaces exposed to the atmospheric conditions and solar radiation. The switchgear constructed in the "H" scheme form will consist of three-poled enclosed busbars, two busbar measuring feeders, one section/coupling feeder, two transformer feeders and two outgoing feeders. The switchgear will be connected to the network and transformers by the overhead connectors Al 300 mm 2 . A block transformer is connected to the current measuring transformers by means of Cu pipes  32 × 2 mm. The designed switchgear satisfies the following minimum demands -modular performance of manufacture completed mounting groups with the possibility of subsequent extension by means of upgrade by additional fields, sections, busbars, circuit breakers, disconnectors and other components without unnecessary dismounting of the equipment's main parts. The switchgear should secure the highest possible security for the workers and others around the switchgear surrounding area under normal operating conditions and during failures (short circuit). In accordance with the conditions at the installation location, the switchgear will be sized for the insulation degree 123 Si 230/550 and will have the following rated values and technical characteristics shown in Table 1. Figure 9 presents the 110 kV SIEMENS SF6 switchgear.

RESULTS OF THE GENERATOR ADEQUACY SIMULATION
The simulation was performed in the program package of DIgSILENT with the power flow calculation [12] for the period of 1 year. The load on the 110 kV side of the power transmission line Nedeljanec is displayed by the annual curve of duration presented in Figure 10. Also, 110 kV busbar outages were considered within the load Nedeljanec. The input data for HIS was used from [12], and for the existing switchgear from the statistics of the plant events HOPS from [13]. The input data for generators in the hydro power plant are given in Table 2. After the generator assessment analysis is performed using Monte Carlo simulation for 100 000 runs, the generator adequacy indices are shown in Figure 11. Figure 11. Total available capacity, available dispatch capacity, grid total demand and total reserve generation  )  2010  5535  2418  793  14  8760  2011  5884  2024  850  2  8760  2012  5834  1980  946  0  8760  2013  6490  1481  651  138  8760  Average  6030,8   The difference in adequacy indices of the new SF6 gas insulated High Integrated Switchgear in relation to the standard existing switchgear for the outdoor usage is obvious. This can only be one of the advantages and additional criteria for choosing the HIS and justifying the reconstruction.

CONCLUSION
This paper presents the generator adequacy assessment connected to 110 kV/10 kV switchgear in the hydro power plant, particularly of the present switchgear for the outdoor use, as well as the new SF6 HIS-High Integrated Switchgear. Markov space-state method was used, together with the generator assessment Monte Carlo simulation using DIgSILENT software. Monte Carlo generator assessment is also performed in consideration to the generator three states model (in work, derated state and in outage). All outage data are taken from the statistics of the plant events for the four years period of time, form 2009 to 2013 year, from HEP d.d.. The information on the equipment and switchgear elements outages has been statisticaly processed. For the new switchgear in the "H" scheme, which was conducted in the HIS SF6 technology, outage data are taken from the relevant literature statistics. All relevant adequacy generator assessment indices of the switchgear have been computed. The generator adequacy indices of the HIS are far better than the standard switchgear for the outdoor use, which of course, aside from other advantages, can be crucial in transition to the High Integrated Switchgears.