Wind Power Low Voltage Ride Through

Wind power generation has taken its place in its share of electric power generation around the globe. Wind power generation normally utilizes induction generators for power production. In this case, these type of generators do not need synchronization when connecting or reconnecting to the electric grid or to any point of interconnection (POI).

When planning wind power integration, power system stability simulations are conducted to ensure system reliability with the wind power connection. In stability analysis, it is customary to analyze synchronization of generators’ rotor angles in the grid to evaluate system or generator stability. This practice is not applicable to wind power with induction generators based from the above discussion.

For power system stability, it is industry accepted to analyze low voltage ride through (LVRT) of a wind power generation rather than rotor angles. LVRT requires wind power plants to remain connected to the grid in the event of a worst short circuit which maybe nearest to the wind power plant. This is evaluated by observing the voltage at the POI of the wind power plant.

The figure below is a copy of Federal Energy Regulatory Commission (FERC) LVRT requirement. The plot shows the beginning of the voltage sag, the condition for the wind power plant to remain connected to the grid and the condition where the wind power plant is allowed not to remain connected to the grid. The voltage magnitude, plus the time duration, at the POI serves as the barometer if the wind power plant must stay connected or not.

Figure below is an illustration of LVRT requirements of different Transmission Operators and Transmission Owners relating to wind power plant interconnection. This plot is taken from this site. The figure tells us that different grids have different requirements for wind power LVRT. This maybe due to the system stiffness or inertia, system demand or wind power penetration. Moreover, some of the LVRT requirements were benchmarked from actual measured LVRT of wind power plants during system fault events.

Figure below is another interesting LVRT requirement in North America at the Western Electricity Coordinating Council (WECC). This plot is taken from this site. This illustration shows not only LVRT but also high voltage ride through (HVRT). Within the no-trip envelope, the wind power plant must stay connected to the grid but outside this condition the wind power plant can be disconnected from the grid.

The Energy Regulatory Commission in the Philippines has yet to develop voltage ride through requirement for wind power plants. There are existing and proposed or queued wind power generation located in the northern part of Luzon that maybe needed to comply with a developed LVRT. Moreover, LVRT policy can be applied not only to wind power generation but also to conventional power plants. Development of LVRT for wind power plants must be conducted for wind power generation to safeguard the stability and reliability of the power grid.

Conflict in Quality Regulation of Distribution Utilities

The Philippine Distribution Code (PDC) requires all User connection points that all Power Quality (PQ) requirements under PDC Section 3.2 be complied with. The PDC section 3.2.3.4 stresses this policy for voltage variations, see below. There are other similar statements for other PQ indices to be complied along section 3.2 of the PDC.

For new distribution system interconnection, the PDC points to the same section 3.2 that new connection points must have PQ within limitations specified in Section 3.2. In this case, the Distribution Utility (DU) must ensure that these PQ limits are complied with. See section 5.2 and further below.

The Energy Regulatory Commission (ERC) under its Work in Progress provides a seemingly conflicting objective with the PDC, as the Section 4 of the Guideline for Monitoring of PQ standards for DUs allows the DU to have 95% of connection points along a given distribution feeder to comply with section 3.2 of the PDC, see figure below. This means that 5% of the connection point can be accepted not to comply with the said PQ performance requirements. Practically, if the DU feeder has 100 connection points, 5 of those connection points can be operated not following the PQ limits specified in the PDC section 3.2.

The conflicting objective in these regulatory documents is clear. One document, the PDC, specifies that the DUs must comply with PQ limitations per section 3.2 in all connection points. On the other hand, the Guideline points that only 95% of those connection points must be ensured by the DU to have PQ level in accordance with the PDC section 3.2.

The ERC and DUs must adhere the PDC since this is the “code” of operating, financing and planning distribution systems. The Guideline must be reviewed as this is a Work in Progress. If the Guideline supersedes the PDC, then the PDC states that a PQ problem exists in the distribution system as section 3.2.1.2 below stresses this point.

Bring Down Power Rates by UP

As a Filipino living outside the Philippines, I have missed some research development like the study presentation by University of the Philippines (UP) professors on Reducing Power Rates in Generation. I have so much respect for the researchers since some of them have been my professors in my unfinished PhD degree in power systems.

Their study focuses on economic generation dispatch focus on MERALCO's power procurement. The study presents different cases or scenarios where MERALCO can procure from different power suppliers; NPC, IPPs, and WESM. In each case, the reduced power cost is reported and highlighted. This study is even utilized by one of our senators.

I agree on the principle of lowering power cost in anyway and anytime. Every effort on lowering power rates is my kind of thing.

But I am not sure if a DU which might have some purchase contracts with some IPPs will agree not to abide by those contracts. And note that these legal contracts go thru regulatory hearings.

I am not sure that the study focusing on rate economics provided a very firm ground on transmission and distribution capacity constraints of power procurement. DUs economic operation is coupled with technical constraints of the power system.

I am not sure the study related to uncertainty of demand and generation availability as they analyzed past data of the power rates. The variation of demand and generation reliability has much impact on power procurement.

I am not also sure that DUs participation in WESM is not beneficial for consumers. Most of the time, there are several hours of the day that the WESM spot price is zero! See example figure below. Thus, it can be beneficial to participate in the WESM.

Moreover, the senator said that the study is reliable since the researchers have developed a computer-simulation model. I am not sure if the senator is well versed on this. Power system engineers doing technical or economic studies have been using computer-simulation models for the past n years. The reliability or usability of a research study is not based on whether it was done using a computer-simulation model or by hand-manual computations but on the basis of its practicality.

There are a vast strategies to lower power procurement cost by DUs, in the end, legal contracts, power system constraints, uncertainty in demand and generation, and regulatory policies must be considered.

Technical Performance of an Electric Cooperative supplied with Wind Power

Wind power supply has further implications on system reliability because of its variability and can impact power quality in its nature of power production since it uses power electronics for power control that can produce harmonics and with its variability which can result to unacceptable voltage variations and voltage flickers. Ilocos Norte Electric Cooperative (INEC) source its power supply at specific times from the NorthWind Power Development Corporation.

INEC sources its power requirement to supply its franchise demand from National Power Corporation (NPC), NorthWind and Wholesale Electricity Spot Market (WESM). This is an economic operations strategy by INEC. INEC is fed by all these power suppliers from the grid via bulk power transformers to its distribution franchise.

National Electrification Administration (NEA) reports Electric Cooperatives (EC) scorecard for the year 2006. From this report, INEC performs very well technically as shown in the figure below. Aside from reliability and project implementation, INEC performed up to the expected par with all the other technical operations performance requirements. The uncoupling of power supply, voltage transformation thru transformers which go together with INEC’s own technical intervention within their distribution franchise, from the wind power to the INEC distribution system aids in delivering power with such a high quality of supply and technical performance. Overall, the analysis shows that an electric distribution system fed by a wind power plant can be operated up to the level of technical criteria required by the NEA or by Energy Regulatory Commission (ERC).

Simple Method for Selecting Overhead Distribution Line Conductor

Distribution Utilities (DUs) when planning for their distribution system, are faced with various options and constraints. One of the DUs’ process in system planning is choosing the right size of overhead distribution conductor for a given load, voltage level and expected load growth just to name a few. Most of the time, DUs have a set of standard sizes of conductors in the stock shop ready to be utilized for meeting additional demand or has a set of policies what size of conductor to be used for level of voltage or type of load. These practices do not follow that given the new conductor installation, DUs comply with various performance regulation.

In the performance regulation of DUs, it is important to meet the following:

• Supply the voltage within allowable limits - normally this is between 0.90 to 1.10 per-unit voltage.
• Minimize losses - to select a conductor is to ensure that as the power is being delivered, lesser line losses are produced.
• Minimize cost of conductor - economic regulation addressed in planning that DUs as much as possible to lower cost and at the same time operate within performance criteria.

In addition, the DUs must maximize power delivered, i.e. - the DU sees to it that the demand will be supplied with the power it needs given any time. Maximizing power transfer enables maximum utilization of the asset.

In this blog, consider a load of 200 amperes at 13.8kV level and selection between conductors ACSR sizes of 1/0 to 4/0 is to be determined for a line length of 1000 meters. The cost of conductors are derived from National Electrification Administration (NEA) website. The variations of voltage versus losses, cost of conductors versus losses and power delivered versus losses are plotted below. The conflicting patterns of voltage, cost of conductors and power delivered versus line losses are evident from the figures. The size of the conductor to be selected must be near the intersection point which minimizes the parameters involved. In this example the appropriate conductor size, the nearest conductor size to the intersection of the curves must be chosen, is ACSR 3/0.

Benefits of System Loss Reduction

Aside from impacting or not impacting electricity rates, system loss reduction has the following benefits:

• Reduction of fuel emissions due to lesser use of fossil-fuel generating plants - this has societal impact as it cover environmental concerns.
• Utility system capacity savings - decrease in losses provides released extra capacity for the distribution lines and transformers.
• Promotion of Energy Efficiency - it will be noted that the Distribution Utility (DU) is an energy-efficient electric company as it tries to decrease its system loss.
• Improvement of system voltage profile - the utility is regulated to supplying a range of voltage level and reduction of losses will produce a marginal system voltage quality that may be acceptable. This will also provide good power quality at the convenience outlets of consumers allowing their electric equipment/appliances to operate without mis-operation or loss of life.
• Increase Utility Commercial Appeal - a DU aiming at system loss reduction gets an added commercial appeal in the restructured power industry. This is important in the changing environment of the power industry, have you seen MERALCO TV commercials?

There is an optimum level of system loss reduction unique for all Distribution Utilities (DUs) where further reduction of losses will not result to further reduction of operation cost but more investment expenses. On the other hand, as demand rises in a distribution service franchise, technical losses and non-technical losses (utility own use) will at some point come with the demand increase. The equation of generation equals losses plus demand holds true. This is revealed by Philippine Power Statistics as shown in the figures below (the values include transmission and distribution losses, but this does not negate the point) :

Finding ways to reducing system loss must be a continuing program since it always provide benefits to the DUs and electric consumers.

Getting Lost with System Loss

The Energy Regulatory Commission (ERC) is calling for comments from various stakeholders on a new resolution they are proposing on reduction of distribution system losses incurred by Distribution utilities (DUs) which are Private Utilities (PUs) and Electric Cooperatives (ECs) in their operation of their service areas. Figure below is an extraction from the ERC document on the resolution.

From the illustration, the regulated system loss caps have been long withstanding for about eight years. The 9.5% and 14% loss caps for PUs and ECs respectively, are about to celebrate their ninth birthday until this new resolution on system loss caps reduction which is aimed to be implemented in January 2009 billing. The proposed 8% cap on the PUs might be tight since presently MERALCO operates its vast distribution system following a 9.5% loss policy. There are other PUs but benchmarking with MERALCO is a sure test if the other PUs can comply with the proposed 8% cap. The proposed 11% system loss cap on ECs is a product of National Electrification Administration (NEA) media declaration on system loss saying that by 2009 the national average for system loss for ECs will be 11%. The 2006 NEA scorecard does not report this 11% but points that about 53 out of a total of 84 ( I thought there are total of 98 ECs) ECs comply perfectly with system loss reduction.

Reduction of system loss caps always provide good benefits for both consumers and the electric distributor, there is no doubt on this and the currently applied caps should have been in Grade 3 in the present school year. But giving pressure to the DUs to comply within five months can be challenging. Given the facts for both PUs and ECs, the proposed resolution will be an expected regulatory debate in the ERC hearings. Aside from decreasing system loss, DUs look into improving system reliability and quality of supply in their planning and operations which I believe are of equal weight with system loss. If this resolution is implemented, DUs might get lost with reducing their system loss.